UNIVERSITY  OF  CALIFORNIA] 
DAVIS 


THE 

NATURAL  HISTORY  OF  PLANTS 


AUTHOR'S  PREFACE 

TO    THE    ENGLISH    EDITION. 


Not  long  ago  two  artisans,  who  had  borrowed  a  copy  of  THE  NATURAL 
HISTORY  OF  PLANTS  from  one  of  the  Vienna  public  libraries  and  had 
studied  its  pages,  called  upon  me,  asking  me  to  show  them  under  the 
microscope  some  of  the  things  there  described. 

It  seems  that  without  any  special  educational  advantages  they  had 
availed  themselves  of  leisure  moments  to  extend  their  knowledge,  and 
had  read  the  work  with  profit.  On  leaving,  they  thanked  me  in  simple 
words  for  the  pleasure,  instruction,  and  stimulus  which  they  had  derived 
from  the  perusal  of  my  book. 

I  confess  that  these  words  gave  me  vastly  more  pleasure  than  many 
of  the  verbose  and  flattering  reviews  that  had  appeared  in  newspapers 
and  scientific  journals,  many  of  which  conveyed  the  impression  of  being 
the  result  of  hasty  skimming  of  copies  sent  by  the  publishers. 

The  satisfaction  which  the  little  incident  gave  me  was  the  greater, 
in  that  it  was  an  assurance  that  I  had  achieved  what  had  been  my 
intention,  namely,  to  write  a  book  which  might  serve  as  a  source  of 
knowledge,  not  only  for  specialists  and  scholars,  but  also  for  the  many 
who,  though  compelled  to  follow  some  practical  calling,  still  take  an 
interest  in  science,  and  who  wish,  each  in  his  own  particular  degree, 
to  obtain  information  of  its  progress. 

Popular  treatises  on  the  results  of  scientific  investigation  are  by  no 
means  rare  with  us  Germans;  but  in  too  many  cases  scientific  problems 
involving  serious  thought  are  touched  superficially,  and,  like  the  stone 
in  a  sweet  fruit,  are  embedded  in  picturesque  and  attractive  accounts 
of  things  purely  of  subordinate  importance.  The  reader,  gratified  by 
the  elegant  phraseology,  passes  by  the  kernel  of  fact,  and  derives  little 
profit  from  the  book.  Books  such  as  these  have  brought  the  art  of 
popular  writing  into  discredit,  and  we  have  arrived  at  the  point  when 
educated  people  but  lightly  esteem,  or  even  ignore,  the  results  of  careful 


vii 


viii  AUTHOR'S  PREFACE. 

and  laborious  investigations  and  the  theories  based  upon  them,  if  they 
are  produced  in  a  popular  manner  rather  than  in  the  conventional 
language  of  science. 

With  the  English,  however,  it  is  otherwise.  I  have  long  regarded 
with  admiration  the  men  of  science  whom  you  number  amongst  your 
countrymen,  who  present  the  results  of  their  studies  in  words  intelligible 
to  all  who  seriously  desire  knowledge. 

To  follow  in  the  path  of  such  men  has  always  been  my  aim  in  my 
work  and  in  my  writings;  and  this  was  particularly  before  me  in  the 
production  of  THE  NATURAL  HISTORY  OF  PLANTS. 

A.  KEENER   VON   MARILAUN. 

VIENNA,  December,  1893. 


EDITOR'S  PREFATORY  NOTES 

TO   THE   FIEST  EDITION. 


VOLUME  I. 

PROFESSOR  KERNER  has  stated  very  succinctly,  in  the  preface  which  he  has  been 
good  enough  to  write  for  the  English  edition  of  Pflanzenleben,  the  main  idea  which 
guided  him  in  the  writing  of  that  book.  Consequently  little  remains  for  me  to  add 
save  a  few  observations  on  the  book  in  its  present  form.  On  the  appearance  of  the 
original,  the  parts  as  they  were  issued  were  widely  scanned,  and  the  work  soon 
enjoyed  a  large  circulation.  Here  was  a  book  at  once  attractive  to  the  ordinary 
reader,  and  retaining  unimpaired  its  value  to  trained  naturalists.  The  scale  of  the 
undertaking  was  such  that  it  was  possible  to  give  a  presentment  worthy  of  the 
subject.  Hitherto,  though  Astronomy,  Geology,  and  other  branches  of  natural 
knowledge  had  been  long  accessible  to  the  ordinary  reader  in  popular  books  of 
the  greatest  value,  this  service  had  not  been  done  for  Botany.  Long  before  the 
issue  of  Pflanzenleben  was  complete,  the  idea  of  an  English  edition  suggested 
itself  to  me  and  to  my  friend,  Mr.  Walter  Gardiner,  of  Cambridge.  It  was  my 
hope  that  we  should,  jointly,  undertake  its  preparation.  To  my  great  regret, 
Mr.  Gardiner  was  prevented  from  co-operating  by  other  duties;  thus  the  whole 
responsibility  of  this  edition  falls  to  my  lot.  To  my  colleagues  in  this  undertaking^ 
Mrs.  Busk  (Lady  Busk)  and  Miss  Ewart  (Mrs.  M.  F.  Macdonald),  the  chief  credit 
is  due  for  this  translation.  Indeed,  without  their  hearty  collaboration,  the  produc- 
tion of  The  Natural  History  of  Plants  would  have  been  impossible.  In  the 
main,  the  original  text  has  been  faithfully  adhered  to.  The  translation,  though 
not  everywhere  precisely  literal,  never  departs  from  the  spirit  of  the  German 
edition.  The  Index  to  the  complete  work,  together  with  a  Glossary,  will  be 
appended  to  the  concluding  volume. 

F.  W.  0. 

KEW,  November,  1894. 

VOLUME  II. 

With  this,  the  second  and  concluding  volume  of  The  Natural  History  of  Plants, 
a  brief  statement  and  explanation  of  my  position  as  editor  is  imperative.  As  stated 
in  my  note  to  Volume  I.  the  English  text  there  followed  that  of  the  original  with 
considerable  fidelity.  In  the  second  volume  I  have  less  consistently  followed  this 
course.  Throughout  I  have  not  hesitated  to  add  or  substitute  new  matter,  though 


ix 


x  EDITOR'S  PREFATORY  NOTE. 

no  overt  indication  of  such  departure  from  the  original  is  given  either  by  different 
type  or  otherwise.  It  is  needless  to  explain  that  these  changes  are  only  such  as 
the  advance  of  botanical  knowledge  has  rendered  necessary  since  the  original  was 
written,  and  that  I  have  never  desired  to  depart  from  the  intention  of  the  author. 
To  the  specialist  these  modifications  will  be  from  time  to  time  apparent;  the 
general  reader  will  perhaps  treat  me  with  indulgence  should  he  think  that  in  this 
matter  my  judgment  has  been  at  fault.  Though  changes  occur  throughout  the 
volume,  I  have  preserved  intact  the  main  conclusions  of  the  author  and  the  facts 
upon  which  they  are  based.  To  have  altered  these  in  any  way,  even  had  I  been  so 
minded,  would  have  been  inconsistent  with  the  duties  of  an  editor  and  translator. 
But  in  the  purely  systematic  portion  of  the  work  I  have  been  restrained  by  no 
such  scruples.  Professor  Kerner  himself  regarded  that  portion  of  his  work  as  but 
tentative,  and  as  it  was  difficult  to  merely  modify,  the  whole  of  this  portion  has 
been  written  de  novo,  from  the  Thallophytes  to  the  end  of  the  Gymnosperms 
(pp.  616-728),  and  in  part  the  Monocotyledons.  The  exigencies  of  the  serial  issue 
of  The  Natural  History  of  Plants  alone  has  prevented  the  re-cast  of  the  Di- 
cotyledons, which  stand  with  little  modification  as  in  the  original.  For  the  portion 
dealing  with  the  class  Gamophycece  up  to  the  end  of  the  Conjugates  (pp.  627-659), 
I  am  indebted  to  my  colleague,  Mr.  A.  G.  Tansley  of  University  College,  who  has 
devoted  considerable  attention  to  the  group  in  question.  To  him  I  now  offer  my 
hearty  thanks.  The  glossary  of  botanical  terms  makes  claim  neither  to  complete- 
ness nor  originality.  Though  a  large  number  of  the  definitions  and  explanations 
have  been  written  specially  for  this  book,  I  have  never  hesitated  to  lay  published 
sources  under  contribution.  The  laborious  task  of  constructing  the  index  has 
fallen  to  Mr.  George  Brebner,  and  to  him  is  due  the  gratitude  of  such  as  gain 
through  it  direct  and  ready  access  to  the  body  of  the  work. 

F.  W.  0. 

KEW,  August,  1895. 


CONTENTS  OF  VOLUME  FIRST. 


INTRODUCTION. 


THE  STUDY  OF  PLANTS  IN  ANCIENT 
AND  IN  MODERN  TIMES. 

Plants  considered  from  the  point  of  view 

of  Utility,    - 
The  Description  and  Classification  of  Plants, 


Page 


Page 
Doctrine  of  Metamorphosis  and  Speculations 

of  Nature-Philosophy,  7 

Scientific  Method  based  on  the  History  of 

Development, 13 

Objects  of  Botanical  Eesearch  at  the  present 

day, 15 


THE  LIVING  PRINCIPLE  IN  PLANTS. 


1.  PROTOPLASTS  CONSIDERED  AS  THE  SEAT 
OF  LIFE. 

Discovery  of  the  Cell:  Researches  of  Swam- 

merdam,  Leeuwenhoek,  and  Unerer,      -     21 
Discovery  of  Protoplasm,     -  -25 

2.  MOVEMENTS  OF  PROTOPLASTS. 

Swimming  and  Creeping  Protoplasts,  -         -     28 
Movements  of  Protoplasm  in  Cell-cavities,  -     32 
Movements  of    Simple   Organisms — Volvo- 
cineae,     Diatomacese,     Oscillariee,     and 
Bacteria,       -         -         -         -  -     37 


3.  SECRETIONS  AND  CONSTRUCTIVE  ACTIVITY 
OF  PROTOPLASTS. 

Cell-sap:  Cell-nucleus:  Chlorophyll-bodies: 

Starch:  Crystals,  -  -  -  -  41 

Construction  of  the  Cell- wall  and  establish- 
ment of  Connections  between  neigh- 
bouring Cell-cavities,  -  -  -  -  42 

4.  COMMUNICATION  OF  PROTOPLASTS  WITH  ONE 

ANOTHER   AND    WITH    THE    OUTER    WORLD. 

The  Transmission  of  Stimuli  and  the  Specific 

Constitution  of  Protoplasm,  -         -         -     47 
Vital  Force,  Instinct,  and  Sensation,  -         -     51 


ABSORPTION   OF  NUTRIMENT. 


1.  INTRODUCTION. 

Classification  of  Plants,  with  reference  to 

Nutrition,    -                                             -  55 

Theory  of  Food- Absorption,         -  57 

2.  ABSORPTION  OF  INORGANIC  SUBSTANCES. 

Nutrient  Gases,  -                                              -  60 

Nutrient  Salts,    -                                              -  66 

Absorption  of  Food-salts  by  Water-plants,  -  75 

Absorption  of  Food-salts  by  Lithophytes,    -  79 

Absorption  of  Food-salts  by  Land-plants,    -  82 
Relations  of  the  Position  of  Foliage-leaves 

to  that  of  Absorbent  Roots,           -         -  92 


3.  ABSORPTION  OF  ORGANIC  MATTER  FROM 
DECAYING  PLANTS  AND  ANIMALS. 

Saprophytes  and  their  Relation  to  Deca vine- 
Bodies,  -  99 

Saprophytes  in  Water,  on  the  Bark  of  Trees, 

and  on  Rocks,  -  -  -  -  -  104 

Saprophytes  in  the  Humus  of  Woods, 

Meadows,  and  Moors,  -  109 

Special  Relations  of  Saprophytes  to  their 

Nutrient  Substratum,  -  -  -  -  113 

Plants  with  Traps  and  Pitfalls  to  ensnare 

Animals, 119 


Xll 


CONTENTS. 


Page    j 

Carnivorous   Plants   which   exhibit   Move- 
ments in  the  capture  of  Prey,        -         -  140 
Carnivorous  Plants  with  Adhesive  Appa- 
ratus,   153 

4.  ABSORPTION  OF  NUTRIMENT  BY 
PARASITIC  PLANTS. 

Classification  of  Parasites,  -  -  -  -  159 
Bacteria:  Fungi,  ...-.-  161 
Climbing  Parasites:  Green-leaved  Parasites: 

Tooth  wort, 171 

Broom-rapes,  Balanophoreae,  Eafflesiacese,  -  183 
Mistletoes  and  Loranthuses,  -  -  204 

Grafting  and  Budding,         -         -         -         -  213 

5.  ABSORPTION  OF  WATER. 

Importance  of  Water  to  the  Life  of  a  Plant,  216 
Absorption  of  Water  by  Lichens  and  Mosses, 
and  by  Epiphytes  furnished  with  Aerial 
Boots, 217 


Page 
Absorption  of  Eain  and  Dew  by  the  Foliage- 

leaves,  .......  225 

Development  of  Absorption-cells  in  Special 

Cavities  and  Grooves  in  the  Leaves,      -  230 


6.  SYMBIOSIS. 


Lichens,       -         • 

Symbiosis    of    Green-leaved    Phanerogams 

with  Fungal  Mycelia  destitute  of  Chloro- 

phyll :  Monotropa,        - 


-  243 


249 


Animals  and  Plants  considered  as  a  great 

Symbiotic  Community,          -  254 

7.  CHANGES  IN  THE  SOIL  INCIDENT  TO 
THE  NUTRITION  OF  PLANTS. 

Solution,  Displacement,  and  Accumulation 
of  particular  Mineral  Constituents  of 
the  Soil  resulting  from  the  Action  of 
Plants,  -  257 

Mechanical  Changes  effected  in  the  ground 

by  Plants,  ......  265 


CONDUCTION  OF  FOOD. 


1.  MECHANICS  OF  THE  MOVEMENT  OF 
THE  BAW  FOOD-SAP. 

Capillarity  and  Boot-pressure,     -        -        -  269 
Transpiration,     -        -        -        -        -         -  273 

2.  BEGULATION  OF  TRANSPIRATION. 

Means  of  accelerating  Transpiration,  -        -  284 
Maintenance  of  a  Free  Passage  for  Aqueous 

Vapour,        -  -  290 

3.  PREVENTION  OF  EXCESSIVE  TRANSPIRATION. 

Protective  Arrangements  on  the  Epidermis,  307 
Form    and    Position    of     the    Transpiring 

Leaves  and  Branches    ...        -  325 


4.  TRANSPIRATION  DURING  VARIOUS  SEASONS 
OF  THE  YEAR:  TRANSPIRATION  OF  LIANES. 

Old  and  Young  Leaves,  -  347 

Fall  of  the  Leaf,-  -  355 

Connection  between  the   Structure  of  the 

Vascular  Tissues  and  Transpiration,      -  362 

5.  CONDUCTION  OF  FOOD-GASES  TO  THE 
PLACES  OF  CONSUMPTION. 

Transmission  of  the  Food-gases  in  Land  and 
Water  Plants  and  in  Lithophytes:  Sig- 
nificance of  Aqueous  Tissue  in  the  con- 
duction of  Food-gases,  -  367 


FOKMATION   OF   OEGANIC  MATTER  FROM  THE  ABSORBED 
INORGANIC  FOOD. 


1.  CHLOROPHYLL  AND  CHLOROPHYLL- 
GRANULES. 

Chlorophyll-granules  and  the  Sun's  Rays,    -  371 
Chlorophyll-granules  and  the  Green  Tissue 
under  the  Influence  of  various  degrees 
of  Illumination, 379 


2.  THE  GREEN  LEAVES. 

Distribution  of  the  Green  Leaves  on  the  Stem,  396 
Belation  between  Position  and  Form  of 

Green  Leaves, 408 

Arrangements  for  retaining  the  Position 

assumed, 424 

Protective  Arrangements  of  Green  Leaves 

against  the  Attacks  of  Animals,    -         -  430 


CONTENTS. 


Xlll 


METABOLISM  AND   TRANSPORT  OF  MATERIALS. 


1.  THE  ORGANIC  COMPOUNDS  IN  PLANTS. 


Page 


Carbon  Compounds,     - 
Metabolism  in  Living  Plants, 


-  452 

-  455 


2.  TRANSPORT  OF  SUBSTANCES  IN  LIVING 
PLANTS. 

Mechanisms  for  Conveyance  to  and  fro,       -  465 
Significance  of  Anthocyanin  in  the  Trans- 


Page 

portations  and  Transformations  of  Ma- 
terials: Autumnal  Colouring  of  Foliage,  483 

3.  PROPELLING  FORCES  IN  THE  CONVERSION 
AND  DISTRIBUTION  OF  MATERIALS. 

Respiration,  -  -  491 
Development  of  Light  and  B  eat,  -  496 
Fermentation, 504 


GROWTH  AND  CONSTRUCTION  OF  PLANTS. 


1.  THEORY  OF  GROWTH. 

Conditions  and  Mechanics  of  Growth,  -         -  510 
Effects  of  Growing  Cells  on  Environment,  -  513 

2.  GROWTH  AND  HEAT. 

Sources  of  Heat:  Transformation  of  Light 

into  Heat,  -  ...  517 

Influence  of  Heat  on  the  Configuration  and 

Distribution  of  Plants,  -  -  -  523 


Measures   for    protecting   Growing  Plants 

from  Loss  of  Heat,        -  -  528 

Freezing  and  Burning,         ....  539 

Estimation  of  the  Heat  necessary  to  Growth,  557 

3.  ULTIMATE  STRUCTURE  OF  PLANTS. 

Hypotheses  as  to  the  Form  and  Size  of  the 
smallest  Particles  employed  in  the  Con- 
struction of  Plants,  -  566 

Visible  Constructive  Activity  in  Protoplasm,  572 


PLANT-FORMS  AS  COMPLETED   STRUCTURES. 


1.  PROGRESSIVE  STAGES  IN  COMPLEXITY  OF 
STRUCTURE  FROM  UNICELLULAR  PLANTS 
TO  PLANT-BODIES,  -  584 

2.  FORM  OF  LEAF-STRUCTURES. 

Definition  and  Classification  of  Leaves,         -  593 
Cotyledons,  -  598 

Scale-leaves,  Foliage-leaves,  Floral-leaves,   -  623 

3.  FORMS  OF  STEM-STRUCTURES. 

Definition  and  Classification  of  Stems :  The 

Hypocotyl :  Stems  bearing  Scale-leaves,  647 


Stems  bearing  Foliage-leaves,  -  -  655 
Procumbent  and  Floating  Stems,  -  661 
Climbing  Stems,  -  669 
Erect  Foliage- stems,  -  -  710 
Resistance  of  Foliage-stems  to  Strain,  Pres- 
sure, and  Bending,  -  724 
The  Floral-stem,  -  736 

4.  FORMS  OF  ROOTS. 

Relation  of  external  and  internal  Structure 

to  Function,  -         -  749 

Definition  of  the  Root,  -  7  04 

Remarkable  Properties  of  Roots,          -         -  767 


ILLUSTRATIONS. 


FROM  ORIGINAL  DRAWINGS  BY  E.  HEYN,  H.  v.  KONIGSBRUNN,  E.  v.  RANSONNET, 
J.  SEELOS,  F.  TEUCHMANN,  0.  WINKLER,  AND  OTHERS. 


Page 

Seedlings  with  Cotyledons  and  Foliage-leaves,  -  9 
Metamorphoses  of  Leaves  as  exhibited  by  the  Poppy,  11 

Goethe's  "Urpflanze", 12 

Vegetable  Cells,  -  -  22 

Protoplasm  inclosed  in  Cells,  -  -  -  -25 
Cell-chambers,  showing  Intercellular  Spaces  and 

Intercellular  Substance,  -  -  -  -  27 

Swimming  Protoplasm, 29 

Pulsating  Vacuoles  in  the  Protoplasm  of  the  large 

Swarm-spores  of  Ulothrix,  -  -  •  -  31 

Creeping  Protoplasm, 32 

Connecting  Passages  between  adjacent  Cell-cavities,  45 
Linaria  Cymbalaria  dropping  its  Seeds  into  Clefts 

in  the  Rocks, 53 

Absorptive  Cells  on  Root  of  Penstemon,  -  87 

Centrifugal  and  Centripetal  Transmission  of  Water,  94 
Irrigation  of  Rain-water  in  Plants,  -  -  -  97 
Aerial  Roots  of  a  Tropical  Orchid  assuming  the 

form  of  straps, 107 

Transverse  section  through  absorption-roots  of 

Saprophytes, 115 

Bladderworts,  ....  -  120 

Traps  of  Utricularia  neglecta,  -  •  •  -  121 
Spinous  Structures  in  the  Pitfalls  of  Carnivorous 

Plants,  -  -  124 

Sarracenia  purpurea,  -  -  125 

Ascidia-bearing  and  Pitcher- plants,  -  -  -  127 
Cephalotus  fotticularis,  -  -  -  -  131 

Young  Nepenthes  plants,  -  -  132 

Nepenthes  destittatoria, 133 

Glandular  structures  in  the  Toothwort,  Bartsia, 

and  Butterwort,  -  -  -  137 

Tentacles  on  leaf  of  Sun-dew,  -  -  -  -  145 
Venus's  Fly-trap  (Dioncea  muscipula),  -  -  148 
Capturing  apparatus  of  the  leaves  of  Aldro- 

vandia  and  Venus's  Fly-trap,  ...  150 

Aldrovandia  vesiculosa, 151 

The  Fly-catcher  (DrosopJiyllum  lusitanicum),  -  155 
Lonicera  ciliosa  in  South  Carolina,  -  -  -  160 
Hyphae  of  Parasitic  Fungi,  -  -  -  -  165 
Parasites  on  Hydrophytes,  -  -  -  -  169 

Seedlings  of  Parasitic  Plants,  -  -  -  -  173 
Cuscuta  Europcea  parasitic  on  a  Hop-stem,  -  -  175 
Bastard  Toad-flax  (Thesium  alpinum),  -  -  177 


Page 
Toothwort    (Lathrcea  Squamaria),  with  suckers 

upon  the  roots  of  a  Poplar,  -  -  -  -  181 
Langsdorffia  h ypogcea,  from  Central  America,  -  187 
Parasitic  PUlanophorese  (ScybaUum  fungiforme 

and  JSalanophora  Hildenbrandtii),  -  -  189 
Parasitic  Balanophoreaa  (Rhopalocnemis  phalloides 

and  Helosis  gujanensis),  -  -  •  -  191 
Parasitic  Balanophoreae  (Lophophytum  rnirabile 

and  Sarcophyte  sanguinea),  -  -  -  -  195 
Cytinus  Hypocistus  and  Cynomorium  coccineum,  -  197 
Rafflesiaceae  parasitic  on  trunks  and  branches,  -  201 
Parasitic  Rafflesiacea  upon  a  Cissus-root,  -  -  202 
Rafflesia  Padma,  parasitic  on  roots  upon  the  sur- 
face of  the  ground,  -  -  -  203 
The  European  Mistletoe  ( Viscum  album),  -  -  206 
Bushes  of  Mistletoe  upon  the  Black  Poplar  in 

winter, 207 

Loranthus    Europceus    and    Mistletoe    ( Viscum 

album) — both  parasitic  on  branches  of  trees, 

and   seen  in  section.     A  piece  of   Fir-tree 

perforated  by  the  sinkers  of  a  Mistletoe,       -  209 

Porous  Cells  of   Fork-moss,   Bog-moss,  and   an 

Orchid  root,         -  -  219 

Aerial  Roots  of  an  Orchid  epiphytic  upon  bark 

of  the  branch  of  a  tree,         -         -  -  221 

Aerial  Roots  with  root-hairs,  ....  224 
Hairs  and  Leaves  which  retain  Dew  and  Rain,  -  228 
Cauline  and  Capitate  Hairs,  -  229 

Absorption  of  Water  by  Foliage-leaves,  -  -  232 
Absorptive  Cavities  and  Cups  on  Foliage-leaves,  233 
Water-receptacles  in  Plants,  -  239 

Gelatinous  Lichens, 244 

Fruticose  and  Foliaceous  Lichens,  -  -  -  245 
Roots  with  Mycelial  Mantle;  Mycelium  entering 

into  the  external  cells,  -  -  -  -  250 
Olive  Grove  on  the  Shores  of  Lake  Garda,  -  -  275 

Transpiring  Cells, 278 

Spongy  Tissue  of  Franciscea  eximia  and  Daphne 

Laureola, -  279 

Corypha  umbraculifera  of  Ceylon,  -  -  -  289 
Stomata  of  Nephrodium  Filix-mas  and  Peperomia 

arifolia, 294 

Protection  of  Stomata  from  Moisture  by  Papilla- 
like  outgrowths  of  the  Surface,    -         -         -  295 


xiv 


ILLUSTRATIONS. 


XV 


Page 
Protection  of  Stomata  from  Moisture  by  Cuticular 

Pegs, 296 

Over-arched  Stornata  of  Australian  Proteaceae,  -  297 
Stomata  in  Pit-like  Depressions,  -  -  -  298 
Stomata  in  the  Furrows  of  Green  Stems,  -  -  299 
Orchids  whose  Stomata  lie  in  Hollow  Tubercles,  -  300 
Transverse  Sections  through  Rolled  Leaves,  -  301 
Vertical  Section  through  a  Rolled  Leaf,  -  303 

Thickened  Stratified  Cuticle,  -  -  -  310 

Caryota  propinqua, 311 

Vertical  Section  of  Leaf  of  Caryota  propinqua,  •  312 
Edelweiss  (Onaphalium  Leontopodium),  -  -  315 
Covering  Hairs  of  various  plants,  -  -  -  321 
Covering  Hairs  of  various  plants,  •  -  -  322 
Flinty  armour  of  Rochea  falcata,  •  -  •  323 
Switch-plants,  -  -  331 

Switch-shrubs,  sections  of  Stems,  -  -  -  332 
Plants  with  Leaf -like  Branches  (Cladodes),  -  333 
Plants  with  Leaf-like  Branches  (Cladodes),  -  335 

Compass  Plants, 337 

Folding  of  Grass-leaves  (Sesleria  tenuifolia),  •  341 
Folding  of  Grass-leaves  (Stipa  capittata  and  Fes- 

tuca  alpestris], 342 

Folding  of  Grass-leaves  (Lasiagrostis  Calama- 

grostis  and  Festuca  Porcii),  .  .  .  343 

Folding  of  Grass-leaves  (Festuca  punctoria),  -  345 
Folding  of  Moss-leaves  (Polytrichum  commune),  -  346 
Unfolding  of  Leaves  of  various  plants,  -  -  349 
Leaf-unfolding  of  the  Tulip-tree,  -  -  -  352 
Unfolding  of  Beech-leaves,  -  353 

Leaf-fall  of  the  Horse-chestnut,  -  -  -  -  361 
Indian  Climbing  Palms  (Rotang),  -  -  363 

Lianes,  Stems  of,  ...  364 

Aroids,  with  cord-like  aerial  roots,  -  -  -  365 
Position  of  the  Chlorophyll-granules  in  the  cells 

of  the  Ivy-leaved  Duckweed  (Lemna  trisulca),  382 
Plan  of  Whorled  Phyllotaxis,  -  -  -  -  397 
Plan  for  Spiral  Phyllotaxis,  -  -  -  -  400 
Plan  of  Five-thirteenths  Phyllotaxis,  -  -  -  401 
Parastichies  of  a  Pine-cone,  ....  403 
Displacement  of  the  leaf-positions  in  consequence 

of  torsion  of  the  stem,  ....  407 

Leaf -mosaic,  Leaf -rosettes,  and  Scale-like  Leaves,  410 
Formation  of  a  Leaf -mosaic,  ....  411 
Spruce  Firs  (Abies  excelsa),  ....  415 
Erect  Leafy  Twig  of  the  Norway  Maple,  -  -  416 
Twi&ting  of  Internodes  and  Leaf-stalks,  -  -  417 
Horizontally  growing  Leafy  Twig  of  the  Paper 

Mulberry-tree  (Broussonetia  papyrifera),  -  418 
Leafy  Twig  projecting  laterally  from  the  Stem  of 

the  Norway  Maple  (A cer  platanoides),  -  -  419 
Leaf-mosaics  of  Unsymmetrical  Leaves,  -  -  420 
Mosaic  of  Leaves  of  unequal  size,  -  -  -  421 
Mosaic  of  Unsymmetrical  Leaves  of  unequal  size,  422 

Leaf-mosaic  (Ivy), 423 

Acantholimon  and  spiny  Tragacanth-shrubs,  -  435 
Group  of  Thistles  (Cirsium  nemorale),  •  -  436 
Acanthus  spinosissimus, 437 


Weapons  of  Plants, 

Weapons  of  Plants, 

Chemical  Diagrams  (three), 

Chemical  Diagram, 

Crystals  and  Crystalloids  in  Plant-cells, 
Various  Forms  of  Starch-grains, 
Portion  cut  from  a  Branch  (diagrammatic), 
Organs  for  Removal  of  Substances,     - 


Page 

-  439 

-  449 

-  453 

-  454 

-  457 

-  459 

-  469 

-  471 
Rhynchosia  phaseoloides,  a  Liane  with  ribbon-like 

Stems, 475 

Transverse  sections  of  Liane  Stems,  -  -  -  477 
Leafless  Branches  of  Tecoma  radicans,  rooted  on 

a  wall, 479 

Elevation  of  a  Block  of  Stone  in  consequence  of 

the  growth  in  thickness  of  a  Larch  Root,  -  515 
Alpine  Willows  with  stems  and  branches  clinging 

to  the  ground, 524 

Periodic  bending  of  Flowers  and  Inflorescences,  -  531 
Alteration  of  Position  of  Leaflets  in  Compound 

Leaves,       .......  533 

Mimosa  pudica  in  day  and  night  positions,  -  -  537 
Mountain  Pines  (Pinus  humilis)  in  the  Tyrol,  -  549 
Detachment  of  special  shoots  of  Potamogeton 

crispus,  for  hibernation  under  water,  -  -  551 
Edible  Lichen  (Lecanora  esculenta)  in  the  desert,  555 
Changes  in  the  Protoplasm  of  the  Cell -nucleus 

during  its  division, 581 

Laminarias  in  the  North  Sea,     ....  688 
Liverworts  with  Cell-nets,  Cell-plates,  and  Cell- 
rows  in  various  transitional  forms,        -         -  591 
Cotyledons,  various  examples  shown  in  detail,     -  599 
Process  of  Development — (Rhizophora  conjugata),  603 
Mangroves  on  the  West  Coast  of  India  at  ebb- 
tide,     605 

Germinating  Seeds  and  Seedlings,  -         -  607 

Liberation  of  the  Cotyledons  from  the  cavity  of 

the  seed  or  fruit  husk,          -         -         -         -  611 
Anchoring  of  the  Water-chestnut  (Trapa),  -         -  617 
The  Boring  of  Fruits  into  the  Ground,  Feather- 
grass and  Stork's-bill,  -         -  619 
Cotyledons  of  various  Plants,      ....  621 
Arrangement  of  Strands  in  the  blades  of  Foliage- 
leaves.     Forms  with  one  main  strand,  -         -  631 
Distribution  of  Strands  in  the  blades  of  Foliage- 
leaves.     Forms  with  several  main  strands,  -  633 
Flowers  of  the  Silver  Lime  and  Arrow-grass,      -  646 
Cotton  Trees  of  the  Brazilian  catingas,        -         -  656 
Agaves  of  the  Mexican  uplands,          -         -         -  657 

Yucca  gloriosa, 659 

Vattisneria  spiralis,   •  '         '         ' ,       '  ^ 

Rotangs  in  Java, 675 

Shoot-apices  of  three  species  of  Rotang,  -  676 

Branches  of  the  New  Zealand  Bramble,  -  -  677 
Palm-stem  used  as  a  support  by  the  lattice-forming 

stems  of  one  of  the  Clusiacese,      -         -         -  681 
Twining  Hop  (ffumulus  Lupulus),  in  detail,        -  688 
Portion  of  a  Liane  stem,  twisted   like  a  cork- 
screw,           689 


XVI 


ILLUSTRATIONS. 


Page 

Stipular  tendrils  of  the  common  Smilax,     -         -  6! 
Leaf-stalk  tendrils  of  Atragene  alpina,         •         •  691 
Branch-tendrils  of  Serjania  gramatophora,  • 
Tendrils  of  the  Bryony  (Bryonia),      -  -  696 

Light-avoiding  Tendrils  of  Vitis  inserta  and  Vitis 

fiQQ 

inconstans, 

Ivy  (Hedera  Helix)  fastened  by  climbing  roots  to 

the  trunk  of  an  Oak, 7°3 

Ficus  with  girdle-like  clasping  roots,  -  -  -  705 
Ficus  Benjamina  with  incrusting  climbing  roots,  707 
Bignonia  argyro-violacea,  from  Brazil,  •  -  709 
Ficus  with  lattice-forming  climbing  roots,  -  -  711 
Bamboos  in  Java, 

The  Oak, 

The  Silver  Fir  (Abies  pectinata), 
Birch  Trunks  with  white  membraneous  bark,      -  721 
Eucalyptus  trees  in  Australia,    -         -         -         -  723 
Diagrammatic  representation  of  various  combined 

girders, 728 


Page 
Transverse  sections  of  erect  foliage-stems  with 

simple  girders  not  fused  together  into  a  tube,  729 
Transverse  sections  of  erect  foliage-stems  with 

simple  girders  fused  into  cylindrical  tubes,  -  730 
Transverse  sections  of  erect  foliage-stems  with 

flanges  developed  as  secondary  girders,          -  731 
Transverse  section  of  the  climbing  stem  of  the 

Atragene  (Atragene  alpina), 

Undulations  of  old  ribbon-shaped  Liane  stems,  -  734 
Transverse  sections  of  a  runner  of  the  Garden 

Strawberry  and  of  the  Water  Milfoil,  -         -  735 
Branch  of  the  Walnut-tree  with  hanging  male 

catkins,  and  a  small  cluster  of  female  flowers,  742 
India-rubber  Tree  (Ficus  elastica)  and  Banyan- 
tree  (Ficus  Indica),     - 

The  Screw  Pine  (Pandanus  utilis),  -  -  -  758 
Stilt-like  and  columnar  roots  of  Mangroves,  -  759 
Bramble-bush  in  which  the  branches  have  taken 

root, 769 


THE  BIOLOGY 
AND  CONFIGURATION  OF  PLANTS 


THE 

NATUKAL  HISTOKY  OF  PLANTS. 


INTRODUCTION. 

THE  STUDY  OF  PLANTS  IN  ANCIENT  AND  IN  MODERN  TIMES. 

Plants  considered  from  the  point  of  view  of  utility. — Description  and  classification  of  plants. — 
Doctrine  of  metamorphosis  and  speculations  of  nature-philosophy.— Scientific  method  based  on 
the  history  of  development. — Objects  of  botanical  research  at  the  present  day. 

PLANTS   CONSIDERED  FROM  THE  POINT  OF  VIEW  OF  UTILITY. 

SOME  years  ago  I  rambled  over  the  mountain  district  of  North  Italy  in  the 
lovely  month  of  May.  In  a  small  sequeotered  valley,  the  slopes  of  which  were 
densely  clad  with  mighty  oaks  and  tall  shrubs,  I  found  the  flora  developed  in  all 
its  beauty.  There,  in  full  bloom,  was  the  laburnum  and  manna-ash,  besides 
broom  and  sweet-brier,  and  countless  smaller  shrubs  and  grasses.  From  every 
bush  came  the  song  of  the  nightingale;  and  the  whole  glorious  perfection  of  a 
southern  spring  morning  filled  me  with  delight.  Speaking,  as  we  rested,  to  my 
guide,  an  Italian  peasant,  I  expressed  the  pleasure  I  experienced  in  this  wealth 
of  laburnum  blossoms  and  chorus  of  nightingales.  Imagine  the  rude  shock 
to  my  feelings  on  his  replying  briefly  that  the  reason  why  the  laburnum  was  so 
luxuriant  was  that  its  foliage  was  poisonous,  and  goats  did  not  eat  it;  and  that 
though  no  doubt  there  were  plenty  of  nightingales,  there  were  scarcely  any  hares 
left.  For  him,  and  I  daresay  for  thousands  of  others,  this  valley  clothed  with 
flowers  was  nothing  more  than  a  pasture-ground,  and  nightingales  were  merely 
things  to  be  shot. 

This  little  occurrence,  however,  seems  to  me  characteristic  of  the  way  in  which 
the  great  majority  of  people  look  upon  the  world  of  plants  and  animals.  To  their 
minds  animals  are  game,  trees  are  timber  and  fire-wood,  herbs  are  vegetables  (in 
the  limited  sense),  or  perhaps  medicine  or  provender  for  domestic  animals,  whilst 
flowers  are  pretty  for  decoration.  Turn  in  what  direction  I  would,  in  every 
country  where  I  have  travelled  for  botanical  purposes,  the  questions  asked  by  the 
inhabitants  were  always  the  same.  Everywhere  I  had  to  explain  whether  the 
plants  I  sought  and  gathered  were  poisonous  or  not;  whether  they  were  efficacious 

as  cures  for  this  or  that  illness;  and  by  what  signs  the  medicinal  or  otherwise 
VOL.  I.  1 


2  THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES. 

useful  plants  were  to  be  recognized  and  distinguished  from  the  rest.  And  the 
attitude  of  the  great  mass  of  country  folk  in  times  past  was  the  same  as  at  the 
present  day.  All  along  anxiety  for  a  livelihood,  the  need  of  the  individual  to 
satisfy  his  own  hunger,  the  interests  of  the  family,  the  provision  of  food  for 
domestic  animals,  have  been  the  factors  that  have  first  led  men  to  classify  plants 
into  the  nutritious  and  the  poisonous,  into  those  that  are  pleasant  to  the  taste  and 
those  that  are  unpleasant,  and  have  induced  them  to  make  attempts  at  cultivation, 
and  to  observe  the  various  phenomena  of  plant-life. 

No  less  powerful  as  an  incentive  to  the  study  of  herbs,  roots,  and  seeds,  and  to 
the  minute  comparison  of  similar  forms  and  the  determination  of  their  differences, 
was  the  hope  and  belief  that  the  higher  powers  had  endowed  particular  plants  with 
healing  properties.  In  ancient  Greece  there  was  a  special  guild,  the  "  Rhizotomoi," 
whose  members  collected  and  prepared  such  roots  and  herbs  as  were  considered 
to  be  curative,  and  either  sold  them  themselves  or  caused  them  to  be  sold  by 
apothecaries.  Through  the  labours  of  these  Rhizotomoi,  added  to  those  of  Greek, 
Roman,  and  Arabic  physicians,  and  of  gardeners,  vine-growers,  and  farmers,  a  mass 
of  information  concerning  the  plant- world  was  acquired,  which  for  a  long  period 
stood  as  botanical  science.  As  late  as  the  sixteenth  century  plants  were  looked 
upon  from  a  purely  utilitarian  point  of  view,  not  only  by  the  masses  but  also 
by  very  many  professed  scholars;  and  in  most  of  the  books  of  that  time  we  find 
the  medicinal  properties,  and  the  general  utility  of  the  plants  selected  for  descrip- 
tion and  discrimination,  occupying  a  conspicuous  position  and  treated  in  an 
exhaustive  manner.  Just  as  men  lived  in  the  firm  belief  that  human  destinies 
depended  upon  the  stars,  so  they  clung  to  the  notion  that  everything  upon  the 
earth  was  created  for  the  sake  of  mankind;  and,  in  particular,  that  in  every  plant 
there  were  forces  lying  dormant  which,  if  liberated,  would  conduce  either  to  the 
welfare  or  to  the  injury  of  man.  Points  which  might  serve  as  bases  for  the 
discovery  of  these  secrets  of  nature  were  eagerly  sought  for.  People  imagined  they 
discerned  magic  in  many  plants,  and  even  believed  that  they  were  able  to  trace 
in  the  resemblance  of  certain  leaves,  flowers,  and  fruits  to  parts  of  the  human  body, 
an  indication,  emanating  from  supernatural  powers,  of  the  manner  in  which  the 
organ  in  question  was  intended  to  affect  the  human  constitution.  The  similarity 
in  shape  between  a  particular  foliage-leaf  and  the  liver  did  duty  for  a  sign  that 
the  leaf  was  capable  of  successful  application  in  cases  of  hepatic  disease,  and  the 
fact  of  a  blossom  being  heart-shaped  must  mean  that  it  would  cure  cardiac  com- 
plaints. Thus  arose  the  so-called  doctrine  of  Signatures,  which,  brought  to  its 
highest  development  by  the  Swiss  alchemist  Bombastus  Paracelsus  (1493-1541), 
played  a  great  part  in  the  sixteenth  and  seventeenth  centuries,  and  still  survives 
at  the  present  day  in  the  mania  for  nostrums.  The  inclination  of  the  masses  is 
now,  as  it  was  centuries  ago,  in  favour  of  supernatural  and  mysterious  rather 
than  simple  and  natural  interpretations;  and  a  Bombastus  Paracelsus  would  still 
find  no  lack  of  credulous  followers.  In  truth,  the  great  bulk  of  mankind  regard 
Botany  as  subservient  to  medicine  and  agriculture,  they  look  at  it  from  the  purely 


THE  STUDY  OF   PLANTS   IN   ANCIENT   AND  MODERN   TIMES.  3 

utilitarian  point  of  view  in  a  manner  not  essentially  different  from  that  of  two 
hundred — or  even  two  thousand — years  ago,  and  it  may  well  be  a  long  time 
before  they  rise  above  this  idea. 

In  addition  to  the  botanical  knowledge  thus  initiated  by  the  necessities  of  life, 
a  second  avenue  leading  to  the  same  goal  was  early  established  by  man's  sense  of 
beauty.  The  first  effect  of  this  was  limited  to  the  employment  of  wild  flowers 
and  foliage  for  purposes  of  ornament  and  decoration.  Later  on,  it  led  to  the 
cultivation  of  the  more  showy  plants  in  gardens,  and  ultimately  to  the  arts  of 
gardening  and  horticulture,  which  at  different  periods  and  in  different  countries 
have  passed  through  such  various  phases,  corresponding  to  the  standards  of  the 
beautiful  which  have  prevailed. 

THE  DESCRIPTION  AND  CLASSIFICATION  OF  PLANTS. 

A  third  path  leading  to  botanical  knowledge  springs  from  the  impulse  which 
actuates  those  who  are  endowed  with  a  keen  perception  of  form  to  investigate 
structural  differences  down  to  their  most  minute  characteristics.  Workers  in  this 
field  arrange  and  classify  all  distinct  forms  according  to  their  external  resemblances, 
give  them  names  appropriate  to  their  position  and  importance,  catalogue  them,  and 
keep  up  the  register  when  once  it  has  been  started.  Many  people  possess,  in  addi- 
tion, the  remarkable  taste  for  collecting,  which  causes  them  to  find  pleasure  in 
merely  accumulating  and  possessing  enormous  numbers  of  specimens  of  the  particu- 
lar objects  on  which  their  fancy  is  fixed. 

This  tendency  of  the  human  mind  has  played  a  very  important  part  in  the 
history  of  botany.  The  first  traces  of  it  can  be  ascribed  with  certainty  to  a  period 
long  before  the  commencement  of  our  era;  for  such  descriptions  and  other  notes  as 
are  contained  in  the  Natural  History  of  Plants,  written  by  Theophrastus  about  the 
year  300  B.C.,  are  founded,  for  the  most  part,  on  the  observations  and  experiments 
of  "Rhizotomoi,"  physicians  and  agriculturists,  and  it  is  obvious  from  the  text  of  the 
book  that  in  some  cases  those  authorities  did  seek  out  plants,  and  learn  to  distinguish 
them  for  their  own  sakes,  and  not  solely  for  their  economic  or  medicinal  value 

At  the  time  of  the  Roman  Empire  and  in  the  Middle  Ages,  it  is  true,  no  one 
troubled  himself  about  plants  other  than  those  known  to  be  in  some  way  useful. 
But  there  was  a  revival  of  the  practice  of  hunting  for  plants  for  the  purpose  of 
describing  and  enumerating  all  distinguishable  forms,  at  that  great  epoch  when  the 
nations  of  the  West  began  to  study  the  treasures  of  Greek  thought,  endeavouring 
to  adopt  the  point  of  view  of  antiquity,  and  to  harmonize  their  own  circumstances 
with  it.  It  was  at  this  same  period  that  art  too  shook  itself  free  from  the  tradi- 
tions of  the  Middle  Ages,  and  became  actuated  by  a  new  ideal  based  on  the  study 
of  the  antique;  but  science,  particularly  natural  science,  has  as  good  a  claim  as 
art  to  regard  that  memorable  time  as  its  period  of  renaissance.  Although  the 
ancient  Greek  writings  on  natural  history,  to  which  people  turned  with  such 
youthful  enthusiasm  in  the  fifteenth  century,  could  not  satisfy  their  thirst  for 


4  THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES. 

knowledge,  yet  there  is  no  doubt  that,  as  in  art,  the  effect  was  to  stimulate  and 
reform;  and  that  this  study  led  up  to  the  source,  so  long  forgotten,  whence  the 
ancients  had  themselves  drawn  their  knowledge,  that  is,  to  the  direct  investigation 
of  nature,  which  has  invariably  given  to  every  branch  of  human  knowledge  new 
and  pregnant  life. 

As  regards  botanical  knowledge  in  particular,  the  study  of  old  Greek  writings 
on  the  part  of  western  nations  in  both  Northern  and  Southern  Europe  had  the 
immediate  effect  of  instituting  an  eager  search  for  all  the  different  kinds  of 
indigenous  plants;  and,  besides  arousing  a  passion  for  investigation,  it  evoked  un- 
tiring industry  in  this  pursuit,  the  results  of  which  preserved  in  a  number  of  bulky 
herbals  still  excite  our  wonder  and  respect.  If  these  folios,  dating  for  the  most  part 
from  the  first  half  of  the  sixteenth  century,  are  perused  in  the  hope  of  their  reveal- 
ing some  guiding  principle  as  a  basis  for  the  arrangement  of  the  subject,  the  reader 
will  no  doubt  be  obliged  to  lay  them  aside  unsatisfied.  The  plants  were  described 
and  discussed  just  as  the  authors  happened  to  come  across  them;  and  it  is  only 
here  and  there  that  we  find  a  feeble  attempt  to  range  together  and  make  groups  of 
nearly-allied  species.  Only  cursory  attention  was  paid  to  the  facts  of  geographical 
distribution.  Plants  native  to  the  soil,  herbs  which  flowered  in  gardens  and  had 
been  reared  from  seed  purchased  from  itinerant  vendors  of  antidotes,  and  plants 
whose  fruits  were  brought  to  Europe  as  curiosities  from  the  New  World  recently 
discovered — all  these  were  jumbled  together  in  a  confused  medley.  The  whole 
endeavour  of  the  time  was  directed  to  the  enumeration  and  description  of  all  such 
things  as  possess  the  power  of  producing  green  foliage  and  maturing  fruit  under 
the  sun's  quickening  rays. 

Owing  to  the  fact  that  researches  were  then  limited  to  the  native  soil  of  the 
student,  most  of  the  botanical  authors  of  that  day  had  but  dark  inklings  of  the 
extent  to  which  the  floras  of  various  latitudes  and  areas  differ.  They  assumed  that 
plants  of  the  Mediterranean  shores,  which  had  been  described  centuries  before  by 
Theophrastus  or  Dioscorides  or  Pliny,  were  necessarily  the  same  as  those  of  their 
own  more  inclement  countries.  The  German  "  Fathers  of  Botany  "  (Brunfels,  born 
about  1495,  died  1534;  Bock,  1498-1554;  Fuchs,  1501-1566,  are  the  best  known) 
applied  the  old  Greek  and  Latin  names  without  scruple  to  the  species  growing  in 
their  own  localities.  They  were  so  firmly  convinced  of  the  identity  of  the  German, 
Greek,  and  Italian  floras  that  even  the  numerous  inconsistencies  occurring  in  the 
descriptions  did  not  disconcert  them,  or  prevent  them  from  discussing  at  great 
length  whether  a  particular  name  was  intended  by  Theophrastus  and  Dioscorides  to 
indicate  this  or  that  plant.  It  was  by  slow  degrees  that  botanists  first  began  to 
abandon  these  fruitless  debates  concerning  the  Greek  and  Latin  names  of  plants, 
with  which  it  had  been  the  custom  to  fill  so  many  pages  of  the  herbals.  Step  by 
step  they  became  conscious  that  although  the  yellow  pages  of  the  ancient  books 
deserved  all  gratitude  for  the  stimulating  influence  they  had  exercised,  yet  the 
green  book  of  nature  should  be  set  above  them.  This  led  to  their  devoting 
themselves  entirely  to  direct  researches  in  the  subject  of  their  native  floras.  The 


THE   STUDY   OF   PLANTS   IN    ANCIENT   AND   MODERN   TIMES.  5 

herbal  of  Hieronymus  Bock,  which  appeared  in  1546,  and  in  which  "the  herbs 
growing  in  German  countries  are  described  from  long  and  sure  experience,"  contains 
a  passage  treating  of  the  controversy  of  the  day  as  to  whether  the  Latin  name 
Erica  was  applicable  to  the  German  Heath  or  not;  and  in  the  midst  of  the  discus- 
sion the  author  expresses  the  opinion  that  "the  plants  we  know  best  were  the  least 
known  to  the  Latins;"  and  at  last  he  exclaims:  "Be  our  heath  the  same  as  Erica 
or  not,  it  is  in  any  case  a  pretty  and  sturdy  little  shrub,  beset  with  numerous  brown 
rounded  branches,  which  are  clothed  all  over  with  small  green  leaves;  and  its 
appearance  is  like  that  of  the  sweet-smelling  Lavender  Cotton."  And  again  in  a 
number  of  other  places,  after  making  lengthy  philological  statements  relating  to  the 
old  names,  he  ends  by  losing  patience  and  declaring  that  the  proper  thing  would  be 
to  lay  aside  all  disputes  concerning  this  nomenclature. 

At  length  a  Belgian,  Charles  de  1'Ecluse  (1526-1609),  whose  name  was  latinized 
into  Clusius,  emancipated  himself  entirely  from  the  hair-splitting  verbal  contro- 
versies of  the  day.  He  was  also  the  first  to  abandon  the  utilitarian  standpoint; 
and  in  his  extensive  work,  which  appeared  at  the  end  of  the  sixteenth  century, 
he  was  guided  solely  by  the  desire  to  become  acquainted  with  every  flowering  thing. 
He  therefore  endeavoured  to  distinguish,  describe,  and  where  possible  to  draw  the 
various  forms  of  plants,  to  cultivate  them,  and  to  preserve  them  in  a  dried  condition. 
It  was  just  at  that  time  that  collections  of  dried  plants  began  to  be  made.  Such  a 
collection  was  at  first  called  a  "  hortus  siccus,"  and  later  on  a  "  herbarium."  All 
museums  of  natural  history  were  forthwith  furnished  with  them.  Moreover, 
Clusius,  actuated  by  the  wish  to  see  with  his  own  eyes  what  the  vegetation  on  the 
other  side  of  the  mountains  looked  like,  was  the  first  man  to  travel  for  the  purpose 
of  botanizing.  In  order  to  extend  his  knowledge  of  plants  he  roamed  over  Europe 
from  the  sierras  of  Spain  to  the  borders  of  Hungary,  and  from  the  sea-coast  to 
the  highlands  of  the  Tyrol.  Journeys  of  this  kind  in  pursuit  of  botanical  know- 
ledge were  by  degrees  extended  to  wider  and  wider  limits,  and  thus  an  abundance  of 
material  was  brought  together  from  all  latitudes  and  from  every  quarter  of  the  globe. 

An  immense  number  of  isolated  observations  were  accumulated  in  this  way,  till, 
at  length,  in  the  first  decades  of  the  eighteenth  century,  the  desirability  of  sifting 
and  arranging  this  chaotic  mass  became  urgent.  When,  therefore,  the  Swedish 
naturalist  Linnaeus  (1707-1778),  by  the  exercise  of  unparalleled  industry,  mastered 
in  a  fabulously  short  space  of  time  the  detailed  results  of  centuries  of  labour,  and 
afforded  a  general  survey  of  all  this  scattered  material,  he  obtained  universal 
recognition.  Linnaeus  introduced  short  names  for  the  various  species  in  place  of 
the  cumbrous  older  designations,  and  showed  how  to  distinguish  the  species  by 
means  of  concise  descriptions.  For  this  purpose  he  marked  out  the  different  parts 
of  a  plant  as  root,  stem,  leaf,  bract,  calyx,  corolla,  stamens,  pistil,  fruit,  and  seeds. 
Again,  he  distinguished  particular  forms  of  those  organs,  as,  for  instance,  scapes, 
haulms,  and  peduncles  as  forms  of  stems,  and  in  addition  also  the  parts  of  each 
organ,  such  as  filaments,  anthers,  and  pollen  in  the  stamens,  and  ovary,  style,  and 
stigma  in  the  pistil;  and  to  each  one  of  these  objects  he  assigned  a  technical  name 


6  THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES. 

(terminus).  With  the  help  of  the  botanical  terminology  thus  formulated  it  became 
possible  not  only  to  abridge  the  specific  descriptions,  but  also  to  recognize  species 
from  such  descriptions,  and  to  determine  what  name  had  been  given  them  by 
botanists,  and  to  what  group  they  belonged. 

Linnaeus  selected  as  a  basis  of  classification  in  the  "  System  "  established  by  him 
the  characteristics  of  the  various  parts  of  the  flower.  In  this  system  the  number, 
relative  length,  cohesion,  and  disposition  of  the  stamens  formed  the  ground  of 
division  into  "  Classes."  Within  each  Class,  "  Orders "  were  then  differentiated 
according  to  the  nature  of  the  pistil,  especially  the  number  of  styles;  and  each 
Order  was  again  subdivided  into  more  narrowly  defined  groups,  which  received 
the  name  of  "  Genera."  To  the  23  classes  of  Flowering  Plants  (Phanerogamia) 
Linnaeas  added  as  a  24th  Class  Flowerless  Plants  (Cryptogamia),  which  were 
divided  into  several  groups  (Ferns,  Mosses,  Algae,  and  Fungi)  in  respect  of  their 
general  appearance  and  mode  of  occurrence. 

This  system  took  immediate  possession  of  the  civilized  world.  Englishmen, 
Germans,  and  Italians  now  worked  in  unison  as  faithful  disciples  of  Linnaeus. 
Even  laymen  studied  the  Linnaean  botany  with  enthusiasm;  and  it  was  recommended, 
especially  to  ladies,  as  a  harmless  pastime,  not  overtaxing  to  the  mind.  In  France 
Rousseau  delivered  lectures  on  botany  to  a  circle  of  educated  ladies;  whilst  even 
Goethe  experienced  a  strong  attraction  to  the  "  loveliest  of  the  sciences,"  as  botany 
was  called  in  that  day.  Linnaeus  had  introduced  for  the  first  time  the  name 
''flora"  to  signify  a  catalogue  of  the  plants  of  a  more  or  less  circumscribed  district. 
He  had  himself  written  a  flora  of  Lapland  and  Sweden,  and  by  doing  so  had 
stimulated  others  to  undertake  the  compilation  of  similar  catalogues;  so  that  by 
the  end  of  the  18th  century  floras  of  England,  Piedmont,  Carniola,  Austria,  &c., 
had  been  produced.  By  this  means  a  certain  perfection  was  attained  in  that  field 
of 'botany  which  has  only  in  view  the  examination  of  the  fully-developed  external 
forms  of  plants,  together  with  the  distinguishing,  describing,  naming,  and  grouping 
them,  and  the  enumeration  of  species  indigenous  to  particular  regions.  Later  on, 
unfortunately,  botanists  lost  themselves  in  a  maze  of  dull  systematizing.  They 
either  contented  themselves  with  collecting,  preparing,  and  arranging  herbaria,  or 
else  devoted  their  energies  to  endless  debates  over  such  questions,  for  instance,  as 
whether  a  plant,  that  some  author  had  distinguished  from  others  and  described, 
deserved  to  rank  as  a  species,  or  should  be  reckoned  as  a  variety  dependent  on  its 
habitat  or  on  local  conditions  of  temperature,  light,  and  moisture.  They  took  delight 
in  now  including  a  group  of  forms  as  varieties  of  a  single  species,  now  dividing 
some  species  as  described  by  a  particular  author  into  several  other  species.  For 
this  purpose  they  did  not  rely  upon  the  only  sure  method,  the  determination  by 
cultural  experiment  of  the  fact  of  the  constancy  or  variability  of  the  form  in 
question;  nor  did  they,  in  general,  adhere  to  any  consistent  principle  to  guide  them 
in  this  amusement. 

Aberrations  of  this  kind  constituted,  however,  no  serious  barrier  to  progress. 
On  the  contrary,  the  passion  for  collecting  continued  to  extend  its  range.  The 


THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES.  7 

vegetation  of  the  remotest  corners  of  the  earth  was  ransacked  by  travelling 
botanists  without  any  material  advantage  being  gained,  though  they  not  infre- 
quently ran  considerable  risk  to  their  health,  and  sometimes  sacrificed  their  lives. 
As  one  generation  succeeded  another  thousands  of  students  of  the  "  scientia  ama- 
bilis"  made  their  appearance  in  every  country.  Swept  along  by  the  prevailing 
current  of  thought  they  devoted  themselves  to  the  examination  of  native  and  foreign 
floras,  or  to  a  detailed  study  of  the  most  insignificant  sections  of  the  vegetable 
kingdom.  Those  who  are  not  under  the  spell  of  this  passion  cannot  conceive  the 
joy  experienced  by  the  discoverer  of  a  hitherto  unknown  moss.  To  such  it  is 
inexplicable  how  anyone  can  devote  the  labour  of  half  a  lifetime  to  a  classification 
of  Algae  or  Lichens,  or  to  a  monograph  of  the  bramble-tribe  or  orchids.  The  pro- 
gress achieved  eventually  in  this  department  of  botany  is  best  appreciated  when 
the  wide  difference  in  the  numbers  of  species  described  in  botanical  works  of 
different  periods  is  considered.  Theophrastus  in  his  Natural  History  of  Plants 
(about  300  B.C.)  mentions  about  500  species,  and  Pliny  (78  A.D.)  rather  more  than 
1000;  whereas,  by  the  time  of  Linnaeus,  about  10,000  were  known;  and  now  the 
number  must  be  all  but  200,000.  It  should  be  remarked,  however,  that  half  the 
plants  described  since  Linnaeus  lived  fall  into  the  category  of  Cryptogams,  or  non- 
flowering  plants,  the  examination  of  which  was  first  rendered  possible  by  the  wide- 
spread use  of  the  microscope  in  recent  times. 

The  microscope  led  also  to  discoveries  concerning  the  internal  architecture 
of  plants.  A  faint  attempt  in  this  direction,  made  200  years  ago,  had  died  away 
without  leaving  any  trace  behind;  but  at  the  commencement  of  this  century  the 
"inward  construction  of  plants"  was  studied  all  the  more  eagerly  by  means  of  the 
microscope.  In  buildings  belonging  to  different  styles  of  architecture  it  is  not 
only  the  forms  of  the  wings,  stories,  rooms,  and  gables  that  differ,  but  also  and 
in  no  less  degree  those  of  the  columns,  pilasters,  and  decorations.  The  same  is  the 
case  with  plants.  They  possess  chambers  at  different  levels,  vaults,  and  passages. 
They  have  pipes  running  through  them,  and  beams  and  buttresses,  some  massive 
and  some  slender,  to  support  them.  The  pieces  of  which  they  are  built  vary  in 
size,  and  their  walls  are  sculptured  in  all  kinds  of  ways.  It  was  the  business  of  the 
vegetable  anatomist  to  dissect  plants,  to  look  into  all  these  structures  under  the 
microscope,  to  describe  the  various  component  parts  as  well  as  the  ground-plan  and 
elevation  of  the  plant-edifice  as  a  whole;  and  to  name  the  different  forms  of  struc- 
ture after  the  manner  of  Linnaeus  when  he  invented  terms  for  the  different  forms 
of  stems  and  leaves,  and  for  the  several  parts  of  the  flower  and  fruit. 

DOCTKINE  OF  METAMOEPHOSIS  AND   SPECULATIONS  OF 
NATUEE-PHILOSOPHY. 

Side  by  side  with  this  immense  volume  of  research,  which  was  directed  to  the 
separation,  description,  and  synoptical  arrangement  of  mature  forms  only,  there 
arose  about  the  year  1600  another  school  which  considered  vegetable  forms  from 


8  THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN    TIMES. 

the  point  of  view  of  their  life-history,  and  endeavoured  to  trace  them  back  to  their 
origin.  Tracing  the  development,  from  one  stage  to  another,  of  all  the  different 
species,  of  the  multitudinous  forms  of  leaves  and  flowers,  and  of  the  various  kinds 
of  cells  and  tissues,  the  student  of  this  school  has  to  detect  identity  in  multiplicity, 
to  show  that  the  connection  between  forms  which  have  arisen  from  one  another  is 
in  accordance  with  fixed  laws,  and  to  express  those  laws  in  definite  formulae. 

The  attention  of  botanists  was  in  the  first  place  directed  to  the  wonderful  series 
of  changes  in  the  form  of  the  leaf  which  occur  in  all  phanerogamic  (i.e.  flowering) 
plants  as  the  delicate  seedling  gradually  turns  into  a  flowering  shoot.  At  the  circum- 
ference of  the  stem  which  constitutes  the  axis  of  the  plant,  foliar  structures  are 
produced  at  successive  intervals.  All  these  structures  are  essentially  the  same;  but 
they  exhibit  a  continuous  modification  of  their  shape,  arrangement,  size,  and  colour, 
according  to  their  relative  altitudes  upon  the  stem.  To  discover  the  causes  of  this 
structural  variation  was  an  attractive  problem,  and  very  diverse  theories  were 
suggested  for  its  solution.  The  earliest  explanation,  which  was  given  by  the  Italian 
botanist  Cesalpino  in  1583,  is  founded  rather  on  superficial  analogies  and  remote 
resemblances  existing  between  tissues  than  on  careful  observation.  According  to 
this  theory  the  stem  is  composed  of  a  central  medulla  highly  endowed  with  vitality, 
and  surrounded  by  concentric  layers  of  tissue,  those  namely  of  the  wood,  the  bast, 
and  the  cortex.  Each  of  the  foliar  structures  put  forth  from  the  axis  is  supposed  to 
originate  in  one  of  the  above-named  tissues,  the  idea  being  that  the  green  foliage- 
leaf  and  calyx  grew  out  from  the  cortical  layer,  the  corolla  from  the  bast,  the 
stamens  from  the  wood,  and  the  carpels  from  the  medulla.  It  was  believed,  also, 
that  the  outer  envelope  of  a  fruit  arose  from  the  rind  of  the  fruit-stalk,  the  seed- 
coats  from  the  wood,  and  the  central  part  of  the  seed  from  the  medulla. 

Early  in  the  eighteenth  century  there  came  to  be  connected  with  this  theory  the 
doctrine  of  so-called  "  prolepsis,"  which  was  founded  on  more  accurate  comparative 
observations.  It  was  thought  that  the  medulla  of  the  stem  breaks  through  the  rind 
at  particular  spots  to  form  at  each  a  bud,  which  subsequently  grows  out  into  a  side 
branch.  Owing  to  this  lateral  pressure  of  the  medulla  the  ascending  nutrient  sap 
becomes  arrested  beneath  the  rudimentary  bud,  and,  in  consequence,  the  cortex 
develops  under  the  bud  into  a  foliage-leaf.  In  the  bud  the  different  parts  of  the 
future  annual  shoot  are  already  shadowed  forth  in  stages  one  above  the  other;  and 
each  is  produced  always  by  the  one  beneath  it.  As  soon  as  vegetative  activity  is 
resumed  after  the  expiration  of  the  winter  rest,  the  bud  sprouts.  If  only  that  part 
of  it  develops  which  constitutes  the  first  year's  rudiment,  a  shoot  furnished  with 
foliage-leaves  is  produced.  But  the  embryonic  structures  belonging  to  succeeding 
years,  which  are  concealed  in  the  bud,  may  also  be  stimulated  to  development;  and 
when  this  happens,  these  premature  products  do  not  appear  as  foliage-leaves,  but 
in  more  or  less  altered  forms  as  bracts,  sepals,  petals,  stamens,  and  carpels.  If  no 
such  anticipatory  activity  has  been  excited,  the  rudiment  which  in  the  previous 
case  would  have  developed  into  a  bract  does  not  appear  till  the  following  year,  and 
then  as  a  foliage-leaf;  whilst  that  which  would  have  formed  a  calyx  in  the  first 


THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES. 


9 


year  lies  dormant  till  the  third  year,  when  it  too  emerges  simply  as  a  leaf.  This 
transformation  of  the  leaves,  or  metamorphosis  as  Linnaeus  called  it,  is,  therefore,  the 
result  of  anticipation;  and  it  was  assumed  by  the  Linnsean  school  that  the  cause  of 
this  metamorphosis  or  hastened  development  was  a  local  decrease  in  the  quantity 
of  nutriment.  The  idea  was,  that  in  consequence  of  the  limited  supply  of  sap  the 
incipient  leaves  were  not  able  to  attain  to  the  size  of  foliage-leaves,  but  remained 


Fig.  1.— Seedlings  with  Cotyledons  and  Foliage-leaves, 
i  Cytisus  Laburnum.    2  Koelreuteria  paniculata.    «  Acer  platanoides. 

rudimentary,  as  is  the  case  with  many  bracts;  and  further,  that  the  axis  was 
no  longer  capable  of  elongating,  so  that  the  leaves  proceeding  from  it  remained 
close  together,  became  coherent,  and  thus  formed  the  calyx.  The  supporters  of  this 
explanation  relied  particularly  on  the  experience  of  gardeners,  that  a  plant  in  good 
soil  with  a  liberal  supply  of  nutriment  is  apt  to  produce  leafy  shoots  rather  than 
flowers;  whereas,  if  the  same  plant  is  transferred  to  a  poorer  soil,  where  its  food  is 
limited,  it  develops  flowers  in  abundance. 

But  yet  a  third  attempt  was  made  to  explain  this  process  of  transformation,  by 
the  theory  that  parts  which  are  identical  so  far  as  their  origin  is  concerned,  subse- 
quently receive  the  stamp  of  distinct  foliar  organs.  The  diversity  in  the  develop- 
ment of  parts,  originally  alike,  was  supposed  to  depend  on  a  filtration  of  the  nutrient 


10  THE   STUDY   OF   PLANTS   IN    ANCIENT   AND   MODERN   TIMES. 

sap,  the  idea  being  that  identical  primordial  leaves  issuing  from  the  axis  of  a  parti- 
cular plant  were  fashioned  with  more  and  more  delicacy  as  the  sap  became  clarified 
and  refined  in  its  passage  through  the  vessels.  This  explanation  of  metamorphosis 
was  first  given  by  Goethe  (1790)  in  a  treatise  which  was  much  discussed,  and  which 
exercised  a  most  important  influence  in  initiating  researches  of  a  similar  nature. 
Goethe's  interpretation  of  metamorphosis  may  be  briefly  reproduced  as  follows.  A 
plant  is  built  up  gradually  from  a  fundamental  organ — the  leaf — which  issues  from 
the  node  of  a  stem.  First  of  all,  the  organs  which  are  called  seed-leaves  or  cotyledons 
(fig.  1)  develop  on  the  young  plant  as  it  germinates  from  the  seed;  they  proceed 
from  the  lowest  node  of  the  stem,  and  are  frequently  subterranean.  They  are  of 
comparatively  small  size,  are  simple  and  unsegmented,  have  no  trace  of  indentation, 
and  appear  for  the  most  part  as  thick,  whitish  lobes,  which  are,  according  to  Goethe's 
expression,  closely  and  uniformly  packed  with  a  raw  material,  and  are  only  coarsely 
organized.  Goethe  explains  these  leaves  as  being  of  the  lowest  grade  in  the  evolu- 
tionary scale.  After  them  and  above  them  the  foliage  leaves  develop  at  the  suc- 
ceeding nodes  of  the  stem;  they  are  more  expanded  both  in  length  and  breadth; 
their  margins  are  often  notched,  and  their  surfaces  divided  into  lobes,  or  even  com- 
posed of  secondary  leaflets;  and  they  are  coloured  green.  "They  have  attained  to 
a  higher  degree  of  development  and  refinement,  for  which  they  are  indebted  to  the 
light  and  air."  Still  further  up,  there  next  appears  the  third  stage  in  foliar  evolu- 
tion. The  structure  called  by  Linnaeus  the  calyx  is  again  to  be  traced  back  to  the 
leaf.  It  is  a  collection  of  individual  organs  of  the  same  fundamental  type,  but 
modified  in  a  characteristic  manner.  The  close-set  leaves,  which  proceed  from 
nodes  of  the  stem  at  what  is,  in  a  certain  sense,  the  third  story  of  the  plant-edifice 
as  a  whole,  and  which  constitute  the  calyx,  are  contracted,  and  have  but  little  variety 
as  compared  with  the  outspread  foliage-leaves. 

On  the  fourth  rung  of  the  ladder  by  which  the  leaf  ascends  in  its  effort  to  perfect 
itself,  appears  the  structure  named  in  the  Linnsean  terminology  the  corolla.  It 
consists,  like  the  calyx,  only  of  several  leaves  grouped  round  a  centre.  If  a  con- 
traction has  taken  place  in  the  case  of  the  calyx,  we  have  now  once  more  an  expan- 
sion. The  leaves  which  compose  the  corolla  are  usually  larger  than  those  of  the 
calyx.  They  are,  besides,  more  delicate  and  tender,  and  are  brightly  coloured;  and 
Goethe,  whose  mode  of  expression  is  here  preserved  as  far  as  possible,  supposes  them 
to  be  filled  also  with  purer  and  more  subtle  juices.  He  conceives  that  these  juices 
are  in  some  manner  filtered  in  the  lower  leaves  and  in  the  vessels  of  the  lower 
region  of  the  stem,  and  so  reach  the  upper  stories  in  a  more  perfect  condition.  A 
more  refined  sap  must  then,  he  says,  give  rise  to  a  softer  and  more  delicate  tissue 
(fig.  2).  Above  the  corolla  and  at  the  fifth  stage  of  development  there  follows  the 
group  of  stamens,  structures  which,  though  not  answering  to  the  ordinary  conception 
of  leaves,  are  yet  to  be  regarded  again  simply  as  such.  In  the  circle  of  the  corolla 
the  leaves  were  expanded,  and  conspicuous  owing  to  their  colour;  on  the  other 
hand,  in  the  stamens  they  are  contracted  to  an  extreme  degree,  being  almost  fila- 
mentous in  part.  These  leaves  appear  to  have  reached  a  high  degree  of  perfection, 


THE   STUDY  OF  PLANTS   IN   ANCIENT  AND  MODERN  TIMES. 


11 


and  in  the  parts  of  the  stamens  termed  anthers  "pollen-grains"  are  developed  "in 
which  an  extremely  pure  sap  is  stored."     Adjoining  these  pollen-producing  leaves, 


Fig.  2.— Metamorphoses  of  Leaves  as  exhibited  by  the  Poppy. 

i  Germinating  plant  with  cotyledons.      2  and  «  The  same  plant   further  developed  and  with  foliage -leaves;  in  » 
cotyledons  and  lowest  foliage -leaves  are  already  withered.     *  The  same  plant  with  a  flower-bud  showing  tli 
sepals.    «  The  bud  open  and  with  petals,  stamens,  and  carpels  (pistil)  developed. 

where  contraction  has  reached  its  extreme  limit,  is  the  sixth  and  last  story,  which 
is  composed  of  leaves,  once  more  less  closely-set,  and  exhibiting  a  final  expansion 
on  the  part  of  the  plant.  These  are  the  carpels,  which  surround  the  highest  part 


12 


THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES. 


of  the  stem  and  inclose  the  seeds,  the  latter  being  developed  from  the  tip  of  the 
stem.  Thus  the  plant  accomplishes  its  life-history  in  six  stages.  It  is  built  up  of 
leaves,  the  "  intrinsic  identity "  of  which  cannot  be  doubted,  although  they  assume 
extremely  various  shapes  corresponding  to  the  six  strides  towards  perfection.  In 
this  process  of  transformation  or  metamorphosis  of  the  leaf  there  are  three  alter- 
nate contractions  and  expansions,  whilst  each  stage 
is  more  perfect  than  the  one  next  below  it. 

Whilst  seeking  to  explain  metamorphosis  in 
this  manner,  and  endeavouring,  with  greater  per- 
spicacity than  all  his  predecessors  and  contem- 
poraries, "  to  reduce  to  one  simple  universal  prin- 
ciple all  the  multifarious  phenomena  of  the  glorious 
garden  of  the  world,"  Goethe  conceived  the  notion 
of  a  typical  plant,  an  ideal,  the  realization  of 
which  is  achieved  in  nature  by  means  of  a  mani- 
fold variation  of  individual  parts.  This  abstract 
notion  of  a  plant's  development  with  its  six  stages 
corresponding  to  "three  wave-crests"  or  expan- 
sions (Leaf,  Petal,  Carpel)  and  "three  wave- 
troughs"  or  contractions  (Cotyledon,  Sepal,  Sta- 
men) is  expressed  graphically  in  figure  3.  It  still 
holds  its  ground  at  the  present  day  under  the 
name  of  Goethe's  "  Urpflanze,"  and  the  credit  of  its 
invention  is  entirely  his.  But  it  is  not  quite  right 
to  claim  for  Goethe,  in  addition,  the  title  of 
founder  of  the  doctrine  of  vegetable  metamor- 
phosis; for  in  reality  he  only  offered  another  inter- 
pretation and  mode  of  representation  of  a  pheno- 
menon already  included  by  Linnaeus  under  the 
term  metamorphosis.  Linnaeus  had  instituted  a 
comparison  between  the  metamorphosis  of  plants  and  that  of  insects;  in  particular, 
he  likened  the  calyx  to  the  ruptured  integument  of  a  chrysalis  and  the  internal  parts 
of  a  flower  to  the  perfect  insect  (Imago).  He  also  made  many  different  attempts  to 
establish  analogies  between  the  development  of  plants  and  that  of  animals;  and  in 
so  doing  he  opened  up  a  wide  field  for  the  speculations  of  the  "nature  philosophers" 
in  the  earlier  part  of  the  nineteenth  century. 

An  extensive  study  of  this  subject  now  commenced;  and  writers  on  nature- 
philosophy  worked  indefatigably  at  the  amplification  and  modification  of  this 
theme,  first  broached  by  Linnaeus. 

"A  plant  is  a  magnetic  needle  attracted  towards  the  light  from  the  earth  into 
the  air.  It  is  a  galvanic  bubble,  and,  as  such,  is  earth,  water,  and  air.  The  plant- 
bubble  possesses  two  opposite  extremities,  a  single  terrestrial  end  and  a  dual  aerial 
end;  and  so  plants  must  be  looked  upon  as  being  organisms  which  manifest  a 


Fig.  3.— Goethe's  "Urpflanze.' 


THE   STUDY   OF   PLANTS   IN   ANCIENT   AND  MODERN  TIMES.  13 

continual  struggle  to  become  earth  on  the  one  hand  and  air  on  the  other,  unmixed 
metal  at  one  end,  and  dual  air  at  the  other.  A  plant  is  a  radius,  which  becomes 
single  towards  the  centre,  whilst  it  divides  or  unfolds  towards  the  periphery;  it  is 
not  therefore  an  entire  circle  or  sphere,  but  only  a  segment  of  one  of  those  figures. 
The  individual  animal,  on  the  contrary,  constitutes  of  itself  a  sphere,  and  is  there- 
fore equivalent  to  all  plants  put  together.  Animals  are  entire  worlds,  satellites  or 
moons,  which  circle  independently  round  the  earth;  whereas  plants  are  only  equal 
to  a  heavenly  body  in  their  totality.  An  animal  is  an  infinitude  of  plants.  A 
blossom  which,  when  severed  from  the  stem,  preserves  by  its  own  movement  the 
galvanic  process  or  life,  is  an  animal.  An  animal  is  a  flower-bubble  set  free  from 
the  earth  and  living  alone  in  air  and  water  by  virtue  of  its  own  motion." 

Page  after  page  of  the  writings  on  Nature-philosophy  of  Oken  (1810)  and 
other  contemporary  naturalists  is  filled  with  interminable  statements  of  the  same 
kind.  At  the  present  day  it  seems  scarcely  credible  that  such  propositions  were 
then  received  with  admiration  as  profound  and  ingenious  utterances,  and  that  they 
were  even  adopted  as  mottoes  for  botanical  and  geological  treatises.  For  example, 
it  is  worthy  of  record  that  as  late  as  the  year  1843  the  Austrian  botanist  Unger 
made  use  of  the  last  of  the  flowers  of  rhetoric  above  quoted  from  Oken's  Nature- 
philosophy  as  a  motto  for  one  of  his  first  works  on  the  history  of  development, 
the  title  of  which  is  Plants  at  the  Moment  of  their  becoming  Animals. 

The  general  divisions  or  systems  of  the  vegetable  kingdom  which  were  evolved 
by  adherents  of  the  school  of  Nature-philosophy  were,  as  may  be  imagined,  just  as 
absurd  as  the  speculations  on  which  they  were  based.  In  his  Philosophical  Systems 
of  Plants  Oken  develops  in  the  first  place  the  idea  that  the  vegetable  kingdom 
is  a  single  plant  taken  to  pieces.  Inasmuch  as  the  ideal  highest  plant  is  composed 
of  five  organs,  there  must  likewise  be  five  classes:  root-plants,  stem-plants,  leaf- 
plants,  flower-plants,  and  fruit-plants.  The  world  is  fashioned  out  of  the  elements: 
earth,  water,  air,  and  fire.  Hereupon  is  founded  a  classification  of  root-plants  into 
earth-plants  or  lichens,  water-plants  or  fungi,  air-plants  or  mosses,  and  light-plants 
or  ferns.  Proceeding  from  the  assumption  that  all  the  groups  are  parallel  and  that 
the  principle  of  classification  for  each  group  is  always  given  by  the  one  preceding 
it,  we  have  next,  to  take  one  instance,  the  second  class — that  of  stem-plants— 
divided  (in  accordance  with  the  subdivision  of  earth  into  earths,  salts,  bronzes,  and 
ores)  into  earth-plants  or  grasses,  salt-plants  or  lilies,  bronze-plants  or  spices,  and 
ore-plants  or  palms. 

SCIENTIFIC    METHOD    BASED    ON    THE    HISTOKY    OF    DEVELOPMENT. 

Though  as  we  see  the  doctrine  of  metamorphosis,  with  its  conception  of  a 
typical  plant,  degenerated  thus  into  the  most  barren  of  fancies,  still  from  it  originated 
the  line  of  research  based  on  the  history  of  development  which  has  since  borne 
fruit  in  every  department  of  botany.  Observers  arrived  at  the  conviction  that 
every  living  plant  undergoes  a  continuous  transformation  which  follows  a  definite 


14  THE   STUDY   OF   PLANTS   IN    ANCIENT   AND   MODERN   TIMES. 

course,  and  that  accordingly  every  species  is  constructed  on  a  plan  fixed  within 
general  limits  and  exhibiting  variation  in  externals  only.  These,  it  is  true,  are  often 
more  conspicuous  at  first  sight  than  the  direction  and  disposition  of  the  parts  which 
are  really  fundamental,  and  secure  the  stability  of  the  entire  structure.  But  in 
order  to  ascertain  the  plan  of  construction  it  was  found  necessary  to  go  back  to  the 
very  first  visible  appearance  of  each  organ;  to  determine  how  the  original  rudi- 
ments of  the  embryo  and  the  beginnings  of  roots,  stems,  leaves,  and  parts  of  the 
flower  are  formed,  and  to  see  what  rudiments  succeed  in  opening  out,  branching  and 
dividing,  and  what  remain  behind  to  perish  and  be  displaced  by  organs  growing 
vigorously  in  close  proximity  to  them. 

These  researches  into  the  course  of  development  of  the  separate  parts  of  flower- 
ing plants,  and  to  a  still  greater  extent  the  observations  of  the  development  of 
cryptogams  or  spore-plants  (rendered  possible  by  improvements  in  the  construction 
of  microscopes),  led  naturally  to  a  study  of  the  history  of  the  elementary  structures 
of  which  all  plants  are  composed.  Previously  three  kinds  of  elementary  organs  had 
been  supposed  to  exist,  utricles,  vessels,  and  fibres.  The  observations  of  Brown  and 
Mohl  (1830-1840)  resulted,  however,  in  the  identification  of  the  cell  as  the  common 
starting-point  of  all  these  elementary  organs.  This  led  to  the  further  discoveries 
that  protoplasm  is  the  formative  and  living  part  of  a  cell,  and  that  each  cell  is 
differentiated  into  a  protoplasmic  cell-body  and  a  cell-membrane.  It  followed 
that  the  envelope  of  the  protoplasmic  body,  the  cell-membrane,  which  had  hitherto 
been  considered  the  primary  formation,  was  in  reality  a  product  of  the  protoplasm 
enveloped  by  it,  and  this  discovery  resulted  in  a  complete  revolution  in  the  con- 
ception of  cells  generally.  Further  investigation  led  to  the  conclusion  that  the 
various  modes  of  growth  and  multiplication  depend  on  definite  laws.  That  even 
in  the  mode  of  juxtaposition  of  daughter-cells  arising  in  reproduction,  a  certain  plan 
of  construction  may  be  distinguished  in  each  species  which  must  stand  ultimately 
in  some  oausal  relation  to  the  structural  system  of  the  whole  plant.  The  progress 
achieved  along  these  lines  in  the  course  of  a  few  decades  has  been  extraordinarily 
great,  no  doubt  due  to  the  peculiar  fascination  which  the  study  of  the  life-histories 
and  transformations  of  living  organisms  and  the  observation  of  mysterious  process* 
invisible  to  the  naked  eye  have  had  for  the  mind  of  the  inquirer. 

In  that  group  of  plants  which  includes  the  forms  classed  together  by  the  earli( 
botanists  under  the  name  of  Cryptogamia  an  altogether  new  world  was  revealec 
An  undreamed-of  variety  was  discovered  to  exist  in  the  processes  of  propagatioi 
and  rejuvenescence  of  these  forms  of  plants  by  means  of  single  cells  or  spoi 
Objects  which,  having  regard  to  their  external  form,  had  been  assigned  to  widely 
different  groups,  were  found  to  be  connected  with  one  another  as  stages  in  the 
development  of  one  and  the  same  species;  and  one  result  of  these  discoveries  was 
the  establishment  in  this  division  of  the  vegetable  kingdom  of  an  entirely  new 
system  of  classification  based  on  life-histories.     The  systematic  arrangement   of 
Flowering-plants  or  Phanerogams  also  underwent  essential  alteration.     The  Linnsean 
system,  founded  on  the  numerical  relations  between  the  different  parts  of  the  flower, 


THE  STUDY  OF  PLANTS   IN   ANCIENT  AND   MODERN  TIMES.  15 

had  indeed  already  been  displaced  by  another  method  of  classification,  that  of  the 
French  observers  Jussieu  (1789)  and  De  Candolle  (1813),  who  framed  systems  said 
to  be  natural  when  contrasted  with  the  artificial  system  of  Linnseus.  At  bottom, 
however,  these  classifications  only  differed  from  the  Linnaean  in  the  fact  that  they 
multiplied  and  widened  the  grounds  of  division.  The  main  division  of  Phanero- 
gamia  into  those  which  put  forth  one  cotyledon  (or  seed-leaf)  on  germinating 
(Monocotyledones)  and  those  whose  seedlings  bear  two  cotyledons  (Dicotyledones) 
is  the  only  one  that  could  serve  as  a  starting-point  for  a  system  based  on  the  history 
of  development;  but  when  we  come  to  the  grouping  of  Dicotyledones  into  those 
destitute  of  corolla  (Apetalse),  those  with  the  corolla  composed  of  coherent  petals 
(Monopetalse),  and  those  with  the  corolla  composed  of  distinct  petals  (Dialy- 
petalse),  we  have  already  to  admit  something  forced,  and  a  reliance  on  characteristics 
merely  external. 

The  system  which  is  the  outcome  of  the  study  of  development  starts  with 
the  idea  that  similarity  between  adult  forms  is  not  always  decisive  evidence  of 
their  belonging  to  the  same  group,  and  that  the  relationships  of  different  plants 
is  much  more  surely  indicated  by  the  fact  of  their  exhibiting  the  same  laws  of 
growth  and  the  same  phenomena  of  reproduction.  Plants  exhibiting  widely 
different  external  forms  in  the  mature  state  are  nevertheless  to  be  looked  upon 
as  closely  allied  if  they  are  constructed  according  to  the  same  plan,  and  vice  versa. 
There  can  be  no  question  that  a  system  based  on  these  principles  means  a  material 
advance.  At  the  same  time  it  cannot  be  overlooked  that  great  difficulties  are 
involved  in  hitting  upon  the  right  selection  from  among  the  number  of  phenomena 
observed  in  the  course  of  a  plant's  development,  and  in  determining  which  of  these 
phenomena  are  to  be  referred  to  a  mode  of  construction  common  to  a  number 
of  plants,  and  therefore  treated  as  fundamental  properties,  and  which  should  be 
esteemed  merely  as  outcomes  of  the  conditions  of  life  affecting  the  existence  of  the 
plant  in  question. 

OBJECTS  OF  BOTANICAL  KESEAECH  AT  THE  PRESENT  DAY. 

DESCRIPTIVE  BOTANY  only  concerns  itself  with  the  configuration  of  a  plant. 
COMPARATIVE  MORPHOLOGY  endeavours  to  trace  back  to  a  single  prototype  the 
extremely  various  forms  exhibited  by  mature  plants.  The  history  of  development 
deals  with  the  growth  and  differentiation  of  such  forms.  But  all  these  paths  of 
research  shirk  the  problem  of  the  biological  significance  of  the  different  forms. 
The  line  of  investigation  starting  from  the  conception  of  a  plant's  life  as  a  series 
of  physical  and  chemical  processes,  and  which  attempts  to  elucidate  the  configura- 
tion of  a  plant  in  the  light  of  its  environment,  could  not  be  developed  with  the 
slightest  prospect  of  success  until  physics,  chemistry,  and  other  allied  sciences  had 
reached  a  high  degree  of  perfection,  and  till  botanists  had  become  convinced  that  the 
phenomena  of  life  are  only  to  be  fathomed  by  means  of  experiment. 

The  earliest  attempts  to  define  the  biological  significance  of  the  several  parts  of 


16  THE   STUDY   OF   PLANTS   IN    ANCIENT   AND   MODERN   TIMES. 

a  plant  do,  it  is  true,  take  one  back  as  far  as  Aristotle  and  his  school;  but  the  ideas 
of  vegetable  life  entertained  at  that  time  are  scarcely  more  than  fantastic  dreams; 
and  the  recognition  now  accorded  to  them  springs  rather  from  a  reverence  for 
antiquity  than  from  any  intrinsic  merit  which  they  possessed.  The  first  experi- 
mental investigations  into  the  vital  phenomena  of  plants  were  published  by 
Stephen  Hales  in  1718;  but  it  was  not  till  a  hundred  years  later  that  this  kind 
of  research  really  came  into  vogue.  It  brought  with  it  the  conception  of  a  cell 
as  a  miniature  chemical  laboratory,  and  looked  for  mechanical  interpretations  of 
the  phenomena  of  nutrition,  sap-circulation,  growth,  movement — in  short,  all  vital 

processes and  for  some  connection  between  these  processes  and  the  external  form. 

Whereas,  in  the  case  of  descriptive  and  speculative  botany,  and  in  the  study  of 
development,  the  entire  plant  was  first  taken  into  consideration,  next  its  several 
parts,  and  lastly  the  cells  and  protoplasm;  in  the  new  department  of  inquiry,  on 
the  contrary,  the  complete  histories  of  the  ultimate  organs  were  studied  first 
of  all,  then  the  significance  of  the  different  forms  of  the  several  members,  and  lastly 
the  phenomena  occasioned  by  the  aggregate  life  of  all  the  various  kinds  of  animals 
and  plants. 

Modern  science,  governed  as  it  is  by  the  desire  to  lay  bare  the  causes  of  all 
phenomena,  is  no  longer  satisfied  with  knowledge  concerning  the  existence  of  cells, 
the  arrangement  of  the  different  forms  of  cell,  the  development  of  their  contents, 
and  the  changes  undergone  by  cell-membranes.  At  the  present  day  we  inquire , 
what  are  the  functions  of  the  various  bodies  which  are  formed  within  the  proto- 
plasm? Why  is  the  cell-membrane  thickened  at  a  particular  spot  in  a  particular 
manner?  What  is  the  meaning  of  all  the  tubes  and  passages  which  exhibit  such 
great  diversity  of  size  and  shape?  What  part  is  played  by  the  peculiar  mouths  of 
these  channels,  and  why  do  they  vary  so  greatly  in  shape  and  distribution  in  plants 
which  are  subject  to  different  external  conditions?  We  are  no  longer  content  to 
determine  in  what  manner  the  rudimentary  organ  of  a  plant  is  produced,  or  how 
it  expands  in  one  case  and  frequently  divides,  or  else  is  arrested  in  its  growth  and 
shrivels  up;  but  we  inquire  the  reason  why  one  rudiment  grows  and  develops 
whilst  another  is  obliterated.  For  us  no  fact  is  without  significance.  Our 
curiosity  extends  to  the  shape,  size,  and  direction  of  the  roots;  to  the  configuration, 
venation,  and  insertion  of  the  leaves;  to  the  structure  and  colour  of  the  flowers; 
and  to  the  form  of  the  fruit  and  seeds;  and  we  assume  that  even  each  thorn, 
prickle,  or  hair  has  a  definite  function  to  fulfil.  But  efforts  are  also  made  to 
explain  the  mutual  relations  of  the  different  organs  of  a  plant,  and  the  relations 
between  different  species  of  plants  which  grow  together.  Lastly,  this  department 
of  research  (the  rapid  growth  of  which  is  due  to  Darwin)  includes  amongst  its 
objects  a  solution  of  the  problem  of  the  ultimate  grounds  of  morphological  variety, 
the  causes  of  which  can  only  be  sought  for  in  a  qualitative  variation  of  protoplasm. 
Specific  relationship  is  explained  by  attributing  it  to  similarity  in  the  constitution 
of  the  protoplasm  of  allied  species,  and  the  affinities  exhibited  by  living  and  extinct 
plants  are  used  as  means  of  unfolding  the  hereditary  connection  between  the 


THE   STUDY   OF  PLANTS   IN   ANCIENT  AND   MODERN  TIMES.  17 

thousands  of  different  sorts  of  forms,  and  of  tracing  the  history  of  plants  and 
vegetable  life  all  over  the  earth. 

The  various  lines  of  botanical  research  described  in  the  foregoing  pages,  with 
their  particular  problems  and  objects,  have  but  slight  connection  one  with  another. 
They  run  side  by  side  along  separate  paths,  and  it  is  only  occasionally  that  a 
junction  is  apparent  which  establishes  a  communication  between  one  path  and 
another.  The  subject-matter,  however,  is  always  the  same.  Whether  we  have 
to  do  with  the  perfected  form  or  with  its  growth,  whether  we  try  to  interpret  the 
processes  of  life  or  to  trace  the  genealogy  of  the  vegetable  kingdom,  we  always 
start  from  the  forms  of  plants ;  and  the  ultimate  result  is  never  anything  more  than 
a  description  of  the  varying  impressions  which  we  receive  at  different  times  from 
the  objects  observed,  and  which  we  endeavour  to  bring  into  mutual  connection. 
All  the  different  departments  of  botany  are  accordingly  more  or  less  limited  to 
description;  and  even  when  we  endeavour  to  resolve  vital  phenomena  into 
mechanical  processes  we  can  only  describe,  and  not  really  explain,  what  happens. 
The  processes  which  we  call  life  are  movements.  But  the  causes  of  those  move- 
ments, so-called  forces,  are  purely  subjective  ideas,  and  do  not  involve  the  concep- 
tion of  any  actual  fact,  so  that  our  passion  for  causality  is  only  ostensibly  gratified 
by  the  help  of  mechanics.  Du  Bois  Reymond  is  not  far  wrong  when  he  follows 
out  this  train  of  thought  to  the  conclusion  (however  paradoxical  it  may  sound) 
that  there  is  no  essential  difference  between  describing  the  trajectory  (or  particular 
kind  of  curve)  in  which  a  projectile  moves  on  the  one  hand,  and  describing  a  beetle 
or  the  leaf  of  a  tree  on  the  other. 

But  even  though  the  ultimate  sources  of  vital  phenomena  remain  unrevealed, 
the  desire  to  represent  all  processes  as  effects,  and  to  demonstrate  the  causes  of 
such  effects — a  desire  which  is  at  the  very  root  of  modern  research — finds  at  least 
partial  gratification  in  tracing  a  phenomenon  back  to  its  proximate  cause.  In  the 
mere  act  of  linking  ascertained  facts  together,  and  in  the  creation  of  ideas  involv- 
ing interdependence  among  the  phenomena  observed,  there  lies  an  irresistible  charm 
which  is  a  continual  stimulus  to  fresh  investigations.  Even  though  we  be  sure 
that  we  shall  never  be  able  to  fathom  the  truth  completely,  we  shall  still  go  on 
seeking  to  approach  it.  The  more  imaginative  an  investigator  the  more  keenly 
is  he  goaded  to  discovery  by  this  craving  for  an  explanation  of  things  and  for 
a  solution  of  the  mute  riddle  which  is  presented  to  us  by  the  forms  of  plants. 
It  is  impossible  to  overrate  the  value  and  efficiency  of  the  transcendent  gift  of 
imagination  when  applied  to  questions  of  Natural  History.  Thus  when  we  inquire 
whether  certain  characters  noted  in  a  plant  are  hereditary,  constant,  and  inalienable, 
or  are  only  occasioned  by  local  influences  of  climate  or  soil,  and  hence  deduce 
whether  the  plant  in  question  is  to  be  looked  upon  as  a  species  or  a  variety;  when 
we  conclude  from  the  fact  of  a  resemblance  between  the  histories  of  the  develop- 
ment of  various  species  that  they  are  related,  and  place  them  together  in  groups 
and  series;  when  we  unravel  the  genealogies  of  different  plants  by  comparing 

forms  still  living  with  others  that  are  extinct;  when  we  try  to  represent  clearly 
VOL.  I.  2 


18  THE   STUDY   OF   PLANTS   IN   ANCIENT   AND   MODERN   TIMES. 

the  molecular  structure  of  the  cell-membrane  by  arguing  from  the  phenomena 
manifested  by  that  membrane;  when  we  investigate  the  meaning  of  the  peculiar 
thickenings  and  sculpturings  of  the  walls  of  cells,  or  when  we  discover  the  strange 
forms  of  flowers  and  fruits  to  be  mechanical  contrivances  adapted  to  the  forms 
of  certain  animals,  and  judge  the  extent  to  which  these  contrivances  are  advan- 
tageous, or  the  reverse,  to  the  plants — in  all  these  and  similar  investigations 
imagination  plays  a  predominant  part.  Experiment  itself  is  really  a  result  of 
the  exercise  of  that  faculty.  Every  experiment  is  a  question  addressed  to  nature. 
But  each  interrogation  must  be  preceded  by  a  conjecture  as  to  the  probable  state 
of  the  case;  and  the  object  of  the  experiment  is  to  decide  which  of  the  preliminary 
hypotheses  is  the  right  one,  or  at  least  which  of  them  approaches  nearest  to  the 
true  solution.  The  fact  that  when  the  imagination  has  been  allowed  to  soar  unre- 
strained, or  without  the  steadying  ballast  of  actual  observations,  it  has  frequently 
led  its  followers  into  error,  does  not  detract  at  all  from  its  extreme  value  as  an 
aid  to  research,  notwithstanding  the  fact  that  it  is  responsible  for  the  wonderful 
fantasies  of  nature-philosophy  of  which  a  few  specimens  have  been  given.  Nor 
should  we  esteem  it  the  less  because  enlargements  of  the  field  of  observation  and 
improvements  in  the  instruments  employed  have  again  and  again  led  to  the  sub- 
stitution of  new  ideas  for  those  which  careful  observers  and  experimentalists  had 
arrived  at  by  collating  the  facts  ascertained  through  their  labours. 

For  the  same  reasons  it  is  unfair  to  regard  with  contempt  the  ideas  of  plant- 
life  formed  by  our  predecessors.  It  should  never  be  forgotten  how  much  smaller 
was  the  number  of  observations  upon  which  botanists  had  to  rely  in  former  times, 
and  how  much  less  perfect  were  their  instruments  of  research.  Every  one  of 
our  theories  has  its  history.  In  the  first  place  a  few  puzzling  facts  are  observed, 
and  gradually  others  come  to  be  associated  with  them.  A  general  survey  of  the 
phenomena  in  question  suggests  the  existence  of  a  definite  uniformity  underlying 
them;  and  attempts  are  made  to  grasp  the  nature  of  such  uniformity  and  to  define 
it  in  words.  Whilst  the  question  thus  raised  is  in  suspense,  botanists  strive  with 
more  or  less  success  to  answer  it,  until  a  master  mind  appears.  He  collates  the 
observed  facts,  gathers  from  them  the  law  of  their  harmony,  generalizes  it,  and 
announces  the  solution  of  the  enigma.  But  observations  continue  to  multiply; 
•scientific  instruments  become  more  delicate,  and  some  of  the  newly-observed  facts 
will  not  adapt  themselves  to  the  scheme  of  the  earlier  generalization.  At  first 
they  are  held  to  be  exceptions  to  the  rule.  By  degrees,  however,  these  exceptions 
accumulate;  the  law  has  lost  its  universality  and  must  undergo  expansion,  or  else 
it  has  become  quite  obsolete  and  must  be  replaced  by  another.  So  it  has  been 
in  all  past  times,  and  so  will  it  be  in  the  future.  Only  a  narrow  mind  is  capable 
of  claiming  infallibility  and  permanence  for  the  ideas  which  the  present  age  lays 
down  as  laws  of  nature. 

These  remarks  on  the  limitations  of  our  knowledge  of  nature,  the  importance 
of  imagination  as  an  aid  in  research,  and  the  variability  of  our  theories  are  made 
with  a  view  to  moderate,  on  the  one  hand,  the  exuberant  hopes  raised  by  the  belief 


THE  STUDY   OF  PLANTS   IN  ANCIENT  AND   MODERN  TIMES.  19 

that  the  great  questions  connected  with  the  phenomenon  of  life  will  be  solved, 
and  to  correct,  on  the  other,  the  habit  of  not  appreciating  impartially  the  various 
methods  which  have  been  and  are  still  employed  by  different  botanists.  In  our 
own  time,  adhering  as  we  do  to  the  principle  of  the  division  of  labour,  it  has  become 
almost  universal  for  each  investigator  to  advance  only  along  a  single,  very  narrow 
path.  But  owing  to  the  fact  that  one-sidedness  too  often  leads  to  self-conceit,  the 
lines  of  study  followed  by  others  are  not  infrequently  despised,  just  as  overweening 
confidence  in  the  infallibility  of  the  discoveries  of  the  present  day  leads  to  deprecia- 
tion of  the  labours  of  former  times. 

For  the  building-up  of  the  science  of  the  Biology  of  Plants  everything  relating 
to  the  subject  has  its  value,  and  is  capable  of  being  turned  to  account.  Whether 
the  materials  are  rough  or  elaborated,  massive,  fragmentary,  or  merely  connective, 
howsoever  and  whensoever  they  have  been  acquired,  they  all  are  useful.  The  study 
of  dried  plants  made  by  a  student  in  a  provincial  museum,  the  discoveries  of  an 
amateur  regarding  the  flora  of  a  sequestered  valley,  the  contributions  of  horticul- 
turalists  on  subjects  of  experiment,  the  facts  gleaned  by  farmers  and  foresters  in 
fields  and  woods,  the  disclosures  which  have  been  wrested  from  living  plants  in 
university  laboratories,  and  the  observations  conducted  in  the  greatest  and  best 
of  all  laboratories— that  of  Nature  herself— all  these  results  should  be  turned  to 
account.  Let  us  take  for  the  motto  of  the  following  pages  the  text: 


"Prove  all  things;  hold  fast  that  which  is  good." 


THE  LIVING  PEINCIPLE  IN  PLANTS. 


1.   PKOTOPLASTS   CONSIDERED   AS   THE    SEAT   OF  LIFE. 

Discovery  of  the  Cell.— Discovery  of  Protoplasm. 

DISCOVERY   OF   THE  CELL. 

What  is  life?  This  ever-interesting  question  has  seemed  to  approach  nearer 
solution  on  the  occasion  of  every  great  scientific  discovery.  But  never  did  the  hope 
of  being  able  to  penetrate  the  great  secret  of  life  appear  better  founded  than  at  the 
time  when,  among  other  memorable  developments  of  science,  it  was  discovered  that 
objects  could  be  rendered  visible  on  an  enlarged  scale  by  the  use  of  glass  lenses,  and 
the  microscope  was  invented.  These  magnifying  glasses  were  expected  to  yield, 
not  only  an  insight  into  the  minute  structure  of  living  beings  which  is  invisible  to 
the  naked  eye,  but  also  revelations  concerning  the  processes  which  constitute  life 
in  plants  and  animals.  The  first  discoveries  made  with  the  microscope,  between 
1665  and  1700,  produced  a  profound  impression  on  the  observers.  The  Dutch 
philosopher  Swammerdam  became  almost  insane  at  the  marvels  revealed  by  his- 
lenses,  and  at  last  destroyed  his  notes,  having  come  to  the  conclusion  that  it  was 
sacrilege  to  unveil,  and  thereby  profane,  what  was  designed  by  the  Creator  to  remain 
hidden  from  human  ken.  The  observations  of  Leeuwenhoek  (1632-1723)  with 
magnifying  glasses  formed  by  melting  fine  glass  threads  in  a  lamp,  were  for  a 
long  time  held  to  be  delusions;  and  it  was  not  till  the  English  observer  Robert 
Hooke  had  confirmed  the  fact  of  the  existence  of  the  minute  organisms  seen  by 
Leeuwenhoek  in  infusions  of  pepper,  and  had  exhibited  them  under  his  microscope 
in  1667  at  a  meeting  of  the  Royal  Society  in  London,  that  doubts  as  to  their  actual 
existence  disappeared.  Indeed  a  special  document  was  then  drawn  up  and  signed 
by  all  those  who  were  satisfied,  on  the  evidence  of  their  own  eyesight,  of  the  accu- 
racy of  the  observation;  and  this  clearly  shows  how  greatly  people  were  impressed 
with  the  importance  of  these  discoveries.  Of  the  different  forms  of  the  tiny 
organisms,  amounting  to  nearly  four  hundred,  which  were  at  that  time  distinguished, 
and  all  included  under  the  name  Infusoria,  because  first  seen  in  infusions  of  pepper- 
corns, some  only  are  at  the  present  day  reckoned  as  animals.  In  many  cases  it 
has  been  ascertained  that  they  are  the  spores  of  plants,  whilst  others  again  belong 
to  the  boundary-land  where  the  animal  and  vegetable  kingdoms  are  merged. 

The  presence  or  absence  of  movement  used  to  be  considered  as  the  most  decisive 
mark  of  the  difference  between  animals  and  plants,  and,  accordingly,  all  the  minute 


21 


22 


DISCOVERY  OF  THE   CELL. 


beings  which  were  seen  bustling  about  in  watery  media  were  described  and  labelled 
as  animals.  No  movement  was  found  in  the  higher  plants  which  were  studied  with 
the  microscope  about  the  same  time  by  Dutch,  Italian,  and  English  observers;  but, 
on  the  other  hand,  these  investigations  led  to  a  recognition  of  the  quite  special 
peculiarities  of  such  structures  as  leaves  and  stem,  wood  and  pith.  These  parts  of 
plants  appeared  under  the  microscope  like  honey-combs,  which  are  built  up  of  a 


2  / 


Fig.  4.— Vegetable  Cells  (from  Grew's  Anatomy  of  Plants). 

»  Longitudinal  section  through  a  young  apricot  seed.    2  Transverse  section  of  the  petiole  of  the  Wild  Clary. 
»  Transverse  section  of  a  pine  branch. 

great  number  of  cells,  some  empty  and  some  full  of  honey.  From  this  similarity 
the  term  "  cell"  arose,  which  later  was  to  play  so  important  a  part  in  botany.  In 
the  drawings  of  parts  of  plants  as  seen  under  the  microscope  the  resemblance  to  a 
honey-comb  is  very  apparent;  indeed,  it  is  sometimes  rather  more  striking  than 
when  seen  in  reality,  as,  for  instance,  is  the  case  in  the  above  reproduction  of  three 
engravings  from  Nehemiah  Grew's  fine  work  published  in  London,  1672.  It  was 
also  noticed  that,  besides  the  structures  which  resembled  honey-comb,  there  were 
little  tubes  and  fibres  which  were  distributed  and  aggregated  in  very  various  ways, 
and  were  bound  up  together  into  strands  and  membranes,  and  into  pith  and  wood; 
further,  all  these  things  were  seen  to  increase  in  size  and  number  in  the  growing 


DISCOVERY   OF  THE   CELL.  23 

parts  of  plants.  How  growth  and  multiplication  took  place,  and  where  exactly  the 
seat  of  a  plant's  life  lay,  remained,  of  course,  obscure.  It  was,  however,  natural  to 
assume  that  the  walls  of  these  small  cells  constituted  the  essential  part  and  living 
substance  of  plants,  that  they  drew  materials  from  the  fluids  which  rose  by  suction 
in  the  tubes,  and  so  increased  in  size  and  were  renewed. 

It  was  as  yet  hardly  suspected  that  the  slimy  substance  which  filled  the  cells 
of  a  plant,  like  honey  in  a  honey-comb,  was  the  basis  of  life.  The  observation  made 
again  and  again  at  the  beginning  of  the  nineteenth  century,  that  the  cell-contents 
of  certain  algae  are  extruded  in  the  form  of  globules  of  jelly,  and  that  each  globule 
moves  independently  and  swims  about  in  the  water  for  a  time,  but  then  comes  to 
rest  and  becomes  the  starting-point  of  a  new  alga,  might  undoubtedly  have  led 
to  this  conclusion.  The  accounts  of  these  occurrences  were,  however,  considered 
incredible  by  the  majority  of  contemporary  observers;  and  it  was  not  till  recently, 
when  Unger  established  the  phenomenon  as  an  indubitable  fact,  that  a  proper 
estimation  of  its  value  was  accorded.  In  the  year  1826  this  botanist  investigated 
under  the  microscope  a  water-weed  found  at  Ottakrinn,  near  Vienna,  which  had 
been  described  by  systematic  writers  as  an  alga,  and  named  Vaucheria  clavata. 
To  the  naked  eye  it  appears  like  a  dense  plexus  of  dark-green  irregularly  branched 
and  matted  filaments.  These  filaments,  when  magnified,  are  seen  to  be  tubular  cells 
which  wither  and  die  away  at  the  base  whilst  growing  at  the  apex,  and  developing 
sac-like  branches  laterally.  (Fig.  25 A.)  The  free  ends  of  these  tubes  are  blunt  and 
rounded.  The  substance  they  contain  is  slimy,  and,  though  itself  colourless,  is 
studded  throughout  with  green  granules;  whilst  near  the  blunt  end  of  each  filament 
these  green  particles  are  so  closely  packed  that  the  entire  contents  of  that  part 
appear  of  a  dark-green  colour. 

Now,  there  comes  a  time  in  the  life  of  every  one  of  these  filaments  when  its 
extremity  swells  and  becomes  more  or  less  club-shaped.  The  moment  this  occurs, 
the  dark-green  contents  withdraw  somewhat  from  the  extremity,  leaving  it  hyaline 
and  transparent.  Almost  simultaneously  the  contents  of  the  swollen  part  of  the 
tube  nearest  the  apex  become  transparent,  whilst  further  down  the  colour  becomes 
very  dark.  (Figure  25 A, a.)  Twelve  hours  after  the  commencement  of  this  change, 
that  portion  of  the  tube's  contents  which  occupies  the  club-shaped  end  separates 
itself  entirely  from  the  rest.  A  little  later,  the  cell-wall  at  the  apex  of  the  tube 
suddenly  splits,  the  edges  of  the  slit  fold  back,  and  the  inclosed  mass  travels 
through  the  aperture  (fig.  c).  This  jelly-like  ball,  having  a  greater  diameter  than 
the  hole,  is  at  first  strangulated  as  it  struggles  forward,  so  that  it  assumes  the  shape 
of  an  hour-glass  and  looks  for  an  instant  as  if  it  would  remain  stuck  fast.  There 
now  arises,  however,  in  the  entire  mass  of  green  jelly  an  abrupt  movement  of 
rotation  combined  with  forward  straining,  and  in  another  instant  it  has  escaped 
through  the  narrow  aperture  and  is  swimming  freely  about  in  the  surrounding 
water  (fig.  d).  The  entire  phenomenon  of  the  escape  of  these  bodies  takes  place 
between  8  and  9  A.M.,  and,  in  any  one  case,  in  less  than  two  minutes.  When  free, 
each  individual  assumes  the  shape  of  a  perfectly  regular  ellipsoid  (fig.  d),  having 


24  DISCOVERY   OF   THE    CELL. 

one  pole  of  a  lighter  green  than  the  other;  it  moves  always  in  the  direction  of  the 
former,  so  that  the  lighter  end  may  be  properly  designated  the  anterior.  At  first 
the  ball  rises  to  the  surface  of  the  water  towards  the  light,  but  soon  after  it  again 
sinks  deep  down,  often  turning  suddenly  half-way  round  and  pursues  for  a  time  a 
horizontal  course.  In  all  these  movements  it  avoids  coming  into  collision  with  the 
stationary  objects  which  lie  in  its  path,  and  also  carefully  eludes  all  the  creatures 
swimming  about  in  the  same  water  with  it.  The  motion  is  effected  by  short  pro- 
cesses like  lashes  or  "cilia,"  which  protrude  all  round  from  the  enveloping  pellicle 
of  the  jelly-like  body  and  are  in  active  vibration.  With  the  help  of  these  cilia, 
which  occasion  by  their  action  little  eddies  in  the  water,  the  whole  ball  of  green 
jelly  moves  in  any  given  direction  with  considerable  rapidity.  But  at  the  same 
time  as  it  pushes  forward,  the  ellipsoid  turns  on  its  longer  axis,  so  that  the  resultant 
motion  is  obviously  that  of  a  screw.  It  is  worthy  of  note  that  this  rotation  is 
invariably  from  east  to  west,  that  is,  in  the  direction  opposed  to  that  of  the  earth. 
The  rate  of  progress  is  always  about  the  same:  a  layer  of  water  of  not  quite  two 
centimetres  (1*76  cm.)  is  traversed  in  one  minute.  Now  and  then,  it  is  true,  the 
swimming  ellipsoid  allows  itself  a  short  rest;  but  it  begins  again  almost  immediately, 
rising  and  sinking,  and  resumes  its  movements  of  rotation  and  vibration.  Two  hours 
after  its  escape  the  movements  become  perceptibly  feebler,  and  the  pauses,  during 
which  there  is  only  rotation  and  no  forward  motion  of  the  body,  become  both  longer 
and  more  frequent. 

At  length  the  swimmer  attains  permanent  rest.  He  lands  on  some  place  or 
other,  preferably  on  the  shady  side  of  any  object  that  may  be  floating  or  stationary 
in  the  water.  The  axial  rotation  ceases,  the  cilia  stop  their  lashing  motion  and  are 
withdrawn  into  the  substance  of  the  body,  and  the  whole  organism,  hitherto  ellip- 
soidal and  lighter  at  its  anterior  end,  becomes  spherical  and  of  a  uniform  dark- 
green  colour.  So  long  as  it  is  in  motion  the  gelatinous  body  has  no  definite  wall. 
Its  outermost  layer  is,  no  doubt,  denser  than  the  rest;  but  no  distinct  boundary  is 
to  be  recognized,  and  we  cannot  properly  speak  of  a  special  enveloping  coat.  No 
sooner,  however,  is  the  ball  stranded,  no  sooner  has  its  movement  ceased  and  its 
shape  become  spherical,  than  a  substance  is  secreted  at  its  periphery;  and  this 
substance,  even  at  the  moment  of  secretion,  takes  the  form  of  a  firm,  colourless,  and 
transparent  membrane.  Twenty-six  hours  afterwards,  very  short  branched  tubes 
begin  to  push  out  from  the  interior,  and  these  become  organs  of  attachment.  In 
the  opposite  direction  the  cell  stretches  into  a  long  tube  which  divides  into  branches 
and  floats  on  the  water.  After  fourteen  days  the  free  ends  of  this  tube  and  of  its 
branches  swell  once  more  and  become  club-shaped;  a  portion  of  their  slimy  contents 
is,  as  before,  separated  from  the  rest  and  liberated  as  a  motile  body,  and  the  whole 
performance  described  above  is  repeated. 


DISCOVERY  OF   PROTOPLASM. 


25 


DISCOVERY  OF  PROTOPLASM. 

The  study  of  Vaucheria  led,  then,  to  the  discovery  that  there  are  plants  which, 
in  the  course  of  their  development,  pass  through  a  motile  stage,  propelling  them- 
selves about  the  water  as  tiny  balls  of  jelly  with  ciliary  processes,  and  giving 
exactly  the  same  impression  as  infusoria.  Hand  in  hand  with  this  discovery  went 
the  further  observation  that  a  portion  of  the  plastic  cell-contents  in  all  plants  lies, 
like  a  lining,  in  contact  with  the  inner  face  of  the  cell-walls,  so  that  we  find  that 
these  latter,  at  a  certain  stage  of  maturity,  are  made  up  of  two  layers  lying  close 


Fig.  5.— Protoplasm  inclosed  in  Cells. 

i  Protoplasm  in  cells  of  Orobanche.    a  Streaming  protoplasm  in  cells  of  Vallitneria.     »  Streaming  protoplasm 

in  cells  of  Elodea. 

together,  the  outer  one  firm  and  the  inner  soft.  The  name  of  "primordial  utricle" 
was  given  to  this  inner  layer.  On  further  investigation  it  turned  out  that  this 
primordial  utricle  belongs  to  a  body  of  gelatinous,  slimy  consistency  which  lives  in 
the  cell-cavity  like  a  mussel  or  a  snail  in  its  shell.  At  first  it  is  shapeless  and  fills 
the  whole  cavity  with  what  appears  to  be  a  homogeneous  mass;  but  later  on  it  is 
differentiated  into  a  number  of  easily- recognizable  parts  —  i.e.  into  the  above- 
mentioned  lining  towards  the  inner  surface  of  the  cell-membrane,  and  into  folds, 
strands,  threads,  and  plates  stretching  across  the  interior  of  the  cell.  (See  fig.  5.) 
Mohl  of  Tubingen,  the  discoverer  of  these  facts,  applied  in  1846  the  name  of  proto- 
plasm to  the  substance  of  which  the  cell-contents  are  composed. 

It  is  possible  for  protoplasm,  under  certain  conditions,  to  exist  for  a  time  without 
any  special  protective  envelope;  but,  as  a  general  rule,  it  secretes  at  once  a  firm, 


26  DISCOVERY   OF   PROTOPLASM. 

continuous  coat,  and,  so  to  speak,  builds  itself  a  little  chamber  wherein  to  live.  We 
may  therefore  distinguish  naked  protoplasm  from  that  kind  which  inhabits  the 
interior  of  a  cell  of  its  own  creation,  and  compare  the  former  to  a  shell-less  snail, 
and  the  latter  to  a  snail  that  constructs  the  house  in  which  its  life  is  spent.  Still 
better  may  we  compare  the  firm  and  solid  cell-membrane  with  which  the  protoplasm 
clothes  itself  to  a  protective  coat,  a  garment  fitted  to  the  body;  and,  following  out 
this  analogy,  the  protoplasm  must  be  designated  the  living  entity  in  the  cell,  and 
the  secreted  envelope  must  be  considered  as  merely  the  skin  of  the  cell.  Conse- 
quently, although  this  cell- wall  was  the  part  which  was  first  revealed  by  magni- 
fying glasses,  and  was  called  a  cell  on  account  of  its  form,  this  is  not  the  essential 
formative  element,  which  has  the  power  of  nourishing  and  reproducing  itself. 
It  is  the  body  within  the  cell,  the  slimy,  colourless  protoplasm  in  full  activity  within 
the  surrounding  membrane  made  by  itself,  which  must  be  taken  to  be  the  essential 
part  of  the  cell  and  the  basis  of  life. 

The  term  cell  had  become  so  naturalized  in  the  science  that  protoplasm  which 
had  escaped  from  a  cell-cavity  was  also  called  a  cell,  and  the  unfortunate  name  of 
"  naked  cell "  was  brought  into  use  to  designate  it.  More  recently  many  of  these 
older  designations  have  been  abandoned  as  unsuitable.  We  now  include  under 
the  term  "protoplasts"  all  these  individual  organisms,  consisting  of  protoplasm, 
which  occupy  little  chambers  made  by  themselves,  living  either  alone  like  hermits  or 
side  by  side  in  sociable  alliance  in  more  or  less  extensive  structures,  able  under 
certain  circumstances  to  leave  their  domiciles,  laying  aside  their  envelopes  and 
swimming  about  as  naked  globules. 

Only  when  the  protoplasts  live  in  innumerable  little  cavities  congregated  close 
together  in  colonies,  and  when  these  cavities  are  bounded  by  even  walls  and  are  for 
the  most  part  uniformly  developed  in  all  directions,  does  the  part  of  a  plant  com- 
posed of  them  look  under  the  microscope  like  a  honey-comb,  and  each  cavity  like  a 
cell.  But  even  in  these  cases  of  external  similarity  there  is  the  essential  difference 
that  in  a  honey-comb  each  of  the  walls  separating  individual  cells  is  common  to  both 
the  adjacent  spaces,  and,  accordingly,  the  cells  of  the  comb  are  like  excavations  in  a 
continuous  matrix;  whereas,  in  sections  of  cellular  plants,  every  cell  possesses  its  own 
particular  and  independent  wall,  so  that  in  them  every  partition-wall  between 
neighbouring  cavities  is  composed,  properly  speaking,  of  two  layers  (fig.  6). 
These  two  layers  are  scarcely  distinguishable  in  the  case  of  delicate  cell-membranes 
newly  secreted  by  the  protoplasts.  Later  on,  however,  they  are  always  to  be  made 
out  clearly  (fig.  6 2 ).  Frequently  the  layers  separate  one  from  another  at  certain 
spots,  and  thus  channels  are  formed  between  the  cells  (fig.  6  *);  these  are  called  "  inter- 
cellular spaces."  One  often  sees  cells,  too,  whose  entire  surfaces  are,  as  it  were, 
glued  together  with  a  kind  of  cement,  and  then  this  substance  which  is  stored 
between  the  two  layers  is  called  "intercellular  substance"  (fig.  63). 

By  loosening  the  intercellular  substance,  where  present,  by  mechanical  or  chemi- 
cal means,  we  can  easily  separate  adjacent  cells  from  one  another;  the  two  layers 
of  the  partitioning  ceJl-walls  come  asunder,  and  then  each  separate  cell  exhibits  a 


DISCOVERY   OF   PROTOPLASM. 


27 


complete  envelope.  The  individual  cell-cavities  are  often  elongated  and  shaped  like 
either  rigid  or  flexible  tubes;  or  the  wall  of  such  a  cavity  may  become  very  thick 
and  encroach  to  such  an  extent  on  the  cavity  that  the  latter  is  scarcely  recognizable. 
Cells  of  this  kind  look  like  fibres  and  threads,  groups  of  them  look  like  bundles 
and  strands,  and  do  not  resemble  even  remotely  the  cells  of  a  honey-comb.  The 
term  "  cellular  "  is  hence  no  longer  suitable  in  the  case  of  these  structures. 

The  expression  "  cellular  tissue "  is  calculated  also  to  occasion  a  wrong  idea  of 
the  grouping  and  connection  of  the  single  cell-cavities.  By  a  tissue  one  would 
surely  understand  a  collection  of  thread-like  elements  so  arranged  that  some  of  the 
threads  run  parallel  to  one  another  in  one  direction,  whilst  similar  threads  crossing 


Fig.  6.— Cell-chambers.    Showing  Intercellular  Spaces  ( 1  and  2 )  and  "  Intercellular  Substance"  (»)  in  the 
Partition-walls  of  the  Chambers. 

the  first  at  right  angles  are  interwoven  with  them.  In  such  a  tissue,  as  of  woven 
silk  or  the  web  of  a  spider,  the  threads  are  held  together  by  intertwining;  but  this 
is  by  no  means  the  case  with  the  collections  of  cells  which  have  been  called  cell- 
tissues.  Even  where  the  parts  of  a  so-called  tissue  of  cells  are  tubular,  thread-like, 
or  fibrous,  they  lie  side  by  side  and  are  joined  as  it  were  by  a  cement,  but  are  never 
crossed  or  twisted  together  like  the  threads  in  a  woven  fabric. 

Again,  cells  have  been  compared  to  the  bricks  of  a  building,  but  this  analogy  is 
not  exact.  The  process  of  formation  of  a  cubical  crystal  from  a  solution  of  common 
salt  may  perhaps  be  compared  to  the  piling  up  of  bricks;  but  when  a  leaf  grows  the 
process  is  not  for  one  layer  of  cells  to  be  superimposed  from  the  outside  upon  another 
previously  deposited.  The  development  of  new  cells  proceeds  in  the  inside  of  exist- 
ing cells  and  ensues  from  the  activity  of  the  protoplasts  inclosed  within  the  cell- 
walls;  and  these  protoplasts  not  only  provide  the  building  materials,  but  are  them- 
selves the  builders.  It  is  in  this  very  fact  indeed  that  we  grasp  the  sole  distinction 
between  organic  and  inorganic  structures,  and  on  this  account  especially  the  above 
analogy  is  inadmissible  and  should  be  avoided. 

Cells  and  cell-aggregates  may  be  conceived  most  clearly  by  considering  their 
analogy  to  the  shells  of  living  creatures,  as  we  have  already  done  more  than  once  in 
the  foregoing  pages.  Protoplasts  are  either  solitary,  inhabiting  isolated  cell-cavities; 
or  else  they  live  in  associated  groups,  the  cells  being  crowded  close  together  in  great 
numbers  and  firmly  attached  to  one  another — each  cavity  being  inhabited  by  one 
such  protoplast.  When  the  latter  is  the  case,  division  of  labour  usually  takes  place 


28  SWIMMING   AND   CREEPING   PROTOPLASTS. 

in  a  plant,  so  that,  as  in  every  other  community,  some  of  the  members  undertake 
one  function,  some  another.  The  older  cells  in  these  plants  often  lose  their  living 
protoplasts,  and  then,  for  the  most  part,  serve  as  an  uninhabited  foundation  to  the 
entire  edifice,  which  may  thus  be  penetrated  by  air  and  water  channels.  The  proto- 
plasts have  meanwhile  erected  new  stories  for  themselves  and  their  posterity  on 
the  old  deserted  foundations,  and  are  pursuing  their  indefatigable  labours  in  the  little 
chambers  of  these  upper  stories.  This  work  of  the  living  protoplasts  consists  in 
absorbing  nutriment,  increasing  their  own  substance,  maturing  offspring,  searching 
for  the  places  which  offer  most  favourable  conditions  with  a  view  to  an  eventual 
transmigration  and  to  colonization  by  their  families;  and  lastly,  securing  the  region 
where  all  these  tasks  are  performed  against  injurious  external  influences.  The 
sequence  of  these  labours  is  always  governed  by  conditions  of  time  and  place. 
Many  of  them  are  only  to  be  observed  with  difficulty  in  their  actual  performance 
and  are  first  recognized  in  their  perfected  products,  while  others  are  attended  by 
very  striking  phenomena  and  are  easily  followed  in  their  progress. 


2.    MOVEMENTS   OF  PROTOPLASTS. 

Swimming  and  creeping  protoplasts. — Movements  of  protoplasm  in  cell-cavities. — Movements 
of  Volvocineae,  Diatomacese,  Oscillarise,  and  Bacteria. 

SWIMMING  AND   CHEEPING  PEOTOPLASTS. 

Among  the  most  striking  phenomena  observed  in  connection  with  living  proto- 
plasts are,  without  question,  the  temporary  locomotion  of  the  protoplast  as  a  whole 
and  the  displacement  and  investment  of  its  several  particles.  The  freest  motion  is 
of  course  exhibited  by  protoplasts  which  are  not  inclosed  in  cell-cavities,  but  have 
forsaken  their  dwelling  and  are  wandering  about  in  liquid  media.  Their  number, 
as  well  as  the  variety  of  their  forms,  is  extremely  great.  These  naked  protoplasts 
are  evolved  by  several  thousands  of  kinds  of  cryptogamic  plants,  at  the  moment  of 
sexual  or  asexual  reproduction  in  these  plants.  The  escape  from  the  enveloping 
cell- wall  alone  takes  place  in  countless  different  ways,  though  the  process,  as  a  whole, 
is  conducted  in  the  manner  already  described  in  the  case  of  Vaucheria  clavata. 
Sometimes  a  single  comparatively  large  protoplast  glides  out  of  the  opened  cell  by 
itself;  at  other  times,  before  the  cell  opens  the  protoplasmic  body  divides  into  several 
parts — often  into  a  great  number — and  then  a  whole  swarm  of  protoplasts  struggle 
out. 

These  swarming  protoplasts  differ  considerably  in  form.  Usually  their  outline 
is  almost  ellipsoidal  or  oval;  but  pear-shaped,  top-shaped,  and  spindle-shaped  forms 
also  occur.  Often  the  body  of  the  protoplast  is  spirally  twisted  like  a  corkscrew, 
and  has  in  addition  one  end  spatulate  or  clavate.  Thread-like  processes,  definite  in 
number  and  dimensions  and  arranged  variously,  according  to  the  kind  of  protoplast, 


SWIMMING   AND   CREEPING   PROTOPLASTS. 


29 


project  from  the  surface  of  its  body.  In  some  instances  the  whole  surface  is  thickly 
covered  with  short  cilia,  as  in  Vaucheria  (fig.  71);  in  others  the  cilia  form  a  close 
ring  behind  the  conical  or  beak-like  end  of  the  pear-shaped  body,  as  in  (Edogonium 
(fig.  72);  and  in  others  again,  one  or  two  pairs  of  long  and  infinitesimally  thin 
threads,  like  the  antennae  of  a  butterfly,  proceed  from  some  spot,  generally  the 
narrow  end  (fig.  7  3  and  7 4).  Many  forms  are  provided  with  a  single  long  lash  or 
flagellum  at  one  extremity  (fig.  77),  and  yet  others  are  spirally  wound  and  are 
beset  with  cilia,  thus  presenting  a  bristly  or  hirsute  appearance  (fig.  7  n). 

These  ciliary  processes  have  a  combined  lashing  and  rotatory  motion,  and  by 
their  means  the  protoplasts  swim  about  in  water.     In  many  cases,  however,  swim- 


rig.  7.— Swimming  Protoplasm. 

i  Vaucheria;  2  (Edogonium;  «  Draparnaldia;  *  Coleochcete;  «  and  7  Botrydium;  «  Ulothrix;  «  Fucus;  •  Funaria; 

10  Sphagnum;  H  Adiantum. 

ming  is  hardly  an  appropriate  expression;  certainly  not  if  one  associates  the  term 
with  the  idea  of  fishes  swimming  with  fins.  In  point  of  fact  there  is,  associated 
with  progression  in  a  particular  direction,  a  continuous  rotation  of  the  protoplast 
round  its  longer  axis,  and  on  this  account  its  motion  may  be  compared  to  that  of  a 
rifle-bullet,  since  in  both  cases  the  movement  of  translation  takes  place  in  the 
direction  of  the  axis  round  which  the  whole  body  spins.  The  movement  in  question 
is  not  unlike  the  boring  of  one  body  inside  another;  according  to  this,  the  soft 
protoplasts  bore  through  the  yielding  water,  and  by  this  action  make  onward 
progress. 

The  microscope  magnifies  not  only  the  moving  body,  but  also  the  path 
traversed;  and  when  one  contemplates  a  protoplast  in  motion,  magnified,  say, 
three  hundred  times,  its  speed  appears  to  be  three  hundred  times  as  fast  as  it 
really  is.  As  a  matter  of  fact,  the  motion  of  protoplasts  is  rather  slow.  The 
swarm-spores  of  Vaucheria,  described  above,  which  traverse  a  distance  of  17 
millimeters  in  a  minute  are  amongst  the  fastest.  The  majority  accomplish  an 
advance  of  not  more  than  5  m.m.,  and  many  only  1  m.m.  per  minute. 


30  SWIMMING  AND   CREEPING  PROTOPLASTS. 

As  was  mentioned  in  the  description  of  Vaucheria  the  locomotion  of  ciliated 
protoplasts  lasts  for  a  comparatively  brief  period.  It  gives  the  impression  of 
beino-  a  journey  with  a  purpose:  a  search,  as  it  were,  for  favourable  spots  for  settle- 
ment and  further  development;  or  else  a  hunt  after  other  protoplasts  moving 
about  in  the  same  liquid.  Green  protoplasts  always  begin  by  seeking  the  light, 
but  after  a  time  they  swim  back  into  the  shadier  depths.  Many  of  these,  especially 
the  larger  ones,  avoid  coming  into  collision,  and  are  careful  to  give  each  other 
a  wide  berth.  If  numbers  are  crowded  together  in  a  confined  space,  and  two 
collide  or  their  cilia  come  into  contact,  the  motion  ceases  for  an  instant,  but  in  a 
few  seconds  they  free  themselves  and  retire  in  opposite  directions. 

Contrasting  with  these  unsociable  protoplasts  are  others,  which  have  a  ten- 
dency to  seek  each  other  out  and  to  unite;  and  protoplasm  acts  in  many  cases 
on  protoplasm  of  identical  or  similar  quality,  perceptibly  attracting  it  and  deter- 
mining the  direction  of  its  motion.  It  is  very  curious  to  watch  the  tiny  pear- 
shaped  whirling  protoplasts  of  Draparnaldia,  Ulothrix,  Botrydium,  and  many 
others,  as  they  steer  towards  one  another  and,  upon  their  ciliated  ends  coming 
into  contact,  turn  over  and  lay  themselves  side  by  side  (fig.  76);  or,  to  see  one 
pursued  and  seized  by  another,  the  foreparts  of  their  bodies  brought  into  lateral 
contact,  and,  finally,  the  two,  after  swimming  about  paired  for  a  few  minutes, 
fusing  together  into  a  single  oval  or  spherical  protoplast  (fig.  7  6).  Even  the 
minute  fusiform  protoplasts  which  are  moved  by  cilia  proceeding  from  the  sides 
of  their  bodies  (fig.  7s),  as  well  as  the  spirally -coiled  forms  (figs.  7 9i  10' n ) 
endeavour  to  unite  with  some  other  protoplast.  They  always  move  towards 
larger  protoplasmic  bodies  at  rest,  cling  to  them  closely,  and  at  last  coalesce  with 
them  into  single  masses  (fig.  7  8). 

As  a  rule  no  striking  change  is  to  be  perceived  in  the  inside  of  motile  proto- 
plasmic bodies  during  the  rotatory  and  progressive  motion  caused  by  their  cilia; 
and  the  granules  and  chlorophyll-corpuscles  dotted  about  in  the  body  of  the 
protoplast  seem  to  remain,  throughout  the  period  of  locomotion,  almost  unchanged 
as  regards  both  position  and  shape.  It  is  only  in  the  vicinity  of  certain  little 
spaces,  called  "vacuoles,"  in  the  substance  of  the  protoplasm,  that  changes  in 
many  instances  are  observed,  which  indicate  that,  during  the  motion  of  the  whole 
apparently  rigid  mass,  slight  displacements  may  also  occur  in  the  interior,  some- 
what in  the  same  way  as,  when  a  man  walks,  the  heart  inside  his  body  is  not  still 
(relatively  to  the  body),  but  continues  to  pulsate  and  cause  the  blood  to  circulate. 
The  changes  observed  in  vacuoles  have,  moreover,  been  described  as  pulsations, 
because  they  are  accomplished  rhythmically  and  manifest  themselves  as  alternate 
expansions  and  contractions  of  the  vacant  space. 

In  each  of  the  motile  protoplasts  of  Ulothrix  (fig.  8)  there  is  found,  near  the 
conical  end,  which  is  furnished  with  four  cilia,  a  vacuole  which  contracts  in  from 
12  to  15  seconds,  and  dilates  again  in  the  succeeding  12  or  15  seconds.  In  the 
swarm-spores  of  Chlamydomonas  and  those  of  Draparnaldia  two  such  vacuoles 
may  be  observed  close  together,  whose  rhythmic  action  is  alternate,  so  that  the 


SWIMMING   AND   CREEPING   PROTOPLASTS. 


31 


systole  (contraction)  of  the  one  always  takes  place  synchronously  with  the  diastole 
(expansion)  of  the  other.  The  contraction  often  continues  until  the  cavity  entirely 
disappears.  It  must  depend,  as  also  does  the  expansion,  on  a  displacement  of  that 
part  of  the  protoplasm  which  immediately  surrounds  the  vacuole.  But  such  a 
motion  as  this  in  the  protoplasmic  substance,  even  if  only  visible  in  a  small  part 
of  the  whole  body,  can  scarcely  be  without  its  effect  on  other  more  distant  parts; 
and  it  may,  therefore,  be  concluded  that  the  interior  of  a  protoplast,  endowed  with 
ciliary  motion,  rotatory  and  progressive,  does  not  remain  quite  at  rest  relatively, 
as  seems  on  cursory  inspection  to  be  the  case. 

Protoplasts  whose  motion  is  effected  by  means  of  cilia  have  no  more  need  of 
their  vibratile  organs  when  once  they  have  reached  their  destination.      The  cilia, 


Fig.  8.— Pulsating  Vacuoles  in  the  Protoplasm  of  the  large  Swarm-spores  of  Ulothrix. 

whether  numerous  or  solitary,  whether  short  or  long,  first  of  all  become  stationary 
and  then  suddenly  disappear.  Either  they  are  drawn  in  or  else  they  deliquesce 
into  the  surrounding  liquid.  Whether  the  motile  protoplasts  have  come  to  rest 
because  they  have  reached  a  suitable  place  for  further  development,  as  happens 
in  Vaucheria,  or  because  they  have  united,  like  with  like,  into  a  single  mass, 
the  form  taken  by  the  resulting  non-motile  body  is  always  spherical.  The  final 
act  is  the  development  around  itself  of  an  investing  cell-membrane,  so  that  its 
soft  and  slimy  substance  may  be  protected  by  a  firm  covering  from  external 
influences. 

Essentially  different  from  the  motion  just  described  is  that  of  certain  proto- 
plasts which  are  unprovided  with  cilia,  but  perpetually  change  their  outlines, 
thrusting  out  considerable  portions  of  their  gelatinous  bodies  in  one  direction  or 
another,  and  at  the  same  time  drawing  in  other  parts.  At  one  moment  they 
appear  irregularly  angular,  shortly  afterwards  stellate;  then,  again,  they  elongate, 
become  fusiform,  and  gradually  almost  round  (fig.  9).  The  protruded  parts 
are  sometimes  delicate,  tapering  off  into  mere  threads;  sometimes  they  are  com- 
paratively thick,  and  have  almost  the  appearance  of  arms  and  feet  in  relation 
to  the  principal  mass.  The  motion  is  not  in  this  case  like  boring,  but  is  best 
described  as  creeping.  As  one  or  a  pair  of  foot-like  appendages  is  thrown  out 


32  MOVEMENTS   OF   PROTOPLASM   IN   CELL-CAVITIES. 

in  one  direction,  others  on  the  opposite  side  are  retracted,  and  the  protoplast  as 
a  whole  glides  over  the  intervening  space  like  a  snail  without  its  shell.  The 
analogy  is  all  the  more  exact  since  the  protoplast,  as  it  glides  onward,  leaves  a 
slimy  trail  in  its  wake,  so  that  the  latter  is  marked  by  a  streak  resembling  the 
track  of  a  snail.  When  two  or  more  of  these  creeping  protoplasts,  or  plasmodia, 
meet,  they  merge  into  one  another,  flowing  together  somewhat  in  the  same  way 
as  two  oil-drops  on  water  coalesce  into  one — leaving  no  distinguishable  boundaries 
between  the  united  bodies.  Thus,  slimy  lumps  of  protoplasm,  which  may  attain 
to  the  dimensions  of  a  closed  or  open  hand,  result  from  the  coalescence  of  great 
numbers  of  minute  protoplasts.  And  it  is  a  very  remarkable  fact  that  these 
plasmodia  can  themselves  change  their  form,  putting  out  lobes  and  threads,  and 


Fig.  9. — Creeping  Protoplasm. 

creeping  about   in  the  same  way  as  the  single   protoplasts   from  whose   fusion 
they  have  arisen. 

Creeping  masses  of  jelly  sometimes  move  in  the  direction  of  incident  light;  at 
other  times  they  avoid  light  and  hide  in  obscure  places,  wriggling  through  the 
interstices  of  heaps  of  bark  or  into  the  hollows  of  rotten  trunks;  or  they  may 
creep  up  the  stems  of  plants,  or  glide  over  the  brown  earth  in  a  viscous  condition. 
On  these  occasions  they  resolve  themselves  not  infrequently  into  bands,  cords,  and 
threads,  which  surround  fixed  objects,  divide,  and  combine  again,  forming  a  net- work 
of  meshes,  or  else  perhaps  frothy  lumps  like  cuckoo-spit.  If  foreign  bodies  of  small 
size  are  enmeshed  by  the  viscous  threads  of  the  reticulum,  they  may  be  drawn 
along  by  the  protoplasm  as  it  creeps;  and  if  they  contain  nutritive  material,  they 
may  be  eaten  up  and  absorbed.  Plasmodia  are,  for  the  most  part,  colourless,  but 
some  are  brightly  tinted;  in  particular  may  be  mentioned  the  best-known  of  all 
plasmoid  fungi,  the  so-called  "Flowers  of  Tan"  (Fuligo  varians),  which  are  yellow, 
and  Lycogala  Epidendron,  which  comes  out  on  old  stumps  of  pines,  and  is  vermilion 
in  colour. 

MOVEMENTS  OF  PROTOPLASM  IN  CELL-CAVITIES. 

In  the  case  of  a  protoplast  which  is  not  naked,  but  clothed  with  an  attached 
cell-membrane,  the  movements  are  limited  to  the  space  included  by  the  membrane, 
that  is  to  say  to  the  cell-cavity.  Until  the  protoplasmic  cell-body  is  differentiated 
into  distinct  individual  portions  no  very  lively  motion  can  in  general  take  place 
in  the  coated  protoplast;  though  it  is  not  to  be  assumed  that  it  abides  completely 


MOVEMENTS   OF   PROTOPLASM    IN    CELL-CAVITIES.  33 

at  rest  at  any  time,  except  perhaps  during  periods  of  drought  in  summer  and  of 
frost  in  winter,  and  in  seeds  during  their  time  of  quiescence.  This  applies  par- 
ticularly to  immature  cells.  In  them  the  protoplast  forms  a  solid  body  whose 
substance  entirely  fills  the  cell-cavity.  The  young  cell,  however,  grows  up  quickly, 
its  cavity  is  enlarged,  and  the  space,  hitherto  filled  by  the  protoplast,  becomes  two 
or  three  times  as  large  as  before.  But  the  increase  of  volume  on  the  part  of  the 
protoplast  itself  does  not  keep  pace  with  the  enlargement  of  its  habitation.  It  is 
true  that  it  continues  to  cling  closely  to  the  inner  face  of  the  cell- wall,  thus  forming 
the  primordial  utricle;  but  the  more  central  part  of  its  body  relaxes,  and  in  it  are 
formed  vacant  spaces,  the  vacuoles  above  mentioned,  wherein  collects  a  watery 
fluid  known  as  the  "cell-sap."  The  portions  of  protoplasm  which  lie  between 
the  vacuoles  resolve  themselves  gradually  into  thin  partitions  bounding  them;  and 
lastly,  these  partitions  split  up  into  bands,  bridles,  and  threads,  which  stretch  across 
the  cell-cavity  from  one  side  of  the  primordial  utricle  to  the  other,  and  are  woven 
together  here  and  there  where  they  intersect.  With  these  protoplasmic  strands  we 
have  already  become  acquainted. 

But  the  protoplasm  in  the  interior  of  a  growing  cell,  whilst  relaxing  and 
breaking  up,  also  becomes  motile  if  the  liquid  attains  a  certain  temperature,  and 
then  the  appearance  presented  is  like  that  of  a  lump  of  wax  melting  under  the 
action  of  heat.  These  movements  may  be  observed  very  clearly  under  the  micro- 
scope in  the  case  of  large  cells  with  thin  and  very  transparent  cell-membranes, 
especially  when  the  colourless,  translucent,  and  gelatinous  substance  of  the  proto- 
plasm— not  always  sharply  defined  in  contour — happens  to  be  studded  with 
minute  dark  granules,  the  so-called  "microsomata."  These  granules  are  driven 
backwards  and  forwards  with  the  stream,  like  particles  of  mud  in  turbid  water,  and 
their  motion  reveals  that  of  the  protoplasm  wherein  they  are  embedded.  Seeing 
particles  gliding  in  all  directions  through  the  cell-cavity,  arranged  irregularly  in 
chains,  rows,  and  clusters  in  the  protoplasmic  strands,  we  are  justified  in  concluding 
that  this  motion  takes  place  in  the  substance  of  the  strands  itself.  The  movement, 
moreover,  is  not  confined  to  isolated  strands,  but  occurs  in  all.  Granular  currents 
flow  hither  and  thither,  now  uniting,  now  again  dividing.  They  often  run  in 
opposite  directions  even  when  only  a  trifling  distance  apart;  sometimes  two  chains 
are  drifted  in  this  way  when  actually  close  together  in  the  same  band  of  proto- 
plasm. The  streams  pour  along  the  primordial  utricle  and  whilst  there  divide  into 
a  number  of  arms,  meeting  and  stemming  one  another  and  forming  little  eddies; 
then  they  are  gathered  together  again  and  turn  into  another  strand  of  the  more 
central  protoplasm.  The  individual  granules  in  the  currents  are  seen  to  move  with 
unequal  rapidity  according  to  their  sizes;  the  smaller  particles  progress  faster  than 
the  larger,  and  the  larger  are  often  overtaken  by  the  less,  and  when  this  happens 
the  result  often  is  that  the  entire  stream  stops.  If  so,  however,  the  crowded 
particles  are  suddenly  rolled  forward  again  at  a  swifter  pace,  like  bits  of  stone  in 
the  bed  of  a  river  as  it  passes  from  a  level  valley  into  a  gorge.  The  course  of  the 
streaming  protoplasm  remains  throughout  sharply  marked  off  from  the  watery  sap 

VOL.  I. 


34  MOVEMENTS  OF  PROTOPLASM   IN   CELL-CAVITIES. 

in  the  vacuoles,  and  none  of  the  granules  ever  pass  over  into  the  cell-sap  from  the 
protoplasm. 

Larger  bodies,  such  as  the  round  grains  of  green  colouring-matter  or  chlorophyll, 
are  in  many  instances  not  carried  forward,  but  remain  stationary,  the  protoplasmic 
stream  gliding  over  them  without  altering  them  in  any  way.  Further,  the  outer- 
most layer  of  the  protoplast,  contiguous  with  the  cell-membrane,  is  not  in  visible 
motion  in  most  vegetable  cells.  On  the  other  hand,  occasionally  the  entire  pro- 
toplast undoubtedly  acquires  a  movement  of  rotation,  and  then  the  larger  bodies 
imbedded  in  its  substance,  i.e.  chlorophyll  corpuscles,  are  driven  along  like  drift- 
wood in  a  mountain  torrent  (fig.  5 2  and  5  3  ).  On  these  occasions  a  wonderful 
circulation  and  undulation  of  the  entire  mass  takes  place:  chlorophyll  grains  are 
whirled  along  one  after  the  other  at  varying  speeds  as  if  trying  to  overtake  one 
another;  and  yet  another  structure,  the  cell-nucleus  presently  to  be  discussed,  is 
dragged  along,  being  unable  to  withstand  the  pressure,  and,  following  the  various 
displacements  of  the  net- work  of  protoplasmic  strands  in  which  it  is  involved,  is  at 
one  moment  pulled  alongside  of  the  cell- wall,  at  another  again  is  taken  in  tow  by  a 
rope  of  central  protoplasm  and  hauled  transversely  across  the  interior  of  the  cell 
(fig.53). 

When  the  rate  of  the  current  itself  is  estimated  by  the  pace  at  which  the  gran- 
ules are  driven  along,  results  which  vary  considerably  are  obtained,  depending  chiefly 
on  a  qualitative  difference  in  the  protoplasm,  but  secondarily  also  on  temperature  and 
other  external  conditions.  A  rise  in  temperature  up  to  a  certain  point  as  a  general 
rule  accelerates  the  rate  of  the  stream.  Particles  of  protoplasm  in  particularly 
rapid  motion  pass  over  10  m.m.  in  a  minute;  others  in  the  same  time  traverse  from 
1  to  2  m.m.;  and  some,  in  still  less  haste,  advance  only  about  a  hundredth  part 
of  a  millimeter.  Larger  bodies,  especially  the  bigger  chlorophyll  grains,  move 
slowest  of  all.  So  it  is  often  hours  before  chlorophyll  grains  lying  near  one  side  of 
a  cell  are  pushed  through  the  protoplasm  over  to  the  other  side,  a  distance  only 
equal  to  a  small  fraction  of  a  millimeter. 

The  minute  granules,  as  well  as  the  larger  grains  of  chlorophyll  and  the  cell- 
nucleus,  are  entirely  surrounded  by  protoplasm;  and  the  protoplasm,  whether  in  the 
form  of  bands  or  threads,  whether  a  peripheral  lining  or  an  indefinite  mass,  must 
be  conceived  as  always  composed  of  two  layers,  the  outer  "ectoplasm"  being  tougher 
and  denser  than  the  inner  "endoplasm,"  which  is  softer  and  somewhat  fluid.  The 
former  is  homogeneous  and  non-granular,  so  that  it  is  the  more  transparent  and 
has  the  effect  of  a  skin  clothing  the  inner,  softer  layer,  which  is  granular  and 
turbid.  It  would  be  incorrect,  however,  to  think  of  this  as  a  very  strongly-marked 
contrast,  sufficient  to  mark  off  one  layer  clearly  from  the  other.  In  reality  there 
are  no  such  sharp  boundaries,  and  the  tougher  ectoplasm  passes  gradually  into  the 
softer  and  more  mobile  endoplasm.  Of  course  the  granules  and  corpuscles  which 
one  sees  drifting  in  streaming  protoplasm  are  situated  within  the  more  yielding 
endoplasm.  It  is  true,  minute  particles  often  appear  to  glide  from  one  side  to  the 
other  upon  a  delicate  protoplasmic  strand  as  if  it  were  a  tight-rope;  but  on  closer 


MOVEMENTS   OF   PROTOPLASM   IN   CELL-CAVITIES.  35 

study  it  is  apparent  that  the  granules  which  seem  to  be  travelling  on  the  proto- 
plasmic thread  are  covered  by  a  delicate  and  transparent  protoplasmic  pellicle 
Thus,  these  granules  imbedded  in  the  substance  of  protoplasts  have  no  independent 
totion,  but  are  pushed  along  by  the  spreading  protoplasm. 

Each  stream  of  protoplasm  is  shut  off  from  its  environment  and  limited  bv 
a  layer  tougher  than  the  rest.     But  this  does  not  prevent  the  currents,  with  their 
crowds  of   drifting  granules,   from   changing  their  direction.      In   fact  we  have 
only  to  follow  for  a  short  time  the  course  of  one  such  granular  stream  to  remark 
ontmuous  series  of  changes:   a  current  from  being  in  a  straight  line  bends 
suddenly  to  one  side,  it  broadens  and  contracts  again,  now  it  runs  close  alongside 
another  channel,  now  breaks  away  once  more,  divides  into  two  little  arms    and 
oses  itself  finally  in  the  primordial  utricle.     On  the  other  hand,  fresh  folds 'start 
rom  the  primordial  utricle,  stretch  and  grow  until  they  have  pushed  across  the 
call-cavity  to  the  other  side  in  the  form  of  bands,  or  the  protoplasm  may  be 
drawn  out  into  threads,  which  elongate  until  they  encounter  other  similar  strings 
and  form  a  junction  with  them.     The  same  processes  then  that  are  observed  in 
free  creeping  protoplasts  take  place  to  some  extent  here.      Imagine  a  protoplast 
captured  whilst  on  its  travels-creeping  along  the  level  ground-and  imprisoned 
m  a  completely  closed  vessel;  it  would  spread  itself  out  over  the  inner  surface 
the  vessel,  would  branch  and  creep  about  and  have  just  the  same  appearance 
the  protoplasts,  just  described,  which  inhabit  cell-cavities  from  their  earliest 
This  is  but  the  converse  of  the  power  possessed  by  a  protoplast  set  free 
from  its  cell,  which  enables  it  to  move,  stretch  out,  and  draw  in  its  various  parts, 
and  so  to  effect  locomotion. 

Another  motion,  differing  from  the   creeping,  gliding,  and   streaming  action 
f  protoplasts,  manifests  itself  in  the  so-called  swarming  of  granules  contained 
in  the  protoplasm.     It  may  be  best  observed  in  the  cells  of  the  genera  Penium 
and    Closterium,    both    of    which    are    shown    in    figure    25A,    i,    k,    though 
the  same  phenomenon  is  to  be  seen  in  many  allied  forms,  living  in  lakes  and 
ponds  either  singly  or  congregated  in  colonies,  and  remarkable  for  their  bright 
green  colour.     The  above-mentioned  genus  Closterium  includes  delicate  unicellular 
orms  having  a  curved  or  scimitar  shape  unusual  in  plants,  whence  one  of  its 
species,  in  which  the  semi-lunar  form  is  most  striking,  has  been  named  Closterium 
lunula.      The  cell-membrane   in  all  these  little  water-plants  is  clear  and  quite 
transparent.      The  greater   part   of    the   cell-contents   consists   of    a   dark-green 
chlorophyll  body  longitudinally  grooved;  but  the  protoplasm  which  is  visible  in 
the   two   sharply   tapering   ends  of  the  cell-cavity  is   colourless,   and  embedded 
within  it  is  a  swarm  of  microsomata.      These  granules  or  microsomata  appear  to 
be  in  a  most  curious  state  of  motion  so  long  as  the  protoplast  lives.     They  are 
to  be  seen  plainly  within  the  limits  of  the  tiny  cavity,  jumping  up  and  down, 
whirling,  dancing,  and  rushing  about  without  really  changing  their  position.     One' 
s  reminded  of  the  apparently  purposeless  journeyings  to  and  fro  within  reach 
of  their  homes  of  ants  or  bees,  and  the  movement  has  been  called  not  inaptly 


36  MOVEMENTS   OF   PROTOPLASM   IN   CELL-CAVITIES. 

"swarming."  It  is  difficult  to  imagine  the  kind  of  motion  possessed  by  the 
protoplasm  in  which  these  swarming  microsomata  are  embedded;  but  however 
closely  it  is  confined,  there  must  be  continual  rapid  displacements  in  its  substance, 
which  is  very  fluid,  and  it  may  be  assumed  that  here  again  it  is  not  so  much 
the  tiny  grains  that  bestir  themselves  as  the  protoplasm  which  holds  them. 
Probably  the  protoplasmic  matter  spreads  and  stretches  out  and  rotates,  and 
individual  granules  are  carried  about  by  it.  This,  of  course,  does  not  exclude 
the  possibility  of  the  granules  possessing  a  vibratory  motion  of  their  own  within 
the  mass  of  protoplasm. 

Similar,  but  not  identical,  is  the  swarming  movement  of  protoplasm  observed 
in  cells  of  the  Water-net  (Hydrodictyon  utriculatum),  and  in  several  other  plants 
allied  to  it.  Hydrodictyon  looks  like  a  net  in  the  form  of  a  sac,  and  composed 
of  green  threads.  The  meshes  of  this  net,  which  are  generally  hexagonal,  consist, 
however,  not  of  filaments  but  of  slender  cylindrical  cells  joined  together  by  threes 
at  their  extremities,  somewhat  in  the  same  way  as  are  the  leaden  frames  of  the 
little  hexagonal  panes  of  glass  in  gothic  windows.  The  protoplasmic  body  of 
one  of  these  cells  in  due  time  breaks  up  into  a  great  multitude  (7000-20,000)  of 
tiny  clots,  which  begin  to  move  and  swarm  within  the  cell-cavity  in  what  appears 
to  be  a  disordered  medley.  In  half  an  hour,  however,  the  excited  mass  is  again 
restored  to  rest:  the  minute  particles  take  form  and  arrange  themselves  in  definite 
order,  each  having  two  others  at  either  extremity,  making  an  angle  of  120°  with 
it;  and,  lastly,  all  unite  to  form  a  single  tiny  net  having  exactly  the  same  shape 
as  the  one  whose  component  cell  constituted  the  arena  of  this  process  of  construc- 
tion. The  miniature  water-net  so  formed  then  slips  out  of  the  cell,  the  latter 
opening  for  the  purpose,  and  in  from  three  to  four  weeks  it  grows  to  the  same 
size  as  the  parent  plant. 

In  the  above  we  have  an  instance  of  a  protoplast  producing  a  whole  colony 
of  cells,  which  are  obliged  to  leave  their  home  for  want  of  space.  In  cases 
previously  considered  we  have  found  the  protoplast  stretching  and  elongating 
in  all  directions,  drawing  itself  out  into  bridles  and  spreading  as  a  delicate  lining 
to  walls,  and  so  endeavouring  generally  to  expand  and  present  the  greatest  surface 
possible.  Again,  we  have  seen  it  wandering  freely,  creeping,  swimming,  and 
rotating,  and  by  this  method  also  covering  as  much  space  as  it  can.  But,  con- 
versely, there  is  a  time  when  a  protoplast  tends  to  the  other  extreme;  the 
expanded  mass  of  its  body  gathers  itself  together  again,  contracts  more  and 
more,  and  at  length  becomes  a  resting  sphere,  that  is  to  say,  it  assumes  the  con- 
figuration which  exposes  the  least  surface  to  the  environment. 

This  process  exhibits  itself  with  particular  clearness  within  the  cell-cavities 
of  the  green  algae  known  by  the  name  of  Spirogyra,  a  species  of  which  is 
represented,  magnified  three  hundred  times,  in  figure  25A,  I.  In  this  alga 
the  protoplasm  in  each  mature  cell-cavity  forms,  as  a  general  rule,  a  very  deli- 
cate parietal  lining  wherein  green  chlorophyll  bodies  are  embedded,  arranged 
in  a  spiral  band.  All  of  a  sudden,  however,  this  lining  strips  itself  off  the  inner 


MOVEMENTS   OF   SIMPLE   ORGANISMS.  37 

face  of  the  cell-wall  and  shrinks  together  so  as  in  a  short  time  to  present  the 
appearance  of  a  sphere  occupying  the  middle  of  the  cell-cavity.  Again,  just  as 
this  contraction  is  an  instance  of  a  special  form  of  protoplasmic  motion,  so  also 
the  further  change  which  the  contracted  protoplast  in  a  cell  of  Spirogyra  under- 
goes is  reducible  to  displacements  in  its  substance,  and  must  be  mentioned  as 
a  special  kind  of  protoplasmic  movement.  For  the  conglomerated  protoplast 
remains  but  a  short  time  in  the  middle  of  the  cell-cavity.  It  leans  almost 
immediately  to  one  side,  thrusting  itself  into  a  protuberance  of  the  cell-mem- 
brane, which  is  concurrently  developed,  and  which,  when  further  developed,  forms 
a  passage  leading  over  into  another  cell-cavity.  Its  body  becomes  longer  and 
narrower,  and  at  last  slips  through  the  passage  into  the  next  cavity,  where  a 
second  protoplast  awaits  it;  and  the  two  then  unite,  fusing  together  into  one 
It  is  not  premature  to  remark  that  all  these  displacements  and  invest- 
ments of  the  protoplasmic  substance  in  cells  of  Spirogyra,  including  the  pheno- 
mena of  contraction,  as  well  as  those  of  pushing  forward,  escape,  and  coalescence, 
are  not  produced  as  the  results  of  a  shock,  impulse,  or  stimulus  from  without^ 
but  are  to  be  looked  upon  as  movements  proper  to  the  protoplasm,  and  resulting 
from  causes  inherent  in  the  protoplasm. 

MOVEMENTS  OF  YOLYOCINE^,   DIATOMACEJE,   OSCILLAELE 

AND  BACTERIA. 

Very  remarkable  is  the  movement  of  those  wonderful  organisms  which  are 
comprised  under  the  name  of  Volvocineae.  One  species,  Volvox  globator,  was 
known  to  so  ancient  an  observer  as  Leeuwenhoek;  but  he,  and  after  him  Linnaeus, 
took  it  to  be  an  animal  on  account  of  its  extraordinary  power  of  locomotion,  and  it 
was  named  the  "globe-animalcule."  A  Volvox-sphere  consists  of  a  large  number  of 
green  protoplasts  living  together  as  a  family  and  arranged  with  great  regularity 
within  their  common  envelope.  They  appear  to  be  disposed  radially,  and  to  be 
linked  together  and  held  firm  by  a  net- work  of  tough  threads,  their  poles  being 
directed  towards  the  centre  and  the  periphery  of  the  sphere  respectively.  From 
the  peripheral  extremity,  which  in  each  protoplast  is  marked  out  by  a  bright  red 
spot,  proceed  a  pair  of  cilia,  and  these  protrude  through  the  soft  gelatinous 
envelope  of  the  whole  sphere,  and  move  rhythmically  in  the  surrounding  water. 
A  Volvox-globe  rolls  along  in  the  water  propelled  by  regular  strokes,  like  a  boat 
manned  by  a  number  of  oarsmen,  as  soon  as  the  protoplasts,  which  form  the  crew 
of  this  strange  vessel,  begin  to  manipulate  their  propellers.  The  effect  is  exceed- 
ingly  graceful,  and  has  justly  filled  observers  of  all  periods  with  astonishment; 
indeed  no  one  seeing  for  the  first  time  a  Volvox-sphere  rolling  along  can  fail  to  be 
impressed  and  delighted. 

Another  plant  allied  to  the  foregoing,  the  so-called  "red-snow,"  has  always 
excited  wonder  in  no  less  degree  from  the  remarkable  phenomena  of  motion  which 
it  exhibits,  but  also  because  of  its  characteristic  occurrence  in  situations  where  one 


38  MOVEMENTS   OF   SIMPLE    ORGANISMS. 

might  suppose  all  vital  functions  would  be  extinguished.  It  was  in  the  year  1760 
that  De  Saussure  first  noticed  that  the  snowfields  on  the  mountains  of  Savoy  were 
tinged  with  red,  and  described  the  phenomenon  as  "red-snow."  Once  on  the  look-out 
for  it,  people  found  this  red-snow  on  the  Alps  of  Switzerland,  Tyrol,  and  the  district 
of  Salzburg,  on  the  Pyrenees,  the  Carpathians,  and  the  northern  parts  of  the  Ural 
Mountains,  in  arctic  Scandinavia,  and  on  the  Sierra  Nevada  in  California.  But  red- 
snow  has  been  seen  on  the  most  magnificent  scale  in  Greenland.  When  Captain 
John  Ross  in  1818  sailed  round  Cape  York  on  his  voyage  of  discovery  to  Arctic 
America,  he  noticed  that  all  the  snow  patches  lying  in  the  gorges  and  gullies  of  the 
cliffs  on  the  coast  were  coloured  bright  crimson;  and  the  appearance  was  so  start- 
ling that  Ross  named  that  rocky  sea-shore  the  "Crimson  Cliffs."  On  the  occasion  of 
later  expeditions  to  the  arctic  regions,  red-snow  was  observed  off  the  north  coast  of 
Spitzbergen,  and  in  Russian  Lapland  and  Eastern  Siberia,  but  never  in  such  sur- 
prising luxuriance  as  on  the  Crimson  Cliffs  of  Greenland. 

If  a  snow -field  coloured  by  red-snow  is  examined  near  at  hand  it  is  found  that 
only  the  most  superficial  layer,  about  50  millimeters  in  depth,  is  tinged.  It  is  also 
present  in  the  greatest  quantities  in  places  where  the  snow  has  been  temporarily 
melted  by  the  heat  of  summer,  particularly  therefore  in  depressions,  whether  big  or 
little,  and  towards  the  edges  of  the  snow-field,  where  the  so-called  snow-dust  or 
Cryoconite  extends  regularly  in  the  form  of  dark,  graphitic  smeary  streaks.  Exam- 
ined under  the  microscope,  the  matter  which  causes  the  redness  of  the  snow 
appears  as  a  number  of  spherical  cells  having  a  rather  substantial  colourless  cell- 
membrane  and  protoplasmic  contents  permeated  by  chlorophyll.  The  green  colour 
of  the  chlorophyll  is,  however,  so  disguised  by  a  blood-red  pigment  that  it  is  only 
possible  to  detect  it  when  the  latter  has  been  extracted,  or  in  cases  where  it  is 
limited  to  a  few  definite  spots  in  the  cell.  These  spherical  cells  do  not  move,  and 
so  long  as  the  snow  is  frozen  they  show  no  sign  of  life.  But  as  soon  as  the  heat  of 
the  summer  months  melts  the  snow,  these  cells  acquire  vitality,  visibly  increasing 
in  size  and  preparing  for  division  and  multiplication  the  moment  they  have 
attained  a  certain  volume.  The  growth,  so  far  as  it  depends  on  nutrition,  takes 
place  at  the  expense  of  carbon  dioxide  absorbed  by  the  melted  snow  from  the 
atmosphere  and  of  the  inorganic  and  organic  constituent  parts  of  the  dust.  We 
shall  frequently  have  occasion  to  return  to  this  dust,  but  at  present  it  is  only  neces- 
sary to  observe,  for  the  comprehension  of  the  drawing  of  red-snow  as  seen  under 
the  microscope  (figure  25A,  e-h)>  that  in  the  Alps,  amongst  the  organic  materials 
which  constitute  the  dust,  pollen-grains  of  conifers  occur  with  great  frequency, 
especially  those  of  the  fir,  arolla,  and  mountain  pine.  These  pollen-grains  have 
been  swept  up  into  the  high  Alps  by  storms,  and  are  already  partially  decayed. 
In  all  the  material  that  I  investigated  I  found  th«  red-snow  cells  mixed  with 
pollen-grains  of  the  above-mentioned  conifers.  The  pollen-grains  are  oval  in  cross- 
section,  of  a  dirty  yellow  colour,  and  swollen  laterally  into  two  hemispherical  wings, 
as  is  shown  in  figure  25A,  e-h. 

As  has  been  stated,  the  red  cells  are  nourished  by  the  constituent  elements  of 


MOVEMENTS   OF   SIMPLE   ORGANISMS.  39 

the  dust,  which  are  dissolved  in  the  melted  snow.  They  grow  and  at  last  divide 
so  as  to  form  daughter-cells,  usually  four  in  number  but  often  six  or  eight  and 
less  frequently  two  only  (figure  2  5  A,  /,  g).  As  soon  as  the  division  is  accom- 
plished, the  daughter-cells,  so  produced,  free  themselves,  assume  an  oval  shape,  and 
display  at  their  narrower  extremity  two  rotating  cilia  by  means  of  which  they 
move  about  in  snow-water  with  considerable  vivacity.  The  interstices  of  the  still 
unmelted,  but  now  granular,  snow,  are  filled  with  water  from  the  melted  parts,  and 
through  these  the  red  cells  swim  away  and  are  thus  diffused  over  the  snow-field. 
At  the  moment  of  escape  and  first  assumption  of  movement  the  cell-body  appears 
to  be  uninclosed.  But  it  soon  clothes  itself  with  an  extremely  delicate,  though 
clearly  discernible  skin,  which,  curiously  enough,  does  not  lie  close  to  the  proto- 
plasm, which  is  withdrawn  slightly  and  inclosed  as  in  a  distended  sac  (see 
figure  2 5 A,  e).  Only  in  front,  where  the  two  cilia  carry  on  their  whirling  motion, 
does  the  skin  lie  close  to  the  body  of  the  cell;  and  it  must  be  presumed  that  the 
cilia,  which  are  simply  extensions  of  the  protoplasmic  substance,  are  projected 
through  the  envelope.  The  swarm-spores  afford  an  example  of  an  unusual  type  of 
protoplasts,  namely  of  those  that  move  about  singly  in  the  water  by  means  of  cilia 
and  at  the  same  time  carry  their  self-made  cell-membranes  with  them. 

How  long  the  motile  stage  lasts  under  natural  conditions  has  not  been  deter- 
mined for  certain.  On  the  mountains  of  central  and  southern  Europe,  where  hot 
days  are  followed,  even  in  the  height  of  summer,  by  bitterly  cold  nights,  causing 
the  melted  snow  which  has  not  run  off  to  freeze  again  in  the  depressions  of  the 
snow,  the  movement  no  doubt  is  often  interrupted.  On  the  other  hand,  in  high 
latitudes,  where  the  summer  sun  does  not  set  for  weeks  together,  such  interruption 
would  be  exceptional.  In  any  case,  however,  the  locomotion  of  the  red  cells  with 
their  hyaline  cell-membranes  is  not  limited  to  so  short  a  period  as  is  that  of  naked 
ciliated  protoplasts.  Moreover  they  have  the  power  of  nutrition  and  growth  like 
the  red  resting-cells  from  which  they  originate,  and  they  have  been  observed,  in  a 
culture,  to  increase  in  size  fourfold  within  two  days.  When  at  last  they  come  to 
rest  they  draw  in  their  cilia,  assume  a  spherical  shape,  thicken  their  cell-membrane, 
which  now  once  more  lies  close  to  the  protoplasmic  body,  and  divide  anew  into  two, 
four,  or  eight  cells  (figure  25A,/,  g).  The  fusion  of  the  protoplasts  of  the  red  cells  in 
pairs,  and  their  sexual  propagation,  which  has  been  observed  in  addition  to  the 
above-described  asexual  multiplication,  will  be  the  subject  of  discussion  later  on. 
At  present  we  need  only  add  with  reference  to  this  remarkable  plant  that  it  was 
named  Sphcerella  nivalis  by  the  botanist  Sommerfelt,  and  that  not  only  in  mode 
of  life,  but  also  in  form  and  colour,  it  most  closely  resembles  a  kind  of  blood-red  alga, 
which  makes  its  appearance  in  Central  Europe  in  little  hollows  temporarily  filled 
with  rain-water  in  flat  rocks  .and  slabs  of  stone,  and  also  inside  receptacles  exposed 
to  the  open.  This  alga  has  received  the  name  of  Sphwrella  pluvialis,  and  also 
that  of  Hcematococcus  pluvialis. 

Lastly,  we  have  to  consider  the  mysterious  movements  exhibited  by  many 
Diatomacese,    and    by    the    filamentous    species    of    Zonotrichia,  Oscillaria,    and 


40  MOVEMENTS   OF   SIMPLE   ORGANISMS. 

Beggiatoa.  As  regards  the  Diatoms,  some  of  them  are  firmly  attached  to  a 
support,  and  are  not  generally  capable  of  locomotion;  but  others  are  almost  in- 
cessantly in  motion,  and  these  little  unicellular  organisms  steer  themselves  about 
with  great  precision  near  the  bottom  of  the  pools  of  water  in  which  they  live. 
Their  cell-membrane  is  transformed  into  a  siliceous  coat,  and  this  coat,  which  is 
hyaline  and  transparent,  but  very  hard,  consists  of  two  halves  shutting  together 
like  the  valves  of  a  mussel.  The  entire  cell  thus  coated  has  the  form  of  a  gondola 
or  little  boat,  with  a  keel  either  straight  or  curved  (Pleurosigma,  Pinnularia, 
Navicula),  and  is  provided  with  various  bands,  ribs,  and  sculpturings  on  its 
siliceous  walls.  Driven  by  inherent  forces,  these  little  protected  cruisers  pursue 
their  way  at  the  bottom  of  the  water  or  over  objects  which  happen  to  be  in  the 
water.  They  either  glide  evenly  over  the  substratum,  or  else  proceed  by  fits 
and  starts  at  rather  long  intervals,  and  apparently  with  difficulty.  For  some 
time  they  may  hold  a  straight  course,  but  not  infrequently  they  deviate  side- 
ways without  apparent  cause,  and  after  deviating  return  again.  They  double 
round  projecting  objects  or  push  them  out  of  the  way  with  one  of  their  hard 
points,  which  are  often  thickened  into  nodules,  and  cause  the  obstructing  objects 
to  slip  by  alongside  the  keel  of  the  little  vessel.  Yet  no  paddles  or  cilia  are  to 
be  seen  projecting  from  it,  as  in  the  case  already  described  of  Yolvocinese;  nor 
does  the  siliceous  coat  exhibit  any  sort  of  motile  processes  whereto  the  move- 
ments might  be  attributed.  But  the  strong  analogy  between  the  structure  of 
these  Diatomaceae  and  that  of  mussels  seems  to  justify  the  assumption  that  the 
two  siliceous  valves,  which  are  fast  shut  during  the  period  of  rest  of  the  Diatoms 
in  question,  move  a  little  apart,  so  that  the  protoplast  living  within  can  push 
out  one  edge  of  its  body  and  creep  along  over  the  substratum  by  means  of  it. 

The  movements  of  the  filaments  of  Beggiatoa,  Oscillaria,  and  Zonotrichia 
are  explained  in  a  similar  manner.  These  filaments  are  made  up  of  a  number 
of  short  cylindrical  or  discoid  cells,  and  are  attached  by  one  end,  but  with  the 
other  execute  most  striking  movements.  They  stretch  themselves  and  then 
contract  again,  coil  up  and  straighten  out  like  snakes,  and,  most  characteristic 
of  all,  make  periodic  oscillations  in  the  water.  The  belief  is  that  the  mechanism 
of  this  motion  is  similar  to  that  of  the  preceding,  that  infinitesimally  fine  fila- 
ments of  protoplasm  inserted  spirally  penetrate  the  cell-walls,  and  that  these  act 
like  the  propeller  of  a  ship. 

On  looking  back  over  the  multifarious  examples  of  movement  that  have  been 
described,  the  conviction  that  the  capacity  for  motion  is  inherent  in  all  living 
protoplasts  is  difficult  to  resist.  In  many  cases,  of  course,  the  displacement  and 
replacement  of  the  substance  no  doubt  takes  place  so  slowly  that  it  is  scarcely 
possible  to  express  its  amount  numerically.  Movement  may  even  entirely  cease 
for  a  time;  but,  as  necessity  arises,  and  under  favourable  external  circumstances, 
the  protoplasmic  mass  always  becomes  mobile  again — the  direction  of  its  motion 
being  determined  by  inherent  forces.  There  is  still  much  to  learn,  no  doubt,  con- 
cerning the  objects  and  significance  of  the  different  movements  of  protoplasm; 


CELL  CONTENTS.  41 

but  in  this  connection  we  are  justified  in  assuming  that  all  these  movements 
have  to  do  with  the  maintenance  and  multiplication  of  the  protoplasts.  For 
instance,  amongst  the  objects  of  the  various  movements  are  the  search  for  food,  the 
elimination  of  useless  material,  the  production  of  offspring,  the  discovery  of  the 
rays  of  sunlight  necessary  to  the  existence  of  chlorophyll-bodies  and  of  suitable 
spots  to  colonize.  This  conception  has  been  brought  out  frequently  in  the  course 
of  the  foregoing  description,  and  will  again  engage  our  attention  in  succeeding 
pages. 


3.   SECRETIONS   AND   CONSTRUCTIVE  ACTIVITY 
OF   PROTOPLASTS. 

Cell-sap.— Cell-nucleus.— Chlorophyll-bodies.— Starch.— Crystals.— Construction  of  the  Cell-wall  and 
Establishment  of  Communication  between  Neighbouring  Cell-cavities. 

CELL-SAP.— CELL-NUCLEUS.— CHLOROPHYLL-BODIES,— STARCH.—  CRYSTALS. 

In  addition  to  the  powers  which  the  living  protoplast  possesses  of  shifting 
its  parts,  of  expanding  and  contracting,  of  dividing  and  of  fusing  like  with  like, 
it  has  also  the  properties  of  adapting  different  parts  of  its  body  to  particular 
functions,  of  building  up  various  chemical  compounds,  and  of  separating  them  out 
when  necessary.  As  the  protoplast  stretches  and  expands,  spaces  and  depressions 
arise  within  it,  and  these  form  ultimately,  when  the  protoplast  is  limited 
to  a  peripheral  layer  lining  the  walls  of  the  cavity,  a  single  central  vacuole. 
In  the  spaces  there  is  secreted,  in  the  first  instance,  the  cell-sap,  a  watery  fluid 
containing  a  variety  of  substances  either  suspended  or  in  solution,  of  which  the 
chief  are  sugar,  acids,  and  colouring  matters.  Moreover,  in  the  interior  of  the 
protoplasm  itself,  structures  with  quite  different  forms  occur,  and  are  easily  recog- 
nizable by  their  contours;  these  are  the  cell-nucleus,  chlorophyll-bodies,  and  starch- 
grains. 

The  principal  feature  of  the  cell-nucleus  is  that,  although  the  substance  of 
which  it  is  composed  is  only  slightly  different  from  the  general  protoplasm  of 
the  cell,  yet  it  is  always  clearly  marked  off  from  the  protoplasm.  In  the  un- 
developed protoplast  the  nucleus  is  usually  situated  in  the  middle,  but  in  mature 
protoplasts  it  is  either  pressed  against  one  wall  of  the  cell  or  suspended  in  a  sort 
of  pocket  of  protoplasmic  filaments  in  the  interior  (fig.  5  l  and  5 3 ).  It  may 
be  pushed  along  by  the  streaming  protoplasm  and  dragged  into  the  middle  of 
the  cell,  and  in  that  case  its  shape  is  sometimes  altered  and  it  becomes  for  a  time 
somewhat  elongated  and  flattened.  The  nuclear  substance,  which,  as  has  been 
already  mentioned,  differs  but  little  from  ordinary  protoplasm,  is  colourless,  and 
studded  with  microsomata,  and  is  liable  to  internal  displacements  similar  to  those 
of  the  entire  cell-body.  When  a  protoplast  divides,  the  nucleus  plays  a  very 


42  THE   CELL-WALL. 

important  part  in  the  process,  and  it  will  be  necessary  later  on  to  discuss  its 
significance  in  this  connection. 

The  chlorophyll-bodies,  mentioned  already  more  than  once  incidentally,  are 
green  corpuscles,  roundish,  ellipsoidal,  or  lenticular  in  shape,  and  grouped  in  a 
great  variety  of  ways  (figure  25A,  i,  k,  I,  m,  p).  They  are  produced  generally 
in  great  numbers  by  the  protoplast  in  special  sac-like  excavations  in  its  bodyy 
but  nowhere  except  where  they  are  necessary,  that  is,  in  those  cells  wherein 
the  transmutation  of  inorganic  food-stuffs  into  organic  matter  takes  place.  This 
transformation,  so  important  to  the  existence  of  the  organic  world,  will  be  con- 
sidered in  detail  later  on.  Chlorophyll-corpuscles  are  not,  as  regards  their  material 
basis,  essentially  different  from  the  substance  of  the  protoplasm  in  which  they 
are  formed,  and  in  which  they  remain  embedded  for  life,  but  their  green  colour 
distinguishes  them  very  clearly  from  their  environment.  This  greenness  is  due 
to  a  colouring  matter  stored  in  the  protoplasmic  substance  of  the  corpuscle;  and 
our  ideas  of  plant-life  are  so  intimately  associated  with  this  remarkable  pigment, 
that  a  plant  that  is  not  green  seems  to  us  to  be  almost  an  anomaly. 

Besides  the  nucleus  and  the  chlorophyll-bodies  or  corpuscles,  protoplasts  pro- 
duce starch-grains,  aleurone-grains,  crystals  of  oxalate  of  lime,  and  drops  of  oil,  all 
of  which  will  be  dealt  with  presently  in  their  proper  place.  They  are  evolved  in 
accordance  with  the  requirements  of  the  moment  and  with  the  position  held  in  the 
edifice  of  the  plant  by  the  cells  concerned.  Moreover,  the  walls  of  the  cells  them- 
selves are  the  work  of  the  protoplasts,  and  it  is  not  a  mere  phrase,  but  a  literal  fact, 
that  the  protoplasts  build  their  abodes  themselves,  divide  and  adapt  the  interiors 
according  to  their  requirements,  store  up  necessary  supplies  within  them,  and,  most 
important  of  all,  provide  the  wherewithal  needful  for  nutrition,  for  maintenance, 
and  for  reproduction. 

CONSTRUCTION  OF  THE  CELL- WALL  AND  ESTABLISHMENT  OF  CONNECTIONS 
BETWEEN  NEIGHBOURING  CELL-CAVITIES. 

Of  all  these  performances,  the  construction  of  the  cell- wall  shows  the  greatest 
variety  from  the  nature  of  the  case.  For  the  envelope  with  which  each  individual 
protoplast  surrounds  itself  serves  at  once  as  a  protection  for  the  delicate  protoplasm, 
and  as  a  firm  support  for  structural  additions;  and,  at  the  same  time,  it  must  not 
impede  the  reciprocal  action  between  the  protoplasts  and  the  external  world,  or  the 
intercourse  between  those  living  in  adjoining  cavities.  These  cell- walls  are  accord- 
ingly very  wonderful  structures,  and  we  shall  often  have  occasion  to  discuss  them, 
especially  with  reference  to  the  significance  of  variations  in  their  structure  in 
particular  cases.  At  present  it  is  sufficient  to  remark  that  the  original  envelope 
which  is  secreted  from  the  body  of  a  protoplast  and  which  appears  at  first  as  a 
delicate  skin,  is  made  of  a  substance  composed  of  carbon,  hydrogen,  and  oxygen, 
belonging  to  the  class  of  carbohydrates. 

The  name  of  cell-membrane,  usually  applied  to  the  original  envelope  formed  b 


THE   CELL-WALL.  43 

the  cell-body,  is  one  quite  suitable  for  the  purpose.  But  this  earliest  covering  under- 
goes many  modifications.  The  protoplast  is  able  to  store  up  in  it  suberin,  lignin, 
silica,  and  water  in  greater  or  smaller  quantities,  and  by  this  means  it  either  makes 
the  envelope  more  flexible  than  it  was  in  the  first  instance,  or  else  hard  and 
stiff,  converting  it  into  a  shell-like  case.  Even  the  shape  is  seldom  preserved  as  it 
was  originally.  The  solitary  protoplast  surrounded  by  its  cell-membrane  is  gener- 
ally in  the  form  of  a  roundish  ball,  and  its  envelope,  which  is  closely  adherent, 
exhibits  a  corresponding  configuration.  Young  cells,  aggregated  together,  have 
outlines  too  which  remind  one  of  crystalline  forms,  such  as  dodecahedra,  cubes, 
and  short  six-sided  prisms.  But  when  a  protoplast  has  produced  its  first  delicate 
covering  it  does  not  come  to  rest,  but  goes  on  working  at  the  membrane,  distending 
and  thickening  it,  transforming  a  cavity  which  was  originally  spherical  or  cubical 
into  one  of  cylindrical,  fibrous,  or  tabular  shape,  and  strengthening  its  walls  with 
pilasters,  borders,  ridges,  hooks,  bands,  and  panels  of  various  kinds.  Where  a 
number  of  protoplasts  work  gregariously  at  one  many-chambered  edifice,  cells  of 
most  diverse  forms  are  produced  in  close  proximity  to  one  another.  These 
varieties  are,  however,  never  without  method  and  design,  but  are  invariably  such 
as  to  adequately  equip  each  cell  for  the  position  it  holds  and  for  the  particular 
task  allotted  to  it  in  the  general  domestic  economy. 

The  volume  attained  by  cell-cavities  in  consequence  of  the  expansion  of  their 
walls  varies  within  very  wide  limits.  The  smallest  cells  have  a  diameter  of  only 
one  micro-millimeter,  i.e.  the  thousandth  part  of  a  millimeter;  others,  as  for  example 
yeast-cells,  measure  perhaps  two  or  three  hundred ths  of  a  millimeter;  and  yet 
others  have  outlines  perceptible  to  the  naked  eye  and  have  a  volume  amounting 
to  one  cubic  millimeter.  Tubular  and  fibrous  cells  often  stretch  longitudinally 
to  such  an  extraordinary  extent  that  some  with  a  diameter  of  scarcely  the  hun- 
dredth part  of  a  millimeter  reach  a  length  of  one,  two,  or  even  as  many  as  five 
centimeters.  An  instance  may  be  seen  in  the  filaments  of  Vaucheria  clavata 
(figure  25A,  a-d),  and  again  in  the  fibrous  cells  from  which  our  linen  and  cotton 
fabrics  are  manufactured. 

The  enlargement  of  a  cell-cavity,  or,  in  other  words,  the  growth  in  area  of 
its  walls,  ensues  in  consequence  of  the  intercalation  of  fresh  particles  between 
those  which,  by  their  mutual  coherence,  form  the  delicate  skin  of  the  protoplast 
— the  earliest  stage  of  the  cell-wall.  When  these  intercalated  particles  are  situ- 
ated in  the  same  plane  as  are  those  already  deposited,  the  cell-wall  resulting 
from  this  method  of  construction  will  increase  in  area  without  adding  to  its 
thickness.  But  when  once  the  cells  are  full-sized,  the  constructive  activity  of 
the  protoplasts  has  to  be  directed  in  many  cases  to  the  strengthening  and  thick- 
ening of  their  walls,  so  that  later  on  they  may  be  able  to  perform  special  duties. 
From  the  appearance  of  this  thickening  one  would  judge  that  a  number  of  layers 
were  deposited  on  the  thin  original  wall  according  to  requirement,  and  in  many 
instances  no  doubt  the  process  corresponds  to  this  appearance;  but,  as  a  rule,  the 
thickness  of  the  wall  is  increased  by  intercalation,  on  the  part  of  the  protoplasts,  of 


44  THE   CELL-WALL. 

additional  material  between  the  original  particles,  a  process  which  has  been  termed 
"  intussusception." 

The  appearance  of  stratification  in  thickened  cell-walls  is  naturally  most  strik- 
ing where  substances  of  different  kinds  have  been  deposited  alternately  in  the 
different  parts  of  the  wall,  and  when  successive  layers  take  up  unequal  quantities 
of  water.  The  thickening  may  at  length  result  in  such  an  extreme  restriction  of 
the  cell-cavity  that  its  diameter  is  less  than  that  of  the  inclosing  wall.  Sometimes 
nothing  remains  of  the  cavity  but  a  narrow  passage,  and  then  the  cells  are  like 
solid  fibres.  Formerly  they  would  not  have  been  classed  with  cells  at  all,  but 
would  have  been  distinguished  under  the  name  of  fibres,  from  the  forms  resembling 
honey-comb  cells.  The  protoplasts  in  these  contracted  cells  languish  and  often  die, 
especially  when  the  walls  of  the  self-made  prison  are  greatly  thickened  and  do  not 
allow  of  intercourse  with  the  world  outside.  But  generally  a  protoplast  takes  care, 
in  constructing  its  dwelling,  not  to  close  itself  in  entirely,  nor  to  cut  itself  off 
permanently  from  the  outer  world.  It  either  makes  from  the  very  beginning  little 
windows  in  the  walls  of  its  house,  leaving  them  quite  open  or  closed  only  by  thin, 
easily-permeable,  membranes;  or  else,  after  constructing  a  completely  closed  enve- 
lope, it  redissolves  a  piece  of  it,  thus  making  an  aperture  through  which  in  due 
time  it  is  able  to  effect  its  escape.  The  scope  of  this  work  does  not  admit  of  an 
exhaustive  treatment  of  the  formative  power  possessed  by  protoplasts  needful  for 
these  results;  it  will  be  sufficient  to  give  a  general  description  of  some  of  the 
more  important  processes  which  have  for  their  object  the  establishment  of  a 
connection  between  adjacent  cell-cavities  and  of  communication  with  the  external 
world. 

The  new  particles  of  material,  or  cellulose,  which  are  to  strengthen  the 
delicate  original  cell-membrane,  are  in  many  instances  not  deposited  or  intercalated 
evenly  over  the  entire  surface  of  the  protoplast.  Little  isolated  spots  are  left 
unaltered,  and  these  may  be  compared  in  a  way  to  the  small  glazed  windows  in  a 
living-room,  or  cabin  port-holes  closed  by  thin  panes  of  glass.  The  part  of  the 
thickened  wall  which  immediately  surrounds  the  little  window,  and  which  so  to 
speak  constitutes  its  frame,  has,  besides,  often  a  very  characteristic  structure, 
being  elevated  so  as  to  form  first  a  ring -like  border,  and  eventually  a  hood, 
arching  over  the  window  and  perforated  in  the  middle  (see  fig.  10  *).  A  comparison 
of  this  structure,  arched  over  the  thin  spots  in  a  cell-wall,  to  the  iris  spread  in 
front  of  the  crystalline  lens  in  an  eye  would  be  still  more  appropriate.  A  similar 
annular  border  projects  likewise  from  the  window  -  frame  on  the  other  side, 
facing  a  neighbouring  cell-cavity,  so  that  the  window  appears  symmetrically 
vaulted  on  both  sides  by  mouldings  with  round  central  apertures  (fig.  10 2). 
Supposing  someone  wanted  to  pass  from  one  cell-cavity  to  the  other  he  would  have 
in  the  first  place  to  go  through  the  hole  in  the  moulding  on  his  side.  He  would 
then  find  himself  in  a  roomy  space,  which  we  will  call  the  vestibule,  and  would 
next  have  to  break  through  the  little  window,  which  is  somewhat  thickened  in 
the  middle,  but  elsewhere  is  as  soft  and  thin  as  possible.  On  the  further  side 


THE   CELL-WALL. 


45 


again  would  be  a  vestibule,  and  it  would  not  be  until  he  had  emerged  from  this 
through  the  aperture  in  the  second  moulding  that  he  would  reach  the  interior 
of  the  adjoining  cell.  Seen  from  in  front,  the  outline  of  one  of  these  windows, 
or  rather  the  outline  of  the  common  floor  of  the  vestibules,  appears  as  a  circle, 
whilst  the  aperture  or  opening  in  the  moulding— which  is  exactly  in  the  centre 
of  this  circle — is  seen  as  a  bright  dot  or  pit  encompassed  by  the  circle  which 
defines  the  limits  of  the  vestibule.  Hence  these  curiously  protected  window 
structures  are  named  bordered  pits.  They  are  shown  in  fig.  10 1  and  10 2,  and 
are  to  be  seen  in  great  perfection  in  the  wood-cells  of  pines  and  firs. 

Whenever  bordered  pits  are  formed,  the  thickening  of  the  cell -membrane  is 
comparatively  slight;  the  frame  of  the  window  in  the  cell- wall  is  never  more  than 


Fig   10.— Connecting  Passages  between  adjacent  Cell-cavities. 

l,  Bordered  pits.  2,  Section  of  a  bordered  pit.  »,  Mode  of  connection  of  adjacent  cells  in  the  bundle-sheath  of  Scolopendrium. 
*,  Sieve-tubes.  «,  Group  of  cells  from  seed  of  Nux-vomica,  the  protoplasts  of  adjoining  cell-cavities  connected  by  fine 
protoplasmic  filaments. 

five  times  as  thick  as  the  window-pane  itself.  In  other  cases,  however,  the  cell-wall 
becomes  twenty  or  thirty  times  as  thick  as  it  was  at  first,  and  the  interior  of  the 
cell  is  thereby  seriously  diminished  in  size.  But  even  if,  little  by  little,  the  cell- wall 
augments  in  thickness  a  hundredfold,  any  spot  where  thickening  has  not  taken  place 
from  the  first,  and  where,  accordingly,  a  little  depression  occurs,  is  not  subsequently 
covered  with  cellulose,  but  is  carefully  kept  open  by  the  protoplast  as  it  builds. 
A  greatly  thickened  wall  of  this  kind  resembles  a  fortification  provided  here 
and  there  with  deep,  narrow  loopholes.  Where  two  cells  thus  provided  adjoin  one 
another,  the  windows  in  the  one  occur,  normally,  exactly  opposite  those  of  its 
neighbour,  and  the  result  is  the  formation  of  canals,  very  long  relatively,  which 
penetrate  through  the  two  adjacent  cell- walls  and  connect  the  neighbouring  cell- 
cavities  together  (fig.  10 3).  A  canal  of  this  kind  is  still  closed,  it  is  true,  in  the 
middle  by  the  original  cell-membrane  as  though  by  a  lock-gate;  but  this  slight 
obstruction  may  be  removed  later  by  solution,  and  the  contiguous  cells  have  then 
perfectly  open  connection  through  the  canal. 

Very  frequently  provision  is  made  in  the  very  first  rudiments  of  a  cell-mem- 


46  THE    CELL-WALL. 

brane,  destined  to  constitute  a  partition-wall,  for  open  communications  such  as  the 
above.  For  segments  of  the  wall  of  various  sizes  are  made  from  the  beginning  with 
sieve-like  perforations,  as  is  shown  in  fig.  10  4,  which  represents  diagrammatically 
portions  of  tubular  cells  called  "  sieve-tubes."  The  pores  are  crowded  close  together 
on  the  perforated  areas  of  the  walls  of  the  sieve-tubes,  and  their  dimensions  are 
relatively  broad  and  short.  Thus,  when  two  neighbouring  protoplasts  reach  out  to 
one  another  through  these  pores,  that  is  to  say,  when  there  is  continuity  of  the 
protoplasm  of  the  two  cell-cavities,  the  connecting  filaments,  which  pass  through  the 
pores  and  which  fill  them  completely,  are  short  and  thick  and  have  the  appearance 
of  pegs  or  stoppers. 

But  in  many  cases  the  pores  through  which  adjoining  cell-cavities  communicate 
are  drawn  out  to  a  great  length,  forming  infinitesimally  slender  passages.  They  are 
situated  close  together  in  great  numbers  and  penetrate  transversely  through  the 
thick  cell- walls  (fig.  10  5).  Neighbouring  protoplasts  may  be  brought  equally  well 
into  mutual  connection  by  means  of  these  canals,  or  perhaps  it  would  be  better  to 
say  that  their  connection  may  be  equally  well  maintained.  For  it  is  very  probably 
the  case  that  in  the  first  rudimentary  partition- wall,  which  is  produced  between  the 
products  of  division  of  a  protoplast,  minute  spots  remain  open  and  are  occupied  by 
connecting  threads  common  to  both  halves  of  the  protoplasm  as  they  draw  apart. 
Then  in  proportion  as  the  partition-wall  between  the  two  protoplasts,  produced  by 
the  division,  becomes  thicker,  the  openings  take  the  form  of  fine  canals,  and  the  con- 
necting filaments  are  modified  into  long  and  exceedingly  fine  threads  which  fill  the 
canals.  These  protoplasmic  threads  pierce  through  the  thickened  cell-wall  in  the 
same  way  as  a  dozen  telegraph-wires  might  be  drawn  through  a  partition  from  one 
room  into  another.  Often  a  number  of  protoplasts  living  side  by  side  and  one 
above  the  other  are  linked  together  by  filaments  of  this  kind,  which  radiate  in  all 
directions. 

This  species  of  connection,  of  which  an  intelligible  idea  is  given  by  fig.  105, 
escaped  the  notice  of  observers  in  former  times  owing  to  the  extraordinary  minute- 
ness of  the  canals,  and  delicacy  of  the  protoplasmic  filaments.  Another  method  of 
communication  between  protoplasts  in  adjoining  cells  has,  on  the  other  hand,  been 
long  known  and  often  described,  its  phenomena  being  very  striking  and  visible  when 
only  slightly  magnified.  The  connection  referred  to  is  that  which  is  afforded  by 
the  formation  of  so-called  "vessels."  By  vessels  the  older  botanists  understood 
tubes  or  utricles,  arising  from  the  dissolution  of  the  partition-walls  between  a  series 
of  cells.  Either  the  partition-walls  in  a  rectilineal  row  of  cells  vanish,  in  which 
case  long  straight  tubes  are  produced;  or  portions  of  the  walls  of  cells  arranged  at 
different  angles  to  one  another  are  dissolved,  and  then  tubes  are  formed  having  an 
irregular  course,  and  sometimes  branching  or  even  uniting,  so  as  to  make  a  net- work. 
In  instances  of  the  first  kind  the  lateral  walls  of  the  series  of  cells  which  are  to  lose 
their  transverse  partitions  are  previously  thickened  and  made  stiff  by  the  proto- 
plasts, which  also  provide  them  with  various  mouldings  and  panellings,  and  above  all 
with  bordered  pits.  This  task  accomplished,  the  protoplasts  forsake  the  tubes,  whose 


TRANSMISSION   OF   STIMULI.  47 

function  thenceforth  it  is  to  serve  as  passages  for  air  and  water;  thus  the  con- 
tinued presence  of  the  protoplasts  is  no  longer  advantageous.  On  the  other  hand, 
in  the  second  class  of  vessels  the  lateral  walls  of  the  cells,  which  have  coalesced 
to  form  them,  exhibit  no  thickening,  but  are  soft  and  delicate,  and  resemble 
flexible  tubing.  These  tubes,  moreover,  are  not  deserted  by  their  protoplasts;  but, 
after  the  coalescence  of  a  number  of  cells  into  a  single  duct  has  taken  place,  the 
protoplasts  in  the  cells  are  themselves  merged  together,  and  the  entire  tube  is 
then  occupied  by  an  uninterrupted  mass  of  protoplasm,  which  generally  persists 
as  a  lining  to  the  wall. 

As  the  initiation  and  construction  of  cell-walls  are  the  work  of  the  living  proto- 
plast, so  also  is  their  removal.  The  home  it  has  made  for  itself  the  protoplast  can 
also  demolish — either  partially  or  completely.  But  this  demolition  is  preluded  by 
the  importation  of  particles  of  water  into  the  portions  of  the  wall  which  are  to  be 
destroyed.  The  introduction  of  water  brings  the  wall  into  a  gelatinous  condition; 
the  cohesion  of  its  constituent  particles  is  loosened,  little  by  little,  and  at  length 
completely  abolished. 


4.  COMMUNICATION    OF  PROTOPLASTS  WITH  ONE  ANOTHER 
AND  WITH  THE  OUTER   WORLD. 

The  transmission  of  stimuli  and  the  specific  constitution  of  protoplasm. — 
Vital  Force,  Instinct,  and  Sensation. 

THE  TRANSMISSION  OF  STIMULI  AND  THE  SPECIFIC  CONSTITUTION 

OF  PROTOPLASM. 

As  has  been  already  intimated,  the  breaking  down  of  individual  cell- walls  and 
the  formation  of  the  various  pits,  sieve-pores  and  fine  canals  in  thickened  mem- 
branes, in  the  manner  described  in  preceding  pages,  are  processes  of  great  import- 
ance to  the  life  of  protoplasts.  In  the  first  place,  many  of  the  resulting  structures 
are  the  means  of  preserving  the  possibility  of  intercourse  with  the  outside  world. 
In  a  space  inclosed  by  evenly  thickened  walls,  the  absorption  of  air,  water,  and 
other  raw  materials  from  the  environment  would  be  very  difficult  if  not  impossible; 
the  protoplast  inside  would  soon  lack  the  provisions  needful  for  further  development, 
and  would  at  last  die  of  starvation,  drought,  and  suffocation.  But  the  little  win- 
dows, whether  open  or  closed  by  thin  permeable  membranes,  enable  it  to  supply 
itself  with  all  necessaries  of  life.  Another  advantage  is  derived,  in  the  case  of  many 
•of  these  structures,  inasmuch  as  the  protoplasts  on  occasion  escape  through  the  open 
doors  and  settle  down  in  some  other  part  of  the  cell-colony,  where  they  are  able 
again  to  make  themselves  useful.  Lastly,  one  of  the  most  important  benefits  of  all 
is  due  to  the  fact  that  mutual  intercourse  between  protoplasts,  living  together  as  a 
commonwealth,  is  rendered  possible  by  the  canals  which  join  them  together.  And 


48  TRANSMISSION   OF   STIMULI. 

such  an  intercourse  must  of  necessity  be  presumed  to  exist.  When  one  considers 
the  unanimous  co-operation  of  protoplasts  living  together  as  a  colony,  and  observes 
how  neighbouring  individuals,  though  produced  from  one  and  the  same  mother-cell, 
yet  exercise  different  functions  according  to  their  position;  and,  further,  how  uni- 
versally there  is  the  division  of  labour  most  conducive  to  the  well-being  of  the  whole 
community,  it  is  not  easy  to  deny  to  a  society,  which  works  so  harmoniously,  the 
possession  of  unity  of  organization.  The  individual  members  of  the  colony  must 
have  community  of  feeling  and  a  mutual  understanding,  and  stimuli  must  be  pro- 
pagated from  one  part  to  another.  No  more  obvious  explanation  offers  than  that 
the  protoplasmic  filaments,  which  run  like  telegraph-wires  through  the  narrow 
pores  and  canals  in  the  cell- walls  (see  fig.  10 5),  serve  to  propagate  and  transmit 
stimuli  from  one  piotoplast  to  another.  These  threads  of  protoplasm  may  indeed 
be  likened  to  nerves  which  convey  impulses  determining  definite  actions  from  cell 
to  cell. 

Imagination  takes  us  further  still,  and  raises  the  cell-nucleus  to  the  position 
of  the  dominant  organ  of  the  cell-body  For  the  nucleus  not  only  determines 
the  activity  of  the  individual  protoplast  within  its  own  cavity,  but  continues  in 
sympathetic  communion  with  its  neighbour  by  means  of  all  the  threads  and  liga- 
ments which  converge  upon  it.  This  last  idea  in  particular  derives  support  from 
indications  that  the  filaments  uniting  neighbouring  protoplasts  have  their  origin 
in  specific  transformations  in  the  substance  of  the  nucleus  itself.  When  a  proto- 
plast living  in  a  cell-cavity  is  about  to  divide  into  two,  the  process  resulting  in 
division  is  as  follows: — The  nucleus  places  itself  in  the  middle  of  its  cell,  and  at 
first  characteristic  lines  and  streaks  appear  in.  its  substance,  making  it  look  like 
a  ball  made  up  of  threads  and  little  rods  pressed  together.  These  threads  gradu- 
ally arrange  themselves  in  positions  corresponding  to  the  meridian  lines  upon  a 
globe;  but,  at  the  place  where  on  a  globe  the  equator  would  lie,  there  then  occurs 
suddenly  a  cleavage  of  the  nucleus — a  partition-wall  of  cellulose  is  interposed  in 
the  gap,  and  from  a  single  cell  we  now  have  produced  a  pair  of  cells.  In  this 
way,  from  the  nucleus,  and  from  the  protoplast  of  which  the  nucleus  is  the  centre, 
two  protoplasts  have  been  produced,  each  having  a  nucleus  of  its  own,  and  they 
thenceforth  live  side  by  side,  each  in  its  own  chamber.  It  has  been  proved  that 
in  this  process  of  division  the  substance  of  the  nucleus  is  not  completely  sundered 
by  the  partition  as  it  grows,  but  that,  as  we  have  already  mentioned,  minute 
pores  are  kept  open  in  the  cellulose  wall,  and  that  the  pair  of  protoplasts  continue 
joined  together  by  threads  running  through  these  pores. 

When  we  realize  that  every  plant  was  once  only  a  single  minute  lump  of 
protoplasm,  inasmuch  as  the  biggest  tree,  like  the  smallest  moss,  has  its  origin 
in  the  protoplasm  of  an  egg-cell  or  a  spore;  and  when  we  consider  how,  by  growth 
and  repeated  bipartition,  thousands  of  cells  are  evolved,  step  by  step,  from  a 
single  one,  whilst  their  protoplastic  bodies  still  remain  united  by  fine  filaments, 
we  arrive  of  necessity  at  the  conclusion  that  the  whole  mass  of  protoplasm,  living 
in  all  the  myriads  of  cells  whose  aggregation  constitutes  a  tree,  really  is,  and 


TRANSMISSION   OF   STIMULI.  49 

continues  to  be,  a  single  individual,  whose  parts  are  only  separated  by  perforated 
sieve-like  partitions.  Every  member  of  this  community  occupies  a  particular 
compartment  or  cavity,  and  is  governed  by  a  central  organ,  the  cell-nucleus;  but 
being  linked  to  its  fellows  by  connecting  threads  of  protoplasm,  a  mutual  under- 
standing is  thus  established  among  them. 

The  physical  basis  of  such  an  understanding  may  in  this  manner  be  represented 
with  tolerable  certainty.  But  it  is  extremely  difficult  to  throw  light  upon  the 
process  of  this  mutual  intelligence,  the  actual  method  whereby  the  cell-nuclei 
not  only  govern  within  their  own  narrow  spheres,  but  also  co-operate  harmoniously 
for  the  good  of  the  whole.  And  yet  the  problem  involved  in  this  unanimity  of 
action,  with  a  view  to  a  systematic  development  of  the  plant  in  its  entirety,  is 
of  such  extreme  importance  that  we  cannot  evade  it  even  if,  in  the  endeavour 
to  solve  it,  we  have  to  move  altogether  in  the  region  of  hypothesis. 

In  every  attempt  at  explanation  of  the  kind  we  must,  at  all  events,  bear  in 
mind  that  the  agreement  in  question,  as  well  as  the  processes  which  take  place 
in  pursuance  of  this  agreement,  such  as  the  nutrition,  growth,  and  the  organization 
of  the  entire  plant,  are  reducible  to  the  subtlest  atomic  agencies  in  the  living 
protoplasm.  They  may  be  resolved  into  the  motion  of  minute  particles,  into 
attractions  and  repulsions,  oscillations  and  vibrations  of  atoms,  and  into  re-arrange- 
ments of  the  atomic  groups  called  molecules.  Again,  these  movements  are  the 
result  of  the  action  of  forces,  especially  of  gravity,  light,  and  heat.  As  regards 
gravity  and  light,  experiment  shows,  however,  that,  when  acting  on  living  proto- 
plasm, they  give  rise  to  varying  effects  even  under  the  same  conditions;  and  this 
fact,  which  will  be  discussed  frequently  later  on,  indicates  that  these  forces  are 
at  any  rate  only  to  be  conceived  as  stimulative  and  not  coercive,  and  that  they 
have  no  power  to  determine  the  kind  of  form.  It  is  characteristic  of  the  processes 
set  up  by  gravity  and  light,  especially  when  they  take  place  in  the  continuous 
protoplasm  of  a  great  cell-community,  that  the  coarser  movements  visible  to  the 
naked  eye  are  often  manifested  in  members  comparatively  remote  from  the  part 
immediately  affected  by  the  stimulus.  We  cannot  well  represent  this  to  ourselves 
except  by  supposing  that  the  stimulus,  which  is  the  cause  of  the  movement,  is 
propagated  through  the  threads  of  protoplasm  from  atom  to  atom,  and  from 
nucleus  to  nucleus.  But  the  great  puzzle  lies,  as  already  remarked,  in  the  circum- 
stance that  the  atomic  and  molecular  disturbances  occasioned  by  such  stimuli  and 
transmitted  through  the  connecting  filaments  are  not  only  different  in  the  proto- 
plasm of  different  kinds  of  plants,  but  even  in  the  same  plant  they  are  of  such 
a  nature,  according  to  the  temporary  requirement,  that  each  one  of  the  aggregated 
protoplasts  in  a  community  of  cells  undertakes  the  particular  avocation  which  is 
most  useful  to  the  whole,  the  effect  of  this  joint  labour  conveying  the  impression 
of  the  presence  of  a  single  governing  power  of  definite  design  and  of  methodical 
action. 

That  a  stimulus  causes  different  occurrences  in  different  species  of  plants,  and, 
more  especially,  that  cell-communities  arising  from  different  egg-cells  develop  into 
VOL  I. 


50  TRANSMISSION   OF   STIMULI. 

different  forms,  though  under  identical  conditions  and  subjected  to  the  same  stimuli, 
are  phenomena  which  have  parallels  in  the  inanimate  world.  A  different  sound 
is  produced  by  striking  the  key  of  a  piano  which  is  connected  to  an  A-string  from 
that  resulting  from  the  transmission  of  a  similar  impulse  to  an  F-string;  and  the 
difference  depends  on  a  difference  of  structure  and  an  inequality  of  tension  in 
the  strings.  Again,  solutions  of  the  sulphate  and  of  the  hyposulphite  of  sodium 
in  similar  glass  vessels  are  indistinguishable  at  sight,  both  being  colourless  and 
transparent.  These  solutions  will  preserve  their  liquid  condition  when  cooled 
down  gradually  to  below  freezing-point  if  they  are  kept  absolutely  still;  but  the 
moment  the  vessels  are  touched  and  a  vibration  thereby  transmitted  to  the  contents, 
they  freeze.  Crystals  are  formed  in  the  apparently  identical  liquids,  but  crystals 
of  different  kinds,  Glauber's  salts  in  the  one  case,  hyposulphite  of  sodium  in  the 
other.  The  variety  of  form  depends  simply  on  the  sort  of  atoms,  and  on  their 
number  and  mode  of  grouping. 

In  a  similar  manner  must  be  explained  the  variety  of  forms  in  many  plant- 
species  developed  under  the  same  conditions  and  affected  by  the  same  stimuli. 
Dozens  of  kinds  of  unicellular  Desmids  and  Diatoms  are  often  developed  at  the 
same  time  in  a  single  drop  of  water  in  close  proximity  to  one  another.  Although 
the  protoplasm  in  the  spores  of  these  different  species  is  absolutely  identical  to 
our  vision,  aided  by  the  best  microscopes,  yet  the  mature  cells  exhibit  a  multiplicity 
of  form  which  is  quite  astonishing  to  the  observer  on  first  inspection.  One  cell 
is  semi-lunar,  another  cylindrical,  a  third  stellate,  a  fourth  lozenge-shaped,  and 
a  fifth  acicular.  In  one  specimen  the  cell-membrane  is  smooth,  in  another  it  is 
beaded;  some  are  provided  with  siliceous  coats,  whilst  others  have  flexible  envelopes. 

The  same  thing  holds  good  with  respect  to  the  vegetable  structures,  which  are 
composed  of  myriads  of  cells,  and  develop  into  huge  shrubs  or  tall  trees.  The 
protoplasm  in  the  egg-cell  of  an  oleander  is  produced  close  to  that  of  a  poplar  on 
the  same  river-bank,  and  under  exactly  the  same  external  conditions.  The  cells 
divide,  and  partition- walls  are  introduced  in  the  proper  direction  in  either  case, 
according  to  a  plan  of  structure  which  is  adhered  to  with  marvellous  precision 
by  the  protoplasts  engaged  in  the  work  of  construction.  In  each  species,  stem, 
branches,  foliage,  and  blossoms  have  invariably  a  particular  form  and  arrangement, 
have  the  same  colour  and  smell,  and  contain  the  same  substances.  How  utterly 
different  are  the  mature  leaf,  the  opened  flower,  and  ripe  fruit  of  the  oleander  from 
the  corresponding  parts  of  a  poplar.  Yet  both  were  nourished  by  the  same  earth, 
were  surrounded  by  the  same  atmosphere,  and  encountered  the  same  rays  of  sun- 
shine. We  cannot  otherwise  explain  it  than  by  the  supposition  that,  in  a  case 
like  this,  the  difference  of  form  in  the  perfected  state  is  based  upon  a  difference 
in  the  self -developing  protoplasm,  and  that  the  atoms  and  molecules  of  this  proto- 
plasm, which  appears  to  us  to  be  uniform,  vary  in  kind,  number,  and  grouping 
in  the  two  species  of  plants.  Consequently,  we  must  assume  that  every  vegetable 
organism,  every  species  of  plant  that  appears  invariably  in  the  same  external 
form  when  mature,  and  develops  according  to  an  invariable  plan,  has  a  protoplasm 


VITAL   FORCE,    INSTINCT,   AND   SENSATION.  51 

of  its  own  of  a  certain  specific  constitution.  And,  further,  we  must  assume  that 
this  specific  protoplasmic  constitution  is  transmitted  from  one  generation  to  another, 
so  that  the  protoplasm  of  the  oleander,  for  example,  had  exactly  the  same  constitu- 
tion thousands  of  years  ago  as  it  has  to-day.  Lastly,  we  must  assume  that  each 
special  kind  of  protoplasm  has  the  power  to  reproduce  its  like,  ever  anew,  from 
the  raw  materials  occurring  in  its  environment. 

VITAL  FOECE,   INSTINCT,   AND   SENSATION. 

The  phenomena  observed  in  living  protoplasm,  as  it  grows  and  takes  definite 
form,  cannot  in  their  entirety  be  explained  by  the  assumption  of  a  specific  con- 
stitution of  protoplasm  for  every  distinct  kind  of  plant;  though  this  hypothesis 
will  again  prove  very  useful  when  we  inquire  into  the  origin  of  new  species. 
What  it  does  not  account  for  is  the  appropriate  manner  in  which  various  functions 
are  distributed  amongst  the  protoplasts  of  a  cell-community;  nor  does  it  explain 
the  purposeful  sequence  of  different  operations  in  the  same  protoplasm  without 
any  change  in  the  external  stimuli,  the  thorough  use  made  of  external  advan- 
tages, the  resistance  to  injurious  influences,  the  avoidance  or  encompassing  of 
insuperable  obstacles,  the  punctuality  with  which  all  the  functions  are  performed, 
the  periodicity  which  occurs  with  the  greatest  regularity  under  constant  condi- 
tions of  the  environment,  nor,  above  all,  the  fact  that  the  power  of  discharging 
all  the  operations  requisite  for  growth,  nutrition,  renovation,  and  multiplication 
is  liable  to  be  lost.  We  call  the  loss  of  this  power  the  death  of  the  protoplasm. 
It  ensues  upon  assaults  from  without  if  they  succeed  in  destroying  the  molecular 
structure  so  entirely  as  to  render  reconstruction  impossible;  but,  furthermore, 
death  may  take  place  without  external  cause. 

If  cells  of  the  blood-red  alga,  previously  mentioned  as  allied  to  the  red-snow, 
are  collected  from  hollows  in  stones,  casually  full  of  rain-water,  and  are  kept 
dry  for  weeks  and  then  again  moistened,  the  water  is  found  to  have  a  very  power- 
ful effect.  The  protoplasm  becomes  mobile,  and  swarm-spores  are  formed  which 
put  forth  vibratile  cilia,  propel  themselves  about  for  a  short  time  in  the  water, 
and  then  settle  down  in  some  favoured  spot,  draw  in  their  cilia,  come  to  rest 
and  divide,  producing  offspring  which  again  are  motile.  This  alga  may  be  kept 
dry  for  months,  nay  even  over  a  year,  and  still  its  cells  exhibit  the  movements 
above  described  when  put  into  water.  But  if  a  mass  of  it  is  preserved  under 
these  same  conditions  for  many  years  and  then  moistened,  the  little  cells  will,  it 
is  true,  take  up  additional  water,  but  motile  cells  are  no  longer  formed.  The 
cells  do  not  move,  nor  grow,  nor  divide,  but  gradually  become  discoloured;  are  first 
disintegrated  and  then  dissolved.  We  say  then  that  in  them  life  could  no  longer 
be  recalled,  and  we  describe  them  as  dead. 

The  same  thing  is  observed  in  great  cell-communities.  The  seeds  of  many  species 
of  plants  preserve  the  capacity  for  germination  for  an  incredibly  long  period,  especially 
when  kept  in  a  dry  place.  If  after  ten  years  such  seeds  are  transferred  into 


52  .  VITAL   FORCE,   INSTINCT,    AND   SENSATION. 

moist  earth,  the  protoplasm  in  the  majority  of  cases  begins  to  bestir  itself  and 
to  move,  and  the  embryo  grows  out  into  a  seedling.  After  twenty  years,  perhaps, 
only  about  five  per  cent  of  the  seeds  preserved  would  germinate.  The  rest  are  not 
stimulated  by  damp  earth  to  further  development;  their  protoplasm  no  longer 
possesses  the  power  of  augmenting  its  volume  by  absorption  of  matter  from  the 
environment,  or  of  developing  a  definite  form,  but  is  disintegrated  by  the  influx  of 
air  and  water  and  breaks  up  into  simpler  compounds.  After  thirty  years  hardly 
one  of  the  seeds  would  sprout.  Yet  all  these  seeds  were  kept  throughout  the  time 
at  one  place  and  under  precisely  the  same  external  conditions;  nor  can  the  slightest 
change  in  their  appearance  be  detected.  Gardeners  express  the  fact  by  saying  that 
the  capacity  for  germination  becomes  extinct  in  from  twenty  to  thirty  years.  But 
what  kind  of  a  force  is  this  which  may  perish  without  a  physical  change  of  the 
substance  concerned  affording  the  basis  of  the  extinction  ?  In  former  times  a  special 
force  was  assumed,  the  force  of  life.  More  recently,  when  many  phenomena  of  plant 
life  had  been  successfully  reduced  to  simple  chemical  and  mechanical  processes, 
this  vital  force  was  derided  and  effaced  from  the  list  of  natural  agencies.  But  by 
what  name  shall  we  now  designate  that  force  in  nature  which  is  liable  to  perish 
whilst  the  protoplasm  suffers  no  physical  alteration  and  in  the  absence  of  any 
extrinsic  cause;  and  which  yet,  so  long  as  it  is  not  extinct,  causes  the  protoplasm 
to  move,  to  inclose  itself,  to  assimilate  certain  kinds  of  fresh  matter  coming 
within  the  sphere  of  its  activity  and  to  reject  others,  and  which,  when  in  full 
action,  makes  the  protoplasm  adapt  its  movements  under  external  stimulation  to 
existing  conditions  in  the  manner  which  is  most  expedient  ? 

This  force  in  nature  is  not  electricity  nor  magnetism;  it  is  not  identical  with 
any  other  natural  force,  for  it  manifests  a  series  of  characteristic  effects  which 
differ  from  those  of  all  other  forms  of  energy.  Therefore,  I  do  not  hesitate  again 
to  designate  as  vital  force  this  natural  agency,  not  to  be  identified  with  any  other, 
whose  immediate  instrument  is  the  protoplasm,  and  whose  peculiar  effects  we 
call  life.  The  atoms  and  molecules  of  protoplasm  only  fulfil  the  functions  which 
constitute  life  so  long  as  they  are  swayed  by  this  vital  force.  If  its  dominion 
ceases,  they  yield  to  the  operations  of  other  forces.  The  recognition  of  a  special 
natural  force  of  this  kind  is  not  inconsistent  with  the  fact  that  living  bodies 
may  at  the  same  time  be  subject  to  other  natural  forces.  Many  phenomena  of 
plant  life  may,  as  has  been  already  frequently  remarked,  be  conceived  as  simple 
chemical  and  mechanical  processes,  without  the  introduction  of  a  special  vital 
force;  but  the  effects  of  these  other  forces  are  observed  in  lifeless  bodies  as  well, 
and  indeed  act  upon  them  in  a  precisely  similar  manner,  and  this  cannot  be  said 
of  the  force  of  life. 

Were  we  to  designate  as  instinctive  those  actions  of  the  vital  force  which 
are  manifested  by  movements  purposely  adapted  in  some  manner  advantageous 
to  the  whole  organism,  nothing  could  be  urged  against  it.  For  what  is  instinct 
but  an  unconscious  and  purposeful  action  on  the  part  of  a  living  organism?  Plants, 
then,  possess  instinct.  We  have  instances  of  its  operation  in  every  swarm-spore 


VITAL   FORCE,   INSTINCT,   AND   SENSATION. 


53 


in  search  of  the  best  place  to  settle  in,  and  in  every  pollen-tube  as  it  grows 
down  through  the  entrance  to  an  ovary  and  applies  itself  to  one  definite  spot  of 
an  ovule,  never  failing  in  its  object.  The  water-crowfoot,  in  deep  water,  fashions 
its  leaves  with  finely  divided  tips,  large  air-passages,  and  no  stomata;  whilst, 
growing  above  the  surface  of  the  water,  its  leaves  have  broad  lobes,  contracted 
intercellular  spaces  and  numerous  stomata.  Linaria  Cymbalaria  (see  fig.  11) 
raises  its  flower-stalks  from  the  stone  wall  over  which  it  creeps  towards  the  light, 
but  as  soon  as  fertilization  has  taken  place,  these  same  stalks,  in  that  very  place 
and  amidst  unchanged  external  conditions,  curve  in  the  opposite  direction,  so  as 


Fig.  11.— Linaria  Cymbalaria  dropping  its  Seeds  into  Clefts  in  the  Rocks. 

to  deposit  their  seeds  in  a  dark  crevice.  The  flower-stalk  of  Vallisneria  twists 
itself  tightly  into  a  screw  and  draws  the  flowers,  which  previously  it  had  borne 
upon  the  surface  of  the  water,  down  to  the  bottom  when  their  stigmas  have  been 
covered  with  pollen-dust  at  the  surface.  These  are  all  cases  of  unconscious  action 
for  a  definite  object,  that  is  to  say,  they  are  the  result  of  instinct. 

If,  however,  we  attribute  instinct  to  living  plants,  it  is  but  a  step  further  to 
consider  them  as  endowed  with  sensation  also.  Feeling  in  animals  is  the  con- 
comitant of  a  condition  of  disturbance  in  nerves  and  brain  caused  by  a  stimulus, 
which  acts  on  the  organs  of  sense,  and  is  conveyed  by  nerves  to  the  central 
•organ.  The  transmission  of  the  stimulus  and  the  excited  state  of  the  brain  and 
nerves  are  only  molecular  movements  of  the  nervous  substance,  or,  let  us  say,  of 
the  protoplasm,  for  nerve-fibres  and  nerve-cells  are  simply  protoplasm  developed 
in  a  particular  manner.  But  the  state  induced  by  the  stimulation  of  protoplasm, 
which  is  what  we  call  sensation,  cannot  be  essentially  different  in  vegetable 
protoplasm  from  what  it  is  in  animal  protoplasm,  since  the  protoplasm  itself, 
the  physical  basis  of  life  in  both  plant  and  animal,  is  not  different.  In  isolated 
plant-cells,  indeed,  it  may  amount  to  such  a  concentration  of  the  condition  of 
stimulation  as  to  be  called  sensation,  for  the  cell-nucleus  is  to  all  appearance 


54  VITAL   FORCE,   INSTINCT,    AND   SENSATION. 

a  central  organ  in  relation  to  the  protoplast  that  lives  in  a  solitary  cell.  It  is 
not  of  course  to  be  supposed  that  within  a  whole  plant-structure,  that  is  in  the 
community  of  live  protoplasts  which  constitutes  an  individual  plant,  such  a  con- 
centration of  stimulation  could  occur  as  is  the  case  with  individual  animals  which 
have  nerve -fibres  all  converging  into  the  brain;  but  between  the  sensation  of 
animals  without  nerves  and  that  of  plants  no  essential  difference  can  exist. 

Hence  we  infer  that  there  is  no  barrier  between  plants  and  animals.  The 
attempt  to  establish  a  boundary-line  where  the  realm  of  plants  ceases  and  the 
animal  world  begins  is  a  vain  one.  If  we  naturalists,  all  the  same,  agree  to 
separate  plants  and  animals,  we  do  so  only  because  experience  shows  that  a 
division  of  labour  conduces  to  a  speedier  attainment  of  our  object.  On  the 
intermediate  ground  where  animals  and  plants  meet,  zoologists  and  botanists 
encounter  one  another,  not,  however,  as  hostile  rivals  with  a  view  to  exclusive 
possession  of  the  field,  but  as  colleagues  with  a  common  interest  in  the  adminis- 
tration and  cultivation  of  this  jointly  tenanted  region. 


ABSOEPTION  OF  NUTRIMENT. 


1.  INTRODUCTION. 

Classification  of  plants  with  reference  to  nutrition.— Theory  of  food -absorption. 

CLASSIFICATION   OF  PLANTS  WITH  REFERENCE  TO  NUTRITION. 

The  object  of  a  plant's  vital  energy,  next  in  importance  to  the  resistance  of  such 
influences  as  are  likely  to  bring  about  the  death  of  the  protoplasm,  is  growth,  i.e.  the 
addition  of  substance  to  its  body,  or,  in  other  words,  the  absorption  of  nutriment. 
A  living  plant,  whether  consisting  of  a  single  cell  or  of  a  vast  community  of  cells, 
takes  up  food  from  its  environment  in  quantities  varying  according  to  the  needs  of 
the  moment.  But  its  method  of  action — how  it  sets  about  acquiring  possession  of 
this  raw  material,  how  it  manages  to  incorporate  the  substances  absorbed  from  with- 
out, how  it  contrives  to  retain  only  such  part  as  is  useful  to  it,  and  to  reject  and  get 
rid  of,  like  ballast,  what  does  not  subserve  its  own  growth — is  infinitely  varied. 
This  variety  in  the  processes  of  food-absorption  corresponds,  on  the  one  hand,  to 
differences  in  the  habitat  of  plants,  and,  on  the  other,  to  the  requirements  of  particu- 
lar species,  which  requirements  in  their  turn  depend  upon  a  specific  constitution  of 
the  protoplasm  in  each  species  concerned.  The  difference  must  be  very  great 
between  this  process  as  manifested  in  plants  which  are  immersed  in  water  during 
their  whole  lives  and  the  same  as  it  occurs  in  plants  which  live  in  desert  sands  and 
are  not  supplied  with  water  for  months  together.  And  again,  absorption  in  those 
fungi  which  grow  luxuriantly  on  damp  timber  in  the  deep  obscurity  of  a  mine  must 
take  place  very  differently  from  the  corresponding  process  in  the  delicate  alpine 
plants  which  on  our  mountain  slopes  are  exposed  periodically  to  the  most  intense 
sunlight,  and  then,  for  weeks  at  a  time,  are  wreathed  in  sombre  mists.  So,  also, 
the  reciprocal  action  between  plants  and  their  environment  must  have  a  character  of 
its  own  in  the  case  of  parasitic  growths  which  absorb  their  food  from  other  living 
organisms,  and  in  those  remarkable  plants,  too,  which  catch  and  devour  small  insects, 
and  in  such  minute  organisms  as  yeast,  the  vinegar  ferment,  and  others,  which  play 
so  important  a  part  in  our  daily  life,  and  lastly,  in  the  gigantic  trees  which  form  our 
forests. 

To  acquire  a  general  notion  of  these  forms,  with  reference  to  their  varieties  as 
regards  nutrition,  it  is  best  to  classify  them  in  the  first  place  in  groups  according  to 
their  habitat,  viz.:  into  water-plants  or  hydrophytes,  stone-plants  or  lithophytes, 
land-plants,  and  epiphytes.  But  here  again  it  is  necessary  to  remark  that  no  sharp 


56  CLASSIFICATION   OF   PLANTS   WITH   REFERENCE   TO   NUTRITION. 

line  of  demarcation  exists  between  these  groups;  all  are  connected  by  numerous 
intermediate  links,  and  there  are  forms  which  belong  to  one  group  at  one  stage  of 
development  and  to  another  at  another  stage. 

The  distinctive  property  of  aquatic  plants  is  that  they  derive  their  nourishment 
either  entirely  or  principally  from  the  surrounding  water.  Some  preserve  their 
freedom,  floating  or  swimming  about  in  the  liquid  medium;  but  the  majority  are 
fixed  somewhere  under  the  water  by  special  organs  of  attachment.  Many  plants 
that  are  rooted  in  the  mud  at  the  bottom  of  pools  are  able  to  derive  their  food  from 
the  water  when  it  is  high,  and  when  it  is  low,  from  the  atmosphere  as  well:  such 
amphibious  organisms  form  a  transitional  group  between  water-plants  and  land- 
plants.  The  number  of  lithophytes  is  comparatively  very  small.  They  include 
those  lichens  and  mosses  which  cling  in  immediate  contact  to  the  surface  of 
stones  and  derive  their  food  in  a  fluid  state  direct  from  the  atmosphere.  All 
lithophytes  are  so  constituted  that  they  can,  without  injury,  dry  up  and  suspend 
their  vitality  for  a  time  when  there  is  a  failure  of  atmospheric  precipitation  lasting 
over  a  long  period  or  when  the  air  itself  is  very  dry.  But  not  every  plant  which 
grows  upon  rocks  is  to  be  regarded  as  a  lithophyte  in  the  narrower  acceptation  of 
the  term.  Those  that  are  rooted  in  earth  in  the  cracks  and  crevices  of  the  rock 
must  be  classed  amongst  land-plants.  To  this  class  indeed  more  than  half  the 
plants  now  in  existence  belong.  Though  surrounded  by  air  as  regards  a  part  of 
their  structure  they  have  another  part  sunk  in  the  soil,  and  from  the  soil  they  take 
up  water  and  inorganic  compounds  in  aqueous  solution.  Plants  which  grow  attached 
to  other  plants  or  to  animals  are  called  epiphytes. 

The  majority  of  plants  are  during  the  period  of  food-absorption  connected  with 
the  foster-earth  and  are  not  capable  of  locomotion.  The  plant  being  fixed  to  one 
spot  must  therefore  sooner  or  later  exhaust  the  ground  in  its  neighbourhood,  and 
must  require  a  further  supply  of  nutritive  substances.  The  parts  specially  devoted 
to  food-absorption  often  lengthen  out  in  these  circumstances  beyond  the  im- 
poverished region,  and  thus  endeavour  to  bring  areas  more  and  more  distant  within 
the  range  of  absorption.  Many  plants  possess  the  faculty,  to  which  reference  has 
already  been  made,  of  alluring  animals  and  of  killing  and  sucking  their  juices.  Not 
only  amongst  saprophytes  and  parasites,  but  also  amongst  aquatic  plants,  instances 
occur  in  which  certain  movements  are  performed  involving  the  whole  body  of  the 
organism,  with  a  view  to  promoting  the  absorption  of  nutriment.  Particularly  striking 
in  this  respect  are  many  plasmoid  fungi  (which  we  may  well  refer  to  here,  not  on 
this  account  alone,  but  also  for  the  additional  reason  that  they  take  in  nourishment 
without  the  intervention  of  a  cell-membrane).  The  naked  protoplasm  in  these  cases, 
which  include  in  particular  the  class  of  Amoebae,  crawls  in  its  search  for  food  over 
the  nourishing  substratum,  and  derives  from  it  immediately  the  materials  needful  for 
growth.  Loose  bodies  are  liable  to  be  seized  by  the  radiating  processes  of  the  proto- 
plasm, which  then  closes  round  them  and  drains  them  completely  of  their  juices  (see 
fig.  9,  the  last  figure  to  the  right).  These  bodies  encompassed  by  the  protoplasm,  if 
small,  are  drawn  inwards  from  the  periphery  and  are  regularly  digested  in  the 


THEORY   OF    FOOD- ABSORPTION.  57 

interior.  Such  parts  of  foreign  bodies  as  are  not  serviceable  for  nutrition  are  sub- 
sequently eliminated  or  are  left  behind  by  the  protoplast  as  it  creeps  onward.  But 
this  method  of  food-absorption  is  limited  to  amoeboid  forms  belonging  to  the 
boundary-land  of  animal  and  vegetable  life.  The  movements  of  other  naked  proto- 
plasts, such  as  those  which  are  carried  about  in  the  water  by  vibratile  cilia,  have 
nothing  to  do  with  the  search  for  food  or  with  its  absorption,  but  are  connected 
rather  with  the  processes  of  distribution  and  propagation. 

THEORY  OF  FOOD-ABSORPTION. 

In  the  case  of  protoplasts  inclosed  in  cell-membranes  the  food  necessary  for 
nourishment  must  always  pass  through  the  cell-membrane  and  peripheral  proto- 
plasmic layer  (ectoplasm)  into  the  interior  of  the  protoplasmic  bodies.  And  so, 
conversely,  such  of  the  substances  absorbed  as  are  of  no  use  in  the  construction 
of  the  organism  or  for  any  other  purpose,  must  be  separated  and  passed  out 
through  these  envelopes.  The  cell-membranes  of  those  protoplasts  which  are 
employed  in  absorbing  food  must  accordingly  have  a  special  structure:  the 
ultimate  particles  must  be  so  arranged  as  to  allow  of  the  passage  of  nutritious 
material  inwards,  and  of  rejected  matter  outwards,  without  prejudice  to  their  own 
stability.  The  passages  in  cell-walls  used  for  this  purpose  are  very  minute,  much 
smaller  at  all  events  than  the  pore-canals  described  above  as  being  occupied  by 
fine  protoplasmic  filaments;  the  dimensions  are  in  fact  so  trifling  as  to  be  invisible 
even  with  the  best  microscopes.  Still  we  are  forced  to  conclude  that  they  exist 
by  a  posteriori  reasoning  from  a  series  of  phenomena,  and  to  assume  that  the  cell- 
membrane,  like  almost  every  other  kind  of  body,  consists  not  of  continuous  matter, 
but  of  minute  particles,  which  are  termed  atoms,  and  are  separated  from  one 
another  by  infinitesimally  small  spaces.  Various  processes  and  appearances  have 
also  led  physicists  and  chemists  to  the  conclusion  that  these  atoms  are  not  aggre- 
gated in  disorder,  but  are  always  combined  together  in  groups  of  two  or  more, 
even  in  the  case  where  all  the  atoms  in  a  body  are  of  the  same  kind,  i.e.  are  the 
same  element.  If  a  body  contains  different  elements  they  are  not  mixed  together 
indiscriminately,  but  are  grouped  in  conformity  to  a  definite  law:  every  group 
includes  atoms  of  all  the  different  elements  concerned,  arranged  in  a  certain  in- 
variable manner,  not  only  as  regards  number,  but  also  as  regards  relative  position. 
Groups  of  atoms  of  this  kind  are  called  "  molecules,"  and  the  spaces  between  them 
are  supposed  to  be  larger  than  those  between  single  atoms.  Further,  it  is  not 
improbable  that  the  molecules  themselves  form  groups,  each  group  consisting  of 
molecules  conglomerated  in  a  definite  manner,  and  that  the  passages  separating 
these  molecular  groups  are  larger  again  than  those  separating  the  single  molecules 
within  each  group.  These  groups  of  molecules  have  been  called  "micellae"  or 
Tagmata,  and  they  also  are  supposed  to  be  aggregated  together  in  definite  order. 

According  to  this  theory  the  cell-membrane  is  analogous  to  a  sieve,  the  pores 
of  which  are  grouped  in  a  definite  manner,  the  broadest  perforations  being  between 


58  THEORY   OF   FOOD-ABSORPTION. 

the  micellae  or  groups  of  molecules,  narrower  apertures  between  the  molecules 
or  groups  of  atoms  in  each  micella,  and  lastly  the  finest  pores  between  the  atoms 
themselves  in  each  molecule.  These  interspaces  are  liable  to  contraction  and 
expansion,  for  the  union  of  the  molecules  is  affected  by  two  forces,  one  of  which 
manifests  itself  as  a  mutual  attraction  between  atoms  and  atomic  groups,  whilst 
the  other  tends  to  drive  atoms  and  molecules  asunder.  Of  these  forces  the  former, 
i.e.  the  attractive  force  existing  in  all  material  particles,  is  called  chemical  affinity 
when  it  causes  atoms  of  different  kinds  to  unite  to  form  a  molecule;  and  it  is  called 
cohesion  when  applied  to  the  mutual  attraction  of  similar  molecules,  and  adhesion 
where  it  holds  together  masses  of  molecular  groups  with  their  surfaces  in  contact. 
The  action  of  heat  is  opposed  to  this  attractive  force,  which  is  only  effective  at 
infinitesimal  distances.  Bodies  are  all  caused  to  expand  by  heat,  their  atoms,  mole- 
cules, and  micellae  being  forced  apart.  Heat  is  believed  to  be  a  vibratory  motion 
of  these  ultimate  particles,  and  it  is  supposed  that  the  greater  the  vibrations  the 
greater  is  the  separation  of  atoms  and  atomic  groups,  the  interspaces  expanding 
and  the  heated  body  increasing  consequently  in  volume.  As  is  well  known,  the 
atoms  and  molecules  may  be  forced  so  far  apart  by  increase  of  temperature  that 
cohesion  is  entirely  overcome,  and  solids  are  converted,  first  into  liquids  and  at 
last  into  gases. 

The  interspaces  or  passages  between  the  molecules  and  molecular  groups  com- 
posing a  cell-membrane  are  penetrable  by  molecules  of  other  substances,  provided 
always,  firstly,  that  the  admitted  molecules  are  not  larger  than  the  passages;  and 
secondly,  that  there  exists  between  the  molecules  of  the  cell- wall  and  those  of  the 
penetrating  body  that  sort  of  attractive  force  which  has  been  designated  chemical 
affinity.  Both  premises  are  satisfied  in  the  case  of  aqueous  molecules,  and  experi- 
ment proves  that  they  are  admitted  into  the  inter-molecular  spaces  of  a  cell- 
membrane  with  great  ease  and  readiness.  The  cell-membrane  saturates  itself  with 
water,  or,  to  use  the  technical  phrase,  it  has  the  tendency  and  ability  to  "imbibe" 
water.  The  force  of  attraction  between  molecules  of  a  cell-membrane  and  water- 
molecules  is  indeed  so  intense  that  the  cohesion  of  the  molecules  in  the  membrane 
is  partially  neutralized,  and  the  imbibed  water  causes  them  to  move  apart.  In 
consequence  of  this,  the  cell-membrane  swells  up  and  its  dimensions  are  increased. 

It  is  also  supposed  that  the  micellae  of  a  cell-membrane  attract  and  admit  water- 
molecules  to  such  an  extent  as  to  surround  themselves  with  watery  envelopes. 
Such  a  condition  would  no  doubt  be  nothing  but  beneficial,  promoting,  as  it  would, 
the  interchange  of  materials  through  the  cell-membrane,  and  the  mixing  of  fluid 
substances  situated  on  either  side  of  the  porous  membrane.  At  all  events  this 
mixing  process  must  ensue  in  the  interspaces  of  the  cell-membrane;  and,  in  the 
particular  case  out  of  which  this  discussion  has  arisen,  viz.  food -absorption,  the 
interacting  substances  are,  on  the  one  hand,  the  compounds  in  the  soil  outside 
the  cell-membrane,  and,  on  the  other,  the  organic  compounds  under  the  control 
of  the  live  protoplast  within  the  cell-membrane.  Both  the  outgoing  and  the  in- 
coming substances  must  be  soluble  in  water,  and  must,  therefore,  have  an  attraction 


THEORY   OF   FOOD- ABSORPTION.  59 

for  water.  But  the  power  of  a  substance  in  aqueous  solution,  whether  without 
or  within  the  cell-membrane,  to  permeate  the  saturated  pores,  and  to  mix  thoroughly 
there,  certainly  depends  also  on  the  degree  of  chemical  affinity  and  of  adhesion 
existing  between  the  molecules  and  micellae  of  the  cell-membrane  on  the  one  hand, 
and  these  infiltrating  substances  on  the  other.  A  very  complex  interaction  of 
forces  takes  place  which  we  cannot  here  investigate  any  further,  as  it  would  take 
us  much  too  far  afield. 

Returning  to  the  explanation  of  food-absorption,  attention  must  be  drawn  to 
the  fact  that  the  mixing  or  diffusion  which  takes  place  through  the  cell-membrane 
differs  from  the  free  diffusion  which  would  occur  if  the  cell-membrane  were  not 
present.  Experiment  has  proved  that  if  one  side  of  a  cell-membrane  is  steeped 
in  a  saline  solution  and  the  other  in  an  equal  volume  of  pure  water,  the  number 
of  saline  particles  which  pass  through  into  the  water  are  many  fewer  than  the 
number  of  water-particles  which  pass  into  the  solution  of  salt;  and,  moreover,  if 
an  organic  compound,  such  as  albumen  or  dextrin,  is  on  one  side,  and  water  on 
the  other,  water  transfuses  to  the  organic  compound,  whereas  no  trace  of  the 
albumen  or  dextrin  (as  the  case  may  be)  passes  through  to  the  water.  Now  this 
phenomenon,  which  is  called  "osmosis"  ("endosmosis  and  exosmosis"),  is  of  great 
importance  for  the  conception  we  have  to  form  of  food-absorption.  It  is  clear  that, 
whilst  water  and  substances  dissolved  in  water  are  brought  under  the  control  of 
the  protoplast  within  a  cell  through  the  cell-membrane,  as  a  consequence  of  the 
action  of  albuminous  and  other  compounds  constituting  the  body  of  the  protoplast, 
and  of  the  salts  dissolved  in  the  so-called  cell-sap  in  the  vacuoles,  there  is  no 
necessity  for  any  part  of  the  cell-content  to  pass  out  through  the  cell-membrane. 
Thus  the  protoplasm  is  able  to  exercise  an  absorptive  action  on  aqueous  solutions 
outside  the  cell-membrane,  and  to  continue  to  absorb  until  the  cell  is  filled.  Indeed, 
the  chemical  affinity  for  water  possessed  by  the  substances  in  a  cell  may  occasion 
so  great  an  absorption  of  water  that,  in  consequence,  the  volume  of  the  cell  is 
enlarged  and  the  cell-membrane  is  subjected  to  pressure  from  within.  The  cell- 
membrane  is  able  to  yield  to  this  pressure  to  the  extent  permitted  by  its  elasticity; 
but  excessive  stretching  of  the  cell-membrane  is  at  length  counteracted  by  cohesion, 
and  thus  a  condition  is  attained  in  which  the  cell-contents  and  the  cell-membrane 
are  subjected  to  mutual  pressure,  a  state  which  is  called  "  turgidity." 

The  process  just  described,  of  the  absorption  of  water  in  large  quantities  into 
the  precincts  of  the  protoplasm  without  any  simultaneous  transmission  of  matter  to 
the  outside,  is  certainly  in  no  respect  an  exchange.  But  it  obviously  does  not 
exclude  the  possibility  of  a  real  exchange  taking  place  between  substances  on  either 
side  of  a  cell-membrane,  i.e.  between  solutions  in  the  soil  and  those  in  the  cell- 
sap  contained  in  lacunae  of  the  protoplasm.  Certain  phenomena  in  fact  put  it 
beyond  doubt  that  on  occasion  a  real  exchange  of  this  kind  does  occur.  But  it 
is  complicated  by  the  circumstance  that  substances  in  process  of  being  exchanged 
have  to  pass  not  only  through  the  cell-membrane  but  also  through  the  primordial 
utricle;  and  the  primordial  utricle  consists  of  molecules  of  a  kind  other  th-*ui 


60  NUTRIENT   GASES. 

those  of  the  cell-wall,  having  different  chemical  affinities,  and  these  molecules 
again  are  differently  grouped;  nor  are  the  passages  for  aqueous  solutions  the  same. 
All  this  cannot  but  have  an  important  bearing  on  the  permeating  capacity  of 
the  substances  that  are  being  interchanged. 

Although  all  these  ideas  concerning  the  molecular  structure  of  cell-membranes 
and  of  protoplasm,  concerning  the  intermixture  and  exchange  of  materials  and 
the  absorption  on  the  part  of  cells  and  their  swelling  up,  have  only  the  value 
of  theories,  still  we  have  good  ground  for  assuming  that  they  are  fairly  near 
the  truth.  They  give  us,  at  all  events,  an  intelligible  representation  of  the  inter- 
action which  takes  place  between  living  protoplasts,  with  their  need  for  food,  and 
the  environment,  which  supplies  the  nutriment. 


2.    ABSORPTION   OF   INORGANIC   SUBSTANCES. 

Nutrient  Gases.— Nutrient  Salts.— Absorption  of  Nutrient  Salts  by  Water-plants,  Stone-plants, 
and  Land-plants. — Relations  between  the  position  of  Foliage-leaves  and  Absorption-roots. 

NUTRIENT  GASES. 

One  of  the  most  important  sources  of  the  nourishment  of  plants  is  carbonic 
acid.  The  living  protoplasts  appropriate  it  from  water  and  from  air,  in  the  latter 
case  chiefly  by  attracting  the  carbon-dioxide.1  This  gas  penetrates  a  cell- wall  satur- 
ated with  water  more  readily  than  the  other  constituent  gases  of  the  atmosphere 
(nitrogen  and  oxygen).  In  the  wall  it  is  converted  into  carbonic  acid,  and  it  then 
passes  on  into  the  cell-sap  contained  in  the  cavities  of  the  protoplast.  Apart  from 
the  effects  of  temperature  and  atmospheric  pressure,  the  quantity  of  carbonic 
acid  absorbed  is  chiefly  determined  by  the  requirements  of  the  cells  whose  nourish- 
ment is  in  question.  These  requirements,  however,  vary  considerably  according 
to  the  specific  constitution  of  the  protoplasm  and  with  the  time  of  day.  During 
daylight  the  need  of  carbon  is  very  great  in  all  green  plants.  As  soon  as  the 
carbonic  acid  reaches  the  cell-sap  it  is  decomposed  and  reduced  by  the  action  of 
sunlight,  and  from  it  are  formed  compounds  known  as  carbo-hydrates.  The 
oxygen  thus  set  free  is,  however,  removed  from  the  cell  precincts,  and  expelled  into 
the  surrounding  air  or  water.  In  this  way  the  gas  when  barely  absorbed  is 
withdrawn,  as  such,  from  the  cell-sap,  the  carbon  alone  being  retained  and  the 
oxygen  eliminated,  and  a  renewed  attraction  of  carbon-dioxide  from  the  sur- 
rounding medium  ensues.  The  fresh  supply  again  is  immediately  worked  up  in  the 
green  chlorophyll-bodies,  so  that  there  is  a  constant  influx  of  carbon- dioxide,  and 
therefore  indirectly  of  carbonic  acid,  from  the  environment  into  the  interior  of 
green  cells  to  the  part  where  its  consumption  takes  place.  Were  it  possible  to  see 

1  The  atmosphere  contains  free  carbon-dioxide  and  not  carbonic  acid.     But  carbonic  acid  is  formed  when  the 
dioxide  is  absorbed  into  water. 


NUTRIENT   GASES.  61 

the  molecules  of  carbon-dioxide  in  the  air,  we  should  observe  how  much  faster  they 
are  impelled  towards  the  leaves  and  other  green  parts  of  plants,  where  the  intense 
craving  for  carbon  is  localized,  than  are  the  other  constituent  particles  of  the  air. 
This  impulsion  and  influx  lasts  so  long  as  the  green  cells  are  under  the  influence  of 
daylight.  The  first  thing  in  the  morning  when  the  first  ray  of  sunshine  falls  upon 
a  plant  the  protoplasts  begin  work  in  their  little  laboratories  decomposing  carbonic 
acid,  and  producing  from  it  sugar,  starch,  and  other  similar  organic  compounds. 
And  it  is  not  till  the  sun  sets  that  this  work  is  suspended,  and  the  influx  of  carbon- 
dioxide  stopped  till  the  following  morning. 

The  green  plants  that  spend  all  their  lives  under  water  are  supplied  with  car- 
bonic acid  by  the  water  surrounding  their  cells,  which  always  contains  some  of  that 
material.  In  the  case  of  unicellular  plants  of  this  class,  absorption  of  carbonic  acid 
takes  place  through  the  whole  surface  of  the  cell-membrane.  Multicellular  plants, 
with  their  cells  arranged  in  filaments  or  plates,  only  take  in  carbonic  acid  through 
those  parts  of  the  walls  of  their  cells  which  are  in  immediate  contact  with  the 
water.  This  applies  also  to  submerged  plants  composed  of  several  layers  of  cells 
and  of  considerable  dimensions.  Thus,  in  plants  of  this  kind,  the  cells  in  contact 
with  the  water  constitute  the  skin.  They  are  always  pressed  closely  together 
and  squeezed  flat,  are  not  thickened  on  the  side  exposed  to  the  water,  and  are 
united  everywhere  edge  to  edge  leaving  no  gaps.  But  in  the  interior  of  these 
water-plants  large  lacunae  and  cavities  are  formed  from  earliest  youth,  owing  to 
the  detachment  of  single  rows  of  cells,  and  the  spaces  so  formed  are  filled  with 
a  quantity  of  nitrogen,  oxygen,  and  carbon-dioxide,  that  is  to  say,  with  a  gaseous 
mixture  not  essentially  different  from  atmospheric  air.  Although  this  organiza- 
tion may  have  as  its  primary  object  the  reduction  of  the  plant's  weight  as  a 
whole,  it  cannot  be  without  a  further  importance  inasmuch  as  carbonic  acid  can 
be  taken  up  from  the  air-spaces  into  adjacent  cells.  But  there  is  no  doubt  that, 
even  in  this  case  (of  water-plants  provided  with  large  internal  air-cavities), 
the  chief  absorption  of  carbonic  acid  is  through  the  epidermis,  or  more  precisely 
through  those  walls  of  the  epidermal  cells  which  are  in  immediate  contact  with 
the  water. 

The  carbonic  acid  taken  up  by  cells,  wholly  or  partially  immersed  in  water, 
is  either  contained  as  such  dissolved  in  the  watery  medium,  or  occurs  in  com- 
bination with  calcium  as  bicarbonate  of  lime.  Part  of  the  carbonic  acid  in  this 
bicarbonate  in  aqueous  solution  is  susceptible  of  being  withdrawn  by  water-plants, 
mono-carbonate  of  lime,  which  is  insoluble  in  water,  being  then  precipitated  on 
the  cell-wall  through  which  the  rest  of  the  carbonic  acid  has  passed  into  the 
cell-interior.  Accordingly,  a  large  number  of  water-plants  are  found  incrusted 
with  lime  in  both  fresh  and  salt  water.  We  shall  return  to  this  important  pheno- 
menon when  we  treat  of  the  influence  of  living  plants  on  that  part  of  the  environ- 
ment which  comes  within  their  sphere  of  action  for  purposes  of  nutrition. 

Lithophytes  obtain  carbonic  acid  from  the  moisture  deposited  upon  them  from 
the  aqueous  vapour  in  the  atmosphere,  and  attract  carbon-dioxide  direct  from  the 


(32  NUTRIENT   GASES. 

air  around  them.  The  chief  members  of  this  class  are  those  mosses,  liverworts,  and 
lichens  which,  though  clinging  to  dry  rocks,  behave  just  like  water-plants  as  regards 
the  absorption  of  carbonic  acid.  There  is  no  reason  to  think  that  these  plants 
absorb  carbonic  acid  in  dry  weather;  for  under  the  influence  of  dry  air  they  lose 
water  fast,  and  meanwhile  receive  no  compensation  from  the  rock  to  which  they 
are  attached,  and  in  a  short  time  they  become  so  dry  that  they  crumble  into 
powder  when  rubbed  between  the  fingers.  Vitality  is  suspended  for  a  time,  and 
it  is  out  of  the  question  that  there  should  be  any  absorption  of  carbon-dioxide 
from  the  atmosphere  under  such  circumstances.  But  the  moment  the  plant  is 
moistened  by  rain  or  dew,  the  cell-walls  directly  exposed  to  the  air  become 
saturated,  and  are  enabled  to  admit  water  into  the  interior.  Then  the  lithophytes 
suck  up  water  very  fast;  the  dry,  apparently  dead,  incrustations  swell  up  again, 
and,  together  with  the  rain  and  dew,  carbonic  acid  is  absorbed,  it  being  contained 
in  all  depositions  of  atmospheric  moisture.  A  tumescent  moss  tuft  can,  in  addi- 
tion, absorb  carbon-dioxide  direct  from  the  atmosphere  through  its  saturated 
superficial  cells;  but  the  quantity  of  carbonic  acid  thus  acquired  by  a  plant  is  in 
any  case  only  secondary.  Many  mosses,  as  for  example  the  widely-distributed  Grim- 
mia  apocarpa,  are  also  able  to  live  just  as  well  under  water  as  in  air;  nor  is  any 
alteration  of  their  leaves  necessary  in  either  condition,  nor  any  special  contrivance 
for  the  absorption  of  carbonic  acid  and  water.  These  substances  reach  the  interior 
by  similar  passage  through  cell-walls  of  identical  construction,  whether  the 
Grimmia  spends  its  life  attached  to  submerged  rocks  or  in  the  open  air  at  the 
top  of  a  mountain;  whence  we  may  infer  that  there  is  a  greater  resemblance 
between  lithophytes  and  water-plants  as  regards  nutrition  than  between  litho- 
phytes and  land-plants. 

Land-plants  satisfy  their  need  of  carbon  almost  exclusively  by  withdrawing 
the  dioxide  from  atmospheric  air.  For  the  purpose  of  this  direct  appropriation, 
specially  adapted  structures  are  found  in  them.  Seeing  that  these  plants  are 
not  able  to  endure  periodic  desiccation  in  times  of  drought,  as  lithophytes  are, 
it  is  necessary  for  them  to  be  secured  against  excessive  loss  of  water.  Accord- 
ingly, the  cell-walls  in  immediate  contact  with  the  air,  that  is  to  say,  the  outer 
walls  of  the  epidermis,  are  thickened  by  a  layer  (cuticle)  which  is  impermeable 
by  air  or  water,  and,  in  general,  they  are  so  organized  that  water  cannot  readily 
escape  from  the  interior  of  the  cells.  Obviously,  however,  a  cell- wall  which  opposes 
a  strong  resistance  to  the  extravasation  of  water  will  not  give  easy  admittance  to  an 
influx  either,  and  the  conditions  for  the  passage  of  gases  through  a  cell-membrane, 
thickened  and  cuticularized  in  this  way,  would  be  far  from  favourable.  As  a 
matter  of  fact  many  of  the  constituent  gases  of  the  atmosphere  permeate  these 
thickened  walls  of  the  epidermal  cells  only  with  great  difficulty,  and  others  not  at 
all.  Carbon-dioxide  alone  has  the  power  of  penetrating,  but  even  in  the  case  of 
this  gas  the  quantity  is  not  always  sufficient  to  satisfy  the  demand.  To  ensure 
that  so  important  a  form  of  plant-food  should  reach  in  proper  amount  those  cells 
lying  under  the  epidermis,  which  are  occupied  by  protoplasts  engaged  in  the  regu- 


NUTRIENT   GASES.  63 

lation  of  nutrition,  there  is  an  adaptation  of  structure  of  the  following  nature. 
Among  the  firmly  connected  epidermal  cells  with  their  thickened  outer  walls  al- 
most impervious  to  air,  other  cells  are  interspersed  at  intervals.  They  are  always 
in  pairs,  are  generally  rather  smaller  than  the  rest,  and  have  a  little  cleft  open 
between  them.  Inasmuch  as  these  apertures  (stomata)  always  exist  where  passages 
and  canals,  the  so-called  intercellular  spaces,  have  arisen  from  the  separation  of 
individual  cells  of  the  sub-epidermal  tissues,  each  stoma  constitutes  the  mouth  of  a 
system  of  channels  ramifying  between  the  thin-walled  cells  of  the  interior.  The 
components  of  the  atmosphere,  especially  carbon-dioxide,  are  able  to  reach  these 
internal  passages  through  the  stomata,  and  in  them  they  travel  to  the  chlorophyll- 
containing  cells.  Through  the  thin,  saturated  walls  of  these  cells  they  are  able  to 
penetrate  with  ease,  and  so  they  reach  the  living  protoplasts,  with  their  equipment 
of  chlorophyll,  whose  daily  work  it  is,  as  already  mentioned,  to  decompose — under 
the  transforming  power  of  light — the  carbonic  acid  as  it  reaches  the  chlorophyll- 
bodies,  to  work  up  the  carbon  and  expel  by  the  same  path  as  they  entered  not  only 
the  oxygen  but  also  all  other  aerial  constituents  which  may  have  penetrated  and  for 
the  moment  find  no  employment. 

These  ventilation-canals,  with  stomata  as  orifices  at  the  epidermis,  have  other  uses 
besides  the  importation  of  carbon-dioxide  (and  therefore  of  carbonic  acid)  and  the 
exportation  of  oxygen.  For  the  same  pores,  passages,  and  lacunae,  as  serve  for  the 
influx  and  exit  of  carbon-dioxide  and  oxygen  respectively,  are  the  channels  of  a 
plant's  respiration.  Moreover,  they  play  a  very  important  part  also  in  the  escape 
of  aqueous  vapour,  the  process  known  as  "transpiration;"  and  as  the  variety  in 
their  structure  is  to  be  interpreted  chiefly  as  an  adaptation  to  the  different  condi- 
tions under  which  transpiration  occurs,  it  cannot  be  profitably  discussed  until  we 
treat  of  that  process. 

Those  saprophytes  and  parasites  which  contain  no  chlorophyll  or  practically 
none,  do  not  absorb  any  free  carbon-dioxide  from  the  atmosphere,  but  supply  them- 
selves with  carbon  from  the  organic  compounds  in  the  nutrient  substratum  on 
which  they  grow.  But  saprophytes  and  parasites,  abundantly  furnished  with 
chlorophyll,  doubtless  do  attract  free  carbon-dioxide  in  addition.  They  may  do  so 
either  after  the  manner  of  water-plants  and  lithophytes,  as  is  the  case  with  Euglense, 
and  with  mosses  growing  on  the  dung  of  mammalia;  or  else  after  the  manner  of 
land-plants,  as  instances  of  which  the  cow-wheat,  yellow-rattle,  and  eye-bright  may 
be  quoted. 

It  is  a  very  remarkable  fact  that  no  plant  is  known  which  takes  up  carbon- 
dioxide  or  carbonic  acid  from  the  earth.  One  might  expect  that  the  roots  of  land- 
plants  at  any  rate,  ramifying  as  they  do  in  a  stratum  of  earth  saturated  with  water 
containing  carbonic  acid  in  solution,  would  suck  up  to  some  extent  so  important  a 
food,  and  that  it  would  be  from  them  conducted  to  the  green -foliage  leaves.  But 
so  far  as  experiments  have  gone,  they  indicate  that  this  is  not  the  case. 

Equally  curious  is  the  circumstance  that  nitrogen,  which  is  an  indispensable 
constituent  of  protoplasm,  and  therefore  a  very  important  means  of  subsistence,  is 


64  NUTRIENT   GASES. 

not  absorbed  from  the  surrounding  air,  although,  as  is  well  known,  the  atmosphere 
contains  nitrogen  to  the  amount  of  79  per  cent  of  its  volume.  There  can  be  no 
doubt  that  though  nitrogen  permeates  the  cell-walls  of  an  air-encompassed  plant 
much  less  readily  and  quickly  than  carbon-dioxide,  yet  it  is  carried  from  the  atmos- 
phere into  the  ventilation-spaces  of  green  foliage-leaves,  and  further  through  the  thin 
cell- walls  into  the  laboratories  of  the  protoplasts,  where  one  would  expect  it  to  be 
worked  up  in  the  same  way  as  carbonic  acid.  The  most  careful  experiments  have 
determined,  however,  that  it  is  not  turned  to  account  in  this  form  by  the  proto- 
plasts, but  that  on  the  contrary  it  is  given  back  unused  to  the  air,  and  only  such 
nitrogen  as  reaches  the  interior  of  plants  in  combination  with  other  substances  is  of 
any  service  there. 

The  principal  sources  of  the  nitrogen  required  by  plants  are  nitrates  and 
ammoniacal  compounds  absorbed  from  the  ground;  but  nitric  acid  and  ammonia 
themselves,  of  which  there  are  traces  in  the  atmosphere  and  in  water,  must  not  be 
overlooked.  The  quantity  of  nitric  acid  in  air  is,  it  is  true,  even  less  than  that 
of  carbon-dioxide;  but  just  as  the  small  amount  of  carbon-dioxide  can  be  absorbed 
from  the  air  with  highly  productive  results,  so  may  also  the  still  smaller  proportion 
of  nitric  acid  be  turned  to  account.  The  sources  of  nitric  acid  are  dead  organic 
bodies  as  they  decompose  and  become  oxidized.  In  many  ways  the  process  of 
formation  of  nitric  acid  from  decaying  bodies  may  take  place  so  as  to  produce 
ammonia  in  the  first  place  and  from  it  nitric  acid.  It  would  seem  possible,  though 
it  is  an  unproved  assumption,  that  in  places  where  dead  bodies  of  plants  and  animals, 
vegetable  mould,  manure,  and  such  things  are  undergoing  oxidation,  that  is  to  say, 
in  woods  and  fields,  the  small  quantities  of  nitric  acid  that  are  given  off  are  imme- 
diately taken  up  by  the  plants  growing  there.  It  must  be  borne  in  mind  that  plants 
behave  with  reference  to  what  is  necessary  or  useful  to  them  like  a  chancellor  of 
the  exchequer  preparing  his  budget;  they  take  these  things  where  they  find  them. 

The  question  has  been  raised,  too,  as  to  the  source  from  which  the  first  plants 
that  appeared  on  the  earth  were  able  to  obtain  nitric  acid.  We  are  obliged  to 
assume  that,  at  that  time  before  the  existence  of  nitrogenous  organisms  to  supply 
nitric  acid  by  oxidation  of  their  dead  bodies,  all  nitric  acid,  and  therefore  all  the 
nitrogen  used  in  the  nourishment  of  plants,  was  generated  by  thunder-storms.  We 
know  that  nitric  acid  is  formed  in  the  air  on  occasion  of  electric  discharges  and  is 
deposited  on  the  earth  together  with  rain  and  dew.  This  source  of  nitric  acid  is 
not  yet  exhausted,  and  even  at  the  present  day  it  no  doubt  plays  the  same  part  as 
in  the  ages  long  past  at  the  commencement  of  all  vegetable  life. 

If  nitric  acid  is  used  by  protoplasts,  in  the  building  up  of  the  highly  important 
albuminous  compounds,  it  is  broken  up  in  a  manner  similar  to  the  decomposition 
of  carbonic  acid  to  form  carbohydrates,  that  is  to  say,  oxygen  is  separated  out. 
In  this  case,  however,  sunlight  and,  therefore,  chlorophyll  are  not  immediately  con- 
cerned. Moreover,  the  oxygen  that  is  set  free  is  not  eliminated,  but  is  used  in  the 
manufacture  of  other  compounds  in  process  of  formation  in  the  plant,  probably  in 
that  of  vegetable  acids. 


NUTRIENT   GASES.  65 

Ammonia  behaves  in  relation  to  plants  just  in  the  same  way  as  carbon-dioxide 
and  nitric  acid.  It  is  disengaged  from  dead  decomposing  organic  bodies,  and  is 
found  in  traces,  either  alone  or  with  equally  minute  quantities  of  carbon-dioxide 
and  carbonic  and  nitric  acids  in  the  air,  in  atmospheric  deposits,  and  in  all  water 
wherein  animals  and  plants  reproduce  their  kind,  the  old  individuals  dying  and 
making  way  for  the  young.  Water-plants  are  all  limited  to  this  source  for  acquisi- 
tion of  nitrogen.  As  regard  lithophytes,  it  stands  to  reason  that  they  must  derive 
their  nitrogen  from  the  ammonia  contained  in  the  air,  in  atmospheric  deposits, 
and  from  nitric  acid.  Whence  otherwise  could  a  crustaceous  lichen  attached  to  a 
quartz  rock  on  a  mountain  supply  itself  with  the  nitrogen  essential  for  the  growth 
of  its  protoplasm?  Moreover,  some  of  the  larger  lithophytes,  especially  mosses, 
seem  to  be  capable  of  absorbing  ammonia  direct  from  the  air.  An  observation 
made  in  the  Tyrolese  Alps  has  some  bearing  on  this  question: — The  ridges  of  the 
Hammerspitze,  a  peak  rising  to  2600  meters  between  the  Stubaithal  and  the 
Gschnitzthal,  is,  in  favourable  weather  in  the  summer,  the  resting-place  of  hun- 
dreds of  sheep,  and  is  consequently  covered  with  an  entire  crust  of  the  excrements 
of  these  animals.  A  highly  offensive  and  pungent  smell  of  ammonia  is  evolved,  and 
renders  a  prolonged  stay  on  this  spot  anything  but  pleasant,  notwithstanding  the 
beauty  of  the  view.  Now,  it  is  worthy  of  note  that  the  mosses,  which  are  produced 
in  abundance  on  the  rocks  above  this  richly-manured  ground,  but  are  not  them- 
selves actually  amongst  the  sheep-droppings,  exhibit  a  luxuriance  unparalleled  on 
any  of  the  neighbouring  summits  belonging  to  the  same  formation  but  unfre- 
quented by  sheep.  The  gaily-coloured  green  carpet  extends  as  far  as  the  ammo- 
niacal  odour  is  perceptible,  and  it  is  natural  to  suppose  that  this  luxuriant  growth 
is  stimulated  by  the  absorption  of  ammonia  direct  from  the  air. 

Land-plants  also  can  take  up  ammonia  from  the  air.  It  has  been  shown  that 
the  glandular  hairs  of  many  plants,  for  instance  those  on  the  leaves  of  Pelargonium 
and  of  the  Chinese  Primrose,  have  the  power  of  absorbing  traces  of  ammonia,  and 
of  sucking  up  carbonate  and  nitrate  of  ammonia  in  water  with  rapidity.  When  we 
consider  that  a  single  one  of  these  primroses  (Primula  sinensis)  possesses  two  and 
a  half  millions  of  absorbent  glandular  hairs  so  placed  as  to  be  able  to  take  up  the 
ammonia  brought  to  the  plant  by  rain,  we  are  unable  to  look  upon  this  process  as 
of  altogether  trifling  importance.  It  is  highly  probable  that  almost  all  ammonia, 
after  its  formation  from  decaying  substances  in  the  ground,  is  at  once  absorbed  by 
the  plants  growing  in  the  immediate  neighbourhood,  and  that  the  relatively  small 
quantity  of  ammonia  in  the  upper  atmospheric  strata  is  referrible  to  this  cause. 
The  splendid  luxuriance  of  the  pelargoniums,  thickly  studded  with  glandular  hairs, 
which  one  sees  in  front  of  cottage  windows  in  mountain  villages  where  a  dung 
heap  is  close  by,  and  in  the  windows  of  stables,  frequently  excites  admiration  and 
surprise.  Whether  it  is  due  to  the  fact  that  in  these  situations  there  is  the  possi- 
bility of  absorbing  an  unusually  large  quantity  of  ammonia  is  a  question  which  we 
will  leave  undecided. 

VOL.  I.  5 


(Jg  NUTRIENT   SALTS. 


NUTRIENT   SALTS. 

If  wood,  leaves,  seeds,  or  any  other  parts  of  plants  are  subjected  to  a  high 
temperature  with  free  access  of  air,  the  first  changes  that  occur  are  in  the  com- 
pounds of  nitrogen  and  of  carbon  contained  in  the  heated  matter.  They  turn 
black,  are  charred  and  burnt,  and  ultimately  the  products  of  combustion  pass  into 
the  atmosphere  in  gaseous  condition.  The  incombustible  part  which  remains 
behind  is  called  the  "  ash."  The  quantity  of  this  ash,  as  well  as  its  composition, 
varies  very  much  in  different  species  of  plants,  and  even  in  different  parts  of  the 
same  plant.  Generally  the  weight  of  ash  is  only  one  or  two  per  cent  of  the  entire 
weight  of  the  plant  in  a  dry  state  before  burning.  The  greatest  relative  proportion 
of  ash  is  that  which  is  obtained  from  the  combustion  of  those  hydrophytes  which 
live  in  the  sea;  and  next  in  quantity  is  the  ash  of  the  family  of  Oraches  which 
abound  on  salt-steppes.  On  the  other  hand,  the  smallest  quantity  is  that  afforded  by 
fungi  and  mosses,  by  Sphagnum  in  particular,  and  with  these  must  be  mentioned 
the  tropical  orchids  living  on  the  barks  of  trees.  Seeds  and  wood  yield  relatively 
much  less  ash  than  leaves.  But,  as  above  remarked,  some  ash  is  formed  upon  the 
combustion  of  any  part  of  a  plant  or  even  of  a  single  cell,  and  this  residue  of  ash 
sometimes  allows  of  our  recognizing  exactly  the  size,  form,  and  outline  of  the  cells. 
The  universal  distribution  of  ash-forming  constituents  permits  us  to  conclude  with 
certainty  that  they  do  not  exist  fortuitously  in  plants,  but  are  essential  to  them. 
That  these  constituents  are  indispensable  may  also  be  proved  directly.  If  an 
attempt  is  made  to  nourish  a  plant  on  filtered  air  and  distilled  water  exclusively,  the 
plant  soon  dies;  but  if  a  small  quantity  of  the  constituents  of  its  ash  are  added  to 
the  distilled  water  in  which  the  roots  are  immersed,  the  plant  grows  visibly  in  the 
solution,  and  develops  leaves  and  flowers  and  even  seeds  capable  of  germination. 

Experiments  of  this  kind  with  cultures  have  been  the  means  of  almost  com- 
pletely establishing  the  division  between  those  constituents  which  are  indispensable 
for  all  plants,  and  those  which  are  only  necessary  under  certain  conditions  and  to 
particular  species,  or,  still  less,  only  beneficial.  Those  elements  must  be  regarded 
as  essential,  which  are  used  by  plants  for  the  process  of  construction,  and  enter 
into  the  composition  of  the  protoplasm  or  of  the  cell-membrane — such,  for  instance 
as  are  essential  constituents  of  proteid  substances,  or  are  in  some  way  necessary 
to  the  formation  of  these  products.  Amongst  these  must  be  included  sulphur, 
phosphorus,  potassium,  calcium,  and  magnesium.  Some  plants,  especially  those 
that  live  in  the  sea,  require  sodium,  iodine  and  chlorine,  and,  for  green  plants,  iron 
is  necessary.  Silicon  is  also  very  important  for  most  plants  in  helping  them  to 
flourish  in  the  wild  state.  Most  of  these  elements  are  taken  into  a  plant,  in  the 
covrse  of  nutrition,  in  a  condition  of  extreme  oxidation,  that  is  to  say  in  combina- 
tion with  a  quantity  of  oxygen;  in  fact,  as  a  general  rule,  they  are  absorbed  in 
the  form  of  salts,  and  we  may  for  the  sake  of  brevity  include  all  the  mineral  food- 
stuffs under  the  name  of  nutrient  salts  or  food-salts. 


NUTRIENT   SALTS.  67 

It  is  obvious  that  food-salts  can  only  pass  through  cell-membranes  and  reach 
the  interior  of  a  plant  in  a  state  of  solution.  On  this  account  the  soluble  sul- 
phates, phosphates,  nitrates  and  chlorides  of  calcium,  magnesium,  potassium  and 
iron,  may  pre-eminently  be  called  food-salts.  Whether  an  essential  element  is 
absorbed  by  a  plant  in  the  form  of  one  of  these  compounds  or  another  appears 
to  be  unimportant;  phosphorus,  for  example,  may  be  proffered  by  the  soil  in  the 
form  either  of  potassium  phosphate  or  of  sodium  phosphate,  with  like  results. 
As  regards  the  importance  of  sulphur  to  plants,  it  is  at  any  rate  established  that 
it  is  necessary  for  the  production  of  proteid  substances.  Phosphorus  appears  to  be 
indispensable  in  the  transformation  of  certain  compounds  of  nitrogen.  Potassium 
is  supposed  to  play  a  part  in  the  formation  of  starch.  Calcium  is  introduced  into 
plants  in  combination  with  sulphuric  acid  as  calcium  sulphate.  This  salt  is  decom- 
posed, the  lime  combining  with  oxalic  acid  to  form  insoluble  calcium  oxalate,  and 
the  sulphur  going  to  form  the  sulphuric  acid  which  is  used  in  the  construction  of 
albuminous  substances  or  proteids.  Lime  is  therefore  important,  inasmuch  as 
it  is  a  medium  of  transport  for  sulphur.  Iron  certainly  participates  in  the  forma- 
tion of  chlorophyll,  even  if  it  does  not  enter  into  its  composition,  as  was  formerly 
supposed.  For,  it  has  been  proved,  by  means  of  artificial  cultures,  that  plants  reared 
in  solutions  free  from  iron  were  white  instead  of  green,  and  died  at  last;  whereas, 
after  the  addition  of  a  small  quantity  of  a  soluble  iron  salt,  such  plants  became  green 
in  a  very  short  time,  and  were  able  to  continue  their  development.  The  utility  of 
most  of  these  elements  does  not  therefore  appear  to  consist  necessarily  in  their 
entering  into  the  composition  of  organic  compounds,  but  in  the  promotion  and 
regulation  of  the  constructive  and  destructive  chemical  processes. 

Silicic  acid,  which  occurs  so  plentifully  in  the  ash  of  many  plants  as  to  con- 
stitute often  more  than  50  per  cent,  has  a  different  function.  If  the  minute 
unicellular  water-plants  known  as  Diatoms  are  incinerated,  or  if  stems  of  Equisetum, 
Juniper-needles,  or  leaves  of  grasses,  &c.,  are  subjected  to  a  red  heat,  white  skeletons 
remain  behind  which  consist  almost  entirely  of  silicic  acid,  and  exhibit  not  only 
the  forms  of  the  cells,  but  even  the  finest  sculpturing  of  the  cell-walls.  In  par- 
ticular, the  stiff  hairs  on  the  leaves  of  grasses  are  preserved,  and  better  still  the 
cell-membranes  of  diatoms.  The  latter  present  very  beautiful  forms  with  their 
outlines  quite  distinct,  and  many  structural  properties  of  the  cell-membranes, 
especially  their  moulding,  striation,  and  the  dots  and  other  excrescences  are  to  be 
seen  much  more  clearly  after  than  before  ignition,  when  the  transparency  was  less 
owing  to  the  protoplast  occupying  the  interior  of  each  cell.  In  order  to  describe 
exactly  the  very  varied  form  of  Diatomacese,  specimens  are  carefully  and  thor- 
oughly ignited,  and  the  descriptions  and  illustrations  of  these  microscopic  plants 
are  for  the  most  part  made  from  siliceous  skeletons  prepared  in  this  way.  These 
skeletons  show  clearly  that  silicic  acid  occurs  only  in  the  cell-membrane,  and  plays 
no  part  as  constituent  of  any  chemical  compound  in  the  protoplasm;  nor  does  it 
appear  to  be  instrumental  in  the  formation  of  any  such  compound.  The  molecules 
of  silicic  acid  are  so  closely  packed  and  so  evenly  distributed  amongst  the  mole- 


68  NUTRIENT   SALTS. 

cules  of  cellulose  that,  even  after  the  removal  of  the  latter,  the  entire  structure  is 
preserved  in  outline  and  in  detail.  They  form,  therefore,  a  regular  coat  of  mail 
which  may  be  looked  upon  as  a  means  of  protection  against  certain  injurious  ex- 
ternal influences. 

For  a  large  number  of  plants  living  in  the  sea,  sodium,  iodine,  and  bromine  also 
are  of  especial  importance  as  food-stuffs.  How  far  fluorine,  manganese,  lithium, 
and  various  other  metals,  which  have  been  detected  in  the  ash  of  some  plants,  are 
of  use  is  not  determined,  for  our  knowledge  is  particularly  incomplete  with  respect 
to  the  various  uses  subserved  in  nutrition  and  growth  by  the  different  mineral 
food-stuffs.  It  is  worthy  of  note  that  alumina,  which  is  so  widely  distributed  and 
easily  accessible  to  plants,  is  only  very  rarely  absorbed.  The  ash  of  Lycopodium 
is  the  only  kind  in  which  this  substance  has  been  identified  with  certainty  in  any 
considerable  quantities. 

Lastly,  amongst  the  sources  of  elements  contained  in  the  food-salts,  we  must 
consider  the  solid  crust  of  the  earth.  But  it  is  only  in  the  case  of  comparatively 
few  vegetable  organisms  that  this  earth-crust  forms  the  immediate  foster-soil. 
The  majority  derive  the  salts  that  nourish  them  from  the  products  of  the  weather- 
ing of  rocks,  from  refuse  and  the  decaying  remains  of  dead  animals  and  plants, 
which,  in  decomposing,  give  back  their  mineral  substances  to  the  ground,  from 
underground  waters  that  filter  through  fissures  in  rocks  and  through  the  interstices 
of  sandy  or  clayey  soils  soaking  with  lye,  the  adjacent  parts  of  the  earth's  crust, 
and,  lastly,  from  the  water  of  springs,  streams,  ponds,  and  lakes,  which  have  come 
to  the  surface  holding  salts  in  solution,  as  also  from  sea- water  with  its  rich  supply 
of  salts. 

The  very  salts  that  are  needed  by  most  plants  are  amongst  the  most  widely 
distributed  on  the  earth's  surface.  The  sulphates  of  calcium  and  of  magnesium, 
for  example,  and  salts  of  iron,  potassium,  &c.,  are  found  almost  everywhere  in  the 
earth,  and  in  water,  whether  subterranean  or  superficial.  At  the  same  time  it  is 
very  striking  that  these  mineral  food-salts  are  not  introduced  into  plants  by  any 
means  in  proportion  to  the  quantity  in  which  they  are  contained  in  the  soil,  but 
that,  on  the  contrary,  plants  possess  the  power  of  selecting  from  the  abundance  of 
provisions  at  their  disposal  only  those  that  are  good  for  them  and  in  such  quantity 
as  is  serviceable.  This  selective  capacity  of  plants  is  manifested  in  many  ways,  and 
we  will  now  briefly  consider  some  of  the  most  important  of  them. 

In  the  first  place  we  have  the  fact  that  plants  reared  close  together  in  the  same 
soil  or  medium  may  yet  exhibit  an  altogether  different  composition  of  ash.  This 
is  particularly  striking  in  water  and  bog-plants,  which,  though  rooted  in  close 
proximity  and  immersed  in  the  same  water,  show  very  considerable  differences  in 
respect  of  mineral  food  absorbed.  The  result,  for  instance,  of  testing  specimens  of 
the  Water-soldier  (Stratiotes  aloides),  the  White  Water-lily  (Nymphcea  alba), 
a  species  of  Stone-wort  (Chara  fatida),  and  the  Reed  (Phragmites  communis),  all 
growing  close  together  in  a  swamp,  was  as  follows  as  regarded  the  potash,  soda, 
lime,  and  silicic  acid,  held  by  them  respectively: — 


NUTRIENT   SALTS. 


69 


Water-soldier. 

Water-lily. 

Stone-wort. 

Reed. 

Potash,  

3Q-82 

14'4 

0'2 

8'6 

Soda,  

27 

29  '66 

O'l 

0'4 

Lime 

10'7 

18'9 

54'8 

5*9 

1*8 

0*5 

0'3 

71'5 

The  other  constituents  of  the  ash  of  these  plants,  in  particular  iron  oxide,  mag- 
nesia, and  phosphoric  and  sulphuric  acids,  exhibited  less  marked  differences;  but 
the  inequality  in  the  amounts  of  potash,  soda,  lime  and  silicic  acid  are  so  great, 
as  only  to  be  explicable  on  the  assumption  of  a  power  of  selection  on  the  part  of 
these  plants.  Various  species  of  brown  and  red  sea- weeds,  which  had  been  attached 
to  the  same  rock  and  developed  in  the  same  sea- water,  showed  similar  variations 
in  the  composition  of  their  ash. 

On  the  mountains  of  serpentine  rock  near  Gurhof,  in  Lower  Austria,  specimens 
of  Biscutella  Icevigata  and  Doi^ycnium  decumbens  were  collected  from  plants 
growing  together,  and  one  above  the  other,  upon  a  declivity  which  they  clothed. 
Their  roots,  interlaced  here  and  there,  were  fixed  in  the  same  ground,  and  drew 
nourishment  from  the  same  store.  The  following  table  gives  the  composition  of 
the  ash  in  these  two  species: — 


Biscutella 
Isevigata. 

Dorycnium 
decumbens. 

Biscutella 
Isevigata. 

Dorycnium 
decumbens. 

Potash 

9*6 

16*7 

13-0 

6'3 

Li  in  6 

14-7 

20'9 

Sulphur,  

5-2 

1-6 

28  -0 

19'6 

15'9 

22-3 

Iron  Oxide,. 

7-8 

2-8 

Carbonic  Acid,  

5'4 

9-7 

The  differences  here  seem  to  be  not  so  great  as  in  the  case  of  the  water-plants 
previously  given,  but  they  are  sufficient  to  prevent  our  regarding  them  as  merely 
the  result  of  chance. 

If,  on  the  other  hand,  we  compare  the  composition  of  the  ash  of  different 
specimens  of  the  same  species,  which  have  been  reared  on  similar  soils,  but  at 
great  distances  from  one  another,  the  discrepancies  are  comparatively  slight. 
Foliage  from  beech-trees  growing  on  the  limestone  mountains  near  Regensburg 
yielded  an  ash  practically  identical  with  that  obtained  from  leaves  of  beeches  on 
the  Bakonyer-Wald  hills  in  Hungary.  The  ash  of  different  individuals  of  a  single 
species  even  exhibits  the  same  constitution,  in  the  main,  when  those  individual 
plants  have  obtained  their  nutriment  from  soils  differing  greatly  in  chemical 
composition.  Only  in  cases  where  the  quantity  of  a  substance  in  one  soil  is 
more  abundant  than  in  the  other  there  is  generally  a  greater  or  less  amount  of  it 
to  be  found  in  the  ash. 

That  under  these  circumstances  certain  substances  may  replace  one  another  is  not 
improbable.  But  such  substitution  must  be  confined  to  those  nearly  allied  com- 
pounds whose  molecules  are  capable  of  being  used  indifferently  by  the  formative 


70 


NUTRIENT   SALTS. 


protoplasm  in  construction,  and  in  the  storage  of  materials.  The  annexed  table, 
which  gives  side  by  side  analyses  of  the  ash  of  branches  of  the  Yew  (Taxus  baccata) 
with  their  leaves  attached,  illustrates  the  replacement  of  calcium  by  magnesium :— 


Ash  from  branches  and  leaves  of  the  Yew  from 

Serpentine. 

Limestone. 

Gneiss. 

3'8 
1-9 
8-3 
2'1 
16-1  )  ^ 
22-7  }  w 
29-6 
14-1 

3-6 
1-6 
5'5 
17 

•K-|«» 

21-8 
23-1 

3-7 
1-9 

4-2 
0-6 

30-6  )  _  Q 
5.7  J363 

27-6 
24-4 

Potash                                       

Trappy  nf  TVTflTijcra'npsfi   Ohlorine   &c 

Totals,  

99-6 

98-5 

98-7 

The  Yew  occurs  in  Central  Europe  on  very  various  mountain  formations,  chiefly  on 
limestone,  but  not  infrequently  on  gneiss,  and  occasionally  on  serpentine  rocks. 
On  comparing  the  quantities  of  calcium  and  of  magnesium  in  the  ash  of  yews,  grown 
on  lime  and  on  gneiss  respectively,  with  those  yielded  in  the  case  of  serpentine  for- 
mation, we  find  that  magnesia  preponderates  considerably  in  weight  over  lime  in  a 
yew  from  serpentine  rocks  (which  are  in  the  main  a  compound  of  magnesia  and 
silicic  acid),  whilst  the  proportion  between  these  two  salts  is  reversed  in  a  yew 
grown  upon  limestone.  The  obvious  inference  from  the  table  is  that,  in  plants  from 
a  serpentine  ground,  lime  is  to  a  great  extent  replaced  by  magnesia.  This  is  fur- 
ther supported  by  the  circumstance  that  if  lime  and  magnesia  are  counted  together 
the  resulting  numbers  are  very  near  one  another,  namely  41 '2  per  cent  of  the  ash 
for  limestone,  38'8  per  cent  for  serpentine  rock,  and  36'3  per  cent  for  gneiss. 

But  all  these  phenomena  observed  in  connection  with  the  selection  of  food-salts 
are  not  nearly  so  surprising  as  the  fact  that  plants  are  also  capable  of  singling  out 
from  an  abundance  of  other  matter  particular  substances,  which  are  of  impor- 
tance to  them,  even  from  a  soil  containing  them  in  barely  perceptible  quantities,  and 
of  concentrating  them  to  a  certain  extent.  As  has  been  shown  above,  nearly  a 
third  of  the  ash  of  the  white  water-lily  is  composed  of  common  salt.  One  might, 
therefore,  suppose  that  the  water  in  which  water-lilies  flourish  contains  a  particu- 
larly large  quantity  of  common  salt.  But  nothing  of  the  kind  is  the  case.  The 
bog  water  which  bathed  the  stem  and  leaves  of  this  specimen  only  contained  0*335 
per  cent  of  common  salt,  and  the  mud  through  which  the  roots  straggled  contained 
only  O'OIO  per  cent. 

No  less  astonishing  is  it  to  find  Diatomacese,  with  cell-membranes,  as  above 
mentioned,  sheathed  in  silicic  acid,  existing  in  water  which  contains  no  trace  of 
silicic  acid.  Above  the  Arzler  Alp,  in  the  Solstein  chain  near  Innsbruck,  there  is  a 
spring  of  cold  water  which  falls  in  little  cascades  between  blocks  of  rock.  The 


NUTRIENT   SALTS.  71 

water  of  this  spring  is  hard,  and  it  deposits  lime  at  a  little  distance  from  the  source. 
Exactly  at  the  spot  where  it  wells  out  of  a  fissure  in  the  rock  its  bed  is  entirely 
filled  by  a  dark-brown  flocculent  mass  which  consists  of  millions  of  cells  of  the 
beautiful  Odontidium  hiemale,  a  species  of  diatom  with  siliceous  coating.  These 
cells  are  ranged  together  in  long  rows,  and  are  present  in  numbers  and  luxuriance 
such  as  are  scarcely  ever  to  be  observed  in  other  situations.  Yet  the  spring  water 
flowing  round  contains  so  little  silicic  acid  that  no  trace  of  this  substance  could  be 
discovered  in  the  residue  from  the  evaporation  of  10  litres. 

An  instance  similar  to  this  of  silicic  acid,  is  afforded  by  the  iodine  in  the  sea. 
Most  of  the  sea-wracks  inhabiting  the  North  Sea  contain  iodine,  many  indeed  in 
considerable  quantity,  and  yet  we  have  not  hitherto  succeeded  in  detecting  iodine  in 
the  water  of  the  North  Sea.  Similar  phenomena,  sometimes  quite  baffling  explana- 
tion, are  exhibited  by  land-plants.  The  clefts  in  the  rocks  of  quartziferous  slate  in 
the  Central  Alps  are,  in  many  places,  overgrown  by  saxifrages  (Saxifraga  Sturmiana 
and  Saxifraga  oppositifolia)  with  leaves  aggregated  together  in  closely-crowded 
rosettes,  which  are  conspicuous  from  afar,  owing  to  their  pale  colouring.  On 
closer  inspection  one  finds  that  the  apices  and  edges  of  these  rosulate  leaves  are 
covered  with  little  incrustations  of  carbonate  of  lime,  a  substance  which  will  be 
frequently  referred  to  in  connection  with  its  importance  to  plants.  But  one  seeks 
in  vain  for  any  lime  compound  in  the  earth  which  fills  the  clefts,  and  the  only 
traces  of  lime  contained  in  the  adjacent  rock  itself  are  those  occurring  in  the  little 
scales  of  mica  scattered  about,  and  these  are  not  readily  decomposable.  Yet  the 
lime  incrusting  the  saxifrage  leaves  can  only  be  derived  from  the  underlying  rock, 
just  as  in  former  instances  the  silicic  acid  in  the  cell-membranes  of  diatoms 
must  be  secreted  from  the  spring  described,  the  iodine  in  sea-weeds  from  the 
sea,  and  the  common  salt  in  water-lilies  from  the  pond  where  they  grow,  although  in 
each  case  the  substance  concerned  is  only  to  be  found,  if  at  all,  in  scarcely  ponder- 
able traces  in  the  soil  or  liquid  serving  as  medium.  Facts  of  this  kind  have  a 
special  interest,  because  they  prove  that  plants  have  the  power  of  appropriating  a 
substance,  if  it  is  important  to  them,  even  when  it  is  only  present  in  extremely 
minute  quantities.  Where  a  plant  is  surrounded  by  liquid,  we  can  well  imagine 
that  fresh  portions  of  the  medium  are  constantly  coming  into  contact  with  its 
surface;  for,  even  in  water  apparently  still,  compensating  currents  are  con- 
tinually being  caused  by  changes  of  temperature.  Thus,  in  the  course  of  a  day, 
thousands  of  litres  of  sea-water  may  flow  over  a  sea-weed  with  a  surface  of 
one  square  meter,  and,  even  if  only  a  small  portion  of  the  substance,  traces  of 
which  we  are  supposing  to  exist  in  the  water,  is  wrested  from  each  litre,  still, 
the  absorbing  plant  might  collect  quite  a  profitable  quantity  in  a  number  of 
days.  The  volume  of  water  flowing  over  a  plant  situated  in  the  source  of  a 
spring  is  still  greater,  and  it  is  readily  conceivable  that  even  the  most  minute 
trace  of  silicic  acid  may  become  of  account  in  course  of  time.  There  is  more 
difficulty  in  understanding  how  plants  with  roots  in  the  earth  set  about  utilizing 
substances  contained  in  the  soil  in  scarcely  appreciable  quantities.  These  plants 


72  NUTRIENT   SALTS. 

must  at  all  events  come  into  contact  with  as  great  a  mass  of  nutrient  soil  as 
possible,  and  this  is  effected  by  means  of  a  widely-ramifying  system  of  roots; 
and,  in  addition,  they  must  assist  in  making  available  desirable  matter  in  the 
soil  by  the  elimination  from  themselves  of  certain  substances. 

In  order  to  explain  the  remarkable  power  that  plants  possess  of  exercising 
a  choice  in  the  absorption  of  certain  food-stuffs  from  amongst  the  whole  number 
presented  to  them,  we  must  in  the  first  place  assume  a  special  structure  to  exist 
in  the  cells  which  are  in  immediate  contact  with  the  nutrient  medium.  To 
reach  the  interior  of  a  cell,  the  salts  must  pass  through  the  cell-membrane  and 
the  so-called  ectoplasm.  We  may  look  upon  these  walls,  that  are  to  be  pene- 
trated, as  filters,  or,  to  abide  by  our  previous  simile,  as  sieves,  which  allow  only 
certain  kinds  of  molecules  to  pass  and  arrest  others.  Moreover,  just  as  the 
structure  of  a  sieve,  especially  the  size  and  shape  of  its  pores,  has  its  effect  in  the 
separation  of  the  particles  of  the  matter  sifted,  so  also  may  the  structure  of  a 
cell-wall  have  a  discriminating  influence  in  the  absorption  of  food-salts.  It  may 
be  supposed  that  the  cell-wall  in  one  species  of  plant  acts  as  a  sieve  capable  of 
letting  through  molecules  of  potash  but  none  of  alumina,  whilst  the  cell-wall  in 
a  second  species  allows  molecules  of  alumina  to  pass  as  well,  but  is  impervious  to 
those  of  chloride  of  sodium.  This  hypothesis  would  also  explain  why  the  absorp- 
tion of  food-stuffs  by  plants  generally  takes  place  through  cell-walls,  and  why 
absorption  into  the  organs  concerned  by  means  of  open  tubes,  which  would  be 
at  all  events  a  much  simpler  method,  is  not  preferred.  It  is,  however,  necessary 
to  investigate  first  the  nature  of  the  force  which  causes  molecules  of  the  various 
salts  to  move  from  the  soil  to  the  cell-membranes,  which  we  suppose  to  be  like 
sieves,  and  through  them  into  the  interior  of  a  plant.  A  force  acting  in  this 
sense  from  without  is  inconceivable,  and  we  must  therefore  look  for  the  motive 
stimulus  in  the  plant  itself. 

As  has  been  already  stated  in  connection  with  the  absorption  of  carbonic  acid, 
it  is  believed  that  the  cause  of  this  movement  is  the  disturbance  of  the  molecular 
equilibrium  in  the  growing  vegetable  organism.  If  at  one  spot  in  the  protoplasm 
of  a  cell  a  particular  substance  is  altered,  and,  let  us  say,  converted  into  an 
insoluble  compound,  the  previous  grouping  of  molecules  appears  to  be  altered,  or 
in  other  words,  the  molecular  equilibrium  is  disturbed.  To  restore  equilibrium, 
there  must  be  a  re-introduction  of  molecules  of  the  material  that  has  been  removed ; 
and  the  attraction  of  them  from  the  quarter  where  they  occur  in  a  fluid,  that  is 
to  say  in  a  mobile  condition,  is  the  more  energetic.  Supposing,  for  instance, 
gypsum  (i.e.  sulphate  of  lime)  is  being  decomposed  within  a  cell,  and  the  lime 
combines  with  the  oxalic  acid  (set  free  in  the  same  cell)  to  form  insoluble  oxalate 
of  lime,  whilst  the  sulphur  combines  with  other  elements  to  form  insoluble 
albuminoids,  this  use  of  the  gypsum  occasions  a  violent  attraction  of  that  sub- 
stance from  the  environment,  or,  to  put  it  another  way,  it  causes  a  movement  of 
gypsum  towards  the  place  of  consumption.  If  this  latter  place  is  a  cell  in  imme- 
diate contact  with  the  nutrient  substratum,  the  absorption  of  the  substance 


NUTRIENT   SALTS.  73 

attracted  is  direct;  but  if  the  cell  in  which  the  material  is  used  up  is  separated 
from  the  substratum  by  intervening  cells,  the  attraction  must  act  through  all  those 
cells  upon  it.  The  substance  consumed  must  be  taken  in  the  first  place  from  the 
cell  adjoining  the  consuming  cell  on  the  side  towards  the  periphery;  this  cell  again 
must  take  it  from  its  neighbour,  which  is  still  nearer  the  periphery,  and  so  on 
until  the  external  cells  themselves  exercise  their  influence  upon  the  nutrient  sub- 
stratum. Thus,  one  may  regard  the  growing  cells  in  which  substances  are  used 
up,  as  centres  of  attraction  with  respect  to  those  substances.  This  also  explains 
why  it  is  that  the  influx  of  food-salts  takes  place  only  so  long  as  the  plant  is  grow- 
ing; and  we  see,  too,  that  the  direction  of  the  current  must  vary  according  to  the 
position  of  the  growing  cells,  and  according  to  the  degree  of  their  constructive 
activity. 

But  that  one  plant  prefers  one  substance  and  another  another — that  one  species 
attracts  iodine,  a  second  sodium,  and  a  third  iron — can  only  be  interpreted  as  a 
result  of  the  specific  constitution  of  the  protoplasm.  The  protoplasm  of  a  growing 
cell  which  contains  no  iodine  does  not  require  that  substance  either,  for  the  pro- 
cesses of  transmutation  and  storage.  A  protoplast  of  this  kind  will  not  therefore 
be  a  centre  of  attraction  for  iodine,  but  will  draw  from  the  environment  with 
great  force  substances  which  are  its  essential  constituents.  Having  gained  this 
conception  of  the  absorption  and  selection  of  food-salts,  we  are  able  to  imagine 
the  possibility  of  a  substance  being  sought  after  by  one  species  whilst  acting  as 
poison  on  another.  Iodine  itself  exercises  a  prejudicial  effect  on  many  plants, 
even  when  present  in  very  small  quantities.  Cell-membranes  in  immediate  contact 
with  a  medium  containing  iodine  are  modified  as  regards  their  structure  by  the 
iodine:  their  pores  are  enlarged,  lose  their  value  as  orifices  adapted  to  the  admit- 
tance of  certain  food-salts  in  limited  quantities,  and  they  no  longer  prevent  the 
influx  of  injurious  substances.  Ultimately  they  die,  and  by  so  doing  the  entire 
plant  suffers.  On  the  other  hand,  plants  to  which  iodine  is  an  indispensable 
constituent  are  not  hurt  in  any  way  by  the  presence  of  small  quantities  of  this 
substance  in  the  nutrient  medium:  their  cell-membranes  are  neither  paralysed 
nor  destroyed,  and  suction  is  able  to  take  place  through  them  in  a  perfectly  normal 
manner.  But  we  must  in  this  case  specially  emphasize  the  condition  of  the  amount 
being  small,  for  a  larger  quantity  of  this  substance  is  positively  injurious  even  to 
plants  which  require  iodine. 

The  general  rule  for  a  great  number  of  plants  is  that  they  thrive  best  when  the 
food-salts  necessary  to  them  are  supplied  in  very  dilute  solutions.  An  increase  in 
the  quantity  of  the  salts  administered  not  only  fails  to  promote  development,  but, 
on  the  contrary,  arrests  it.  This  is  the  result  even  if  the  salts  are  such  as  are 
absolutely  necessary  in  small  quantities  to  the  plants  in  question.  A  very  minute 
amount  of  an  iron  salt  is  indispensable  to  all  green  plants;  but,  if  a  certain 
measure  is  exceeded,  iron  salts  have  a  destructive  effect  on  the  cell-membranes  and 
protoplasm,  and  cause  the  plant  to  die.  But  at  what  point  the  boundary  lies 
between  salubrious  effects  and  the  reverse,  where  the  beneficial  action  of  particular 


74  NUTRIENT   SALTS. 

substances  ceases  and  detrimental  action  begins,  is  not  known  more  precisely  than 
has  been  stated.  We  only  know  that  different  plants  behave  very  differently  in 
this  respect.  Suppose,  for  example,  that  we  scatter  wood-ash  over  a  field  which  is 
overgrown  by  grasses,  mosses,  and  various  herbs  and  shrubs.  The  result  is  that  the 
mosses  die;  in  the  case  of  the  grasses  growth  is  somewhat  increased;  whilst  some  of 
the  herbs  and  shrubs,  notably  polygonaceous  and  cruciferous  plants,  exhibit  a  strik- 
ingly luxuriant  growth.  If  we  scatter  gypsum  instead,  the  development  of  clover 
is  enhanced,  and,  on  the  other  hand,  there  are  certain  ferns  and  grasses  that  die 
earlier  when  gypsum  is  supplied,  or,  at  least,  are  considerably  stunted  in  their 
growth. 

The  fact  that  certain  plants  predominate  on  calcareous  and  others  on  siliceous 
ground  has  been  the  subject  of  very  thorough  investigation;  and  these  researches 
were  regarded  as  justifying  the  assumption  that  particular  species  require  a  more  or 
less  considerable  quantity  of  lime  for  food,  whilst  others  require  similarly  silicic 
acid.  Hereupon  was  founded  a  division  of  plants  into  those  which  required  and 
were  tolerant  of  lime,  and  into  such  as  required  and  tolerated  silica.  The  explana- 
tion given  of  these  facts  does  not  seem,  however,  to  be  satisfactory,  at  any  rate  in 
the  case  of  siliceous  plants.  It  is  much  more  probable  that  the  so-called  silica- 
loving  plants  are  produced  on  ground  composed  of  quartz,  granite,  or  slate,  not  by 
reason  of  the  abundance  of  silicic  acid,  but  because  of  the  absence  of  lime  in  any 
large  quantity,  such  as  would  be  liable  to  injure  plants  of  the  kind;  for  only  traces 
of  lime  are  found,  and  its  presence  to  this  extent  is  absolutely  necessary  for  every 
plant.  This  is  not  of  course  inconsistent  with  the  fact  that  individual  species 
require  larger  quantities  of  particular  food-salts  and  only  flourish  luxuriantly  when 
these  nutritive  salts  are  not  meted  out  too  sparingly.  In  the  case  of  oraches, 
thrifts,  wormwood  species,  and  cruciferous  plants,  alkalies,  in  comparatively  large 
quantities,  are  necessary  for  hardy  development.  The  proper  habitat  for  these 
plants,  therefore,  is  on  soils  which  contain  an  abundance  of  easily  soluble 
alkaline  compounds,  in  places  where  the  ground  is  regularly  saturated  by  saline 
solutions,  and  where  crystals  of  salt  effloresce  on  the  drying  surface.  Such  places 
are  the  sea-shore,  the  salt  steppes,  and  the  neighbourhood  of  salt-mines.  The 
above  plants  not  only  flourish  in  these  localities  in  great  abundance  and  perfection, 
but  they  supplant  all  other  species  on  which  the  excessive  provision  of  soluble 
alkaline  salts  is  not  beneficial.  If  the  seeds  of  such  plants  happen  to  fall  upon  the 
salt  ground  they  germinate,  but  only  drag  out  a  miserable  existence  for  a  short 
time,"  and  in  the  end  are  crowded  out  by  the  luxuriant  oraches  and  crucifers. 
Plants  which  only  flourish  abundantly  on  soils  rich  in  alkaline  salts  are  called 
halophytes.  The  same  name  has  also  been  applied  to  plants  which  only  thrive  in 
sea-water.  Most  of  the  species  used  by  us  as  edible  vegetables,  as,  for  instance, 
cabbages,  turnips,  cress,  &c.,  are  really  descended  from  halophytes,  and  accordingly 
require  a  soil  that  contains  a  comparatively  rich  supply  of  alkalies.  An  oppor- 
tunity will  occur,  later  on,  of  returning  to  the  question  as  to  how  far  agriculture 
has  gained  by  all  these  discoveries,  and  of  considering  what  processes,  based  upon 


ABSORPTION   OF    FOOD-SALTS   BY   WATER-PLANTS.  75 

the  results  of  scientific  research,  have  been  introduced  into  practice.  Amongst 
these  processes  may  be  mentioned  the  rotation  of  crops,  the  artificial  application  of 
manure  to  exhausted  land,  and  the  restitution  of  the  mineral  food-salts  which  the 
particular  plants  last  cultivated  have  withdrawn  from  the  land  under  tillage. 

ABSOEPTION  OF  FOOD-SALTS  BY  WATEK-PLANTS. 

It  is  usual  to  designate  all  plants  that  grow  in  water  as  hydrophytes  or  water- 
plants.  But  in  their  narrower  sense  these  names  are  only  applicable  to  those  plants 
which,  during  their  entire  lives,  vegetate  under  water  and  derive  their  nutriment, 
especially  carbonic  acid,  direct  from  the  water.  A  number  of  plants  have  widely 
ramifying  roots  fixed  in  the  earth  at  the  bottom  of  water,  and  the  lower  parts  of 
their  stems,  either  temporarily  or  throughout  life,  immersed  in  water,  whilst  the 
upper  parts  of  their  stems  and  their  upper  leaves  are  exposed  to  the  air  and  take 
carbonic  acid  direct  from  the  atmosphere,  and  these  should  be  regarded  as  marsh- 
plants  and  classed  with  land-plants  so  far  as  regards  food-absorption.  Reeds  and 
rushes,  water-fennel  and  water-plantain,  the  yellow  water-lily,  even  the  amphibious 
Polygonum  and  the  white  water-lily,  are  marsh-plants  and  not  true  hydrophytes. 
It  is  characteristic  of  all  these  marsh-plants,  that  if  they  are  entirely  submerged 
for  any  length  of  time  they  die,  whereas  they  are  not  injured  if  the  water's  level 
at  the  place  where  they  grow  sinks  so  as  to  expose  the  lower  portions  of  the  stem. 
In  places  formerly  submerged,  but  from  which,  in  course  of  time,  the  water  has 
retreated,  so  that  they  have  been  turned  into  meadows,  one  may  come  across  not 
only  clumps  of  reeds  and  rushes  but  even  yellow  and  white  water-lilies,  flourishing 
perfectly  on  the  moist  earth. 

Water-plants,  or  hydrophytes  in  the  proper  acceptation  of  the  term,  perish 
if  they  are  kept  for  a  length  of  time  out  of  their  proper  medium  and  exposed  to 
the  air.  In  most  of  them  death  ensues  quickly,  for  their  delicate  cell-membranes 
are  not  able  to  prevent  the  exhalation  of  water  from  the  interior  of  their  cells; 
and,  there  being  no  provision  for  a  replacement  of  the  evaporated  fluid,  the 
whole  plant  dries  up.  If  one  supplies  aquatic  plants,  thus  desiccated,  with 
water,  though  it  is  indeed  absorbed  it  no  longer  has  the  power  of  reviving  them. 
Those  hydrophytes  which  occur  in  the  sea,  near  the  shore,  are  able  to  stand 
exposure  to  the  air  for  a  comparatively  long  time,  and  they  are  regularly  sub- 
ject to  it  during  ebb-tide.  Sea- wracks  which  at  high-tide  were  floating  in  the 
water  are  then  seen  lying  on  the  dry  rocks  or  sand  of  the  shore.  But  the  mem- 
branes of  the  cells  forming  the  outermost  layer  in  all  these  sea- wracks  is  very  thick. 
They  retain  water  staunchly  and  prevent  the  plants  from  drying  up,  at  least  until 
high-tide  occurs  again,  when  they  are  once  more  submerged. 

Amphibious  plants  in  which  the  lower  leaves  are  like  those  of  aquatics  and  the 
upper  like  those  of  land-plants  so  far  as  desiccation  is  concerned  (e.g.  several  kinds 
of  pond- weed — Potamogeton  heterophyllus  and  P.  natans — and  a  few  white-flowered 
Ranunculi— Ranunculus  aquatilis  and  R.  hololeucus),  exhibit  a  transition  stage  from 


76  ABSORPTION   OF   FOOD-SALTS   BY   WATER-PLANTS. 

aquatic  plants  to  land-plants.  When  the  water  sinks  and  they  are  finally  left  lying 
exposed  on  the  mud  or  wet  sand,  to  which  they  appear  to  be  firmly  attached  by 
their  abundant  roots,  it  is  only  the  previously  submerged  leaves  that  dry  up.  That 
part  of  the  foliage  which  floated  on  the  surface  and  was  consequently  always  in 
contact  with  the  air  continues  to  thrive,  and  any  fresh  leaves  that  may  be  developed 
adapt  themselves  completely  to  the  new  environment.  Similar  behaviour  is  ob- 
served in  many  of  the  plants  which  float  freely  on  the  surface  of  water.  Such,  for 
instance,  is  the  case  with  some  species  of  duckweed  (Lemna  minor  and  L. 
polyrrhiza),  with  Azolla,  Pontederia  and  Pistia;  they  do  not  die  when  the  water 
sinks,  leaving  them  stranded,  but  absorb  food-stuffs  from  the  wet  earth  through 
their  roots,  and  in  this  condition  are  not  to  be  distinguished  from  land-plants. 

Hydrophytes  in  the  narrow  sense,  i.e.  plants  which  are  entirely  submerged  and 
die  if  they  are  surrounded  by  air  instead  of  wrater  for  any  length  of  time,  are  for 
the  most  part  fixed  to  some  support  beneath  the  water.  In  many  cases  the 
characteristic  method  of  reproduction  consists  in  the  separation  of  special  cells, 
which  then  swim  about  for  a  time  in  the  water.  Sooner  or  later,  however,  they 
re-attach  themselves  to  some  seemingly  suitable  spot,  and  the  further  phases  of  their 
development  are  again  stationary.  Comparatively  few  permanently  submerged 
species  are  freely  suspended  in  the  liquid  medium  in  every  stage  of  development. 
Such  free  plants  are  liable  to  be  shifted  by  currents  in  the  water,  but  the  extent  of 
their  displacement  is  never  very  great,  owing  to  the  fact  that  submerged  species  of 
this  kind  occur  almost  exclusively  in  still  water.  As  instances  may  be  mentioned 
the  ivy-leaved  duckweed  (Lemna  trisulca),  the  water- violet  (Hottonia  palustris), 
the  various  species  of  horn  wort  (Geratophyllum),  in  all  of  which  roots  are  absent; 
and  in  addition  amongst  the  lower  or  cryptogamic  plants  Riccia  fluitans,  and 
many  of  the  Desmidiaceae,  Spirogyras  and  Nostocinese. 

Some  of  these  aquatic  plants  periodically  rest  on  the  bottom  of  the  pond  or 
lake  in  which  they  live.  An  example  is  afforded  by  the  remarkable  plant  known 
as  the  water-soldier  (Stratiotes  aloides),  which,  as  is  indicated  by  its  Latin  name, 
is  not  unlike  an  aloe  in  appearance.  During  the  winter,  this  plant  rests  at  the 
bottom  of  the  pond  it  inhabits.  As  April  draws  near,  the  individual  plants  rise 
almost  to  the  surface  and  remain  floating  there,  producing  fresh  sword-shaped 
leaves  and  bunches  of  roots  which  arise  from  the  abbreviated  axis,  and  finally  flowers 
which,  when  the  summer  is  at  its  height,  float  upon  the  surface.  When  the  time  of 
flowering  is  over,  the  plant  sinks  again  to  mature  its  fruit  and  seeds,  and  develop 
buds  for  the  production  of  young  daughter-plants.  Towards  the  end  of  August, 
it  rises  for  the  second  time  in  one  year.  The  young  plants  that  have  meantime 
grown  up  resemble  their  parent  completely,  except  that  their  size  is  smaller. 
They  grow  at  the  end  of  long  stalks  springing  from  amongst  the  whorled  leaves, 
and  the  stately  mother-plant  is  now  surrounded  by  them  like  a  hen  by  her  chickens. 
During  the  autumn,  the  shoots  connecting  the  daughter-plants  with  their  parent  rot 
away,  and,  thus  isolated,  each  little  rosette,  as  well  as  the  mother-plant,  sinks  once 
more  to  the  bottom  of  the  pond  and  there  hibernates. 


ABSORPTION   OF   FOOD-SALTS   BY   WATER-PLANTS.  77 

Altogether  the  number  of  submerged  plants  which  live  suspended  in  water  is 
very  small.     As  has  been  said  before,  by  far  the  greater  number  are  attached  some- 
where.    Seed-bearing  plants  or  Phanerogamia,  such  as  Vallisneria,  Ouvirandra 
Mymophyllum,  Najas,  Zannichellia,  Ruppia,  Zostera,  Elodea,  Hydrilla,  and  several 
species  of  Potamogeton  (P.  pectinatus,  P.  pusillus,  P.  lucens,  P.  densus,  P.  crispus); 
as  also  Cryptogams,  such  as  the  various  species  of  Isoetes  and  Pilularia  and  sub- 
merged mosses,  are  fastened  in  the  mud  under  water  by  means  of  attachment -roots 
or  of  rhizoids,  whilst  the  almost  illimitable  host  of  brown  and  red  sea-weeds  are 
ixed  by  special  cells  or  groups  of  cells,  which  are  often  root-like  in  appearance. 
The  sea- weeds  choose  rocks  and  stones,  by  preference,  for  their  support,  but  they 
also  make  use  of  animals  and  plants.     The  shells  of  mussels  and  snails  are  often 
completely  overgrown  by  brown  and  red  sea-weeds.     Larger  kinds  of   Fucacese, 
especially  the  species  of  Sargassum  and  Cystosira,  which  form  regular  submarine5 
forests,  bear  upon  their  branches  numerous  other  small  epiphytes,  chiefly  Floridese, 
and   these  again  are  themselves  covered   by  minute  Diatomacese.      Many  of  the 
huge  and  lofty  brown  sea-weeds  which  raise  themselves  from  the  bottom  of  the 
sea,  remind  one  forcibly  of  tropical  trees  covered  with  Orchidese  and  Bromeliacese, 
whilst  the  latter  are  themselves  overgrown  by  Mosses  and  Lichens.    These  epiphytes 
are  for  the  most  part,   however,  neither  parasitic  nor   saprophytic.     In   general 
hydrophytes   attached   by   means   of  single   cells   or  groups    of   cells   derive   no 
nutriment,  i.e.  no  food-salts,  from  the  support  they  rest  upon.    When  loosened  from 
the  substratum  they  continue  to  live  in  the  water  for  a  long  time;  they  increase  in 
size,  and  if  they  come  into  contact  with  a  solid  body  are  apt  to  attach  themselves  to 
it.    In  this  connection  it  is  well  worthy  of  remark  that  certain  Crustacea  have  their 
carapaces  entirely  covered  by  hydrophytes  of  this  kind,  and  that  it  takes  a  very 
short  time  for  the  plants  to  establish  themselves  upon  them.     For  instance,  some 
species  of  crabs,  such  as  Maja  verrucosa,  Pisa  tetraodon  and  P.  armata,  Inachus 
scorpioides  and  Stenorrhyncus  longirostris,  cut  off  bits  of  Wracks,  Floridese,  Ulvse, 
&c.,  with  their  claws,  and  place  them  on  the  top  of  their  carapaces,  securing  them 
on  peculiar  spiky  or   hooked   hairs.     The  fragments  grow    firmly   to   the   crabs' 
chitinous  coats,  and  far  from  being  harmful  to  the  animals  are,  on  the  contrary, 
an  important  means  of  protection.      The  crabs  in  question  escape  pursuit  in  con- 
sequence of  this  disguise,  and  it  is  to  be  observed  that  each  species   chooses  the 
very  material  which  makes  it  most  unrecognizable  to  plant  upon  the  exterior  of 
its  body:  those  species  which  live  chiefly  in  regions  where  Cystosiras  are  indigenous 
deck  themselves  in  Cystosiras,  whilst  those  which  inhabit  the  same  places  as  Ulvse, 
carry  Ulvse  on  their  backs.     This  phenomenon  has  for  us  a  special  interest  in  that 
it  shows  that  the  water-plants  we  are  discussing  draw  no  food-salts  from  their 
place  of  attachment,  and  that  accordingly  the  chemical  composition  of  the  support 
is  a  matter  of  utter  indifference  to  all  these  Fucacese,  Floridese,  Ulvse,  &c. 

There  is  no  doubt  that  food-salts  are  absorbed  by  these  hydrophytes  from  the 
surrounding  water  through  their  whole  surface.  Accordingly  the  structure  of  their 
peripheral  cells  is  much  simpler  than  is  the  case  in  land-plants.  In  the  latter  very 


78  ABSORPTION   OF   FOOD-SALTS   BY   WATER-PLANTS. 

complicated  adaptations  are  necessary  for  the  extraction  of  food-salts  from  the 
earth.  In  particular,  the  portions  which  are  exposed  to  the  air  above  ground  exhibit 
a  number  of  special  structures  connected  with  this  extraction.  These  structures 
(cuticle,  stomata,  &c.)  are  superfluous  in  the  case  of  aquatic  plants,  for  there  is  with 
them  no  necessity  for  raising  and  conducting  food-salts  into  the  parts  where  they 
can  be  used  up.  Moreover  the  absorption  of  nutritious  matter  is  much  simpler, 
inasmuch  as  it  is  not  necessary  for  the  absorbent  parts  to  search  for  a  perpetual 
source  of  the  requisite  substances.  The  roots  of  land-plants  have  often  to  range 
over  a  wide  area  in  order  to  find  sufficient  nourishment  in  the  earth,  and  frequently 
they  have  then  to  liberate  it,  i.e.  bring  it  into  a  state  of  solution.  This  is  not  the 
case  with  water-plants.  They  are  completely  surrounded  by  a  medium  which 
is  itself  to  a  large  extent  a  solution  of  food-salts,  and  no  sooner  are  substances 
withdrawn  by  the  absorbent  cells  from  the  layers  of  water  immediately  bounding 
them  than  those  substances  are  again  supplied  from  the  more  remote  environ- 
ment. Constant  compensating  currents  occur  in  water,  and  there  is,  therefore, 
scarcely  an  aquatic  plant  towards  which  there  is  not  a  perpetual  flow  of  the  food- 
salts  it  requires  in  a  form  suitable  for  absorption.  In  connection  with  this  kind  of 
food-absorption  there  is  also  the  fact  that  the  parts  by  which  hydrophytes  attach 
themselves  to  a  support  are  relatively  small  in  area.  Fucoids,  as  large  as  hazel 
trees  in  height  and  girth,  are  fixed  to  submerged  rocks  by  groups  of  cells  perhaps 
only  1  cm.  in  diameter. 

The  quantity  of  food-salts  absorbed  by  hydrophytes  is  very  considerable  com- 
pared with  the  amounts  absorbed  by  other  plants.  As  has  been  mentioned  before, 
soda  and  iodine  play  a  very  important  part  in  the  thousands  of  different  varieties 
which  live  in  the  sea.  If  Floridese  are  transferred  from  the  sea  into  pure  distilled 
water,  common  salt  and  other  saline  compounds  diffuse  out  of  the  interior  of  the 
cells  through  the  cell-membranes  into  the  fresh  water  around.  The  red  colouring 
matter  of  these  Florideae  also  passes  through  the  cell- walls  into  the  water,  proving 
that  the  molecular  structure  of  the  membrane  is  adapted  to  the  agency  of  salt 
water  in  the  osmotic  processes  of  food-absorption. 

Plants  living  in  fresh,  or  in  brackish  water,  likewise  absorb  relatively  large 
quantities  of  food-salts;  and  this  accounts  for  the  fact  that  water  which  is  very 
poorly  provided  with  nutriment  of  the  kind  contains  only  very  few  vegetable 
species. 

One  would  expect  that  exceedingly  abundant  vegetation  would  be  evolved  in 
running  water,  provided  the  latter  contained  food-salts  in  solution,  however  small 
they  might  be  in  quantity.  For,  in  such  a  situation,  it  is  not  necessary  to  wait  for 
the  salts  withdrawn  by  the  plants  from  their  immediate  environment  to  be  restored 
by  the  slow  processes  of  mixture  and  equilibration;  the  water  which  has  been  drained 
of  nutriment  is  replaced  the  next  moment  by  other  water  bearing  fresh  food-salts. 
Experience  shows,  however,  that  flowing  water  is  not  so  favourable  to  the  develop- 
ment of  hydrophytes  as  is  the  still  water  of  pools,  ponds,  and  lakes.  This  may 
partly  depend  on  the  fact  that  running  water  is  always  poorer  in  food-salts-,  and 


ABSORPTION   OF    FOOD-SALTS   BY   LITHOPHYTES.  79 

partly  also  on  the  circumstance  that  mechanical  difficulties  are  opposed  to  the  taking 
up  of  saline  molecules  from  water  in  rapid  motion.  There  are  only  a  few  plants 
that  are  able  to  absorb  under  these  conditions,  and  these  choose,  by  preference,  the 
very  spots  where  they  are  most  exposed  to  the  dash  of  the  water.  Thus,  certain 
Nostocineae  (Zonotrickia,  Scytonema)  are  to  be  found  constantly  in  waterfalls  at 
the  parts  where  the  most  violent  fall  occurs.  Lemanea,  Hydrurus,  and  many 
mosses  and  liverworts,  grow  by  preference  in  the  foaming  cascades  of  rapid 
torrents.  Amongst  flowering  plants  we  only  know  of  the  Podostemacese  as  choosing 
a  habitat  of  this  kind.  Podostemacese  are  exceedingly  curious  little  plants,  which 
at  first  glance  one  would  take  for  mosses  or  liverworts  without  roots.  Some  of 
them,  e.g.  the  Brazilian  species  of  the  genus  Lophogyne  and  the  various  species  of 
Terniola  growing  in  Ceylon,  exhibit  no  differentiation  into  stem  and  leaves,  but  are 
only  represented  by  green  fissured  and  indented  lobes  attached  to  stones.  They 
belong  without  exception  to  the  tropical  zone,  and  occur  there  in  the  beds  of  streams, 
attached  to  rocks,  over  which  the  foaming  water  rushes. 

ABSORPTION   OF  FOOD-SALTS  BY  LITHOPHYTES. 

Nothing  would  seem  more  natural,  as  to  the  absorption  of  mineral  salts  by 
lithophytes,  than  that  the  stone  which  constitutes  their  support  should  yield  the 
salts,  and  that  the  attached  plants  should  suck  them  up;  but,  generally  speaking, 
the  case  is  not  so  simple.  There  are  mosses  and  lichens  which  cling  to  the  surfaces 
of  rocks  on  mountain  tops.  These  rocks  are  sometimes  composed  of  perfectly  pure 
quartz,  and  yet  the  plants  in  question  contain  very  little  silica;  they  contain,  on 
the  other  hand,  a  number  of  substances  entirely  wanting  in  the  composition  of  the 
underlying  rock,  and  which  could  not,  therefore,  have  been  derived  from  that 
source.  For  many  of  these  lithophytes  the  rock  is,  in  the  main,  only  a  substratum 
for  attachment,  and  in  no  way  a  nutrient  soil;  just  as,  in  the  case  of  many  aquatic 
plants,  the  stones  to  which  they  cling  by  their  discs  of  attachment  are  anything 
but  sources  of  nourishment. 

From  what  source,  then,  do  stone-plants  of  this  kind  derive  the  food-salts  which 
are  wanting  in  their  substratum  ?  It  may  sound  paradoxical,  but  it  is  nevertheless 
the  fact,  that  they  obtain  those  salts  from  the  air  through  the  medium  of  atmospheric 
precipitation.  Rain  and  snow  not  only  absorb  carbon  dioxide,  sulphuric  acid,  and 
ammonia — which  occur  in  air  universally,  although  in  extremely  minute  quantities 
—but  they  also  collect,  as  they  fall,  floating  particles  of  dust.  The  opinion  is  widely 
entertained  that  although  the  atmosphere  is  full  of  dust  in  the  neighbourhood  of 
cities  and  human  settlements  generally,  where  the  soil  is  laid  bare  and  ploughed 
up,  and  roads  and  paths  have  been  made  for  purposes  of  traffic,  and  perhaps  also 
over  steppes  and  deserts  where  large  areas  of  ground  are  destitute  of  vegetation, 
yet  that  there  is  no  dust  in  the  air  over  land  remote  from  places  of  that  kind  or  in 
the  air  of  marshes,  lakes,  or  seas.  This  notion  has  certainly  some  warrant  if  we 
regard  as  dust  only  the  coarser  particles  which  are  raised  from  loose  earth  and 


80  ABSORPTION   OF   FOOD-SALTS   BY   LITHOPHYTES. 

whirled  into  the  air  by  the  wind.  Moreover,  the  quality  of  the  dust  will  no  doubt 
be  characteristically  affected  by  the  vicinity  of  areas  of  industry.  One  has  only  to 
look  at  the  sooty  leaves  and  branches  of  trees  in  parks  near  manufactories  to 
convince  oneself  of  the  reality  of  this  influence.  But  it  would  be  quite  erroneous  to 
suppose  that  the  air  in  regions  far  from  land  that  has  been  cultivated  or  otherwise 
opened  up  is  free  from  dust.  It  contains  dust  everywhere.  There  is  dust  in  the  air 
of  the  extensive  ice-fields  of  arctic  regions  and  of  high  mountain  glaciers,  and  there 
is  dust  in  the  air  of  great  forests  and  over  the  boundless  sea. 

If  the  rays  of  the  setting  sun  fall  obliquely  through  a  gap  between  two  peaks  in 
a  wood-clad  mountain  valley,  sun-motes  may  be  seen  floating  up  and  down  and  in 
circles,  just  as  they  do  in  a  room  when  the  last  rays  before  sunset  fall  through  the 
window.  These  motes  are  of  course  not  usually  visible,  and  they  are  moreover 
much  smaller  than  the  particles  of  dust  which  are  raised  by  the  wind  from  roads 
and  then  again  deposited.  Now,  when  rain  falls,  it  takes  the  sun-motes  from  the 
air  and  brings  them  down  to  earth,  and  the  air  is  thus  washed  to  a  certain  degree 
of  purity.  This  happens  still  more  completely  in  the  event  of  snow.  The  latter 
acts  not  unlike  a  mass  of  gelatine  used  to  purify  cloudy  liquids,  its  effect  being  to 
drag  down  with  it  all  the  particles  to  which  the  turbidity  is  due,  leaving  the  upper 
part  of  the  liquid  quite  clear.  Similarly,  falling  snow-flakes  filter  the  air;  and, 
mixed  with  fallen  snow,  there  are  accordingly  innumerable  particles  of  dust. 
If  afterwards  the  snow  gradually  melts,  it  dissolves  some  of  the  dust,  which  then 
drains  away  into  chinks  and  depressions;  but  a  portion  remains  behind  undissolved. 
This  portion  is  gradually  consolidated,  and  then  appears  lying  on  the  parts  of  the 
snow  that  are  still  unmelted  in  the  form  of  dark  patches,  streaks,  and  bands;  often 
also  it  forms  a  smeary  graphitic  covering  so  widely  spreading  over  the  last  remnants 
of  melting  snow  that  the  latter  resemble  lumps  of  mud  rather  than  snow.  Accord- 
ingly we  find  it  everywhere  —  in  regions  cultivated  and  uncultivated,  in  tilled 
lowlands  and  on  high  grassy  plains  above  forest  limits,  where  no  tilled  land  is  to  be 
seen  in  any  direction,  and  lastly  in  arctic  regions  in  the  middle  of  glaciers  several 
miles  across. 

All  this  snow  dust  is  not  invariably  deposited  as  a  result  of  the  filtering  of  the 
air  by  falling  snow-flakes;  an  additional  supply  is  brought  by  the  winds  which 
blow  across  the  snow-fields.  It  is  not  of  rare  occurrence  in  the  Alps  for  snow- 
fields  to  exhibit  suddenly,  after  violent  storms,  an  orange-red  coloration.  On  closer 
inspection  one  finds  that  the  surface  of  the  snow  is  strewn  with  a  layer  of  powder, 
infinitesimally  fine  and  for  the  most  part  brick-red,  which  has  been  brought  by  the 
gales.  Investigation  of  this  "  meteoric  dust "  shows  that  it  is  composed  chiefly  of 
minute  fragments  of  ferruginous  quartz,  felspar,  and  various  other  minerals. 
Mixed  with  these  there  are,  however,  sometimes  remnants  of  organic  bodies,  such 
as  bits  of  dead  insects,  siliceous  skeletons  of  diatoms,  spores,  pollen-grains,  tiny 
fragments  of  stems,  leaves,  and  fruits,  and  the  like.  Once,  after  a  south  wind  had 
prevailed  for  several  days,  the  snow-fields  of  the  Solstein  range  near  Innsbruck 
were  covered,  at  a  height  of  from  two  to  three  thousand  meters  above  the  sea-level, 


ABSORPTION   OF    FOOD-SALTS   BY   LITHOPHYTES.  81 

with  millions  of  a  species  of  Micrococcus,  which  lent  a  rosy  hue  to  vast  expanses 
of  snow. 

Most  of  the  dust  in  the  atmosphere  originates,  doubtless,  from  our  earth.  The 
air  that  blows  in  waves  over  the  earth  can  carry  along  with  it  not  only  dead  and 
detached  portions  of  plants,  but  also  loose  particles  of  rock,  sand,  earth,  and  dried 
mud.  If  one  draws  one's  palm  across  the  weather  side  of  a  dry  rock  composed  of 
dolomitic  limestone,  gneiss,  trachyte,  or  mica-schist,  the  surface  of  the  stone  always 
feels  dusty,  and  the  slightest  movement  of  the  hand  is  sufficient  to  detach  a  number 
of  particles  which  were  already  separate  from  the  rock  and  only  held  in  loose  con- 
nection with  it.  This  dust  is  liable  to  be  detached  and  carried  away  by  any  strong 
gust  of  wind.  Larger  and  heavier  particles  are  not,  it  is  true,  lifted  much  above  the 
ground;  they  are  rolled  and  pounded  along  and  thereby  reduced  to  a  still  finer 
powder.  This  finer  dust  may  then  be  scattered  afar  by  gales  blowing  horizontally, 
or  even  ascend  into  higher  atmospheric  strata.  The  finest  dust  in  particular,  how- 
ever, is  carried  up  into  the  higher  layers  of  the  air  by  the  currents  which  ascend 
from  the  earth  in  calm  weather;  and  this  applies  not  only  to  the  tropics  but  to  the 
temperate  zones  as  well,  and  even  to  the  frigid  regions  of  the  arctic  zone.  When, 
therefore,  this  dust  is  brought  back  by  rain  or  snow  from  the  upper  aerial  strata  to 
the  earth,  it  but  completes  a  circuit.  Indeed  it  is  highly  probable  that  the  particles 
of  dust  restored  to  earth  by  means  of  atmospheric  deposits  recommence  their  aerial 
travels  as  soon  as  they  are  thoroughly  dry  again,  and  that  there  is  thus  a  circulation 
of  dust  analogous  to  that  of  water. 

There  is  of  course  no  inconsistency  in  the  fact  that  meteoric  dust,  which  is 
often  drifted  along  in  surprisingly  large  quantities,  may  originate  quite  suddenly 
during  volcanic  eruptions;  nay,  it  is  even  possible  that  cosmic  dust  reaches  our 
atmosphere  and  thence  falls  to  the  earth.  Chemical  investigation  of  aerial  dust 
has,  no  doubt,  yielded  in  most  cases  only  sulphuric  and  phosphoric  acids,  lime,  mag- 
nesia, oxide  of  iron,  alumina,  silica,  and  traces  of  potash  and  soda,  that  is  to  say,  the 
most  widely  distributed  constituents  of  the  solid  crust  of  our  earth;  but  cobalt  and 
copper  have  also  been  found  in  it,  over  and  over  again,  and  it  has  hence  been 
inferred  that  the  dust  in  these  cases  was  of  cosmic  origin. 

In  relation  to  the  question  which  we  have  here  to  answer  the  above  is,  after  all, 
almost  a  matter  of  indifference.  The  only  important  facts  are  that  dust  in  a  state  of 
extremely  fine  division  is  blown  about  in  the  air,  that  this  dust  contains  the  salts 
required  by  plants  for  their  food,  that  it  is  carried  for  the  most  part  mechanically 
by  drops  of  water  and  flakes  of  snow,  condensed  in  the  atmosphere,  and  is  partially 
dissolved,  that  the  atmospheric  deposits  supply  lithophytic  plants  with  a  sufficient 
quantity  of  nutrient  salts,  and  that  the  aqueous  solution  so  supplied  is  rapidly 
absorbed  by  the  whole  surface  of  the  plants  in  question.  We  must  not  omit  to 
mention  here  that  the  demand  of  lithophytes  for  mineral  food-salts  is  not  very  great. 
In  particular  the  protonemse  and  even  the  leafy  shoots  of  Grirtimice,  Rhacomitrice, 
Andreceacece  and  other  rock  mosses,  and  the  Collemacece  and  most  crustaceous 
lichens  only  contain  very  minute  quantities  of  these  substances.  Water  containing 

VOL.  I.  6 


82  ABSORPTION    OF   FOOD-SALTS   BY    LAND-PLANTS. 

the  usual  mineral  salts  in  about  such  proportion  as  is  necessary  for  the  cultivation 
of  cereals  in  fields  has  actually  an  injurious  effect  on  these  lithophytes  and  soon 
kills  them. 

At  the  end  of  this  section  we  shall  consider  what  happens  to  dust  which  is 
brought  to  earth  from  the  air  by  rain  and  snow  but  is  not  dissolved,  and  the 
important  part  it  plays  in  clothing  the  naked  ground  and  in  changes  of  vegetation. 
Here,  however,  it  must  be  noted  that  most  lithophytes  are  true  dust-catchers,  that  is 
to  say,  they  are  able  to  retain,  mechanically,  dust  conveyed  to  them  by  wind,  rain, 
and  snow,  and  to  use  it  in  later  stages  of  development  by  extracting  nutriment  from 
it.  Many  mosses  are  completely  lithophytic  in  early  stages  of  development  whilst 
later  they  figure  as  land-plants. 

ABSORPTION  OF  FOOD-SALTS  BY  LAND -PLANTS. 

In  no  class  of  plants  is  the  absorption  of  mineral  food-salts  accomplished  in 
so  complicated  a  manner  as  in  land -plants.  Moreover,  this  absorption  is  by  no 
means  uniform  in  different  forms  of  plants,  and  we  must  beware  of  generalizing 
with  regard  to  processes  which  have  only  been  traced  and  studied  in  isolated 
groups — perhaps  only  in  the  commonly  distributed  cultivated  plants.  On  the  other 
hand,  with  a  view  to  synoptical  representation,  it  is  not  desirable  to  enter  into  too 
great  detail  or  to  attempt  to  describe  all  the  various  differences  minutely. 

At  the  outset,  it  is  difficult  to  give  an  accurate  account  of  the  soil  which 
constitutes  the  source  of  nutriment  in  the  case  of  land -plants.  From  the  dark 
graphitic  mass  composed  of  sun-motes,  which  is  deposited  in  the  place  of  a  melted 
layer  of  snow,  to  coarse  gravel,  there  is  an  unbroken  chain  of  transition  stages; 
loam,  sand  and  gravel  are  only  specially-marked  members  of  this  chain.  Again, 
just  as  earth  varies  in  respect  of  the  size  of  its  component  parts,  so  also  it 
varies  in  the  mineral  salts  it  contains,  in  the  amount  of  admixture  of  decaying 
vegetable  and  animal  remains,  in  the  nature  of  the  union  of  its  constituents, 
and  in  its  capacity  to  absorb,  to  retain,  or  to  yield  up  water.  Compare  the  sand 
composed  of  quartz  on  the  bank  of  a  mountain  stream  with  that  of  calcareous 
origin  which  is  found  impregnated  with  salt  on  the  sea-shore,  or  with  the  sand 
at  the  foot  of  mountains  of  trachyte,  which  has  an  efflorescence  of  soda-salts. 
Or  compare  the  granite  bed  of  a  desert,  bare  of  soil,  with  the  loam  on  the  granitic 
plateaus  of  northern  regions  where  there  is  an  intermixture  of  the  remains  of  a 
vegetation  for  centuries  active.  How  great  is  the  difference  in  each  case!  But 
whatever  the  kind  of  earth,  it  is  only  of  value  as  a  source  of  nutriment  for  a 
plant  when  the  interstices  of  its  various  particles  are  filled  with  watery  fluid 
for  the  time  during  which  the  plant  is  engaged  in  the  construction  of  organic 
substances. 

But  how  is  the  earth  supplied  with  water? 

"  Das  hat  nicht  East  bei  Tag  und  Nacht, 
1st  stets  auf  Wanderschaft  bedacht." 


ABSORPTION   OF   FOOD-SALTS  BY  LAND-PLANTS.  83 

Streams  fall  into  lakes,  rivers  into  the  sea,  and  hence  the  water  ascends  into  the 
atmosphere  in  the  form  of  vapour,  and  returns  once  more  to  earth  as  snow,  rain, 
and  dew.  Through  porous  earth  it  percolates  until  it  has  filled  all  the  interspaces. 
If  its  further  descent  be  impeded  by  impervious  strata,  it  spreads  literally  as  sub- 
terranean water,  or  else  comes  up  at  some  special  spot  as  a  spring.  Earth  which  is 
richly  endowed  with  decaying  vegetable  remains  is  able  to  absorb  vapour  in  addition 
from  the  atmosphere.  When  this  occurs,  carbonic  and  nitric  acids  are  always 
absorbed  along  with  the  aqueous  vapour.  These  are  contained,  as  has  been  mentioned 
before,  in  atmospheric  deposits,  and  another  source  of  these  acids  is  afforded  by  the 
decay  of  dead  parts  of  plants.  Water  precipitated  from  the  atmosphere,  and  con- 
taining carbonic  and  nitric  acids,  is  able  by  their  means  to  decompose  the  compounds 
in  all  the  rocks  which  come  in  its  way  as  it  percolates  through  the  ground,  especially 
when  its  action  is  long  continued.  The  siliceous  compounds  or  so-called  silicates — 
felspars,  mica,  hornblende,  and  augite  in  particular — and  quartz,  the  anhydride  of 
silicic  acid,  which  form  the  preponderant  mass  of  the  rocks  of  the  solid  crust  of  our 
earth,  either  contain  a  great  quantity  of  silica,  alumina,  and  alkalies,  or  if  they  are 
relatively  poor  in  silica  they  may  be  rich  in  iron.  The  former  are  found  chiefly 
in  granite,  gneiss,  mica-schist,  and  argillaceous  slate;  the  latter  preponderate  in 
serpentine,  syenite,  melaphyr,  dolerite,  trachyte  and  basalt.  First  the  felspars  are 
decomposed  by  the  acid  water.  Their  alkalies  combine  with  the  carbonic  and  nitric 
acids  forming  soluble  salts,  and  the  alumina  and  silica  remain  behind  as  clay.  Iron 
is  also  converted  into  soluble  salts.  The  most  difficult  substances  to  decompose  are 
the  mica  and  quartz,  and  it  is  on  that  account  that  they  so  often  appear  in  the 
form  of  glittering  scales  and  angular  nodules  mixed  with  the  clay  produced  from 
the  decomposition  of  felspar.  But,  ultimately,  even  they  are  unable  to  withstand 
the  continuous  action  of  the  acidulated  water.  The  result  of  these  chemical 
changes  is  an  earth,  which,  according  to  the  nature  of  the  parent  rock,  contains 
a  preponderating  amount  of  clay,  of  quartzose  sand  or  of  mica,  which  is  coloured 
in  various  ways  by  iron  compounds.  Of  substances  useful  to  plants  these 
earths  yield  generally  on  analysis  the  following:  potash,  soda,  lime,  magnesia, 
alumina,  ferrous  and  ferric  oxides,  manganese,  chlorine,  sulphuric  acid,  phosphoric 
acid,  silica,  and  carbonic  acid,  sometimes  one  sometimes  another  in  greater 
proportion  relatively,  and  traces  of  many  substances  often  so  slight  as  hardly 
to  be  detected. 

It  is  true  that  limestone  and  dolomite,  which,  next  to  the  above-mentioned 
rocks,  enter  most  largely  into  the  composition  of  the  solid  crust  of  the  earth, 
consist  chiefly  of  carbonate  of  lime  and  magnesium  carbonate  respectively;  but 
wherever  they  occur  in  extensive  strata  and  piles,  they  always  contain  in  addition 
an  admixture  of  alumina/ silicic  acid,  ferrous  oxide,  manganese,  traces  of  alkalies 
in  combination  with  phosphoric  and  sulphuric  acids,  &c.  Of  the  carbonates  of 
lime  and  magnesia  a  great  part  is  gradually  dissolved  and  carried  away  upon  the 
invasion  of  water  containing  carbonic  and  nitric  acids,  and  a  proportion  also  of 
the  substances  mixed  with  them,  as  above  mentioned,  is  lixiviated.  What  remains 


84  ABSORPTION   OF   FOOD-SALTS   BY   LAND-PLANTS. 

behind  then  consists  of  an  argillaceous,  loamy  mass,  variously  coloured  by  iron  and 
very  similar  in  appearance  to  the  clay  formed  from  the  decomposition  of  felspar. 
According  to  the  quantity  of  the  substances  mixed  with  the  carbonate  of  lime  in 
the  rock,  the  loamy  earth  formed  from  limestone  is  either  abundant  or  only  in 
restricted  layers,  bands  and  pockets  lying  on,  or  intercalated  within,  the  unde- 
composed  debris  of  the  stone.  Chemical  analysis  has  resulted  in  the  discovery 
that  there  are,  as  a  rule,  in  loamy  earth  of  this  kind  the  same  ingredients  avail- 
able for  plants  as  have  been  identified  in  earth  produced  from  silicates;  and  we 
are  led  to  believe  that  earths,  collected  in  widely  different  places  and  covering 
rocks  of  most  various  kinds,  are  much  more  uniform  qualitatively  than  has  been 
supposed.  Only,  the  relative  proportions  of  the  substances  forming  the  mixture 
are  usually  different.  Silica  and  the  alkalies  are  less  conspicuous  in  earth  derived 
from  limestone,  and  carbonate  of  lime  in  that  which  is  formed  from  silicates. 
This  difference  is  particularly  striking  in  instances  where  the  rock  consisted 
almost  entirely  either  of  quartz  and  mica  or  of  nearly  pure  carbonates  of  lime 
and  magnesium.  In  these  cases  the  earth  formed  is  not  argillaceous,  but  of  loose 
consistence,  very  abundant,  and  composed,  according  to  the  kind  of  rock,  of 
quartzose  sand  and  mica  scales  or  calcareous  and  dolomitic  sand. 

The  conversion  of  rocks  into  earths  by  the  action  of  water  from  the  atmosphere 
containing  carbonic  and  nitric  acids  is,  besides,  materially  modified  by  the  disrup- 
tions which  ensue  from  changes  of  temperature,  more  particularly  by  the  freezing 
of  water  within  the  pores  of  rocks.  It  is  also  affected,  though  more  remotely,  by 
the  mechanical  action  of  water  and  air  in  motion,  and,  lastly,  by  the  plants  them- 
selves, which  penetrate  with  their  roots  into  the  narrowest  crevices  and  mingle  their 
dead  remains  with  the  portions  of  the  rock  that  are  decomposed,  broken  up,  or 
abraded  by  chemical  and  mechanical  agencies.  The  substance  produced  from  a 
rock  in  the  manner  explained  is  called  earth-mould,  or  simply  earth.  The  matter 
resulting  from  the  decomposition  of  plants  and  animals  is  designated  by  the  term 
"  humus."  Earth  which  includes  an  abundance  of  decomposed  fragments  of  plants, 
i.e.  has  a  large  admixture  of  humus,  is  called  vegetable  mould. 

Every  kind  of  earth,  but  especially  earth  rich  in  humus  and  clay,  has  the  power 
of  retaining  gases,  and  especially  water  and  salts.  When  water  containing  salts  in 
solution  is  poured  over  a  layer  of  dry  vegetable  mould,  it  percolates  into  the  spaces 
between  the  particles  of  earth,  and  speedily  drives  out  of  them  the  air  which  has 
but  slight  adhesion,  and  which  then  ascends  in  bubbles.  It  is  not  till  all  the  inter- 
spaces are  full  of  water,  whilst  a  fresh  supply  is  constantly  maintained  from  above, 
that  any  of  the  liquid  oozes  out  from  beneath  the  stratum  of  earth.  The  water 
remaining  in  the  interstices  is  held  there  by  adhesion  to  the  particles  of  earth,  and 
we  must  conceive  each  of  these  particles  as  surrounded  by  an  adherent  film  of 
water.  The  inorganic  salts,  infiltrating  with  the  water,  are  held  with  still  greater 
energy.  The  water  which  trickles  from  the  bottom  of  the  earth  always  contains  a 
much  smaller  proportion  of  salts  in  solution  than  that  which  was  poured  on  above, 
whence  we  conclude  that  the  latter  are  in  part  absorbed  by  the  earth. 


ABSORPTION   OF   FOOD-SALTS   BY   LAND-PLANTS.  85 

The  salts  are  to  be  regarded  as  forming  an  extremely  delicate  coating  round 
minute  particles  of  earth  where  they  are  forcibly  retained.  If  a  plant  rooted  in 
the  earth  is  to  take  in  these  salts  it  has  to  overcome  the  force  by  which  their 
molecules  are  detained.  This  is  effected,  however,  by  means  of  a  very  powerful 
attraction  exerted  by  the  protoplasts  of  the  plant  as  they  grow,  carry  on  the  work 
of  construction,  and  use  up  material.  What  actually  happens  is  an  energetic  suction 
by  the  cells  that  are  in  close  contact  with  particles  of  earth.  This  suction  depends, 
however,  upon  the  chemical  affinity  between  the  substances  in  the  interior  of  the 
cells  and  the  salts  adhering  to  the  earth-particles,  as  well  as  upon  the  consumption 
of  food-salts  for  the  manufacture  of  organic  compounds  within  the  green  cells.  It 
is  supposed  that  whenever  salts  are  abstracted  from  soil-particles  by  suction,  a 
restitution  of  like  salts  immediately  takes  place,  particles  still  unresolved  in  the 
immediate  neighbourhood  being  dissolved,  and  a  fresh  influx  taking  place  from  the 
environment.  Consequently  the  concentration  of  the  solution  retained  by  the  earth 
is  always  approximately  the  same,  or,  at  any  rate,  equilibrium  is  very  quickly 
restored.  One  advantage  of  this  is  that  the  cells  in  immediate  contact  with 
particles  of  earth,  and  their  adherent  liquid,  can  only  meet  with  a  saline  solution  of 
constant  weak  concentration,  and  are  therefore  secure  from  injury  such  as  would 
result  in  the  case  of  most  plants,  from  contact  with  a  very  concentrated  solution. 
In  other  words,  the  absorptive  power  of  earth  acts  as  a  regulator  of  the  process  of 
absorption  of  food-salts  by  plants,  and  is  the  means  of  keeping  the  saline  solution 
in  the  earth  always  at  the  degree  of  strength  best  suited  to  the  plants  concerned. 

Naturally,  the  passage  of  salts  from  the  earth  to  the  interior  of  a  plant  is 
dependent  on  the  aid  of  water  containing  both  the  substances  composing  cell- 
contents  and  the  food-salts  in  solution.  The  cell-membranes,  through  which 
absorption  takes  place,  are  saturated  with  this  solution.  The  aqueous  films  adhering 
to  the  particles  of  earth,  the  water  saturating  the  cell-membrane,  and  the  liquid 
inside  the  cells  are  really  in  unbroken  connection,  and  along  this  continuous  water- 
way the  passage  of  salt  molecules  in  and  out  can  take  place  easily. 

The  absorption  of  food-salts  directly  from  the  earth  by  green  cells  occurs  very 
rarely.  The  protonema  of  Polytrichum,  which  spreads  its  threads  over  loamy  earth 
and  wraps  it  in  a  delicate  green  felt,  and  that  of  the  famous  Cavern  Moss  (Schis- 
tostega),  whose  long  tubular  lower  cells  penetrate  the  earth  in  the  recesses  of  caves, 
do  undoubtedly  suck  up  their  necessary  food-salts  by  means  of  cells  containing 
chlorophyll.  A  drawing  of  the  latter  is  given  in  figure  2  5 A,  p. 

The  majority  of  land-plants  have,  however,  special  absorptive  cells  for  the 
taking-up  of  salts  in  solution.  These  cells  are  imbedded  amongst  or  lodged  upon 
the  earth-particles,  and  are  usually  in  intimate  connection  with  portions  of  them. 
Any  part  of  a  plant  that  penetrates  into  the  earth  or  lies  upon  it,  may,  if  it  performs 
the  function  of  absorption,  be  equipped  with  cells  of  the  kind.  Plagiothecium 
nekeroideum,  a  delicate  moss  belonging  to  the  flora  of  Germany,  and  growing  on 
earth  under  overhanging  rocks,  where  it  is  not  exposed  to  rain,  and  therefore  cannot 
receive  any  food-salts  through  that  agency,  develops  absorption-cells  on  the  apices 


86  ABSORPTION   OF   FOOD-SALTS   BY    LAND-PLANTS. 

of  its  green  leaflets.  So  also  does  Leucobryum  javense,  a  species  native  to  Java. 
Several  delicate  ferns  of  the  family  of  the  Hymenophyllacece  exhibit  them  on  their 
subterranean  stems.  Many  liverworts  and  the  prothalli  of  ferns  bear  them  on  the 
under  surfaces  of  their  flat  thalli  which  lie  outspread  on  damp  earth.  But  most 
commonly  of  all  are  they  to  be  found  close  behind  the  growing  tips  of  roots.  Their 
form  does  not  vary  very  much.  On  the  roots  of  plants  fringing  the  sources  of  cold 
mountain-springs,  as  on  those  of  many  marsh-plants  in  low-lying  land,  they  are  in 
the  form  of  comparatively  large,  oblong,  flattened,  closely  united  cells,  with  thin 
walls  and  colourless  contents.  In  some  conifers,  whilst  having  in  the  main  the 
shape  just  described,  they  differ  in  that  they  are  arched  outwards  so  as  to  form 
papillae;  but  in  most  other  phanerogams  the  external  cell- wall  projects  outwards, 
and  the  whole  absorptive  cell  develops  into  a  slender  tube,  set  perpendicularly  to 
the  longitudinal  axis  of  the  root  (fig.  12  *). 

Seen  with  the  naked  eye,  or  but  slightly  magnified,  these  delicate  tubes  look  like 
fine  hairs,  and  have  received  the  name  of  "root-hairs."  The  end  of  a  root  often 
appears  to  be  covered  with  velvety  pile,  and  the  absorptive  cells  are  then  very 
closely  packed;  more  than  four  hundred  per  square  millimeter  have  been  occa- 
sionally counted.  In  other  cases,  however,  there  are  hardly  more  than  ten  on  a 
square  millimeter.  When  in  such  small  numbers  they  are  usually  elongated  and 
clearly  visible  to  the  naked  eye.  Their  length,  for  the  most  part,  varies  from  the 
fraction  of  a  millimeter  to  three  millimeters,  and  their  thickness  between  O'OOS  m.m. 
and  O14  m.m.  It  is  only  exceptionally  that  one  meets  with  plants,  rooted  in  mud, 
possessing  root-hairs  5  m.m.  or  more  in  length.  The  absorptive  cells  of  phanero- 
gams are  almost  always  simple  epidermal  cells  of  the  particular  part  of  the  plant 
that  bears  them,  and  are  not  partitioned  by  any  transverse  walls.  In  mosses  and 
fern  prothalli,  on  the  other  hand,  the  absorption-cells  are  generally  segmented  by 
transverse  septa  and  are  usually  greatly  elongated.  In  those  liverworts  which 
belong  to  the  genus  Marchantia  they  form  a  thick  felt  on  the  under  side  of  the 
leaf -like  plant,  or  rather,  on  such  part  of  it  as  is  turned  away  from  the  light,  and 
some  of  these  tangled  rhizoids  attain  a  length  of  nearly  2  c.m.  The  stems  of  many 
mosses  also  are  wrapped  in  a  regular  felt.  This  property  is  rendered  very  striking 
in  the  species  of  Barbula,  Dicranum,  and  Mnium,  and  especially  in  such  forms  as 
have  bright  green  leaves,  by  the  reddish-brown  colour  of  the  cells  in  question. 
Sometimes  the  long  capillary  cells  of  which  the  felt  is  composed  are  twisted 
together  spirally  like  the  strands  of  a  rope.  A  good  instance  of  this  is  Polytri- 
chum.  These  fine,  hair-like,  segmented  and  branched  structures,  found  on  mosses, 
variously  matted  and  intertwisted,  are  called  rhizoids.  But  only  those  cells  which 
come  into  contact  with  the  earth-particles  are  truly  absorbent.  The  rest  do  not 
serve  to  imbibe  from  the  ground,  but  to  conduct  the  aqueous  solution  of  food-salts, 
after  it  has  been  taken  up  by  the  absorptive  cells,  to  the  stem  and  to  the  leaves. 

The  tubular  cells  resulting  from  the  development  of  a  root's  epidermis  are  placed, 
as  before  observed,  at  right  angles  to  its  longitudinal  axis.  They  only  grow,  how- 
ever, in  earth  that  is  very  damp,  and  even  then  their  course  is  not  always  a  straight 


ABSORPTION    OF    FOOD-SALTS   BY    LAND-PLANTS. 


87 


line,  for  as  a  rule  they  describe  a  spiral  as  they  elongate.  Their  movement  seems 
as  though  it  were  for  discovering  the  most  favourable  parts  of  the  earth  for  absorp- 
tion and  attachment.  In  this  manner  they  penetrate  into  the  interspaces  in  the 
earth  which  are  filled  with  air  and  water.  They  also  have  the  power  of  thrusting 
aside  minute  particles  of  earth,  especially  if  the  latter  consists  of  loose  sand  or  mud. 
If  they  strike  perpendicularly  a  solid  immovable  bit  of  earth,  they  bend  aside 
and  grow  round  it  with  their  surfaces  closely  adpressed  to  that  of  the  obstacle  until 
they  reach  the  opposite  point  on  the  other  side,  when  they  once  more  resume  their 
original  direction  (fig.  12 3).  When  they  encounter  large  grains  of  earth  they 


Fig.  12.— Absorptive  Cells  on  Root  of  Penstem&u. 

i  Seedling  with  the  long  absorptive  cells  of  its  root  ("root-hairs")  with  sand  attached,  a  The  same  seedling;  the  sanci 
removed  by  washing.  3  Root-tip  with  absorptive  cells ;  x  10.  *•  Absorptive  cells  with  adherent  particles  of  earth.  «  Sectiom 
through  the  root- tip ;  x  60. 

sometimes  stop  and  swell  up  to  the  shape  of  a  club.  The  club  divides  into  two  or 
more  arms,  which  grasp  and  cling  to  the  granule  like  the  fingers  of  a  hand.  Many 
fragments  of  earth  remain  thus  in  the  grasp  of  finger-like  processes,  whilst  others 
are  held  fast  in  the  knots  and  spirals  of  corkscrew-shaped  root-hairs  which  are 
often  found  tangled  together.  But  the  retention  of  most  of  the  earth-particles 
which  adhere  to  a  plant,  including  fragments  of  lime,  quartz,  mica,  felspar,  &c.,  as  well 
as  plant-residues,  is  due  to  the  fact  that  the  outermost  layer  of  the  absorptive  cells 
is  sticky,  it  being  altered  into  a  swollen  gelatinous  mass  which  envelops  the 
particles.  When  this  sticky  layer  becomes  dry  it  contracts  and  stiffens,  and  the 
granules  partially  imbedded  in  it  are  thereby  cemented  so  tightly  to  the  absorptive 
cells  that  even  violent  shaking  will  not  dislodge  them. 

In  the  case  of  most  seedlings,  and  in  that  of  grasses,  the  absorptive  cells  which 
proceed  from  the  roots  and  which  are  especially  numerous  in  the  latter,  are  generally 
thickly  covered  with  particles  of  earth  (see  fig.  12  4).  If  such  a  root  is  pulled  out 
of  sandy  soil  it  appears  to  be  completely  encased  in  a  regular  cylinder  of  sand  (fig. 
12 a).  A  root  of  Clusia  alba,  taken  from  coarse  gravel,  had  its  root-hairs  so  tightly 


88  ABSORPTION   OF   FOOD-SALTS   BY   LAND-PLANTS. 

adherent  to  bits  of  gravel  that  several  little  stones,  weighing  T8  grms.,  were  found 
clinging  to  it  when  it  was  lifted.  The  gelatinous  mass,  resulting  from  the  swelling- 
up  of  the  external  coat  of  the  cell,  does  not  in  any  way  hinder  absorption  or  the 
passage  of  food-salts  in  solution.  Nor  does  the  inner  coat,  the  thickness  of  which 
varies  between  0*0006  m.m.  and  O'Ol  m.m.,  constitute  any  impediment  to  imbibition. 

In  addition  to  the  absorption  of  nutritive  salts  by  root-hairs,  there  is  also,  in 
many  cases,  an  interchange  of  materials;  that  is  to  say,  not  only  do  substances 
infiltrate  from  the  earth  into  the  absorption-cells,  and  wo  onward  into  the  tissues 
of  a  plant,  but  others  pass  out  of  the  plant  through  the  absorptive  cells  into 
the  earth.  Amongst  these  eliminated  substances,  carbonic  acid,  in  particular, 
plays  an  important  part.  A  portion  of  the  earth-particles  adhering  to  root-hairs  is 
decomposed  by  it,  and  food-salts  in  immediate  proximity  to  those  cells  are  hereby 
rendered  available  and  pass  into  the  plant  by  the  shortest  way. 

Having  now  seen  that  land-plants  take  in  food-salts  by  means  of  special 
absorptive  cells,  it  is  natural  to  find  that  each  of  these  plants  develops  its 
absorption-cells,  projects  them,  and  sets  them  to  work  at  a  place  where  there  is 
a  source  of  nutritive  matter.  The  parts  that  bear  absorptive  cells  will  accord- 
ingly grow  where  there  are  food-salts  and  water,  which  is  so  necessary  for  their 
absorption.  The  Marchantias  and  fern  prothalli  spread  themselves  flat  upon  the 
ground,  moulding  themselves  to  its  contour.  From  their  under-surfaces  they 
.send  down  rhizoids  with  absorptive  cells  into  the  interstices  of  the  soil.  Roots 
provided  with  root-hairs  behave  similarly.  If  a  foliage-leaf  of  the  Pepper-plant 
•or  of  a  Begonia  be  cut  up,  and  the  pieces  laid  flat  on  damp  earth,  roots  are 
formed  from  them  in  a  very  short  time.  The  roots  on  each  piece  of  leaf  proceed 
from  veins  near  the  edge,  which* is  turned  away  from  the  incident  light,  and 
grow  vertically  downwards  into  the  ground. 

It  is  matter  of  common  knowledge  that  roots  which  arise  upon  subterranean 
parts  of  stems,  like  those  formed  on  parts  above-ground,  grow  downward  with  a 
force  not  to  be  accounted  for  by  their  weight  alone.  This  phenomenon,  which  is 
called  positive  geotropism,  is  looked  upon  as  an  effect  of  gravitation.  The  idea  is 
that  an  impetus  to  growth  is  given  by  gravity  to  the  root-tip,  and  that  a  trans- 
mission of  this  stimulus  ensues  to  the  zone  behind  the  tip  where  the  growth  of  the 
root  takes  place.  It  is  noteworthy  that  if  bits  of  willow  twigs  are  inserted  upside 
down  in  the  earth,  or  in  damp  moss,  the  roots  formed  from  them,  chiefly  on  the  shady 
side,  after  bursting  through  the  bark,  grow  downwards  in  the  moist  ground,  pushing 
aside  with  considerable  force  the  grains  of  earth  which  they  encounter.  The 
appearance  of  a  willow  branch  thus  reversed  in  the  ground  is  all  the  more  curious 
inasmuch  as  the  shoots,  which  are  developed  simultaneously  with  roots  from  the 
leaf -buds,  do  not  grow  in  the  general  direction  of  the  buds  and  branches,  but  turn 
away  immediately  and  bend  upwards.  Thus  the  direction  of  growth  of  roots  and 
shoots  produced  on  willow-cuttings  remains  always  the  same,  whether  the  base  or  the 
top  of  the  twig  used  as  a  cutting  is  inserted  in  the  earth.  A  similar  phenomenon  is 
observed  if  the  leafy  rootless  shoot  of  a  succulent  herb  (e.g.  Sedum  reflexum)  is  cut 


ABSORPTION    OF    FOOD-SALTS   BY   LAND-PLANTS.  89 

off  and  suspended  in  the  air  by  a  string.  Whether  it  hangs  with  the  apex  upper- 
most, ^.e.  in  the  position  in  which  it  grew  naturally,  or  with  the  apex  towards  the 
ground,  it  always,  in  a  short  space  of  time,  produces  roots  which  spring  from  the 
axis  between  the  fleshy  foliage-leaves  and  bending  sharply  grow  to  the  earth.  Thus 
in  the  former  case  their  direction  is  contrary  to  the  apex  of  the  shoot;  in  the  latter 
curiously  enough,  it  is  in  the  same  direction.  If  the  height  at  which  the  shoot  is 
suspended  is  only  2  c.m.  above  the  earth,  the  roots  growing  towards  the  ground 
develop  their  root-hairs  2  c.m.  from  their  place  of  origin.  But  if  the  shoot  is  at  a 
distance  of  10  c.m.,  the  roots  only  develop  their  root-hairs  when  they  have  attained  a 
length  of  10  c.m.  The  rule  is,  therefore,  for  the  roots  to  grow  until  they  reach  the 
nutrient  soil  without  developing  absorption-cells,  and  only  to  provide  themselves 
with  them  when  they  are  in  the  earth.  It  is  to  be  observed  that  these  roots  are 
produced  on  the  suspended  shoot  at  places  where,  under  normal  conditions  (i.e.  if 
the  shoot  were  not  cut  off  and  hung  up),  no  roots  would  be  developed.  Subject 
to  abnormal  conditions  and  liable  to  starvation,  the  plant  sends  out  these  roots  for 
self-preservation. 

Phenomena  of  this  kind  force  one  to  conclude  that  a  plant  discerns  places  which 
offer  a  supply  of  nutriment,  and  then  throws  out  anchors  for  safety  to  those  places. 
This  power  of  detection  may,  undoubtedly,  be  explained  by  the  influence  which 
conditions  of  moisture,  in  addition  to  the  action  of  gravitation,  have  on  the  direction 
taken  by  growing  roots.  The  root-hairs  can  only  obtain  food-salts  when  the  ground 
is  thoroughly  moist;  and  whenever  roots,  or  rather  their  branches,  have  to  choose 
between  two  regions,  one  of  which  is  dry  and  the  other  wet,  they  invariably  turn 
towards  the  latter.  If  seeds  of  the  garden-cress  are  placed  on  the  face  of  a  wall  of 
clay  which  is  kept  moist,  the  rootlets,  after  bursting  out  of  the  seeds,  grow  at  first 
downwards,  but  later  they  enter  the  wall  in  a  lateral  direction.  The  longitudinal 
growth  of  the  roots  is  greater  on  the  dry  side  than  on  the  wet  side,  and  this  results 
in  a  bending  of  the  whole  towards  the  source  of  moisture,  in  this  instance  the  damp 
wall.  It  has  been  established  that  the  tip  of  a  rootlet  is  very  sensitive  to  the 
presence  of  moisture  in  the  environment.  Where  there  is  a  moist  stratum  on  one 
side  and  a  dry  stratum  on  the  other,  a  root-tip  receives  a  stimulus  from  the  unequal 
conditions  in  respect  of  moisture;  the  stimulus  is  propagated  to  the  growing  part  of 
the  root,  which  lies  behind  the  tip,  and  the  result  is  a  curvature  of  the  root  towards 
the  moist  side.  Thus,  the  presence  of  absorbable  nutriment,  or  rather  of  moisture, 
in  the  ground  explains  the  divergence  of  roots  from  the  direction  prescribed  by 
gravity. 

The  extent  to  which  the  direction  taken  by  roots  in  their  search  for  food  is 
dependent  upon  the  presence  of  that  food,  and  the  fact  that  roots  grow  towards 
places  that  afford  supplies  of  nutritious  material,  are  strikingly  exhibited,  also, 
by  epiphytes  growing  on  the  bark  of  trees,  such  as  tropical  orchids  and 
Bromeliacece ;  and  again  by  plants  parasitic  on  the  branches  of  trees,  of  which  the 
Mistletoe  and  other  members  of  the  Loranthacece  afford  examples.  Although  the 
absorption  of  food  by  these  plants  will  not  be  thoroughly  discussed  till  a  later 


90  ABSORPTION   OF   FOOD-SALTS   BY   LAND-PLANTS. 

stage,  this  is  the  proper  place  to  mention  the  fact  that  in  them  positive  geotropism 
appears  to  be  completely  neutralized.  The  growing  rootlets  which  spring  from  the 
seed,  and  the  absorptive  cells  produced  from  minute  tubercles,  grow  upwards  if 
placed  on  the  under  surface  of  a  branch,  horizontally  if  placed  on  the  side,  and 
downwards  if. on  the  upper  surface.  Thus,  whatever  the  direction,  they  grow 
towards  the  moist  bark  which  affords  them  nourishment. 

Positive  geotropism  seems  to  be  quite  abolished  also  in  those  marsh-plants 
which  live  under  water.  When,  for  instance,  the  seed  of  the  Water-chestnut 
(Trapa  natans)  germinates  under  water  in  a  pond,  the  main  root  emerges  first  from 
the  little  aperture  of  the  nut  and  begins  by  growing  upwards.  Soon  the  smaller 
scale-like  cotyledon  is  put  forth,  whilst  the  other,  which  is  much  larger,  remains 
within  the  nut.  The  whole  plant  so  far  is  standing  on  its  head,  as  it  were,  and 
is  growing  upwards  with  its  principal  root  directed  towards  the  surface  of  the 
water.  Gradually  the  leafy  stem  emerges  from  the  bud  between  the  two  coty- 
ledons, and  likewise  curves  upwards  and  grows  towards  the  surface,  whilst  an 
abundance  of  secondary  roots  is  developed  at  the  same  time  from  the  main  root. 
Their  function  is  to  absorb  nutritive  substances  from  the  water  around,  now  that 
the  materials  for  growth  stored  in  the  seed  are  exhausted.  Finding  an  aqueous 
solution  of  food-salts  everywhere  these  roots  grow  in  all  directions,  upwards, 
downwards,  or  horizontally  to  right  or  left,  forwards  or  backwards,  only  they 
carefully  avoid  touching  one  another  or  interfering  with  each  other's  sphere  of 
absorption.  It  is  not  till  much  later  that  the  main  root  changes  the  direction  of 
its  apex  and  bends  downward.  New  roots  are  then  produced  from  the  stem;  but 
this  subject  has  no  further  bearing  on  the  problems  at  present  before  us. 

The  movements  of  roots,  as  they  grow  in  earth,  suggest  that  they  are  seeking- 
for  nutriment.  The  root-tip  traces,  as  it  progresses,  a  spiral  course,  and  this 
revolving  motion  has  been  compared  to  a  constant  palpitation  or  feeling.  Spots 
in  the  earth  which  are  found  to  be  unfavourable  to  progression  are  avoided  with 
care.  If  the  root  sustains  injury,  a  stimulus  is  immediately  transmitted  to  the 
growing  part,  and  the  root  bends  away  from  the  quarter  where  the  wound 
was  inflicted.  When  the  exploring  root-tip  comes  near  a  spot  where  water 
occurs  with  food-salts  in  solution,  it  at  once  turns  in  that  direction,  and,  when  it 
reaches  the  place,  develops  such  absorptive  cells  as  are  adapted  to  the  circum- 
stances. 

As  has  been  mentioned  before,  the  roots  of  most  land-plants  bear  root-hairs  on  a 
comparatively  restricted  zone  behind  the  growing  point  (see  fig.  12 3),  and  these 
hairs  have  only  an  ephemeral  existence.  As  the  root  grows  and  elongates,  new 
hairs  arise  (always  at  the  same  distance  behind  the  tip),  whilst  the  older  ones 
collapse,  turn  brown,  and  perish.  In  ground  which  contains  on  every  side  food-salts 
in  quantities  adequate  to  the  demand,  and  sufficient  water  to  act  as  solvent  and  as 
medium  for  the  transmission  of  the  salts,  the  absorptive  cells  are  rarely  tubular,  but 
exhibit  themselves,  as  already  described,  in  the  form  of  flat  cells  destitute  of  outward 
curvature.  This  is  the  case,  for  instance,  with  those  Alpine  plants  which  grow  i 


' 


ABSORPTION    OF    FOOD-SALTS   BY   LAND-PLANTS.  91 

ever-moist  hollows  and  depressions  in  proximity  to  springs  (e.g.  Saxifraga  aizoides 
and  many  others).  But  wherever  the  substances  to  be  absorbed  are  not  so  easily 
obtained,  the  surfaces  of  the  absorptive  cells  are  increased  by  means  of  a  protrusion 
of  the  outer  cell-wall,  the  whole  cell  being  converted  into  a  tube.  These  tubular 
absorptive  cells  are  most  elongated  in  mossy  forests,  where  rather  large  gaps  occur 
not  infrequently  in  the  soil.  When  a  root  in  the  course  of  growth  reaches  one  of 
these  lacunae,  filled  with  moist  air,  its  root-hairs  often  lengthen  out  to  an  extraordi- 
nary extent,  and  sometimes  attain  to  twice  the  length  of  those  which  are  in  compact 
soil.  The  absorptive  cells  on  the  roots  of  the  Water-hemlock  (Gicuta  vvrosa)  and 
the  Sweet  Flag  (Acorus  Calamus)  do  not  project  at  all  if  the  earth  in  which  they 
grow  is  muddy;  whilst,  if  the  earth  is  only  slightly  damp,  and  an  increase  of  surface 
is  therefore  advantageous,  the  absorptive  cells  become  tubular.  Plants  which  grow 
in  ground  liable  to  periodic  drought,  and  which  at  these  times  must  secure  all  the 
moisture  retained  by  the  earth  to  save  their  aerial  portions  from  death  by  desiccation, 
endeavour  to  obtain  as  great  an  area  of  absorption  as  possible  by  the  development 
of  long  tubular  cells. 

The  fact  must  not  be  overlooked,  however,  that  the  form  and  development  of 
absorptive  cells  depend  partly  on  the  quantity  of  water  that  is  given  off  from  the 
aerial  parts  of  the  plant,  that  is  to  say,  by  the  transpiration  of  the  foliage-leaves. 
Plants  which  lose  a  great  deal  of  water  in  this  way  must  provide  for  abundant  resti- 
tution. They  must  absorb  from  as  large  an  area  as  possible,  and  enlarge  their  absorp- 
tive surfaces  adequately  by  pushing  out  the  cells  into  long  tubes.  For  this  reason 
all  plants  with  very  thin,  delicate,  expanded  foliage-leaves,  which  transpire  readily 
and  abundantly,  have  numerous  long  tubular  root-hairs.  Examples  are  afforded  by 
Viola  biflora  and  the  various  species  of  Impatiens.  On  the  other  hand,  plants  with 
stiff,  leathery  leaves,  being  protected  by  a  thick  epidermis  from  excessive  transpira- 
tion, as,  for  instance,  the  Date-palm,  exhibit  flat,  non-protuberant  absorptive  cells, 
because  there  is  a  very  limited  amount  of  evaporation  from  these  plants,  and  the 
quantity  of  water  to  be  absorbed  to  replace  what  is  lost  is  therefore  small.  The 
same  thing  holds  in  the  case  of  evergreen  Conifers,  in  which,  owing  to  the  structure 
of  the  stiff  needles  and  to  the  peculiar  formation  of  the  wood,  water  is  conducted 
very  slowly  from  the  roots  to  the  transpiring  green  organs.  It  has  been  ascertained 
that  they  exhale  from  six  to  ten  times  less  vapour  than  do  ashes,  birches,  maples, 
and  other  flat-leaved  trees  growing  on  the  same  ground. 

We  shall  presently  return  to  the  question  of  the  substitution  for  absorptive  cells 
in  many  coniferous  and  angiospermic  trees  and  in  evergreen  Daphnacece,  Ericacew, 
Pyrolacece,  Epacridece,  &c.,  of  the  mycelium  of  fungi,  and  shall  treat  also  of  the 
importance  of  the  form  of  the  absorptive  cells,  and  of  the  roots  which  bear  them, 
in  relation  to  the  mechanism  of  striking  root  in  the  ground. 


92  RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS. 


KELATIONS   OF  THE  POSITION  OF  FOLIAGE-LEAVES   TO  THAT  OF 

ABSORBENT   BOOTS. 

Anyone  who  has  ever  taken  refuge  from  a  sudden  shower  under  a  tree  will 
remember  that  the  canopy  of  foliage  afforded  protection  for  a  considerable  time,  and 
that  the  ground  underneath  was  either  not  wet  at  all,  or  only  slightly  so.  No  doubt 
some  of  the  rain  flows  down  the  bark  of  the  trunk,  and  in  many  species,  as,  for 
instance,  the  Yew  and  the  Plane-tree,  the  volume  of  water  conducted  down  the 
trunk  is  considerable;  but  in  the  case  of  most  trees  the  rain-water  which  reaches 
the  earth  in  this  manner  is  not  abundant,  and  in  comparison  with  that  which  drips 
from  the  peripheral  parts  of  the  foliage  its  quantity  is  negligeable.  This  phenome- 
non is  dependent  upon  the  position  of  the  foliage-leaves  relatively  to  the  horizon. 
In  almost  all  our  foliage-trees — in  limes  and  birches,  apple  and  pear  trees,  planes 
and  maples,  ashes,  horse-chestnuts,  poplars,  and  alders — these  organs  slope  out- 
wards, and  are  so  placed  one  above  the  other  that  rain  falling  upon  a  leaf  uii  one 
of  the  highest  branches  flows  along  the  slanting  surface  to  the  apex,  collects  there 
in  drops,  and  then  falls  on  to  a  lower  leaf  whose  surface  is  also  inclined  outwards. 
Here  it  coalesces  with  the  water  fallen  directly  upon  this  leaf;  and  so  it  goes  from 
one  tier  to  another,  lower  and  lower,  and  at  the  same  time  further  and  further 
from  the  axis,  till  a  number  of  little  cascades  are  formed  all  round  the  tree.  From 
the  under  and  outermost  leaves  of  the  entire  mass  of  foliage  the  water  falls  in 
great  drops  to  the  ground,  and  after  every  shower  of  rain  the  dry  area  at  the 
foot  of  the  tree  is  surrounded  by  a  circular  zone  of  very  wet  earth,  It  is  only 
necessary  to  dig  at  these  places  to  convince  one's  self  that  the  tree's  absorptive  roots 
penetrate  the  earth  precisely  to  the  wet  zone.  When  a  tree  is  young,  its  roots  lie 
in  a  small  circle,  and  the  crown  too  is  not  extensive,  so  that  the  damp  zone  is 
proportionately  restricted.  But  as  the  latter  is  enlarged  there  is  a  corresponding 
elongation  of  the  roots  in  their  search  for  moisture,  and  thus  roots  and  foliage 
progress  pari  passu  in  peripheral  increase.  It  seems  not  improbable  that  the 
custom  amongst  gardeners  and  foresters  of  trimming  the  foliage  and  roots  of  trees 
when  the  latter  are  transplanted  is  to  be  attributed  to  the  phenomenon  above 
described.  For  the  rule  is  observed  that  the  branches  of  the  trunk  and  those  of  the 
root  must  be  about  equally  shortened,  and  accordingly  the  suction  -  roots,  as  they 
develop,  reach  the  zone  of  drip  of  the  growing  crown. 

A  similar  method  of  carrying  off  water  is  to  be  observed  in  coniferous  trees. 
Take,  for  example,  the  Common  Pine.  The  lateral  branches  are  horizontal  near 
the  main  trunk;  the  secondary  branches  curve  upwards  like  bows  The  needles 
near  the  tip  of  each  of  the  latter  slant  obliquely  upwards  from  the  axis,  whilst 
the  older  needles,  situated  on  the  under  side  of  the  part  of  the  branch  which  is 
almost  horizontal  and  at  some  distance  from  its  extremity,  are  directed  obliquely 
downwards  and  outwards.  Rain-drops  striking  the  upturned  needles  glide  down 
them  to  the  bark  of  the  branch  in  question,  and  thence  to  other  needles  whose 


RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS.  93 

inclination  is  downwards  and  outwards.  On  their  apices  great  drops  are  gradually 
formed,  which  finally  detach  themselves  and  fall  on  to  the  mass  of  needles  be- 
longing to  a  lower  branch.  Thus  transmitted,  the  rain-water  travels  through 
the  foliage  lower  and  lower  and  at  the  same  time  further  from  the  axis.  This 
is  also  the  case  with  larches.  The  drops  of  rain  which  fall  upon  the  erect  needles 
of  the  tufted  "short  branches"  collect  and  gradually  descend  to  the  needles  of 
the  drooping  "long  branches"  on  lower  boughs.  Large  drops  are  always  to  be 
seen  on  their  drooping  apices,  whence  they  drip  to  the  earth.  Owing  to  the 
pyramidal  form  of  larches,  and  to  the  circumstance  that  the  long  shoots  on  each 
branch  are  terminal,  almost  all  the  water  which  falls  upon  one  of  these  trees 
reaches  the  long  shoots  hanging  down  from  the  lowest  branches,  which  discharge 
most  of  all.  Although  larches  with  their  tender  needles  do  not  look  at  all  as 
though  they  would  be  any  protection  against  rain,  the  ground  underneath  them 
keeps  dry  nevertheless,  the  principal  part  of  the  water  falling  upon  them  being 
conducted  to  the  periphery.  Indeed,  the  larch  belongs  to  the  number  of  trees 
which  conduct  almost  all  the  rain  that  falls  upon  them  to  a  certain  distance  from 
the  axis  where  the  absorbent  roots  lie,  and  only  allow  a  little  to  trickle  down 
the  bark  of  the  main  trunk. 

Many  shrubs  and  perennial  herbs  also  transmit  the  water,  which  falls  on 
their  upturned  laminae,  to  parts  of  the  ground  where  their  absorbent  roots  are 
embedded;  or,  rather,  the  roots  send  forth  their  branches  bearing  absorptive  cells 
to  the  area  which  is  kept  moist  by  drippings  from  the  leaves.  Particularly  striking 
in  this  respect  are  the  species  of  the  two  genera  of  Aroids  CoLocasia  and 
Caladium.  A  specimen  of  the  latter  is  figured  below  (fig.  13 1).  If  one  digs 
about  individuals  of  this  genus  cultivated  on  open  ground,  one  invariably  finds 
that  the  tips  of  the  lateral  roots,  which  proceed  in  a  horizontal  direction  from 
the  bulbous  root-stock,  are  buried  under  the  point  of  the  great  leaves  which  slope 
obliquely  outwards.  We  must  not  omit  to  mention,  in  addition,  that  the  stalks 
of  leaves  which  conduct  the  rain  centrifugally  are  not  channelled  on  the  upper 
surface;  they  are  round,  and  comparable  to  wires  supporting  at  their  upper  extremities 
the  laminae  in  an  outward  and  downward  direction.  As  instances  we  may  quote 
the  Horse-chestnut,  Maple,  and  Lime,  and  many  shrubby,  suffruticose,  and 
herbaceous  plants,  such  as  Sparmannia,  Spircea,  Aruncius,  and  Corydalis,  and  also 
climbing  and  trailing  plants  (e.g.  Menispermum,  Banisteria,  Aristolochia,  Hoy  a, 
Zanonia,  and  Tropceolum).  Whenever  a  system  of  grooves  is  developed  on  the 
surface  of  an  outward  sloping  leaf,  the  channels  run  along  the  veins  and  terminate 
at  the  apex  of  the  leaf,  or  at  the  apices  of  the  leafs  lobes,  and  invariably  cause 
the  water  to  travel,  not  to  the  basal  part,  but  to  a  spot  on  the  margin  whence 
it  will  detach  itself  in  the  form  of  a  drop,  and  fall  upon  the  leaves  situated 
immediately  below  and  at  a  greater  distance  from  the  axis. 

A  striking  contrast  to  these  trees  and  shrubs,  climbing  and  trailing  plants, 
and  suffruticose  and  herbaceous  species,  with  their  absorptive  roots  lying  in  one 
plane,  and  usually  spreading  at  but  little  depth,  is  afforded  by  plants  which  possess 


94 


RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS. 


bulbs  or  short  root-stocks  with  deep-reaching  suction-roots,  and  those  which  have 
tap-roots  descending  vertically  in  continuation  of  the  main  stem,  and  whose  second- 
ary roots  are  short  and  travel  only  a  little  distance  from  their  places  of  origin. 
This  other  extreme  in  root-structure,  which  is  represented  in  fig.  13 2,  has  its 
counterpart  above-ground  in  the  form  and  direction  of  the  laminae  upon  which 
the  rain  falls.  In  all  these  plants  the  surfaces  of  the  leaves  are  not  directed 


^^^^ 

Fig.  13.— Centrifugal  and  Centripetal  Transmission  of  Water, 
i  By  a  Caladium.     *  By  a  Rhubarb  plant. 

outwards,  but  slope  obliquely  towards  the  central  axis.  Their  upper  sides,  more- 
over, are  concave  and  exhibit  a  system  of  grooves,  which  conveys  the  water  collected 
by  the  leaf  towards  the  stem,  and  therefore  also,  towards  the  tap-root  and  suction- 
roots.  The  leaves  of  bulbous  plants,  such  as  the  Hyacinth  and  Tulip,  all  stand 
up  obliquely,  and  their  upper  surfaces  are  concave  and  often  deeply  channelled. 
Along  the  grooves  the  rain  flows  centripetally  downwards,  and  so  directly  reaches 
the  part  of  the  earth  where  the  bulbs  and  suction-roots,  which  proceed  in  a  tuft 
from  underneath  the  bulbs,  are  situated.  The  young  leaves  of  Cannacese  and  of  the 
Lily-of-the-valley  are  coiled  up  like  a  trumpet;  and  rain,  falling  from  above 
upon  the  expanded  portion,  is  led  along  the  coiled  surface,  describing  a  helix  as 


RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS.  95 

it  goes,  to  the  earth  in  the  neighbourhood  of  the  absorptive  roots,  which  proceed 
from  the  short  root-stock.  When  the  leaves  of  plants  furnished  with  tap-roots 
are  arranged  in  whorls,  and  are  without  internodes,  and  the  rosette  rests  upon 
the  ground,  as  is  the  case  in  the  Mandrake,  the  Dandelion,  and  several  species  of 
Plantain  (M andr agora  ojficinalis,  Taraxacum  officinale,  Plantago  media),  there 
are  always  one  or  more  main  grooves  on  the  upper  surfaces  of  the  leaves,  and 
the  leaves  have  always  such  form  and  position  as  compel  the  rain  which  falls 
upon  them  to  flow  centripetally,  i.e.  towards  the  tap-root  growing  vertically 
beneath  the  centre.  Plants  with  petiolate  leaves,  which  conduct  rain  centri- 
petally, always  have  on  the  upper  side  of  each  leaf -stalk  an  obvious  groove,  the 
depth  of  which  is  frequently  increased  by  the  development  of  green  or  (in  many 
cases)  membranous  ridges  on  the  two  lateral  edges.  Grooves  of  this  kind  are 
to  be  seen  particularly  well  on  the  petioles  of  the  radical  leaves  of  the  Rhubarb 
(see  fig.  13 2 ),  Beet-root,  Funkias,  and  most  Violets. 

Far  more  complicated  in  structure  than  the  radical  leaves  just  described,  are 
cauline  leaves.  Leaves  proceeding  from  the  stem  high  above  the  ground,  and 
forming  receptacles  for  rain-water,  like  those  of  the  Rhubarb,  are  best  fitted  to 
preserve  their  proper  direction  when  they  have  no  stalks  and  the  base  fits  directly 
on  to  the  stem  or  passes  into  it.  Cup-shaped  laminae,  if  borne  on  long  erect 
petioles,  necessitate  a  great  expenditure  on  supporting-cells,  and  they  are,  there- 
fore, on  the  whole,  rare.  Of  the  plants  we  know,  only  certain  Stork's-bills, 
Pelargonium  zonale,  P.  heterogamumt  &c.,  afford  examples  of  cup-shaped,  cauline 
leaves  of  the  kind,  borne  on  long,  rigid  petioles.  In  most  cases,  therefore,  cauline 
leaves  which  conduct  water  centripetally  are  either  sessile  or  very  shortly  petiolate, 
have  their  bases  close  to  the  stem,  and  even  extend  their  edges  down  it  more  or 
less  in  the  form  of  wings  and  ridges,  or  surround  it  in  the  form  of  collars,  lobes, 
and  auricles,  as  in  the  case  of  so-called  amplexicaul  leaves. 

When  the  leaves  are  in  pairs  opposite  one  another  and  the  alternate  pairs  at 
right  angles,  an  arrangement  known  as  decussate,  the  surplus  water  is  usually 
conveyed  through  two  grooves,  which  run  down  the  intervening  piece  of  stem 
from  one  pair  of  leaves  to  the  next.  Each  of  these  grooves  begins  in  an  indenta- 
tion between  the  margins  of  the  bases  of  a  pair  of  leaves,  and  terminates  above  the 
midrib  of  one  of  the  leaves  belonging  to  the  next  pair.  Now,  water  trickling 
down  such  a  groove  falls  precisely  on  that  part  of  a  lower  leaf  where  the  rain 
retained  by  the  surface  of  that  leaf  is  collected;  and  so  the  stream  of  water 
becomes  more  and  more  copious  as  it  approaches  the  ground.  These  grooves 
may  be  seen  in  many  species  of  ringent  Labiatce,  Scrophulariacece,  Primulacece, 
Gentianacece,  Rubiacece,  and  Willow-herbs;  the  best-marked  instances  are  found 
in  the  Knotty  Fig-wort  (Scrophularia  nodosa),  the  Yellow-rattle  (Rhinanthus), 
the  meadow-gentians  (Gentiana  germanica,  Rhcetica,  &c.),  and  the  Centaury 
(Erythrcea).  The  grooves  always  possess  the  property  of  being  wetted  by  water, 
whereas  the  ungrooved  parts  of  the  same  stem  are  not  wetted.  Sometimes  the 
grooves  are  fringed  with  hairs  which  absorb  the  water  like  the  threads  of  a 


96  RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS. 

wick.  By  means  of  both  contrivances  advantage  is  ensured  in  that  the  water 
only  oozes  quite  gradually  down  the  moistened  grooves,  or  else  is  conducted  by 
the  hairy  fringes  to  the  base  of  the  stem,  and  does  not  rebound  at  any  spot  in 
the  form  of  drops.  Irregularly  bounding  drops  would  be  liable  to  fall  on  the 
ground  at  spots  where  no  absorptive  organs  awaited  them. 

In  cases  where  foliage-leaves,  adapted  to  a  centripetal  conduction  of  rain,  are 
arranged  upon  a  spiral  line  down  the  stem,  instead  of  in  pairs  opposite  one  another, 
the  water  leaks  away  along  the  spiral  from  one  leaf  to  the  next,  and  finally  to 
the  bottom.  Then,  again,  there  are  often  grooves  in  the  stem  along  which  the 
water  trickles,  as,  for  instance,  in  the  Common  Whortleberry  (  Vaccinium  Myrtillus). 
The  erect  leaves  of  this  plant  conduct  the  drops  as  they  fall  to  the  branches, 
which  are  deeply  furrowed.  The  water  travels  through  the  furrows  into  those 
of  lower  branches,  and  finally  along  those  of  the  main  stem  of  the  whole  bush 
down  to  the  earth.  In  Veratrum  album  each  of  the  concave  cauline  leaves  has, 
on  the  upper  surface,  a  number  of  deep  longitudinal  grooves,  which  all  discharge 
together  at  the  base  of  the  leaf.  The  water  collected  there  at  length  overflows 
and  runs  down  the  round  stem  in  no  particular  channel. 

The  descent  of  rain-water  along  a  spiral  line  may  be  very  clearly  traced  in 
many  plants  of  the  Thistle  tribe.  If  tiny  shot-grains  are  substituted  for  rain- 
drops in  a  stiff-leaved  plant,  the  course  designed  for  the  drops  in  that  particular 
species  may  be  followed  with  ease.  When  strewn  on  a  mature  plant  of  the 
Safflower  (Carthamus  tinctorius)  or  of  Alfredia  cernua  (fig.  14 ]),  the  grains 
of  shot  roll  down  the  somewhat  channelled  surface  of  the  highest  cauline  leaf, 
which  stands  up  obliquely,  and  dash  against  the  stem.  The  latter  is  half  encom- 
passed by  the  leaf-base,  and  the  shot  then  roll  over  one  of  the  basal  lobes  of 
the  leaf  and  travel  out  of  the  range  of  that  leaf,  falling  on  to  the  middle  of 
the  one  next  below.  For  the  amplexicaul  foliar  bases  are  so  placed  that  each 
leaf  has  one  of  its  basal  lobes  above  a  concave  part  of  the  next  lower  leaf.  In 
precisely  the  same  way  the  shot  descend  from  the  second  leaf  to  the  third,  and 
so  on  until  they  reach  the  earth  quite  close  to  the  stem.  The  descent  reminds 
one  of  the  game  in  which  a  little  ball  is  made  to  roll  along  a  spiral  groove  on 
to  a  board  furnished  with  numbered  holes.  Rain-drops  falling  upon  thistle-like 
plants  of  this  kind  naturally  follow  the  same  course  as  the  shot.  Only,  the 
additional  fact  must  be  taken  into  account  that  not  only  the  highest  but  all  the 
leaves  are  adapted  as  receptacles  for  the  rain  as  it  falls,  and  that  consequently 
the  drops  falling  from  leaf  to  leaf  are  augmented  by  new  tributaries,  and  become 
greater  and  greater  as  they  descend. 

A  somewhat  different  method  of  water-conduction  from  that  which  occurs  in 
the  Safflower  and  in  the  nodding  Alfredia  is  observed  in  the  Milk  Thistle 
(Silybum  Marianum),  in  the  Cotton  Thistle  (Onopordon),  and  in  the  Mullein 
(Verbascum  phlomoides).  The  upper  leaves,  which  have  two  semi-amplexicaul 
lobes,  are  as  nearly  erect  as  those  of  the  Safflower  and  the  nodding  Alfredia, 
and  lead  the  rain  off  in  exactly  the  same  way.  But  the  leaves  in  the  middl 


RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS.  97 

of  the  stem  are  only  erect  for  about  three-quarters  of  their  length;  the  upper- 
most third,  including  the  apex,  is  bent  obliquely  outwards  and  downwards.  Drops 
of  rain  falling  on  this  upper  third  of  a  leaf  would  flow  in  a  centrifugal  direction, 
and  do,  as  a  matter  of  fact,  drip  down  from  the  apex.  Now  the  leaves  in  all 


Fig.  14— Irrigation  of  Rain-water, 
i  In  Atfredia  cernua.    a  In  a  Mullein  (  Verbascum  phlomoides). 

these  plants  are  shorter  the  higher  their  position  upon  the  stem,  so  that  the  total 
contour  of  the  plant  may  be  described  as  a  slender  pyramid.  In  consequence  of 
this,  water  dropping  from  the  outward-bent  and  drooping  apices  of  superior  leaves 
is  arrested  by  that  part  of  an  inferior  leaf  which  shelves  towards  the  stem,  and  is 
thereby  conducted  centripetally.  Thus  all  the  rain-water  received  by  a  plant 
of  this  kind  at  last  reaches  the  immediate  neighbourhood  of  the  tap-root,  and  is 

VOL.  I. 


98  RELATIONS   OF   FOLIAGE-LEAVES   TO   ABSORBENT   ROOTS. 

a  source  of  nutriment  to  the  absorption -roots  which  proceed  from  it.  In  the 
Milk  Thistle  (Silybum  Marianum)  the  margins  of  the  cauline  leaves  are  very 
much  waved,  and,  in  consequence  of  this  undulation,  three  or  four  depressions  exist 
on  each  side,  through  which  part  of  the  rain,  when  there  is  a  heavy  downpour, 
flows  off  sideways.  But  even  this  water,  falling  laterally,  drops  upon  parts  of 
lower  leaves,  which  conduct  centripetally,  and  so  coalesces  with  the  streamlets 
otherwise  produced. 

It  is  very  rare  for  plants  which  convey  water  centripetally  to  have  their  leaves 
arranged  in  two  rows.  The  most  striking  example  of  this  class  is  the  Japanese 
Tricyrtis  pilosa.  Its  leaves  are  situated  on  the  fully -developed  stem  very  regularly, 
one  above  the  other,  in  two  series.  Each  leaf  has  two  lobes  embracing  the  stem, 
but  the  base  is  fixed  somewhat  obliquely,  so  that  one  of  the  lobes  is  fixed  higher 
than  the  other.  Moreover,  the  higher  lobe  is  closely  adpressed  to  the  stem,  whilst 
the  lower  forms  a  channel  which  discharges  exactly  above  the  concave  surface  of 
the  next  lower  leaf  belonging  to  the  other  side.  When  rain  falls  on  this  plant, 
the  water,  collected  by  one  leaf,  flows  through  the  broad  exit-channel  on  to  the 
leaf  below  on  the  other  side.  Thence  a  somewhat  augmented  stream  falls  upon 
a  leaf  of  the  first  series,  and  so  on,  a  peculiar  cascade  resulting,  which  falls  in 
a  zigzag,  from  leaf  to  leaf,  until  it  reaches  the  bottom,  close  to  the  stem. 

It  would,  however,  be  wrong  to  suppose  that  the  above  explanation  sets  forth 
the  only  significance  to  be  assigned  to  the  various  arrangements  described.  To 
many  plants  it  is  a  matter  of  indifference  in  what  direction  rain-water  falls  from 
the  leaves.  Such,  for  instance,  is  the  case  with  all  marsh -plants  with  roots 
buried  in  mud  under  water,  inasmuch  as  the  rain,  as  it  drops,  only  goes  into  the 
water  in  the  pond  or  marsh,  and  could  not  be  conveyed  to  a  definite  spot  for 
the  sake  of  the  absorbent  roots.  In  the  Water-plantain,  the  Flowering-rush,  and 
the  Arrow-head  (Alisma,  Butomus,  Sagittaria),  accordingly,  no  relationship  between 
the  form  and  direction  of  the  leaves  and  the  position  of  the  absorbent  roots  is 
to  be  discovered. 

On  the  other  hand,  in  arundinaceous  plants  (Arundo,  Phragmites,  Phalaris) 
an  arrangement  has  been  hit  upon  which  is  obviously  designed  to  prevent  rain- 
water from  collecting  between  the  haulm  and  the  leaf.  As  is  the  general  rule 
with  grasses,  so  also  in  the  above-named  kinds  of  reeds,  the  stem  or  haulm  is 
furnished  with  nodes,  and  from  each  node  proceeds  a  leaf  the  lower  part  of  which 
encases  the  haulm  in  the  form  of  a  tube  or  sheath,  whilst  the  upper  part  is  expanded 
and  presents  a  flat,  strap-shaped  or  concave  surface,  standing  well  away  from 
the  stem.  The  leaves  may  be  folded  round  the  haulm  like  banners.  At  the  place 
where  the  sheath  passes  into  the  part  of  the  leaf  which  stands  away  from  the 
axis  at  an  obtuse  angle,  one  observes  on  the  edge  of  the  leaf  close  to  the  angle, 
two  distinct  depressions  which  represent  conduits  and  convey  part  of  the  rain  from 
the  lamina.  There  is  also  a  very  neat  contrivance  here  in  the  form  of  an  erect  dry 
membrane  which  acts  as  a  dam,  the  so-called  "  ligule."  This  membrane,  inserted 
upon  the  leaf -sheath,  is,  like  the  sheath,  in  contact  with  the  haulm.  When  rain- 


SAPROPHYTES   AND   THEIR    RELATION   TO   DECAYING   BODIES.  99 

water  flows  down  to  this  place  it  is  stemmed  by  the  membrane,  as  by  a  dam,  and 
diverted  right  and  left  into  the  two  grooves.  In  this  way  water  is  prevented 
from  accumulating  between  the  leaf -sheath  and  haulm,  where  it  might  do  damage. 
In  many  reeds  the  contrivances  for  irrigation  are  even  more  complete  than  this. 
Sometimes  hairs  depend  from  the  margin  of  the  membrane  in  the  direction  of  the 
grooves  and,  like  a  wick,  lead  the  water  in  the  proper  direction. 

An  opportunity  will  occur  later  on  of  showing  how  the  conduction  of  rain  to 
particular  spots  has  an  important  bearing  on  the  phenomenon  of  absorption  by 
aerial  parts  of  plants:  and  also  in  the  regulation  of  transpiration;  and  how,  by 
means  of  the  apparatus  for  water-irrigation,  not  only  absorptive  cells  at  the 
extremities  of  roots  in  the  earth,  but  special  organs  on  the  foliage-leaves  as  well, 
are  often  supplied  with  water. 


3.  ABSORPTION  OF  ORGANIC  MATTER  FROM  DECAYING 
PLANTS  AND  ANIMALS. 

Saprophytes  and  their  relation  to  decaying  bodies. — Saprophytes  in  water,  on  the  bark  of  trees,  and 
on  rocks. — Saprophytes  in  the  humus  of  woods,  meadows,  and  moors. — Special  relations  between 
Saprophytes  and  the  nutrient  substratum. — Plants  with  traps  or  pitfalls  for  animals. — Insecti- 
vorous plants  which  perform  movements  for  the  capture  of  prey. — Insectivorous  plants  with 
adhesive  apparatus. 

SAPBOPHYTES  AND  THEIE  KELATION  TO  DECAYING  BODIES. 

Whenever  plants  which  take  up  organic  compounds  formed  in  the  process  of 
decay  are  the  subject  of  discussion,  the  first  examples  that  occur  to  everyone  are 
members  of  the  great  family  of  Fungi,  specimens  of  which  make  their  appearance 
wherever  dead  animals  or  plants  are  undergoing  decomposition.  We  recall  the 
moulds,  plasmodia,  puff-balls,  and  mushrooms,  which  grow  from  dead  organic  bodies, 
and  are  associated  with  the  unpleasant  mouldy  and  cadaverous  smell  always 
perceptible  in  their  neighbourhood. 

Many  of  these  organisms  do,  in  fact,  belong  to  the  class  of  Saprophytes.  Indeed, 
one  group  of  them  is  itself  the  cause  of  the  chemical  decomposition  of  dead  plants 
and  animals  called  decay.  Their  elongated  thin- walled  cells,  the  so-called  "hyphae", 
thread  themselves  through  dead  bodies,  and  unite  to  form  strands,  bundles,  net- 
works, and  membranes,  the  whole  constituting  a  structure  to  which  the  term 
"mycelium"  is  applied.  These  mycelia  are  often  to  be  seen,  with  the  naked  eye, 
covering  large  areas.  For  instance,  in  damp  cellars,  mines,  and  railway-tunnels,  any 
old  rotten  wood-work  is  clothed  with  delicate,  whitish  reticula  and  membranes. 
The  heaps  of  grape-skins,  stalks,  and  other  refuse  piled  up  in  the  open  air  by  the 
side  of  vineyards  after  a  vintage,  are  usually  so  completely  overgrown  by  mycelia 


100         SAPROPHYTES  AND  THEIR  RELATION  TO  DECAYING  BODIES. 

that  their  colour  is  quite  altered.  The  so-called  "mushroom-spawn",  used  in  the 
cultivation  of  mushrooms,  is  also  nothing  but  a  mycelium,  which  entirely  invests 
the  manure  employed  in  the  cultivation  of  that  fungus,  and  gives  it  a  white 
mottled  appearance. 

In  addition  to  Fungi,  however,  a  number  of  Mosses,  Liverworts,  Ferns,  Lycopods, 
and  Phanerogams  take  up  organic  compounds  from  the  products  of  decay  to  serve 
as  their  food. 

In  deciding  whether  a  plant  takes  up  only  the  mineral  substances  rendered 
soluble  by  the  decomposition  of  the  soil,  or  only  organic  substances  disengaged 
by  the  decay  of  dead  plants  and  animals,  we  depend  generally  on  the  condition 
and  appearance  of  the  nutrient  substratum,  and,  in  particular,  on  its  composition, 
i.e.  whether  it  is  exclusively  or  predominantly  organic.  But  such  observations 
give  a  very  uncertain  indication.  For,  on  the  one  hand,  it  is  possible  for  plants 
rooted  in  a  substratum  of  decaying  matter  to  take  nothing  but  mineral  salts  (i.e. 
inorganic  compounds)  from  it;  and,  on  the  other  hand,  it  frequently  happens  that 
sand  or  clay,  apparently  uncontaminated  with  organic  matter,  is  saturated  by 
water  which  oozes  from  a  layer  of  humus  in  the  vicinity,  and  brings  with  it 
organic  compounds  in  solution.  The  following  facts  are  instructive  with  reference 
to  the  former  of  these  two  phenomena.  Maize,  barley,  and  other  cereals  may  be 
reared  in  fluids,  so  prepared  as  to  contain  a  small  quantity  of  mineral  food-salts 
dissolved  in  distilled  water  (12  mg.  potassium  phosphate,  12  mg.  sodium  phosphate, 
27  mg.  calcium  chloride,  40  mg.  potassium  chloride,  20  mg.  magnesium  sulphate, 
10  mg.  ammonium  sulphate,  and  a  few  drops  of  iron  chloride  in  a  litre  of  distilled 
water),  all  organic  compounds  being  carefully  excluded.  When  the  plants  germinate, 
they  develop  roots  which  descend  in  the  liquid  and  absorb  from  it  mineral  salts 
according  to  their  requirements.  They  produce  stems,  leaves,  flowers,  and,  ulti- 
mately, seeds  capable  of  germination.  Other  plants  of  maize  or  barley  reared 
simultaneously  in  richly-manured  ground  develop  likewise  leaves,  flowers,  and  fruit. 
Moreover,  analysis  of  the  ash  in  both  cases  reveals  the  fact  that  the  plants  which 
took  their  nutriment  from  the  manure  contain  the  same  salts  as  those  reared  in  the 
made-up  solution  of  salts  free  from  organic  compounds.  Hence,  the  conclusion  may 
be  drawn  that  a  plant  of  this  kind  is  capable  of  obtaining  an  adequate  supply  of 
food-salts  equally  well,  either  from  earth  free  from  humus  and  manure,  or  from 
humus  or  manure  themselves.  The  experiment  further  shows  that,  in  the  latter 
case,  organic  compounds  need  not  necessarily  be  absorbed,  in  addition  to  the  mineral 
constituents  of  humus  or  manure  which  are  disengaged  during  decomposition. 

We  must  next  refer  to  a  fact  in  connection  with  the  second  point  above  men- 
tioned, viz.  that  plants  rooted  in  sand  or  loam  devoid  of  humus  may  yet  hav( 
organic  compounds  brought  to  them  by  water  filtering  through  a  stratum  of  humus 
near  at  hand.  The  fact  in  question  is,  that  the  very  water  which  one  would  least 
expect  should  contain  organic  compounds,  that,  for  instance,  of  cold  mountain 
streams,  does  very  generally  include  traces  of  such  compounds.  On  looking  through 
analyses  of  mineral  springs,  one  finds  for  the  most  part,  amongst  their  constituents, 


SAPROPHYTES  AND  THEIR  RELATION   TO   DECAYING  BODIES.  101 

combustible  bodies  arising  from  the  dissolution  of  organic  matter.  Even  the  acid 
formerly  designated  by  Berzelius  by  the  name  of  "spring-acid",  is  doubtless  a  pro- 
duct of  the  decay  of  fragments  of  plants  in  the  place  where  the  water  of  the  spring 
collects.  So  also  is  humic  acid,  a  compound  produced  by  decay.  The  nature  of 
this  acid  is  not  yet,  it  is  true,  thoroughly  known,  and  it  may  be  a  mixture  of  several 
acids.  We  know,  however,  that  it  is  easily  soluble  in  water,  and  that  it  forms 
soluble  compounds  with  alkalies.  Brooks  running  through  woods  or  meadows, 
small  mountain  lakes  adjoining  peat-beds,  and  pools  in  actual  peat,  consist  of 
water,  brown  in  colour,  which  gives  an  acid  reaction,  and  contains  invariably 
organic  substances  in  solution. 

The  following  observations  are  of  great  interest  in  connection  with  this  subject. 
In  the  salt-mine  at  Hallstatt  (Upper  Austria)  one  of  the  galleries,  which  is  hewn 
through  rock  and  contains  no  wood-work  of  any  kind,  exhibited  (spread  out  upon 
its  smooth  limestone  roof)  the  mycelium  of  a  fungus  (an  Omphalia),  which 
certainly  required  organic  nutriment.  There  were  no  decaying  animal  or  vegetable 
remains  anywhere  in  the  gallery,  and  the  mycelium  derived  nourishment  solely 
from  water  oozing  from  above  through  a  few  narrow  cracks  in  the  stone  whereby 
the  surface  of  the  latter  was  kept  moist.  This  water  came  from  a  meadow  lying 
high  above  the  mine.  Between  the  two  was  a  thick  stratum  of  limestone  with 
a  deep  layer  of  earth  resting  upon  it.  The  water  was  clear  and  colourless,  and 
contained  a  certain  amount  of  lime,  but  no  perceptible  trace  of  organic  substances. 
Yet  this  water  must  have  brought  organic  matter  from  the  meadow  above  into  the 
mine,  and  the  minute  quantity  so  introduced  sufficed  to  enable  the  fungus  mycelium 
to  grow  luxuriantly. 

In  the  Volderthal,  near  Hall,  in  Tyrol,  there  is  a  spring  of  cold  clear  water 
rising  out  of  slate  at  a  height  of  1000  metres  above  the  sea-level,  which  is  filled 
at  its  source  with  a  dark  thick  felt.  The  felt  may  be  lifted  out  in  pieces  the  size 
of  one's  hand,  and  it  is  the  mycelium  of  a  fungus,  probably  a  Peziza.  It  clings 
to  slabs  of  slate,  between  which  the  water  trickles  abundantly,  and  its  nutriment 
can  only  be  derived  from  this  water.  There  are  pine- woods  and  meadows  in 
the  neighbourhood,  but  no  greater  amount  of  vegetation,  humus,  or  rotten  timber 
than  is  found  near  other  springs. 

These  instances  satisfactorily  prove  that  even  the  clearest  mountain  springs 
contain  organic  substances  in  quantities  sufficient,  however  minute,  to  nourish 
fungi.  When  the  origin  of  springs  is  taken  into  account,  this  result  is  not  really 
surprising.  They  are  fed  by  deposition  from  the  atmosphere.  The  water  thus 
deposited  percolates  into  the  ground,  passing,  in  the  first  place,  through  a  layer  of 
earth-mould  which  is  covered  by  vegetation,  and  contains  more  or  less  humus  in  its 
upper  strata.  A  small  quantity  of  the  products  of  decay  is  inevitably  absorbed, 
and  even  if  they  are  partially  withdrawn  again  in  lower  strata  of  the  earth,  traces 
are  still  retained  by  the  water  in  its  descent  to  greater  depths,  and  re-ascent  to  the 
surface  in  the  form  of  springs.  The  characteristics  of  the  great  veins  of  water 
which  ascend  in  this  way  are  no  doubt  common  to  the  smaller  veins  which  originate 


102  SAPROPHYTES   AND  THEIR   RELATION   TO   DECAYING   BODIES. 

in  the  vegetable  mould  saturated  by  snow  and  rain  on  the  ground  of  forests  or  in 
the  humus  covering  meadows,  and  which  percolate  through  into  the  sand  or  loam 
beneath.  Plants  whose  roots  ramify  in  this  deeper  layer  of  earth  derive  thence  the 
organic  compounds  conveyed  by  the  water,  and  have  the  additional  advantage  of 
being  able  to  satisfy  at  the  same  time  their  requirements  as  regards  mineral  sub- 
stances. This  circumstance  is  of  importance  not  only  to  flowering-plants  but  also 
to  many  fungi,  as,  for  instance,  to  all  species  of  Phallus,  they  having  need  of  a 
great  deal  of  lime.  An  explanation  is  thus  afforded  of  the  fact,  formerly  difficult  to 
understand,  that  in  forests  and  meadows  not  only  the  upper  black  or  brown  humus 
layer,  but  also  the  underlying  yellow  loam,  or  pale  sand,  neither  of  which  latter 
contains  any  humus,  has  mycelia  of  fungi  running  through  it  in  every  direction, 
and  weaving  their  threads  over  little  fragments  of  rock.  Indeed,  it  sometimes 
happens  that  the  lower  layer  of  earth  is  more  abundantly  penetrated  with  plexuses 
of  hyphse  than  is  the  upper  layer,  consisting  of  vegetable  mould.  The  greatest 
number  of  saprophytes  is  to  be  found  therefore  at  places  where  the  humus  layer  is 
not  too  thick  and  loam  or  sand  occurs  at  no  great  depth;  but  where  decaying 
vegetable  remains  are  piled  metres  high,  as  on  moors,  for  example,  instead  of  fungi 
being  produced  in  extraordinary  abundance,  as  one  might  expect,  only  a  few  occur. 
Pure  peat  is  by  no  means  a  favourable  soil  for  fungi,  a  circumstance  which  may  be 
partly  due  to  the  antiseptic  action  of  certain  compounds  developed  in  it. 

It  follows  from  the  foregoing  observations  that  a  sure  conclusion  as  to  the 
nature  of  plants  rooted  in  a  particular  substratum  cannot  possibly  be  derived  from 
the  mere  appearance  of  the  substratum.  Moreover,  the  conditions  necessary  for  the 
growth  of  plants  requiring  organic  products  of  decay  as  nutriment  appear  to  be  of 
much  wider  occurrence  than  one  would  suppose  upon  a  cursory  observation  of  the 
conditions  existing  in  fields  and  forests,  or,  if  one  considers  exclusively  instances  of 
cultivated  plants  reared  on  arable  land,  which  is  manured  and  constantly  turned 
over.  The  great  variety  of  plants  produced  on  a  limited  area  is  also  now 
intelligible.  From  the  same  soil  some  absorb  organic  compounds,  others  mineral 
substances  only;  whilst  others  again  take  some  organic  and  some  mineral  food- 
salts.  The  determining  factor  is  not  the  amount  of  a  given  substance  present 
in  the  substratum,  but  rather  the  special  needs  of  each  species,  and  ultimately  the 
specific  constitution  of  the  protoplasm  in  each  one  of  the  plants  which  thus,  side  by 
side,  nourish  themselves  in  totally  different  ways. 

If,  then,  neither  the  appearance  of  the  ground  nor  its  richness  in  respect  of 
humus  affords  any  certain  indication  as  to  whether  a  particular  plant  lives  on 
organic  products  of  decay  or  not,  the  question  may  perhaps  be  solved  by  the  fact 
of  the  plant's  containing  or  not  containing  green  chlorophyll-corpuscles.  We  may 
take  it  as  proved  by  many  results  of  investigation,  that  the  decomposition  of  the 
carbon-dioxide  absorbed  by  a  plant  from  the  air,  and  the  formation  of  the  organic 
compounds  of  carbon,  hydrogen,  and  oxygen  known  as  carbohydrates  (which  play 
so  important  a  part  in  vegetable  economy),  only  take  place  in  organs  possessing  the 
green  pigment  known  as  chlorophyll.  We  shall  return  to  a  discussion  of  these 


SAPROPHYTES   AND   THEIR   RELATION   TO   DECAYING   BODIES.  103 

processes  in  detail  later  on,  but  the  fact  must  be  taken  into  consideration  here. 
One  would  suppose,  accordingly,  that  plants  able  to  obtain  ready-made  organic 
compounds  from  a  nutrient  substratum  could  spare  themselves  the  trouble  of 
building  them  up,  so  that  the  presence  of  chlorophyll  would  be  superfluous. 
This  conjecture  is  in  fact  supported  by  the  absence  of  chlorophyll  in  fungi,  which 
are  typical  instances  of  saprophytes.  But,  on  the  other  hand,  some  plants  appear 
to  negative  this  assumption,  or  at  any  rate  to  deprive  it  of  general  application. 
In  mountain  districts,  where  cattle  continually  pass  to  and  from  the  meadows  and 
alps,  one  notices  on  their  halting  grounds,  and  along  their  tracks,  moss  of  a  con- 
spicuous green  colour  growing  on  circumscribed  spots.  On  closer  examination  we 
find  that  we  have  here  an  example  of  the  remarkable  group  of  the  SplachnacecB, 
and  that  it  has  selected  the  cow-dung  to  be  its  nutrient  substratum.  Each  growth 
of  emerald  green,  Splachnum  ampullaceum,  is  strictly  limited  to  the  area  of  a 
lump  of  dung;  no  trace  of  it  is  to  be  seen  elsewhere.  All  the  stages  of  development 
of  this  moss  follow  one  another  upon  the  same  substratum.  First  of  all  the  lumps 
of  dirt  which  are  kept  moist  by  rain  or  by  standing  water,  become  enveloped  in 
a  web  of  protonemae,  and  their  surfaces  acquire  thereby  a  characteristic  greenish 
lustre.  Later,  hundreds  of  little  green  stems,  thickly  clothed  with  leaves,  emerge, 
and  the  spore-cases,  which  resemble  tiny  antique  jars,  and  are  amongst  the 
prettiest  exhibited  by  the  world  of  mosses,  become  visible  as  well.  Just  as 
Splachnum  ampullaceum  is  produced  on  the  dung  of  cattle,  so  is  Tetraplodon 
angustatus  on  that  of  carnivorous  animals,  and  there  can  be  no  doubt  that 
these,  and  in  general  all  Splachnacece,  are  true  saprophytes-  A  similar  remark 
holds  with  regard  to  the  green  Euglence  which  escape  from  Hormidium-cells,  and 
fill  the  foul-smelling  liquor  in  dung-pits  and  puddles  near  cattle-stalls  in  mountain 
villages,  and  which  multiply  to  such  an  extent  that  in  a  few  days  the  liquid 
changes  colour  from  brown  to  green. 

Thus  plants  do  exist  containing  chlorophyll  although  absorbing  from  the 
substratum  organic  compounds  alone,  and  containing  it,  indeed,  in  such  quantities 
that  its  presence  cannot  be  looked  upon  as  accidental.  It  follows,  firstly,  that 
absence  of  chlorophyll  is  not  the  distinguishing  mark  of  saprophytic  plants;  and, 
secondly,  that  the  organic  nutriment  of  the  plants  above  mentioned  cannot  be  used 
forthwith  unaltered  in  the  building  up  and  extension  of  their  structures,  but,  like 
inorganic  material,  must  undergo  various  changes,  that  is,  must  be  to  a  certain 
extent  digested  before  being  used  for  construction.  The  probability  is  that  green 
saprophytes  take  carbon  from  their  substratum  in  a  form  unfitted  for  the  manu- 
facture of  cellulose  and  other  carbohydrates.  Saprophytes  that  are  not  green 
must  obtain  carbon  from  the  substratum  in  the  form  of  a  compound,  the  direct 
absorption  of  which  could  be  dispensed  with  if  chlorophyll  were  present;  but  it 
does  not  necessarily  follow  that  all  the  organic  compounds  absorbed  by  non-green 
saprophytes  are  capable  of  immediate  service  as  materials  for  construction  without 
any  preliminary  alteration. 

Impartial  consideration  of  the  above  facts  forces  us  to  conclude  that  there  is  no 


104  SAPROPHYTES   IN   WATER,  ON   THE   BARK   OF   TREES,  AND   ON   ROCKS. 

well-marked  boundary  line  between  plants  which  absorb  organic  compounds  and 
those  which  absorb  inorganic  compounds  from  their  respective  substrata;  and  that 
there  undoubtedly  exist  plants  capable  of  taking  up  both  kinds  of  material  at  the 
same  time.  This  conviction  is  strengthened  still  further  by  the  circumstance, 
which  has  been  repeatedly  confirmed  by  experiment,  that  plants  susceptible  of 
being  successfully  reared  in  artificial  solutions  of  mineral  salts — to  the  exclusion 
of  organic  compounds — do  not  entirely  reject  organic  compounds  when  the  latter 
are  tendered  to  them,  but  unquestionably  assimilate  some  of  them  (urea,  uric  acid, 
glycocoll,  &c.)  and  work  them  up  into  constituents  of  their  own  frames. 

But,  in  spite  of  the  impossibility  of  drawing  a  sharp  line  of  demarcation 
between  the  two  groups,  it  is  convenient  to  treat  of  the  absorption  of  organic 
compounds  separately,  because  this  division  of  the  subject  affords  the  best 
opportunity  of  inspecting  in  detail,  and  of  surveying  generally,  the  conditions  of 
food-absorption,  the  comprehension  of  which  is  otherwise  difficult.  In  order  to 
determine  in  each  individual  case  whether  a  given  plant  lives  either  exclusively 
or  principally  upon  organic  food,  derived  from  decaying  animal  or  vegetable 
remains,  reliance  must  be  placed  on  experiments  with  cultures;  and,  in  the  absence 
of  better  vantage-ground,  the  results  of  the  rougher  experiments  made  by 
gardeners  should  not  be  neglected,  always  providing  that  they  are  accepted 
subject  to  possible  correction  by  subsequent  exact  experiment. 

SAPROPHYTES  IN  WATER,   ON  THE  BAEK  OF  TREES,   AND   ON  ROCKS. 

Of  the  special  cases  of  absorption  of  organic  compounds  from  decaying  bodies, 
we  have  first  of  all  to  consider  those  occurring  amongst  water-plants.  In  the  sea, 
wherever  there  is  an  abundance  of  animal  and  vegetable  life  there  is  also  plenty 
of  refuse,  for  there  death  and  decay  hold  a  rich  harvest.  The  quantity  of  organic 
matter  dissolved  in  the  water  is  naturally  greater  in  these  places  than  where 
vegetation  and  animal  life  are  less  conspicuous.  There  is  a  much  more  varied 
flora  and  fauna  to  be  met  with  in  the  sea  near  its  coasts,  especially  in  shallow 
inlets,  than  at  a  greater  distance  from  the  shore;  and  the  number  of  dead  organisms 
is  also  greater  near  the  coast.  A  mass  of  organic  remains  is  thrown  up  by  the 
tide,  and  by  waves  in  stormy  weather.  This  mass  rots  during  the  ebb.  Part  of  it 
is  dragged  out  to  sea  again  by  the  next  high  tide,  and  then  flung  up  once  more; 
so  that  the  beach  is  always  strewn  with  dead  remains,  and  the  sea  near  the  shore 
contains  more  products  of  decomposition  than  in  the  open. 

In  the  immediate  neighbourhood  of  seaports,  moreover,  or  wherever  people 
live,  the  volume  of  refuse  is  considerably  increased,  and  the  water  in  harbours  and 
stagnant  inlets  behind  breakwaters,  and  at  the  mouths  of  canals  and  sewers,  contains 
such  a  large  quantity  of  organic  refuse  in  a  state  of  decomposition  that  its  presence 
is  revealed  by  the  odour  emitted.  Now  it  is  just  at  these  places  that  an  abundant 
vegetation  of  hydrophytes  is  developed.  Not  only  the  bottom  of  shallows,  but 
stones,  stakes,  quays,  buoys,  and  even  the  keels  and  planks  of  boats  long  anchored 


SAPROPHYTES   IN   WATER,  ON   THE   BARK   OF   TREES,  AND   ON   ROCKS.  105 

in  harbour  are  overgrown  by  Ulvce,  wracks,  filamentous  alg»,  and  Florid**.  Not 
a  few,  as,  for  instance,  the  so-called  sea-lettuce  ( Ulva  lactuca),  several  species  of 
Qehd^m,  Bangia,  and  Ceramic,  and  the  great  Cystosira  barbata,  thrive  best 
and  m  greatest  abundance  in  polluted  water  of  the  kind;  and  there  can  be  no 
doubt  that  this  is  to  be  accounted  for  by  the  presence  of  a  greater  quantity  of 
organic  compounds  in  that  water. 

It  is  not  only  in  contaminated  sea-water,  but  also  in  other  collections  of  water 
which  contain  products  of  putrefaction  in  solution,  that  we  find  a  characteristic 
vegetation.     We  have  already  alluded  to  the  presence  of  Euglence  in  the  liquor 
of   manure-pits.      They   occur  also   at   the   foot   of   shady   walls,   in   dirty   back 
streets  in  towns,  in  the  puddles,  and  on  ground  which  is  saturated  with  urine  and 
impurities  of  every  kind.     These  places  are  the  home  of  a  number  of  other  minute 
plants,  which  stain  the  polluted  ground  after  rain  with  the  gayest  colours      There 
ide  by  side  with  black  patches  of  Oscillaria  antliaria  and  verdigris-coloured  films 
Oscillaria  tenuis,  are  blood-red  patches  of  Palmetto,  cruenta,  and  brick-red 
patches  of  Chroococcus  cinnamomeus.     Equally  characteristic  is  the  vegetation 
which  covers  the  earth  at  the  mouths  of  drains,  and  is  bathed  by  the  trickling 
sewage.     Large  areas  here  are  overgrown  by  the  green  Hormidium  murale,  which 
weaves  itself  over  the  mire,  and  by  the  dark,  actively-oscillating  Oscillaria  limosa; 
id,  above  all,  the  curious  Beggiatoa  versatilis  makes  itself  conspicuous,  sending 
out  from  a  whitish  gelatinous  ground  mass  long  oscillating  filaments,  which  emerge 
,fter  sundown,  and  next  day  split  up  into  innumerable  little  bacteria-rods.     The 
red-snow  alga,  too  (represented  in  fig.  25A),    lives  at  the  expense  of  the  pollen- 
grains,  bodies  of   insects,  and  other  decaying  matter   blown  on   to   snow-fields; 
whilst   the   nearly  allied  blood-red  alga  (Hcematococcus  pluvialis  or  Sphcerella 
pluvialis)  lives   in  the  water  in   hollow  stones  where  all  sorts  of  animal  and 
vegetable   remains   collect.     Leaves    blown    into   deep    pools,   and    lying  rotting 
at  the  bottom,  are  everywhere  overgrown  by  green  (Edogonium,  by  Pleurococcus 
angulosus,   and   by   the    amethyst-coloured    Protococcus    roseo-persicinus.      The 
bottoms  of  ditches  on  peat-bogs,  which  are  full  of  brownish  water  containing  an 
abundance  of  compounds  of  humic  acid  in  solution,  are  covered  with  this  amethyst 
Protococcus,  whilst  a  profusion  of  small  filamentous  algae,  Oscillariae  and  so  forth 
(Bulbochccte  parvula,  Schizochlamys  gelatinosa,  Sphcerozosma  vertebrata,  Micro- 
cystis  ichthyloba,  &c.),  as  well  as  a  group  of  dusky  mosses  (Hypnum  giganteum,  H. 
sarmentosum,  H.  cordifolium),  all  have  their  home  exclusively  in  still  water  richly 
supplied  with  organic  compounds.     When  we  include  also  the  curious  mould-like 
Saprolegnice  produced  on  dead  bodies   floating  in  waier—Saprolegnia  ferax  and 
Achlya  prolifera  on  flies  and  fishes— some  idea  is  obtained  of  the  great  variety 
of  saprophytes  living  in  fresh  water,  as  well  as  of  those  inhabiting  the  sea. 

A  much  more  agreeable  and  attractive  picture  than  that  of  these  aquatic  sapro- 
phytes is  afforded  by  plants  whose  sole  habitat  is  the  bark  of  trees.  The  dead 
bark  does  not  constitute  the  nutrient  base  of  all  the  plants  which  grow  from 
trunks  and  branches,  or  climb  up  them  in  the  form  of  clinging  and  twining  lianas. 


106  SAPROPHYTES   IN   WATER,  ON   THE   BARK   OF   TREES,  AND   ON   ROCKS. 

Often  the  trees  only  serve  as  supports,  by  means  of  which  the  plants  in  question 
raise  themselves  out  of  darkness  into  light.  Such  food-salts  as  they  require  they 
take,  not  from  their  support,  but  from  the  earth,  into  which  they  send  absorptive 
roots.  As  years  go  by,  a  quantity  of  inorganic  dust  collects  in  the  forks  of 
branches  and  in  the  little  rents  and  fissures  in  the  bark  of  old  trees,  and  this  dust 
gets  mixed  with  crumbled  particles  of  bark.  The  clefts,  therefore,  are  more  or 
less  full  of  vegetable  mould,  and  this  forms  an  excellent  foster-soil  for  a  large 
number  of  plants.  But  it  is  not  necessarily  the  case  that  all  plants  rooting  in  this 
mould  take  up  organic  compounds  from  it.  Thus,  one  finds  not  infrequently  in  the 
angles  of  bifurcation  of  the  trunks  of  old  limes  and  other  trees,  little  gooseberry 
and  elder  bushes,  and  bitter-sweet  plants,  which  have  germinated  there  from  fruits 
brought  by  black-birds,  thrushes,  and  other  frugivora.  These  shrubs,  in  the  forks 
of  limes  and  poplars  hardly  take  any  organic  compounds  from  the  mould  in  which 
they  are  rooted,  but  confine  themselves  to  the  absorption  of  such  mineral  salts  as 
they  may  require. 

But,  with  the  exception  of  instances  of  that  kind,  the  great  majority  of  plants, 
nestling  in  the  mould  in  crevices  of  bark,  do  take  nutriment  from  this  their 
substratum  in  the  form  of  organic  compounds.  In  cold  regions  the  plants  living 
in  the  mould  of  bark  are  for  the  most  part  mosses  and  liverworts.  They  cover 
trunks  and  branches  of  old  ashes,  poplars,  and  oaks,  with  a  thick  green  mantle,  and 
grow  especially  on  the  weather-side  of  the  trees.  In  the  tropics,  on  the  other  hand, 
the  fissured  bark  of  trees  is  a  rallying  ground  not  only  for  delicate  mosses  and 
moss-like  Lycopodia,  but  also  for  a  whole  host  of  ferns  and  vivid  flowering  plants. 
The  number  of  small  ferns  which  develop  and  unroll  their  fronds  from  chinks  in 
the  bark  of  trees  is  so  great  that  old  trunks  appear  wrapped  in  a  regular  foliage  of 
fern-fronds.  Of  Phanerogams,  in  particular,  the  Aroidece,  Orchidacece,  Bromeliacece, 
Dorstenice  Begoniacece,  and  even  Cactacece  (species  of  the  genera  Cereus  and 
Rhipsalis)  bury  their  roots  in  the  mould  of  bark.  It  is  to  be  remarked  that  the 
rosettes  of  Bromeliacece  ornament  chiefly  the  forks  of  trunks,  whilst  Dorstenice, 
Orchidece,  and  the  various  species  of  Rhipsalis  grow  on  the  upper  side  of  branches 
that  ramify  horizontally;  whilst,  lastly,  Aroidece  and  Begonice  take  root,  for  the 
most  part,  on  the  surfaces  of  huge  erect  trunks. 

Besides  the  mould  collected  in  crevices  and  fissures  of  bark,  the  bark  itself,  that 
is,  the  cortical  layer,  dead  but  not  yet  crumbled  and  mouldered  into  dust,  forms 
a  nutrient  substratum  for  a  whole  series  of  plants  of  most  various  affinity. 
Many  fungi  and  lichens  penetrate  deeply  the  compact  bark,  and  their  hyphal 
filaments  ramify  between  its  dead  cells.  Other  plants,  instead  of  piercing  through 
the  substance  of  the  bark,  lay  themselves  flat  upon  its  surface,  and  grow  to  it  so 
firmly  that  if  one  tries  to  lift  them  away  from  the  substratum,  either  part  of  the 
latter  breaks  off,  or  the  adnate  cell-strata  are  rent,  but  there  is  no  separation  of  the 
one  from  the  other.  If  a  tuft  of  moss  (e.g.  Orthotrichum  fallax,  0.  tenellum,  or 
0.  pollens),  growing  on  bark,  or  a  liverwort  (e.g.  Frullania  dilatata)  closely 
adherent  to  a  similar  basis,  is  forcibly  removed,  little  fragments  of  the  bark  may  be 


SAPROPHYTES  IN  WATER,  ON  THE  BARK  OF  TREES,  AND  ON  ROCKS.     107 

always  seen  torn  off  with  the  rhizoids  at  the  places  where  they  issue  from  the 
stemlets.  The  same  thing  occurs  in  the  case  of  the  roots  of  tropical  orchids 
growing  to  the  tree-trunks  which  constitute  their  habitat.  The  majority  of  these 
tree-orchids  nestle,  no  doubt,  in  mould-filled  crevices  of  the  bark,  and  nourish  them- 


Fig.  15.— Aerial  Roots  of  a  Tropical  Orchid  (Sarcanthus  restrains)  assuming  the  form  of  straps. 

selves,  besides,  by  means  of  special  aerial  roots  which  hang  down  in  white  ropes 
and  threads,  like  a  mane,  from  the  places  where  the  plants  are  situated  upon  the 
trees,  and  which  will  presently  be  described  in  detail.  But  a  small  section  develops 
strap-shaped  roots  as  well,  which  adhere  firmly  to  the  bark  with  their  flat  surfaces. 
This  phenomenon  is  most  strikingly  exhibited  by  the  splendid  Phalcenopsis 


108  SAPROPHYTES   IN   WATER,  ON   THE   BARK   OF   TREES,  AND   ON   ROCKS. 

Schilleriana,  a  native  of  the  Philippine  Islands;  its  roots  are  rigid,  compressed, 
and  about  1  c.m.  in  breadth;  the  surface  turned  away  from  the  trunk  is  slightly 
convex,  and  has  a  granular  structure  and  metallic  glitter  like  a  lizard's  or  chame- 
leon's tail.  The  surface  towards  the  trunk  is  flat  and  without  metallic  glitter, 
and  upon  it,  close  behind  the  growing  point,  there  is  a  whitish  fur  consisting  of 
short,  thickly  packed,  absorptive  cells.  When  the  tip  of  one  of  these  roots  comes 
into  contact  with  the  bark  it  grows  so  firmly  to  the  substratum  by  means  of  the 
absorption-cells,  that  it  is  easier  to  detach  superficial  bits  of  the  bark  itself  than 
the  root.  The  latter,  once  fixed,  flattens  out  still  more  and  becomes  strap-shaped, 
whilst  creeping  outgrowths  proceed  from  it,  forming  strips  which  may  ultimately 
attain  a  length  of  1J  metres.  The  sight  of  a  trunk  covered  with  these  long 
metallic  bands  is  one  that  never  fails  to  excite  wonder  even  in  the  midst  of  the 
world  of  orchids,  wherein,  as  is  well  known,  there  is  much  to  marvel  at. 

In  other  species  of  tropical  orchids,  e.g.  in  Sarcanthus  rostratus  (fig.  15),  the 
roots  are  not  flat  from  the  beginning,  but  become  so  when  they  come  into  con- 
tact with  the  bark.  A  root  is  often  to  be  seen  which  arises  as  a  cylindrical  cord 
from  the  axis,  then  lays  itself  upon  the  bark  in  the  form  of  a  band,  and  further  on 
lifts  itself  once  more,  resuming  at  the  same  time  the  rope  form,  as  is  shown  in  the 
illustration.  Here  also  complete  coalescence  takes  place  between  the  bands  and  the 
bark,  and  the  union  is  extremely  close.  Similar  conditions  have  been  observed  to 
hold  in  many  Aroidece  living  on  the  bark  of  trees.  The  plants  in  question  lie  with 
their  stems,  leaves,  and  roots  flat  against  the  trunks,  so  that  they  suggest  a  covering 
of  drapery.  Taking,  for  instance,  the  Marcgravice  (Marcgravia  paradoxa,  M. 
umbellata),  one  might  at  first  sight  suppose  that  they  adhere  to  the  bark  not  only 
by  the  roots,  but  also  by  the  large  discoid  leaves,  which  are  arranged  in  two  rows. 
A  very  remarkable  fact  also,  in  connection  with  these  plants,  is  that  they  only 
grow  on  very  smooth  and  firm  bark.  When  transferred  to  a  soft  substratum,  such 
as  mould  or  moss,  they  languish,  because  their  roots  are  unable  to  enter  into  close 
union  with  a  support  of  such  loose  texture.  This  is  also  true  of  most  tropical 
orchids  living  on  bark.  When  their  seeds  are  transferred  to  loose  earth  devoid  of 
humus,  they  do  indeed  germinate,  but  then  perish;  whereas  when  sown  on  the  bark 
of  a  tree,  they  not  only  germinate,  but  grow  up  with  ease  into  hardy  plants. 

Where  steep  rocks  occur  near  clumps  of  trees  it  is  not  uncommon  for  the  same 
species  of  plants  to  grow  on  both.  Allusion  is  not  here  made  to  kinds  which,  like 
ivy,  have  their  roots  in  the  earth  at  the  foot  of  rocks  and  trees,  and  creep  up  the 
one  or  the  other  indifferently,  using  both  merely  for  support  and  not  as  sources  of 
nutriment,  and  clinging  to  them  by  means  of  special  attachment-roots.  The  remark 
is  applicable  also  to  plants  which  live  on  the  products  of  the  decay  of  organic 
bodies,  for  example  many  tropical  Orchidece,  Dorstenice,  Begonice,  and  Ferns;  and  in 
cooler  parts  a  number  of  Mosses  and  Liverworts.  It  is  not  difficult  to  explain  this 
phenomenon  in  the  case  of  species  which  derive  their  food  from  vegetable  mould. 
The  crannied  wall  of  rock  is,  in  a  certain  way,  analogous  to  the  rugged  bark  of  a 
tree.  The  holes  in  the  rock  are  filled  in  course  of  time  with  black  vegetable  mould, 


SAPROPHYTES   IN   THE   HUMUS   OF  WOODS,   MEADOWS,   AND   MOORS.  109 

and  plants  with  foliage,  flowers,  and  fruit  of  a  form  adaptable  to  cracks  and  holes 
are  able  to  establish  themselves  in  the  mould  there,  just  as  well  as  in  that  collected 
in  crevices  of  bark.  In  one  respect,  indeed,  they  are  even  more  favourably  situated. 
For  the  humus  in  bark  gets  quite  dry  in  long  periods  of  drought,  because  no  water 
is  yielded  to  the  bark  by  the  wood  of  a  tree,  even  though  the  latter  be  abundantly 
supplied  with  sap;  whereas,  in  the  case  of  rocks  the  probability  is,  the  clefts  being 
very  deep,  that  even  when  the  top  layers  of  humus  filling  them  yield  up  their  water 
to  the  air,  a  certain  restitution  of  moisture  takes  place  from  the  deeper  parts,  which 
are  never  quite  dry.  Moreover,  plants  growing  in  the  mould  of  rock  crevices  are 
able  to  send  their  roots  down  to  much  deeper  strata  than  is  possible  in  the  case  of 
bark.  This  is  another  reason  why  deep  cracks  in  rocks,  filled  with  humus,  exhibit 
a  richer  flora,  as  a  rule,  than  do  the  much  shallower  crevices  in  the  bark  of  trees, 
although,  as  has  been  said  before,  the  two  habitats  have  many  plants  in  common. 

It  is  more  difficult  to  explain  how  it  happens  that  plants  which  derive  their 
sustenance,  not  from  the  mould  in  crevices,  but  from  the  substance  of  the  bark 
itself,  and  which  lie  flat  against  its  surface,  are  also  found  adhering  to  walls  of 
rock.  As  an  example  take  Frullania  tamarisci,  a  Liverwort  with  small  brown 
bifurcating  stems,  which  bear  double  rows  of  leaves  and  are  of  dendritic  appearance. 
This  plant  grows  equally  well  on  the  bark  of  pines  or  on  the  face  of  adjacent  gneiss 
rocks.  At  first  sight  it  would  seem  scarcely  possible  that  a  plant  of  this  kind, 
clinging  to  the  unfissured  surface  of  rock,  should  be  in  a  position  to  obtain  organic 
compounds  from  its  substratum.  This  is  nevertheless  the  case.  Closer  inspection 
reveals  the  fact  that  the  Liverwort  does  not  adhere  to  blank  rock,  but  to  a  part 
formerly  clothed  by  rock-lichens.  This  inconspicuous  incrustation  of  dead  lichens 
is  a  complete  substitute  for  the  superficial  layer  of  bark,  and  it  is  into  it  that  the 
Frullania  tamarisci  sinks  its  roots.  Another  way  by  which  food  is  supplied  to 
plants  adherent,  like  the  above,  to  vertical  and  unfissured  rocks  will  be  discussed 
later  on. 


SAPROPHYTES  IN  THE  HUMUS  OF  WOODS,  MEADOWS,  AND  MOOES. 

Damp  shady  woods,  especially  pine  woods,  are  particularly  well  furnished  with 
saprophytes.  Here  again  we  find  representatives  of  the  same  families  as  choose  the 
bark  of  trees  for  their  habitat.  On  the  ground  of  woods,  the  most  characteristic 
forms  are  mosses,  fungi,  lycopods,  ferns,  aroids,  and  orchids.  The  dark -brown 
huinus,  produced  from  dropped  and  decaying  needles,  is  first  of  all  covered  by  a 
rich  carpet  of  mosses,  such  as  the  widely  distributed  Hylocomium  splendens, 
Hypnum  triquetrum,  and  Hypnum  Grista-castrensis.  The  mouldered  dust  of 
dead  trees  has  a  clothing  of  Tetraphis  pellucida  and  of  Webera  nutans,  and 
decaying  trunks  are  overgrown  by  the  cushions  of  species  of  Dicranum  (Dicranum 
scoparium,  D.  congestum,  Dicranodontium  longirostre),  pale  feathery  mosses 
(Hypnum  uncinatum  and  H.  reptile)  and  various  liverworts.  Everywhere  above 
the  soft,  ever-moist  carpet  of  moss  rise  green  fronds  belonging  to  broad-leaved  ferns. 


110  SAPROPHYTES   IN   THE   HUMUS   OF   WOODS,   MEADOWS,   AND   MOORS 

Woods  are  also  the  special  abode  of  fungi,  and  the  damp  ground  is  covered  towards 
autumn  by  innumerable  quantities  of  their  curious  fructifications.  Dropped  needles 
and  cones,  leaves  and  sticks  strewn  upon  the  ground,  fallen  trunks,  and  even  the 
dark  amorphous  dust  arising  from  the  mouldering  of  these  bodies  and  of  the 
numerous  roots  ramifying  in  the  ground,  appear  to  be  perforated  by  and  wrapped 
in  the  protoplasmic  threads  of  plasmoid  fungi,  or  similarly  invested  by  a  plexus 
of  filaments,  the  so-called  mycelia  of  other  forms  of  fungi.  Amongst  the  scaly 
fragments  of  bark,  peeling  from  the  trees,  they  appear  in  the  form  of  slimy  strings, 
or  as  a  dark  trellis  and  net- work,  inserted  between  the  bark  and  wood  of  the 
rotting  tree;  on  the  stripped  white  trunk  they  are  in  dark  zigzag  lines  like 
those  of  forked  lightning;  and  between,  the  white  mycelia  of  huge  toadstools  and 
tremellas  are  woven  in  all  directions.  Here  and  there  large  areas  of  the  brown 
decaying  soil  are  flecked  and  speckled  by  these  mycelia,  and  even  the  dead  stems 
of  the  mosses  on  the  ground  are  festooned  with  white  fleece,  and  wrapped  round 
by  hyphse. 

It  is  worth  while  to  glance  too  at  the  reciprocal  relations  of  these  woodland 
plants.  We  find  mosses,  lycopods,  and  various  ferns  and  phanerogams  living 
upon  the  fallen  twigs  and  needles,  and  on  the  mouldering  roots  of  pines  and  fir- 
trees.  The  dead  remains  of  those  plants  afford  sustenance  to  the  fungi,  which  lift 
their  fructification  above  the  bed  of  moss.  In  their  turn  the  rotting  fructifications 
of  the  larger  fungi  form  a  nutrient  substratum  for  smaller  fungi,  which  cover  the 
decaying  caps  and  stalks  with  a  dark-green  velvet.  Lastly,  these  little  fungi,  too, 
fall  a  prey  to  corrupting  bacteria,  and  are  resolved  into  the  same  simple  inorganic 
compounds  as  were  absorbed  from  the  air  and  earth,  in  the  first  instance,  by  the 
pines  and  fir-trees.  In  the  depths  of  forests  there  is  going  on,  for  the  most  part 
unseen  by  us,  a  mysterious  stir  and  strife,  accompanied  by  an  uninterrupted  process 
of  exchange  between  the  living  and  the  dead,  and  a  marvellous  transformation  of 
those  very  substances  whose  secret  we  have  only  partially  succeeded  in  solving. 

The  results  of  cultivation  have  proved  that  in  the  group  of  flowering-plants 
belonging  to  the  woodlands  of  Central  and  Northern  Europe,  which  derive  sus- 
tenance partially  or  entirely  from  the  organic  compounds  afforded  by  the  humus, 
are  to  be  included,  amongst  others,  the  various  species  of  coral-wort  (Dentaria 
bulbifera,  D.  digitata,  D.  enneaphyllos),  Circcea  alpina,  Galium  rotundifolium, 
and  Linncea  borealis,  and  above  all  a  large  number  of  orchids.  Of  these,  Dentaria 
prefers  mould  produced  from  the  beech  leaves,  and  Circcea,  Galium,  and  Linncea 
appertain  to  the  mould  of  pine- woods.  Of  the  orchids  some  are  provided  with 
green  leaves,  as,  for  instance,  the  delicate  little  Listera  cordata,  Goodyera  repens 
remarkable  for  its  villous  petals,  and  the  various  species  of  Cephalanthera,  Epi- 
pactis,  and  Platanthera;  others,  such  as  Limodorum  abortivum,  the  bird's-nest 
orchis,  the  coral-root,  and  Epipogium  aphyllum  have  none.  Limodorum  abortivum 
belongs  rather  to  the  warmer  districts  of  Central  Europe.  It  has  fleshy  root- 
fibres,  twisted  and  twined  into  an  inextricable  ball,  and  a  slender  steel-blue  stem, 
over  half  a  metre  in  height,  bearing  a  lax  spike  of  fairly  large  flowers,  which 


SAPROPHYTES   IN   THE    HUMUS   OF   WOODS,   MEADOWS,   AND   MOORS.  Ill 

subsequently  become  paler  in  colour.  The  bird's-nesfc  orchis  (Neottia  Nidus-avis) 
is  of  wide  distribution  both  in  forests  of  pines  and  in  those  composed  of  angio- 
spermous  trees.  Its  stem  and  flowers  are  of  a  light-brown  colour,  unusual  in  plants, 
but  somewhat  like  that  of  oak-wood.  The  flowers  have  no  scent,  and  the  numerous 
roots,  issuing  from  the  subterranean  part  of  the  stem  and  imbedded  in  humus, 
remind  one  in  form  and  colour  of  earth-worms,  and  together  constitute  a  strange 
tangled  mass  as  large  as  a  fist.  The  latter  has  been  thought  to  resemble  a  bird's 
nest,  and  to  this  is  due  the  name  of  the  plant.  The  coral-root  (Corallorhiza  innata), 
unlike  the  bird's-nest  orchis,  has  no  root  at  all;  but,  on  the  other  hand,  the  sub- 
terranean portion  of  the  stem,  the  so-called  rhizome,  possesses  a  distant  resemblance 
to  the  root -tangle  of  Neottia.  Pale -brownish  branches  of  this  rhizome,  which 
bifurcate  repeatedly  at  their  obtuse  and  whitish  extremities,  looking  as  if  they 
had  been  subjected  to  pressure  for  a  time,  and  all  the  short  lobe-shaped  branchlets 
thereby  spread  out  into  one  plane,  lie  closely  crowded  together,  sometimes  crossing 
one  another,  and  so  form  a  body  which  vividly  recalls  the  appearance  of  a  piece  of 
coral.  This  underground  coral-like  stem-structure  develops  each  year  pale  greenish 
shoots  which  rise  above  the  ground  and  bear  small  flowers  speckled  with  yellow, 
white,  and  violet,  and  exhaling  a  scent  of  vanilla;  later,  green  fruits  of  a  com- 
paratively large  size  develop,  turning  brown  when  they  ripen. 

The  fourth  mentioned  of  these  pale  wood-orchids,  the  Epipogium  aphyllum,  is 
at  once  the  rarest  and  most  curious  of  them  all.  Like  the  coral-root  it  has  no  true 
roots.  Its  rhizome  so  closely  resembles  the  latter's  that  it  is  easy  to  mistake  the 
one  for  the  other;  but  they  may  be  distinguished  by  the  fact  that  in  the  case  of 
Epipogium  the  rhizome  sends  out  long  filiform  shoots,  which  swell  up  like  tubers 
at  their  tips,  and  may  be  regarded  as  subterranean  runners.  The  swollen  extremity 
becomes  the  point  of  origin  of  a  new  coral-like  structure,  which  develops  at  about 
the  distance  of  a  span  from  the  old  one;  whilst  the  latter,  usually  exhausted  after 
flowering,  gradually  perishes.  This  coral -like  stem  lives  of  course  underground, 
and  is  not  visible  till  one  lifts  away  the  moss  from  the  mould  on  the  ground.  It  is 
often  completely  imbedded  in  sandy  loam,  lying  immediately  beneath  the  black 
mould.  Many  years  frequently  go  by  without  the  Epipogium  producing  flowers. 
The  plant  meanwhile  lives  entirely  underground.  In  the  course  of  a  summer  in 
which  it  has  not  flowered,  anyone  not  having  previous  exact  knowledge  of  its  where- 
abouts might  pass  by  without  dreaming  that  the  bed  of  moss  and  humus  on  his 
path  concealed  this  strange  growth.  The  flowering  stems  which  at  length  emerge, 
when  there  is  a  warm  summer,  are  right  above  the  place  where  they  branch  off 
from  the  subterranean  rhizome.  They  are  thickened  in  a  fusiform  manner,  and 
have,  for  the  most  part  on  one  side,  a  reddish  or  purplish  tinge.  Everything 
connected  with  them  is  tense,  smooth,  full  of  sap,  and  almost  opalescent.  The 
few  flowers  that  are  borne  by  the  stem  are  comparatively  large,  and  emit  a  strong 
perfume  resembling  that  of  the  Brazilian  genus  of  orchids  Stanhopea.  The  colour- 
ing, too,  a  dull  yellowish  white  with  touches  of  pale  red  and  violet,  reminds  one  of 
these  tropical  orchids. 


112  SAPROPHYTES   IN   THE   HUMUS   OF   WOODS,   MEADOWS,   AND   MOORS. 

The  sight  of  the  pale-coloured  plants  lifting  their  heads,  at  flowering  time,  from 
the  tumid  carpet  of  moss  has  all  the  stranger  effect  because,  as  a  rule,  no  other 
flowering  plants  are  visible  in  any  direction.  The  flowers  are  suspended  by  delicate 
drooping  pedicels,  and  owing  to  their  peculiar  colour,  fleshy  consistence,  and  form 

the  erect  concave  petal  like  a  Phrygian  cap  or  helmet,  and  the  others  stretched 

out  like  prehensile  limbs — remind  one  of  the  opalescent  medusae  which  float  on 
the  blue  sea  waves.  The  propriety  of  the  analogy  is  enhanced  by  the  fact 
that  the  form  and  colour  of  other  saprophytes  produced  near  Epipogium  in 
woods  have  a  striking  resemblance  to  the  animals  and  wracks  which  inhabit  the 
sea-bottom.  The  fungi,  known  by  the  name  of  club-tops,  much-branched,  flesh- 
coloured,  yellow  or  white  Clavarice,  which  often  adorn  whole  tracts  of  ground  in  a 
wood,  imitate  the  structure  of  corals;  Hydnece  are  like  sea-urchins,  and  Geaster 
like  a  star-fish,  whilst  the  various  species  of  Tremella,  Exidia,  and  Guepinia,  which 
are  flesh-pink,  orange,  or  brownish  in  colour,  and  the  white  translucent  Tremellodon 
gelatinosum,  resemble  gelatinous  sponges.  The  small  stiff  toad-stools  (Marasmius), 
which  raise  their  slender  stalks  on  fallen  pine-needles,  remind  one  of  the  rigid 
Acetabularice.  Other  toad-stools,  with  flat  or  convex  caps  exhibiting  concentric 
bands  and  stripes,  such  as  the  different  species  of  Craterellus,  have  an  appearance 
similar  to  the  salt-water  alga  known  by  the  name  of  Padina.  Dark  species  of 
Geoglossum  imitate  the  brown  Fucoidece;  and  one  may  fancy  the  red  warts  of 
Lycogala  Epidendron,  a  plasmoid  fungus  inhabiting  the  rotten  wood  of  dead 
weather-beaten  trees,  to  be  red  sea-anemones  with  their  tentacles  drawn  in, 
clinging  to  gray  rocks.  However  far-fetched  this  comparison  between  the  two 
localities  may  seem  at  first  sight,  everyone  who  has  had  an  opportunity  of 
thoroughly  observing  the  characteristic  forms  of  vegetable  and  animal  life  in 
woods,  and  at  the  bottom  of  the  sea,  will  inevitably  be  convinced  of  its  accuracy. 

Meadow-land,  rich  in  humus,  is  much  more  sparsely  occupied  by  saprophytes 
than  the  soil  of  woods.  There  is  no  lack  of  the  strange  forms  of  toad-stools  and 
puff-balls,  whose  fructifications  often  spring  up  in  thousands,  especially  in  the 
autumn,  in  company  with  the  meadow-saffron;  but  in  numbers  they  are  not  to  be 
compared  with  those  which  occur  in  the  mould  of  woods.  Amongst  ferns  and 
phanerogams,  the  following  species  are  dependent  upon  the  organic  compounds 
arising  from  the  decomposition  of  the  humus:  Moon  wort  (Botrychium  Lunaria), 
numerous  orchids,  blue  and  violet-flowered  gentians,  the  famous  Arnica,  Poly- 
galacese,  and  more  especially  several  grasses,  chiefly  the  Matweed  (Nardus  stricta) 
which,  when  once  it  has  struck  root  in  the  humus,  extends  in  dense  masses  over 
large  areas.  Several  plants,  too,  adorning  alpine  pastures,  and  belonging  for  the 
most  part  to  the  same  families  as  the  species  mentioned  above,  are  to  be  regarded 
as  humus-plants.  Such  are  the  Alpine  Club-moss  (Lycopodium  alpinum),  the 
dark-flowered  Nigritella  nigra,  and  several  other  sub-alpine  orchids;  a  number  of 
small,  sometimes  tiny,  gentians  (Gentiana  nivalis,  G.  prostrata,  G.  glacialis, 
G.  nana,  Lomatogonium  Carinthiacum),  Valeriana  celtica,  the  Scottish  asphodel 
(Tqfieldia  borealis)  of  the  north,  a  few  grasses,  sedges,  and  rushes  (e.g.  Agrostis 


RELATIONS   OF   SAPROPHYTES   TO   THEIR   NUTRIENT   SUBSTRATUM.  113 

alpina,  Carex  curvula,  Juncus  trifidus),  various  anemones,  campions,  umbelliferous 
plants,  violets  and  campanulas  (e.g.  Anemone  alpina,  Silene  Pumilio,  Meum 
Mutellinat  Viola  alpina,  Campanula  alpina)  and  several  mosses  (e.g.  Dicranum 
elongatum  and  Polytrichum  strictum)  which  clothe  the  humus  on  stretches  of  turf 
and  in  inclosures. 

Many  of  the  plants  also  that  are  native  on  the  black  graphitic  soil  in  hollows 
of  high  mountain  ridges  take  up  organic  food  from  their  substratum.  These 
include  Meesia  alpina  and  various  other  mosses  produced  exclusively  in  places  of 
the  kind;  and,  above  all,  numerous  Primulaceae  and  Gentianeae  (Primula  glutinosa, 
Soldanella  pusilla,  Gentiana  Bavarica).  It  seems,  moreover,  to  be  by  no  means  a 
matter  of  indifference  to  these  plants  at  what  temperature,  and  in  what  state  of  the 
air,  in  respect  of  moisture,  the  decomposition  of  humus  takes  place.  If  species  which 
grow  abundantly  in  these  localities  are  dug  up  and  transferred,  together  with  the 
black  earth  in  which  their  roots  are  imbedded,  into  a  garden,  and  are  there 
cultivated  in  such  a  way  that  the  external  conditions  are  as  nearly  as  possible  those 
of  the  original  habitat;  or  if  young  plants  are  reared  from  seed  in  the  same  black 
humus-filled  earth,  they  thrive  only  for  a  short  time,  soon  begin  to  fade,  and  within 
the  space  of  a  year  are  dead;  whereas,  alpine  plants  belonging  to  the  same  altitude 
above  the  sea,  but  rooted  in  loamy  or  sandy  earth,  flourish  excellently  in  gardens 
as  well.  Various  moor-plants  (e.g.  Lycopodium  inundatum,  JSriophorum  vagin- 
atum,  Trientalis  Europcea)  only  live  a  short  time  in  a  garden  even  though  the 
clods  of  peat,  in  which  their  roots  are  imbedded,  are  transplanted  with  them.  This 
fact  can  scarcely  be  explained  except  by  supposing  that  the  organic  compounds, 
produced  by  the  decay  of  vegetable  remains  on  alpine  heights  and  moors,  are 
essentially  different  from  those  evolved  by  similar  matter  under  the  changed 
conditions  of  temperature  and  moisture  occurring  in  a  garden  at  a  lower  level. 
Gardeners  say  that  the  peat  and  black  graphitic  soil  from  the  slopes  of  snowy 
mountains  turn  sour  in  gardens,  and  they  may  be  to  this  extent  right,  that  in  all 
probability  the  humic  acids  produced  under  altered  circumstances  are  different. 

SPECIAL  EELATIONS  OF  SAPKOPHYTES  TO  THEIR  NUTEIENT  SUBSTRATUM. 

In  the  plants  under  discussion,  the  cells  which  absorb  organic  compounds  are, 
taken  all  in  all,  very  similar  to  those  which  absorb  mineral  food-salts.  Where  there 
is  no  cell-membrane,  as  in  the  case  of  Plasmodia  and  Euglense,  the  food  diffuses 
through  the  so-called  ectoplasm,  or  outer  layer  of  the  protoplasm,  into  the  interior  of 
the  cell.  Saprophytic  marine  and  fresh- water  algae  are  able  to  absorb  the  products 
of  decay  in  the  water  around  by  means  of  their  superficial  layers  of  cells.  The 
mycelia  of  fungi  have  the  power  of  taking  in  nourishment  with  special  rapidity. 
Each  hypha,  or  more  accurately,  each  long,  delicate-walled  cell  of  a  mycelium  is,  to 
a  certain  extent,  an  absorptive  cell;  its  entire  surface  is  capable  of  exercising  the 
function  of  suction  and  of  withdrawing  from  the  environment,  along  with  water, 

the   very   substances   which   are   needed.      The   coral-like   underground   stem   of 
VOL  I.  8 


114  RELATIONS   OF   SAPROPHYTES   TO   THEIR   NUTRIENT   SUBSTRATUM. 

Epipogium  aphyllum,  as  well  as  that  of  the  "  Coral-root",  which  is  entirely  destitute 
of  roots,  develop  fascicles  of  absorptive  cells  on  their  ramifications,  and  on  special 
little  swellings;  and  the  white  subterranean  stem  structures  of  Bartsia  alpina  are 
also  provided  with  long  absorptive  cells.  The  white,  fusiform,  tuberously  thickened, 
underground  stems  of  the  Alpine  Enchanter's  Nightshade  (Circcea  alpina)  exhibit 
no  roots  during  autumn  and  winter,  nor  until  such  time  as  new  leafy  stems  sprout 
from  them  and  lift  themselves  into  the  daylight;  they  only  have  scattered  club- 
shaped  absorptive  cells.  Yet  it  is  inconceivable  that  the  few  absorptive  cells  meet 
the  entire  requirements  of  these  plants  at  the  season  of  the  development  of  their 
aerial  stems.  Food  is  absorbed  in  these  cases  also  by  the  epidermal  cells  of  the 
entire  tuber,  underground  stem,  or  coral-like  rhizome,  as  the  case  may  be.  The 
epidermal  cells  of  these  subterranean  caulomes  which  lie  immediately  in  contact 
with  the  black  mould  or  humus  on  the  ground  of  forests,  have  such  thin  and  tender 
walls  that  they  are  quite  as  well  adapted  to  the  absorption  of  nutriment  as  are  the 
projecting  absorptive  cells;  indeed  the  club-shaped  absorptive  cells  on  the  small 
tubers  of  Enchanter's  Nightshade  exhibit  somewhat  thicker  walls  than  those 
forming  the  general  epidermis  of  these  tubers. 

We  may  compare  food-absorption  as  performed  by  these  coral-like  and  tuberous 
structures,  imbedded  in  decaying  plant  residues,  with  the  action  of  tape-worms  in 
process  of  sucking  in  through  their  entire  epidermis  the  fluid  filling  the  intestines 
they  inhabit.  The  epidermal  cells  of  the  thick  tortuous  root -fibres  of  Neottia 
Nidus-avis  are  all  capable  of  absorbing  nutriment,  though  they  do  not  project  as 
tubes,  but  are  tabular,  and  have  their  outer  walls,  which  are  in  immediate  contact 
with  the  nutrient  soil,  only  slightly  arched  outwards  (see  fig.  16  2).  The  green  leafy 
orchids  rooted  in  the  vegetable  mould  of  woods  and  meadows  are,  on  the  contrary, 
furnished  with  very  long  tubular  absorption  cells;  and  these  cells  do  not  wither 
and  collapse  forthwith  when  the  root  elongates,  but  long  retain  their  vigour  and 
activity.  Whereas  in  the  case  of  land  plants  adapted  to  mineral  food-salts,  the 
tubular  absorption  cells  ("root-hairs")  are  limited  to  a  narrow  zone  behind  the 
growing  point  of  the  root  and  always  die  comparatively  soon;  in  the  case  of  orchids, 
having  cylindrical  roots  imbedded  in  vegetable  mould,  these  structures  appear  to  be 
beset  from  end  to  end  with  long  scattered  tubular  absorption  cells,  which  are 
retained  even  through  the  drought  of  summer  or  the  frost  of  winter  right  into  the 
next  period  of  vegetative  activity;  and  these  cells  occur  most  abundantly  in  parts  of 
the  ground  where  there  happens  to  be  a  bed  of  humus  or  mouldering  remains 
particularly  amenable  to  their  purpose.  Similar  relations  are  found  to  exist  in  the 
case  of  the  dichotomously-branched  roots  of  the  Club-moss.  They  are  twisted  in 
spirals  and  bore  into  the  vegetable  mould  like  corkscrews,  and  their  absorption 
cells  form  in  some  places  regular  tassels,  which  are  completely  cemented  over  with 
fine  black  mould.  The  roots  of  grasses  which,  like  the  Mat-grass,  live  on  the 
decomposition-products  of  vegetable  mould,  are  also  distinguished  by  strikingly 
long  absorption  cells,  which  grow  in  black  or  brown  humus  and  there  undergo  the 
strangest  bends  and  contortions.  When,  for  instance,  a  fragment  of  a  dead  root  or 


RELATION  OF  SAPROPHYTES  TO  THEIR  NUTRIENT  SUBSTRATUM.       115 

underground  stem,  peculiarly  suitable  for  absorption,  is  encountered,  it  is  regularly 
embraced  by  the  suction  cells,  and  as  great  an  absorbent  surface  as  possible  is  thus 
brought  into  contact  with  the  nutritious  fragment.  Indeed,  the  development  of 
suction  cells  on  the  roots  of  many  gentians  (viz.  Gentiana  ciliata,  G.  germanica,  G. 
Austriaca,  and  G.  Rhcetica)  is  confined  to  the  parts  of  the  root-branches,  which,  in 
the  course  of  their  passage  through  the  vegetable  mould,  have  come  into  contact 
with  a  particularly  nutritious  portion  of  it.  Wherever  there  is  contact,  the  root  is 
thickened,  and  absorption  cells  project  unilaterally  from  the  epidermis  and  grow 
into  the  decaying  fragment  of  wood  or  bark  which  is  to  be  drained  of  its  nutrient 


Fig.  16. —Transverse  section  through  absorption-roots  of  Saprophytes, 
i  Gentiana  Rhcetica.  a  The  Bird's  Nest  Orchis  (Neottia  Nidus-avis). 

material  (see  fig.  16 1).  Roots  of  this  kind  remind  one  of  the  root-structures  of 
parasites  which  are  furnished  with  so-called  "haustoria",  and  which  will  be 
discussed  more  in  detail  in  subsequent  pages.  But  they  are  different  in  that  they 
absorb  food  not  from  living  but  from  decaying  parts  of  the  nutrient  substratum. 

Most  plants  that  grow  on  the  vegetable  mould  of  alpine  meadows,  and  the  black 
earth  deposited  by  snow-drifts  in  mountainous  regions,  develop  flat  instead  of 
tubular  epidermal  cells  as  suction  cells,  and  in  this  resemble  marsh-plants.  In 
many  of  these  cases  the  roots  are  so  abundantly  and  minutely  ramified  that  they 
form  a  plexus  investing  the  humus.  This  is  likewise  true  of  the  absorptive  cells  on 
the  rhizoids  of  mosses. 

Plants  which  lie  flat  against  the  bark  of  trees  and  have  no  connection  with  the 
ground,  so  that  they  are  unable  to  derive  nutriment  from  it,  have  a  very  peculiar 
method  of  maintaining  themselves.  Their  roots,  rhizoids,  or  hyphae,  as  the  case  may 
be,  either  grow  straight  into  the  bark  or  are  merely  adnate  to  its  surface.  In 
the  latter  case  they  are  exposed  on  one  side  to  the  open  air,  and  form  more  or 
less  projecting  lines  and  ridges  ramifying  in  all  directions,  often  constituting  a 
regular  trellis- work  cemented  to  the  bark.  Sometimes,  too,  they  are  represented 


116  RELATIONS   OF   SAPROPHYTES   TO   THEIR   NUTRIENT   SUBSTRATUM. 

by  thicker  ropes  or  bands  which  run  longitudinally  down  or  encircle  the  trunk. 
These  structures  certainly  serve  as  instruments  of  attachment,  but  at  the  same  time 
they  also  absorb  nutriment  from  the  substratum,  the  decaying  bark  upon  which  the 
plant  is  epiphytic.  In  periods  of  drought  the  absorption  of  food  by  plants  of  this 
kind  is,  in  general,  interrupted  and  suspended.  But  when  the  rainy  season 
commences  and  there  is  a  long  duration  of  wet  weather,  water  trickling  over  the 
surface  of  boughs  and  trunks  washes  the  bark,  cleanses  it  as  it  were,  and,  falling 
lower  and  lower,  brings  down  not  only  tiny  loosened  particles  of  bark  but  mineral 
and  organic  dust  which  has  been  blown  into  it  by  the  wind;  it  dissolves  all  the 
soluble  matter  it  finds  on  its  way,  and  so  reaches  the  roots,  rhizoids,  and  hyphse 
which  adhere  to  the  bark,  in  the  form  of  a  solution  of  mineral  and  organic 
compounds,  chiefly  the  latter.  The  trickling  water  is  in  some  measure  stopped  by 
the  projecting  ridges  of  these  adnate  structures;  here  and  there  also  it  deposits 
particles  mechanically  suspended  in  it,  and  so  it  conveys  to  these  curious  epiphytes 
the  requisite  nourishment. 

In  the  same  way,  no  doubt,  epiphytes  which  grow  upon  other  epiphytes  are 
nourished.  In  more  inclement  regions,  the  green  bark,  stem,  and,  less  frequently, 
the  green  leaves  of  the  mistletoe  are  found  to  be  beset  by  iposses  and  lichens;  and, 
in  the  tropics  it  is  a  common  phenomenon  for  mosses,  liverworts,  and  even  small 
kinds  of  Bromeliacese  to  settle  on  the  green  and  still  living  leaves  of  Bromeliacese, 
Orchidese,  and  Loranthaceae,  although  they  are  certainly  not  properly  parasitic,  and 
only  use  their  absorption  cells  for  the  purpose  of  clinging  to  the  thick  epidermis  of 
the  living  leaves  or  stems  which  support  them.  The  principal  part  of  the  liquid 
substances  absorbed  by  these  plants  is  conveyed  to  them  by  the  rain-water  that 
washes  over  the  substratum. 

The  species  of  plants  also  which  have  been  mentioned  as  sometimes  growing  on 
smooth  vertical  faces  of  rock,  though  the  bark  of  trees  is  their  usual  habitat,  are 
able  to  obtain  their  food-materials  in  a  similar  way.  If  the  summit  of  a  cliff  is 
covered  by  a  continuous  carpet  of  plants,  or  if  ledges  and  terraces  projecting 
somewhat  from  its  face  support  sods  of  grass,  tufts  of  moss,  and  various  small 
kinds  of  bushes,  it  must  inevitably  happen  when  there  is  an  abundant  fall  of  rain 
that  the  water  flowing  down  the  declivity  conveys  with  it  organic  compounds  in 
solution.  First  the  sods  of  grass  and  moss  on  the  ledges  and  on  the  top  of  the  cliff 
are  wetted,  then  the  humus,  which  is  their  substratum,  becomes  saturated,  and  such 
part  of  the  water  as  cannot  be  retained  by  this  humus,  or  does  not  percolate  into 
the  cracks  and  crevices  of  the  rock,  trickles  down  from  the  ledges  and  moistens  the 
face  of  the  rock  as  it  soaks  down  to  the  bottom.  A  rocky  declivity  is  thus  washed 
in  the  same  way  as  is  the  bark  of  trees,  and  small  fragments  of  organic  and 
inorganic  bodies  must  of  necessity  be  rinsed  out  and  carried  down  by  the  trickling 
water,  and  then  again  be  deposited  in  heaps  where  projecting  obstacles  are 
encountered.  It  is  just  in  the  tracks  along  which  the  water  flows  down  steep 
rocks  of  the  kind  that  the  plants  of  which  we  have  made  mention  are  situated. 

Associated  with  the  above  are  generally  a  number  of  other  plants,  for  the  most 


RELATIONS    OF   SAPROPHYTES   TO   THEIR   NUTRIENT   SUBSTRATUM.  117 

part  microscopic,  all  of  which  cannot  be  classed  as  saprophytes,  but  which,  in  order 
to  be  able  to  thrive  in  the  tracks  of  trickling  water,  must  have  the  capacity  of 
surviving  desiccation  for  weeks,  and  even  months,  on  the  barren  rock  after  having 
been  previously  supplied  with  copious  moisture  for  a  time.  In  the  case  of  lichen- 
growths  in  particular  these  are  very  favourite  sites;  and  when  the  lichens  cover 
a  large  area  they  attract  one's  attention  from  afar.  In  limestone  ranges,  the 
light-gray  rock  of  steep  declivities,  interrupted  by  ledges  covered  with  grass  and 
low  brushwood,  is  extensively  coloured  by  dark  vertical  bands  and  streaks,  and  the 
effect  is  the  same  as  if  a  dye  had  flowed  from  the  ledges  over  the  face  of  the  rock. 
These  dark  streaks  indicate  the  course  of  the  water  which  oozes  from  the  humus 
and  renders  possible  the  existence  of  numberless  minute  plants  on  the  precipitous 
face,  in  particular  several  dark  crustaceous  lichens  (Acarospora  glaucocarpa, 
Aspicilia  flavida,  Lecidea  fuscorubens,  Opegrapha  lithyrga,  &c.). 

The  quantity  of  organic  compounds  brought  down  in  solution  by  the  water 
which  filters  from  the  layers  of  humus  on  rocky  ledges,  and  that  which  trickles 
down  the  bark  of  trees,  is,  however,  very  small.  Still,  it  is  amply  sufficient  to 
meet  the  requirements  of  the  plants  occurring  at  the  spots  in  question.  The  claims 
made  by  them  upon  their  nutrient  source  are  very  moderate.  We  may  here  recall 
the  instances  previously  mentioned  of  mycelia  of  fungi  which  have  been  found 
satisfied  with  the  scarcely  perceptible  quantities  of  organic  compounds  in  water 
filtering  into  the  shaft  of  a  mine,  and  in  the  pure  water  of  a  mountain  spring 
respectively.  To  these  instances  must  here  be  added  the  production  of  mycelia 
in  the  wooden  pipes  through  which  the  clear  water  of  mountain  springs  is  con- 
veyed. After  these  pipes,  which  are  made  from  the  trunks  of  pines,  have  been 
used  as  conduits  for  years,  and  their  inner  layers  of  wood  have  long  since  been 
washed  out,  the  mycelium  of  the  fungus  Lenzites  sepiaria  is  not  infrequently 
developed  within  them,  and  in  such  luxuriance,  indeed,  that  it  forms  great  yellowish- 
gray  flocculent  masses,  which  issue  from  the  pipe's  inner  surface,  and  float  in  the 
stream  of  running  water.  In  time  these  flocculent  masses  increase  in  the  clear 
spring- water  to  such  a  degree  that  the  pipes  become  completely  blocked,  and  the 
flow  of  water  is  arrested.  And  yet  the  water  conducted  through  the  pipes  is  so 
pure,  where  it  enters  into  and  issues  from  them,  that  the  residue  obtained  by  the 
evaporation  of  hundreds  of  litres  afforded  no  trace  of  any  organic  matter. 

Seeing  that  most  saprophytes  absorb  only  such  a  comparatively  small  amount 
of  organic  matter,  one  is  all  the  more  surprised  to  notice  that  a  large  number  of 
them  fall  suddenly,  at  certain  times,  into  the  opposite  extreme.  People  speak  of 
things  rapidly  produced  in  abundance  as  "  mushroom-growths  ",  and  as  "  shooting  up 
like  fungi ".  The  fructifications  of  many  fungi  are  in  fact  developed  with  a  rapidity 
which  borders  on  the  miraculous.  The  various  species  of  Coprinus  living  on  dung 
produce  their  long-stalked,  cap-shaped  fructifications  during  the  night,  and  by  the 
evening  of  the  next  day  the  caps  have  already  fallen  to  pieces,  and  are  in  a  state 
of  decomposition,  and  nothing  is  to  be  seen  in  their  place  but  a  black  deliquescent 
mass  like  a  blot  of  ink.  The  weight  of  this  fructification,  thus  matured  within 


118  RELATIONS   OF   SAPROPHYTES   TO   THEIR   NUTRIENT   SUBSTRATUM. 

twenty-four  hours,  is  certainly  many  times  as  great  as  that  of  the  entire  mycelium 
which  produced  it;  and  it  is  quite  incomprehensible  how  this  mycelium,  which  for 
weeks  only  achieves  a  moderate  development,  and  adds  but  little  to  its  dimensions, 
is  in  a  position  suddenly,  and  in  so  short  a  time,  to  supply  the  amount  of  water  and 
organic  compounds  requisite  for  the  building  up  of  the  fructification.  Epipogium 
aphyllum  exhibits  a  similar  property.  After  producing  nothing  for  two  years 
excepting  a  few  branches  on  its  subterranean  stem,  it  develops  all  at  once  and  in 
a  very  short  space  of  time  fleshy  stems  with  large  flowers,  and  one  asks  with 
astonishment  how  the  relatively  small  coral-shaped  stock  sets  about  obtaining  the 
quantity  of  nutrient  materials  necessary  for  the  construction  of  these  flowering 
stems.  We  are  here  confronted  again  with  the  great  mystery  of  periodicity,  the 
solution  of  which  we  must  for  the  present  forego. 

Saprophytes  are  much  more  fastidious  as  regards  the  quality  of  their  nutriment 
than  one  might  expect.  It  is  true  that  certain  fungi  are  produced  wherever  there 
are  plants  in  a  state  of  decomposition,  and  to  them  it  is  quite  indifferent  whether 
the  mouldered  dust,  which  serves  as  a  nutrient  soil  for  their  mycelia,  has  arisen 
from  one  species  or  another.  Also  in  the  case  of  orchids  imbedded  in  vegetable 
mould,  and  in  that  of  most  of  the  mosses  and  liverworts  adherent  to  the  barks  of 
trees,  it  is,  as  a  rule,  of  no  consequence  whether  the  tree  constituting  the  substratum 
is  a  conifer  or  a 'dicotyledon.  But  a  large  number  of  species  are  associated  with  the 
decaying  remains  of  particular  plants  or  animals  only.  For  example,  certain  small 
species  of  Marasmius,  belonging  to  the  group  of  the  Agarici,  occur  only  on  moulder- 
ing pine-needles;  another  small  fungus,  Antennatula pinophila,  is  found  exclusively 
on  fallen  needles  of  the  Silver  Fir;  Hypoderma  Lauri,  which  resembles  small  black 
type  on  rotting  laurel  leaves,  and  the  tiny  Septoria  Menyanihis  on  leaves  of  the 
Bog -bean  (Menyanthes  trifoliata)  lying  under  water  in  a  state  of  decay.  The 
cinnamon -coloured  receptacles  of  Lenzites  sepiaria  only  grow  from  prostrate 
trunks  of  conifers,  and  the  black  fuliginous  fructifications  of  Bulgaria  polymorpha 
only  on  those  of  oaks.  A  small  discoid  fungus  named  Poronia  punctata,  white 
with  black  spots  on  the  top,  is  only  found  on  cow-dung;  another  fungus,  Gymnoascus 
uncinatus  on  that  of  mice,  and  Ctenomyces  serratus  on  decaying  goose  feathers. 

That  many  mosses  are  also  very  fastidious  in  the  selection  of  their  substratum 
has  already  been  intimated.  Just  as  in  the  Alps  Splachnum  ampullaceum  is  only 
found  growing  on  the  putrefying  dung  of  cattle,  so  in  arctic  regions  the  splendid, 
large-fruited  Splachnum  luteum  and  8.  rubrum  occur  exclusively  on  that  of  rein- 
deer. Tetraplodon  urceolatus  is  met  with  on  mountains  always  with  decaying 
excrements  of  chamois,  goats,  or  sheep  for  a  substratum,  whilst  Tetraplodon 
angustatus  chooses  the  excrements  of  carnivorous  animals,  and  Tayloria  serrata 
is  only  seen  near  cow-chalets  on  decomposing  human  faeces.  The  circumstances 
of  the  occurrence  of  another  moss  belonging  to  the  Splachnacese,  i.e.  Tayloria 
Rudolfiana  is  also  very  interesting.  It  grows  usually  on  the  branches  of  old 
trees,  especially  maples  in  sub-alpine  regions,  and  one  is  tempted  to  believe  that  in 
respect  of  its  nutrient  substratum  it  is  an  exception  to  the  rule  of  the  rest  of  the 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS.  119 

Splachnacese.  But  on  closer  examination  there  is  convincing  evidence  that  this 
moss  also  lives  only  on  animal  dung  undergoing  putrefaction.  For  remains  of 
broken  mouse  and  bird  bones  are  invariably  to  be  discovered  in  the  substratum, 
and  there  can  be  no  doubt  that  the  Tayloria  chooses  for  its  site  boughs  of  old 
trees  upon  which  birds  of  prey  have  dropped  their  excrements.  Of  the  mosses 
living  on  the  bark  itself,  one  instance  is  also  worth  mentioning.  Whereas  in  the 
case  of  most  species  of  the  genus  Dicranum,  the  mouldering  residues  of  conifers 
constitute  the  favourite  substratum;  there  is  one  species,  viz.  Dicranum  Sauteri, 
which  is  found  only  on  the  bark  of  the  beech.  The  weather-worn  bark  of  this 
tree  is  seen,  in  sub-alpine  districts,  covered  with  the  most  brilliant  emerald-green 
films  of  the  above-named  moss;  whilst  on  adjacent  pines  and  fir-trees  no  trace  of  it 
can  be  found. 

PLANTS  WITH  TRAPS  AND  PITFALLS  TO  ENSNARE   ANIMALS. 

A  number  of  plants  exhibit  contrivances  which  obviously  have  for  their  object 
the  capture  and  retention  of  such  small  creatures  as  may  fly  or  creep  on  to  their 
leaves;  and  it  has  been  ascertained  by  searching  experiments  that  the  majority  of 
these  plants  use  the  animals  they  capture,  in  one  way  or  another,  as  sources  of 
nutriment.  For  the  most  part  the  animals  that  are  caught  are  insects,  and  hence 
the  term  "insectivorous  plants"  has  been  applied  to  the  class  in  question.  The 
flesh  of  the  insect  being  the  part  of  it  principally  serviceable  for  food,  the  name 
"carnivorous"  or  "flesh-eating",  or  better,  perhaps,  "flesh-consuming"  plants  has 
also  been  used;  and  seeing  that  the  most  important  part  of  the  whole  process  is- 
really  the  digestion,  or  taking  in  of  organic  compounds  from  the  captured  animals 
after  they  are  dead,  we  might  call  those  plants  which  are  furnished  with  organs- 
for  the  absorption  of  the  dissolved  flesh  of  animals  ensnared  by  them,  "  flesh-digest- 
ing "  plants  as  well.  As  will  appear  from  the  following  discussion  of  the  subject,. 
no  one  of  these  names  completely  covers  the  wonderful  phenomena  in  question, 
and  it  is  scarcely  possible  to  find  a  short  and  not  too  cumbrous  expression  which 
shall  henceforward  exclude  all  misconceptions. 

In  round  numbers  we  may  estimate  the  plants  which  capture  animals  and 
demolish  them  for  food  at  five  hundred.  Within  this  comparatively  small  range, 
however,  the  variety  of  the  mechanism  for  seizure  and  absorption  of  nutritive 
matter  is  so  great  that  in  order  to  give  a  general  picture  of  them  it  is  necessary  to 
classify  them  into  several  sections  and  groups.  In  the  first  section  we  have  a  series 
of  plant-forms  wherein  chambers  are  developed,  which  admit  of  the  entrance  of 
small  animals,  but  not  of  their  escape.  The  organs  of  capture  and  digestion  of  the 
plants  belonging  to  this  section  exhibit  no  external  movements  of  any  kind,  and 
are  thereby  differentiated  from  the  forms  belonging  to  the  second  section,  which 
perform  definite  movements,  in  response  to  a  stimulus  caused  by  the  contact  of  the 
animals,  with  the  object  of  covering  the  prey  with  as  great  a  quantity  of  digestive 
fluid  as  possible.  Lastly,  there  is  a  third  section  wherein  the  individual  forms  are 


120 


PLANTS   WITH   TRAPS   AND    PITFALLS   TO   ENSNARE   ANIMALS. 


neither  provided  with  pitfalls  nor  capable  of  performing  special  movements,  but 
have  leaves  converted  into  lime-twigs  and  on  them  animals  stick  and  are  also 

digested. 

The  first  and  most  extensive  group  included  in  the  first  section  is  that  of 
Utricularise  or  Bladderworts.  Their  capturing  apparatus  consists  of  little  bladders 
with  orifices  closed  in  each  case  by  a  valve,  which  permits  objects  to  penetrate  into 


Fig.  17.— Bladderworts. 
In  the  foreground  Utricularia  Graflana;  in  the  background  Utricularia  minor. 

the  cavity  of  the  bladder,  but  not  to  issue  out  of  it.  The  Utriculariae  are  rootless 
plants  which  live  suspended  in  water,  and,  according  to  the  season  of  the  year, 
either  sink  down  to  the  bottom  or  ascend  to  just  below  the  surface.  Upon  the 
approach  of  winter,  when  animal  life  is  gradually  disappearing  in  the  chilled  and 
freezing  upper  layers  of  water,  the  leaves  at  the  extremities  of  the  floating  stems 
are  enlarged  and  form  spherical  winter  buds;  the  older  parts  of  the  stems  together 
with  the  leaves  die,  their  cavities  hitherto  occupied  by  air  are  filled  with  water,  and 
they  sink  to  the  bottom  drawing  down  with  them  the  winter  buds.  After  the 
winter  these  buds  elongate,  detach  themselves  from  the  old  stems  and  ascend  near 
the  surface,  where  innumerable  little  aquatic  animals  are  swimming  to  and  fro,  and 
there  develop  two  rows  of  lateral  branches  in  rapid  succession.  Either  all  of  these 
are  thickly  covered  with  leaves  which  are  divided  into  thread-like,  repeatedly 


PLANTS   WITH  TRAPS  AND   PITFALLS  TO   ENSNARE   ANIMALS.  121 

bifurcating,  segments,  or  else  only  half  of  them  are  thus  clothed  with  leaves  whilst 
the  other  half  bear  the  before-mentioned  bladders.     The  former  is  the  case  in 
Utmculama  minor,  the  plant  represented  in  the  background  of  the  figure  on  p  120- 
and  the  latter  in   Utricularia  Grafiana,  which  is  drawn  in  the  foreground       In 
instances  of  the  former  kind  obliquely  ellipsoidal  bladders  are  to  be  seen  on  short 
stalks  on  the  principal  segments  of  the  leaves,  usually  quite  near  their  angles  of 
bifurcation.    In  the  smaller  species,  such  as  Utricularia  minor,  they  have  a  diameter 
of  about  2  mm.     In  individuals  of  the  latter  kind  the  bladders  have  longer  stalks 
and  are  about  5  mm.  in  diameter.     They  are  always  pale-green  and  partially  trans- 
parent.    Each  bladder  is  somewhat  flattened  at  the  sides  and  exhibits  a  markedly 
convex  dorsal  surface  and  slightly  curved  lateral  surface.    An  orifice,  whose  border 
is  fringed  with  peculiar  stiff  tapering  bristles,  leads  into  the  interior  of  each  of 
these   stalked  bladders.      The  aperture  has  four  rounded   angles  and  is  framed 
as    it    were,    by    a 
pair   of    lips.      The 
under  lip  is  strong- 
ly   thickened,     and 
is  furnished  with  a 
solid  cushion  projec- 
ting  into  the  inte- 
rior of  the  bladder. 
From  the  upper  lip 
hangs  a  thin  trans- 
parent,     obliquely- 
placed  valve  (see  fig.  18  2),  the  free  edge  of  which  rests  upon  the  inner  surface  of 
the  cushion  before  referred  to,  and  closes  the  entire  orifice.     This  valve  is  very 
elastic  and  yields  easily  to  any  pressure  from  outside.     A  tiny  animal  is  able,  by 
pressing  against  it,  to  force  a  way  without  difficulty  from  the  nether  lip  into  the 
interior  of  the  bladder,  and  to  slip  in  through  the  opening  thus  made.     But  as  soon 
as  the  animal  has  got  inside,  and  ceases  to  press  upon  the  valve,  its  elasticity  brings 
it  back  upon  the  under  lip  again.     It  cannot  be  opened  by  pressure  from  within; 
for,  resting  as  it  does  upon  the  projecting  cushion,  it  is  impossible  for  the  little 
prisoner  to  force  it  over  the  latter  in  an  outward  direction. 

The  whole  apparatus  forms  a  trap  for  small  aquatic  animals,  they  being  able,  as 
before  observed,  to  slip  into  the  bladder  but  not  to  get  out  again.  Most  animals 
that  enter  make,  it  is  true,  efforts  to  escape,  but  they  are  all  in  vain.  Many  perish 
in  a  short  time — about  twenty-four  hours — others  live  from  two  to  three,  or,  in 
some  cases,  even  as  much  as  six  days.  But  in  the  end  they  must  suffer  death  by 
suffocation  or  starvation,  and  they  then  decay,  and  the  products  of  their  decomposi- 
tion are  sucked  in  by  special  absorption  cells  developed  within  the  bladder.  These 
absorption  cells  (see  fig.  18 3)  are  linear-oblong  and  somewhat  like  little  rods  in 
shape,  and  they  line  the  whole  internal  surface  of  the  cavity  of  the  bladder.  They 
are  arranged  in  fours,  each  group  of  four  forming  a  cross  and  being  united  by  a 


Fig.  18.— Traps  of  Utricularia  neglecta. 

i  A  bladder  magnified  (  x  4).    2  Section  of  a  bladder,    s  Absorption-cells  on  the  internal 
surface  of  the  bladder  (  x  250). 


122  PLANTS    WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS 

common  basal  cell.  The  basal  cells  themselves  are  intercalated  amongst  the  cells 
lining  the  bladder.  The  organic  substances  from  the  decaying  bodies  of  captured 
animals  are  sucked  up  by  these  stellate  groups  of  cells,  and  from  them  pass  into  the 
basal  cells,  and  later,  into  the  other  adjacent  cells  of  the  bladder  and  those  of  the 
plant  at  large. 

The  majority  of  the  animals  caught  by  the  bladders  are  crustaceans.  It  is 
principally  larvae  and  adult  individuals  of  small  species  of  Cypris,  Daphnia,  and 
Cyclops  that  fall  into  the  trap;  but  larvae  of  gnats,  and  various  other  small 
insects,  little  worms,  and  infusoria,  are  also  not  infrequently  met  with  imprisoned 
in  the  bladders.  The  number  of  animals  captured  is  comparatively  large.  In 
single  bladders  the  remnants  of  no  less  than  twenty-four  small  crustaceans  have 
been  observed.  The  prey  secured  by  Utricularia  minor  (fig.  17),  which  lives  in 
little  pools  of  still  water  in  peat-bogs,  is  very  abundant.  The  North  American 
Utricularia  clandestina  seems  also  to  use  its  capturing  apparatus  with  great 
success. 

What  it  is  that  induces  the  animals  to  press  upon  the  valves  and  so  fall  into  the 
trap  is  not  fully  explained.  We  may  suppose  that  they  expect  to  find  food  in  the 
bladder-cavity,  or  that  they  hope  it  will  afford  a  shelter  where  they  can  rest  for 
a  time  and  be  protected  from  their  pursuers.  The  last  suggestion  is  especially 
supported  by  the  circumstance  that  the  approach  to  the  valve-covered  orifice  of 
the  bladder  is  guarded  against  the  intrusion  of  larger  animals  by  stiff  sharp  bristles 
which  stick  out  from  it  (fig.  IS1).  Only  very  small  animals,  which  can  easily  slip 
in  between  the  relatively  large  bristles,  reach  the  inside  of  the  bladder,  whilst 
larger  creatures,  which  would  injure  the  whole  apparatus,  are  prevented  from 
coming  near  it.  Thus,  the  most  probable  explanation  is  that  lesser  animals  pursued 
by  greater  take  refuge  in  the  hiding-places  behind  the  bristles,  and  so  fall  into  the 
trap.  Another  very  striking  fact  is  that  the  bladders  of  Utriculariae,  living  in  still 
water,  look  delusively  like  certain  Ostracoda,  especially  species  of  the  genus 
Daphnia.  The  bladder  itself  resembles  the  shell-covered  body  in  size  and  form, 
and  the  bristles  the  antennae  and  swimmerets  of  one  of  these  crustaceans.  Whether 
there  is  any  significance  in  this  curious  similarity  of  outward  appearance  must  be 
left  undecided. 

The  majority  of  Utriculariae  live  in  pools  of  water  beside  foot- tracks  on  moors 
and  in  the  little  collections  of  water  between  clumps  of  reeds  in  peat-bogs;  and 
these  are  precisely  the  haunts  of  the  little  creatures  that  are  to  fall  into  the  traps. 
Every  handful  of  water  that  one  scoops  up  contains  hundreds  of  midge -larvae, 
water-fleas,  Ostracoda,  and  one-eyed  Cyclops,  which  rush  about  promiscuously, 
pursuing  and  seizing  one  another.  One  species  of  these  plants  lives  in  the  moun- 
tains of  Brazil  in  the  rain-filled  receptacles  of  Tillandsia  plants.  The  Tillandsia 
is  allied  to  the  pine-apple,  and  has  rosettes  of  concave  leaves,  the  latter  resting  one 
upon  the  other  in  such  a  way  as  to  form  a  niche  or  cavity  in  front  of  each  leaf 
which  fills  with  rain  like  a  cistern.  Many  different  kinds  of  small  animals  are 
always  swimming  about  in  these  little  cisterns,  and  almost  every  one  of  the  latter 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS.  123 

is  the  sphere  of  activity  of  an  individual  Utricularia  nelumbifolia.  This  plant 
is  remarkable  also  from  the  fact  that  long  runners  are  thrown  out  from  its  stems, 
which  grow  across,  in  wide  arches,  from  its  cistern  to  a  neighbouring  Tillandsia, 
where  it  selects  one  of  the  reservoirs  in  the  rosettes  as  a  new  site  and  dips  down 
into  the  water — a  fantastic  method  of  propagation  of  which  we  shall  speak  again 
on  another  occasion. 

A  few  Utricularise  do  not  live  in  water  at  all,  but  grow  amongst  mosses,  liver- 
worts, and  lycopods,  in  the  vegetable  mould  filling  the  clefts  and  crevices  of  rocks, 
and  the  bark-fissures  of  old  trees.  Of  this  habit,  for  example,  is  the  pretty 
Brazilian  Utricularia  montana,  which,  in  spite  of  the  difference  of  its  habitat,  is 
provided  with  an  apparatus  for  capturing  animals  agreeing  in  all  essential  respects 
with  the  description  already  given.  The  bladders  used  by  these  plants  for  pur- 
poses of  prey  are  produced  on  subterranean  filiform  stems  which  thread  their  way 
in  the  vegetable  mould  and  wefts  of  decaying  moss-stems,  and  here  and  there  swell 
into  tubers.  The  bladders  are  hyaline  and  transparent,  and  are  filled  with  watery 
liquid,  sometimes  also  with  air.  They  are  only  1  millimeter  in  diameter,  but  are 
present  in  large  numbers.  The  entrance  into  these  bladders  is  much  more  con- 
cealed than  in  the  species  that  live  in  water.  The  dorsal  surface  of  the  bladder 
being  still  more  strongly  curved,  the  position  of  the  orifice  is  altered  so  as  to  be 
quite  close  to  the  little  stalk  of  the  bladder.  In  addition,  the  orifice  is,  as  it  were, 
roofed  over,  and  thereby  protected  against  the  possibility  of  being  stopped  up  by 
particles  of  earth,  and  the  passage  leading  to  it  is  very  narrow.  That,  in  spite  of 
the  difficulty  of  entrance,  a  number  of  minute  animals  do  seek  a  hiding-place  here 
is  proved  by  the  circumstance  that,  besides  various  infusoria,  rhizopoda,  and 
creatures  of  that  kind  inhabiting  damp  earth,  species  of  Acarus  and  larvae  of 
other  animals  have  been  found,  both  dead  and  alive,  in  the  bladders. 

With  this  first  group  of  insectivorous  plants,  wherein  the  capturing  apparatus 
includes  a  valve  to  prevent  the  egress  of  such  animals  as  fall  into  the  trap,  is 
associated  in  the  first  section  a  second  group,  viz.  that  of  the  ascidia-bearing  or 
pitcher-plants,  in  which  the  foliage -leaves  are  converted  into  pitfalls,  and  the 
escape  of  the  captured  prey  prevented  by  a  number  of  points  lining  the  inner 
wall  of  the  cavity,  and  directed  from  the  aperture  towards  the  closed  bottom. 
There  is  an  extraordinary  variety  in  the  form  of  the  pitfalls.  Sometimes  they 
are  tubular,  utricular,  or  funnel-shaped  cavities,  sometimes  mug  or  pitcher-shaped, 
or  urceolate;  in  some  cases  these  are  straight,  in  others  bowed  like  sickles,  or 
spirally  twisted.  They  always  arise  from  the  part  of  the  petiole  upon  which  the 
lamina  immediately  rests.  The  lamina  is  always  relatively  small,  being  represented 
in  the  majority  of  the  traps  by  a  scale  or  lobe,  and  it  only  appears  to  be  an 
appendage  of  the  large  expanded  and  hollowed -out  petiole.  In  many  pitcher- 
plants  the  little  lamina  looks  like  a  lid  placed  over  the  orifice  to  the  pitfall,  as, 
for  instance,  is  shown  in  the  illustration  (fig.  21  4),  whilst  in  others  (Nepenthes 
ampullaria  and  N.  vittata)  it  has  the  form  of  a  handle  or  stalk,  and  serves  as  a 
place  for  animals  visiting  the  pitchers  to  alight  upon. 


124 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 


In  each  pitfall  there  are  always  three  kinds  of  contrivance  to  be  distinguished: 
first,  a  device  for  the  allurement  of  animals;  secondly,  an  arrangement  for  entrap- 
ping the  prey  enticed,  which  at  the  same  time  prevents  individuals  once  imprisoned 
from  returning  and  escaping  through  the  entrance  hole;  and  thirdly,  a  structure 
for  causing  the  decay  or  dissolution  of  the  dead  animals  at  the  bottom  of  the  pit- 
falls, and  for  rendering  possible  the  absorption  of  the  products  of  decomposition 
as  nutriment.  The  means  of  allurement  are  similar  to  those  which  cause  the  visits 
of  small  creatures  to  flowers,  that  is  to  say,  principally  honey  and  bright  and 
varied  coloration,  whereby  the  nectar -secreting  spots  are  recognized  from  afar, 
especially  by  flying  insects.  The  escape  of  animals  when  they  have  once  entered 
the  cavity  of  a  petiole  is  prevented,  as  has  been  already  mentioned,  by  a  fringe 
of  sharp  hairs  pointed  downwards,  or  by  various  spinous  structures  on  the  inner 


Fig.  19.— Spinous  Structures  in  the  Pitfalls  of  Carnivorous  Plants. 

1  Genlisea;  a  piece  of  the  tube  seen  from  inside.  2  Heliamphora  nutans;  spines  on  the  walls  of  pitfalls.  »  Sarracenia 
purpurea;  a  piece  of  the  lining  of  the  pitcher  near  the  orifice  seen  from  inside.  •*  Sarracenia  purpurea;  longitudinal 
section  through  the  membrane  covered  with  spinous  bristles  in  the  lower  part  of  the  pitcher,  &  Nepenthes  hybrida; 
fringe  of  spines  at  the  orifice  of  the  pitcher.  l,  2,  *,  6  greatly  magnified ;  s  slightly  magnified. 

surface  of  the  cavity.  The  decomposition  and  dissolution  of  the  prey  are  effected 
by  fluids  secreted  by  special  cells  at  the  bottom  of  the  utricles  and  pitchers. 

But  although  in  respect  of  the  consecutive  and  co-ordinate  operation  of  these 
three  kinds  of  contrivance,  all  ascidia- bearing  and  pitcher -plants  resemble  one 
another,  there  are  considerable  individual  divergences  as  to  structure  and  function 
that  it  is  well  worth  while  to  study  in  some  detail  the  most  noticeable  of  them. 

One  of  the  most  noteworthy  is  the  genus  Genlisea,  which  is  nearly  related  to 
Utriculariacese  in  the  structure  of  its  flowers  and  fruit.  It  is  composed  of  a  dozen 
species  growing  in  water  and  marshy  places.  Of  these  one  is  a  native  of  tropical 
and  southern  Africa,  whilst  others  are  found  in  Brazil  and  the  West  Indies.  In 
addition  to  ordinary  leaves,  which  in  them  are  spatulate,  most  of  the  Genlisese 
possess  leaf-structures  metamorphosed  so  as  to  constitute  pitfalls.  Each  pitfall 
consists  of  a  long,  narrow,  cylindrical  utricle,  which  at  its  blind  end  is  enlarged 
into  a  bladder,  whilst  at  the  narrow  orifice  at  the  opposite  end  are  placed  two 
peculiar  ribbon-shaped  processes  twisted  spirally.  The  orifice  of  the  utricle  is  set  with 
very  small  sharp  teeth  bent  inwards;  and  the  tubular  part  of  the  utricle  has  its 
inner  surface  lined  throughout  with  innumerable  little  bristles,  which  arise  from  rows 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 


125 


of  cells  forming  inwardly  projecting  ridges,  and  have  their  sharply-pointed  tips 
directed  downwards  (see  fig.  19 l).  Amongst  these  needles  are  also  found,  scattered 
over  the  whole  internal  surface,  roundish  wart-like  glands  or  papillae,  composed  of 
four  or  eight  cells.  The  bottom  of  the  bladder-like  cavity  in  which  the  utricle 
terminates  is  destitute  of  bristles,  and  provided  only  with  glands  arranged  in  rows. 
Small  worms,  mites,  and  other  segmented  animals  which  enter  through  the  orifice 
of  the  utricle  can  easily  reach  the  enlarged  base.  But  as  soon  as  they  try  to  corn- 


Fig.  20. — Sarracenia  purpurea. 

mence  the  return  journey  they  are  opposed  by  the  points  of  a  thousand  bristles. 
Thus  caught  they  die,  and  the  products  arising  from  the  decay  of  their  bodies  are 
absorbed  by  the  glands  situated,  as  above  mentioned,  at  the  bottom  of  the  bladder 
and  on  the  walls  of  the  utricle. 

As  types  of  a  second  series  of  carnivorous  plants  belonging  to  the  group  of 
pitcher-plants  may  be  taken  Heliamphora  nutans,  a  native  of  moorlands  on  the 
mountains  of  Koraima,  on  the  borders  of  British  Guiana,  and  Sarracenia  purpurea 
(see  fig.  20),  which  is  widely  distributed  in  the  marshes  of  eastern  North  America 
from  Hudson's  Bay  to  Florida.  In  both  instances  the  leaves  metamorphosed  into 
ascidia  are  arranged  in  rosettes,  rest  their  bases  on  damp  earth  and  thence  curve 
upwards.  They  are  somewhat  inflated,  like  bladders,  at  about  their  middle,  but 
contract  again  at  the  orifice  where  they  pass  into  the  relatively  small  laminae. 
The  latter  are  threaded  by  red  streaks  like  blood-vessels,  have  the  form  of  valves, 


126  PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 

and  turn  their  concave  surfaces  towards  falling  rain.  They  serve,  moreover,  at 
least  in  Sarracenia  purpurea,  to  catch  the  drops  of  rain,  which  then  flow  down 
into  the  bottom  of  the  ascidia  and  fill  them  more  or  less  writh  water.  There  is 
very  little  evaporation  from  the  hollow  pitchers;  and  even  when  there  has  been  no 
rain  for  a  week,  one  always  finds  some  of  the  previously-collected  water  at  the 
bottom.  The  inner  surface  of  a  pitcher  is  lined  by  cells  arranged  like  the  scales 
of  enamel  on  a  pike's  back  (see  fig.  19  2).  The  internally-projecting  wall  of  each 
of  these  scales  is  transformed  into  a  stiff  decurved  point,  and  the  lower  the  position 
of  the  cells  the  longer  do  the  points  become.  The  shell-like  lamina  again,  above 
the  contracted  orifice,  bears  glandular  hairs  which  exude  honey,  so  that  the  parts 
surrounding  the  aperture  are  covered  by  a  thin  film  of  sweet  juice. 

Many  animals  are  attracted  by  this  honey.  Some  are  winged  and  alight  from 
flying;  others,  being  wingless,  make  use  of  a  peculiar  ridge,  which  projects  on  the 
concave  side  of  the  utricle,  to  help  them  to  creep  up  the  latter.  If  these  honey- 
eaters  happen  to  travel  away  from  the  lamina  to  that  part  of  the  pitcher  which 
is  lined  with  the  smooth  and  slippery  decurved  cells,  they  are  as  good  as  lost. 
They  slip  down  over  the  brink,  every  attempt  to  climb  up  again  being  rendered 
futile  by  the  downwardly-pointing  needles  which  clothe  the  lower  part  of  the  wall; 
and  ultimately  they  fall  into  the  water  collected  at  the  bottom,  where  they  are 
drowned  and  their  bodies  putrefy.  The  products  of  decay  are  absorbed  as 
nutriment  by  the  epidermal  cells  in  this  region.  The  number  of  animals  meeting 
with  this  fate  is  often  so  great  that  an  offensive  odour,  arising  from  the  decaying 
bodies,  is  emitted  by  the  utricles  and  is  noticeable  at  a  considerable  distance.  In 
the  wild  state,  the  ascidiform  utricles  are  often  half -full  of  drowned  animals  and 
it  is  stated  that  in  these  circumstances  birds  also  put  in  an  appearance  and  pick 
some  of  the  dead  remains  out  of  the  utricles. 

Whether  the  liquid  filling  the  bottom  of  the  pitchers  consists  simply  of  rain- 
water, or  whether  the  latter  is  modified  by  a  secretion  originating  in  the  gland- 
like  groups  of  cells  there  (see  fig.  28 7),  is  still  uncertain.  A  centipede  over 
4  centimeters  long  having  fallen  into  a  utricle  of  Sarracenia  purpurea  in  the 
night  was  found  only  half  immersed  in  the  water.  The  upper  half  of  the  creature 
projected  above  the  liquid,  and  made  violent  efforts  to  escape;  but  the  lower  part 
had,  after  a  few  hours,  not  only  become  motionless  but  had  turned  white  from  the 
effect  of  the  surrounding  liquid;  it  appeared  to  be  macerated,  and  exhibited 
alterations  which  are  not  produced  in  so  short  a  time  in  centipedes  immersed  in 
ordinary  rain-water.  When  a  number  of  captured  animals  are  undergoing  putre- 
faction at  the  same  time  in  a  pitfall,  the  liquid  turns  brown  and  has  the  appearance 
of  manure-liquor. 

There  is  a  great  difference  between  the  utricles  of  Sarracenia  purpurea  and  the 
apparatus  adapted  to  the  capture  of  prey  in  the  plants  of  which  we  have  chosen  as 
examples,  Sarracenia  variolaris,  a  native  of  the  marshes  of  Alabama,  Florida,  and 
Carolina,  and  the  JDarlingtonia  Californica,  found  growing  at  a  height  of  from 
300  to  1000  meters  above  the  sea  on  Calif ornian  uplands  from  the  borders  of 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO    ENSNARE   ANIMALS.  127 

Oregon  to  Mount  Shasta.  In  both  of  these  the  liquid  with  an  acid  reaction,  which 
fills  the  bottom  of  each  utricle,  is  certainly  only  secreted  by  the  cells  in  the  interior 
of  the  cavity  itself,  and  it  is  quite  impossible  that  a  single  drop  of  the  rain  or  dew 
deposited  upon  the  plant  should  reach  the  interior  of  the  cavity.  The  hollow 
petiole  is  in  both  plants,  above  mentioned,  utricular  or  tubular,  and  only  slightly 


kFig.  21.—  Ascidia-bearing  and  Pitcher-plants. 
Sarracenia  variolaris.     2  Darlingtonia  Cali/ornica.    »  Sarracenia  laciniata.    *  Nepenthes  villosa,  reduced  to  one-half 
natural  size, 
irged  towards  the  top.     The  dorsal  side  of  each  leaf  is,  however,  at  its  upper 
end   hollowed  out  like  a  cowl  or  a  helmet,  and   forms  a  cupola  as  is  shown  in 
fig.  21  *  and  21 2.     The  orifice  or  entrance  into  the  utricle  is  consequently  covered 
over  and   is   reduced   to   a   slit  or  hole  under  the   hood.     The   lamina  is   trans- 
formed into  a  lobe,  which  in  Sarracenia  variolaris  is  small  and  roofs  over  the 
orifice  of   the   utricle,  and   in   Darlingtonia   is   shaped  like   the   tail  of   a   fish, 
and  hangs  down  in  front  of  the  aperture.     The  lower  part  of  the  utricle  is  of  a 
uniform  green  colour,  but  the  upper  part  (i.e.  the  cupola  and  lobe-like  appendage) 
has  red  ribs  and  veins,  and  here  and  there  is  quite  purple.     Between  the  veins  the 


128  PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 

leaf  is  thin,  translucent,  and  pale-green  or  whitish;  and  these  clear  translucent 
patches,  framed  by  purple  or  green  ribs,  look  as  if  they  were  little  windows, 
especially  when  seen  from  within  the  utricle.  The  mixture  of  green,  red,  and  white 
gives  the  upper  parts  of  the  leaves  such  a  gay  appearance  that,  from  a  distance,  they 
might  be  mistaken  for  flowers. 

Insects  are  doubtless  attracted  by  these  bright  colours,  and  both  round  the 
orifice,  and  on  the  inner  surface  of  the  cupola,  they  find  exudations  of  honey  which 
they  suck  or  lick  up  with  avidity.  In  Sarracenia  variolaris,  honey  is  to  be  seen 
besides,  on  the  edge  of  a  broad  free  border  which  is  decurrent  along  the  utricle,  and 
extends  from  the  ground  to  the  orifice.  This  border  forms  a  favourite  path  for 
wingless  insects,  especially  ants,  which  are  particularly  eager  in  their  quest  for 
honey.  For  them  it  is  a  sure  way  to  destruction,  for  when  they,  gradually 
following  the  honey-baited  pathway,  arrive  at  the  orifice  to  the  utricle  and  pass 
through  it,  they  inevitably  get  upon  the  smooth  decurved  points  of  the  epidermal 
cells,  constructed  just  like  those  in  Sarracenia  purpurea,  and  then,  unable  to  stop 
themselves,  slip  down  to  the  bottom  of  the  pitcher.  When  small  winged  insects 
alight  from  flying  and  fall  down  the  slide  into  the  interior,  they  make  use  of  their 
wings  in  the  hope  of  saving  themselves,  but  they  never  succeed  in  finding  the 
aperture  by  which  they  entered,  as  it  slants  downwards  and  is  situated  in  shadow. 
They  invariably  try  to  escape  through  the  cupola,  mistaking  the  thin  portions, 
through  which  the  light  penetrates  into  the  interior,  for  gaps  permitting  egress. 
But  just  as  flies  in  rooms  dash  against  the  windows  hoping  to  pass  through  them 
into  the  open  air,  so  the  small  insects  in  the  utricles  of  Sarracenia  variolaris  and 
Darlingtonia  Califomica  knock  against  these  windowed  cupolas,  in  their  desire 
to  save  themselves  by  flying  through.  They  always  fall  down  again  to  the  bottom 
of  the  utricle  as  though  into  a  cistern.  If  they  are  immersed  in  the  liquid  there 
secreted,  or  only  in  partial  contact  with  it,  they  are  stupefied,  but  not  immediately 
killed.  They  often  live  incarcerated  for  two  days,  and  it  would  therefore  be 
erroneous  to  suppose  that  the  fluid  in  the  pitchers  acts  on  the  prey  as  a  deadly 
poison.  But  it  assists  the  decay  and  dissolution  of  the  captives  as  they  die  of 
starvation  and  suffocation,  and,  as  in  the  case  of  the  utricle-plants  previously 
described,  a  brown  liquor  of  very  unpleasant  odour  is  produced,  and  there  is  a 
residue  of  solid  pieces  of  skeleton  difficult  to  decompose,  such  as  the  wing-cases, 
claws,  and  thoraces  of  various  beetles,  lice,  ants,  and  other  small  insects  which  have 
shared  the  same  unlucky  fate. 

The  number  of  animals  captured  is  very  considerable.  The  pitchers  of 
Sarmcenia  variolaris,  which  attain  to  a  length  of  30  cm.,  are  usually  found,  when 
growing  in  their  natural  habitat,  filled  to  a  height  of  from  8  to  10  cm.  with  animal 
remains,  and  even  a  heap  15  cm.  high  has  been  observed.  We  must  here  remark 
that  in  the  ascidia,  of  Sarracenia  variolaris,  wingless  insects,  which  creep  about  the 
earth,  are  found  to  predominate,  whilst  in  Darlingtonia,  on  the  contrary,  most  of 
the  insects  are  winged.  The  cause  of  this  is  easily  understood.  The  former  plant 
has  honey  exuding  on  the  flap  or  ridge  running  down  from  the  orifice  to  the 


PLANTS   WITH  TRAPS   AND   PITFALLS  TO   ENSNARE   ANIMALS.  129 

ground,  and  many  wingless  insects  are  thus  induced  to  climb  up  the  alluring  path 
and  to  enter  the  cavity  of  the  pitcher.  Darlingtonia,  on  the  other  hand,  is 
destitute  of  honey  on  its  decurrent  ridge,  and  only  provides  the  sweet  meal  at  the 
top  in  the  vicinity  of  the  orifice,  where  it  is  available  for  flying  insects,  which,  as  a 
rule,  only  visit  nectar-secreting  flowers.  The  purplish-red  scale,  shaped  like  a  fish's 
tail,  and  hung  out  like  the  sign-board  of  an  inn  in  front  of  the  entrance  to  the 
pitcher,  constitutes  an  instrument  for  the  attraction,  from  afar,  of  these  winged 
creatures,  which  are  endowed  with  a  vivid  sense  of  colour;  and,  as  experience 
shows,  it  does  not  fail  in  its  object. 

What  significance  is  to  be  attributed  to  the  spiral  torsion  of  Darlingtonia 
leaves  (see  fig.  21 2)  it  is  difficult  to  say.  Perhaps  the  escape  of  animals  once 
imprisoned  in  the  depths  of  a  pitfall  is  hereby  rendered  more  remote.  It  would  at 
all  events  be  much  more  difficult  for  an  insect  trying  to  escape  by  the  use  of  its 
wings  to  ascend  a  canal  which,  in  addition  to  being  lined  with  decurved  points, 
was  spirally  wound,  than  a  similar  canal,  straight  and  widened  towards  the  top. 
We  must  not  omit  to  mention  that  a  few  flies  and  a  small  moth  have  selected  as 
their  ordinary  habitat  the  pitchers  of  both  the  plants  just  described,  in  spite  of  their 
being  so  fatal  to  most  insects.  The  grubs  of  a  blow-fly  (Sarcophaga  Sarracenice), 
in  particular,  live  in  large  numbers  amidst  the  heaps  of  decaying  insect  bodies  at 
the  bottom  of  the  pitchers,  and  are  there  nourished  just  as  are  the  grubs  of  allied 
species  in  the  rotten  flesh  of  birds  and  mammals.  When  mature,  the  grubs  quit 
the  environment  of  dead  remains,  passing  through  holes  which  they  bore  in  the 
side  wall  of  the  pitcher,  and  turn  into  chrysalises  in  the  earth.  But  the  fly  itself 
can  without  danger  pass  in  and  out  of  the  pitfalls,  which  are  so  perilous  in  the  case 
of  other  insects,  and  it  is  enabled  to  do  this  by  means  of  the  special  structure  of  its 
feet.  On  the  last  joint  of  each  foot  it  has  a  long  claw  and  sole-like  attachment-lobe, 
and  it  is  able  to  push  these  appendages  between  the  sharp,  slippery,  decurved  hairs 
lining  the  inner  surface  of  the  pitcher,  and  so  to  hook  itself  to  the  deeper  strata  of 
the  wall.  This  apparatus  may  be  likened  to  the  grapple-like  climbing  irons  of 
Tyrolese  mountaineers,  and,  thus  armed,  the  fly  is  in  a  position  to  ascend  the  inner 
wall  of  a  pitcher  unscaleable  by  other  insects.  The  case  of  the  small  moth 
Xanthoptera  semicrocea  is  similar.  The  tibise  of  this  insect  are  armed  with  long, 
sharp  spurs,  one  pair  on  each  of  the  two  middle  legs,  and  two  pairs  on  each  of 
the  two  hindermost  legs;  and,  by  the  help  of  these  spurs  it  likewise  is  able  to. 
tread  uninjured  over  the  dangerous  surface  of  the  wall.  Its  caterpillars,  too,  cover 
the  sharp  slippery  hairs  with  a  web,  and  so  render  them  harmless. 

The  presence  of  these  animals  in  the  death-traps  of  Sarracenias  is  of  special 
interest,  inasmuch  as  it  shows  that  the  animals  which  perish  at  the  bottom  of  the 
pitchers  are  not  exactly  digested.  If  maggoty  flesh  enters  the  stomach  of  a 
carnivorous  animal,  not  only  the  flesh  itself  but  the  maggots  as  well  (which,  indeed, 
immediately  die  on  reaching  the  stomach)  are  speedily  dissolved  by  the  action  of 
the  gastric  juice.  Such  is  also  the  case  with  several  animal-capturing  plants  to  be 
described  in  the  next  pages.  But  the  fluid  secreted  in  the  pitchers  of  Darlingtonia 


VOL.  I. 


130  PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 

and  Sarracenia  variolaris  cannot  exercise  this  digestive  action,  for  if  it  did  the 
maggots  in  the  heap  of  rotting  insects  could  not  remain  alive  and  well.  Its 
action  is  limited  to  the  promotion  of  decay  and  the  formation  of  a  foul  liquor,  in 
other  words,  a  liquid  manure,  which  is  absorbed  as  nutriment  by  the  epidermal  cells 
at  the  bottom  of  the  pitchers. 

Another  series  of  pitcher-plants  comprises  forms  in  which  the  petioles  are 
converted  into  symmetrical  sacs  with  apertures  at  the  top,  and  the  laminae  spread 
out  over  them  like  lids  for  protection.  Most  frequently  the  pitfalls  hi  plants  of 
this  kind  are  shaped  like  pitchers,  jars,  urns,  cups,  or  funnels;  and  the  lid  over  the 
orifice  of  each  cavity  is,  for  the  most  part,  so  placed  as  to  prevent  rain-drops  from 
falling  in,  but  not  to  hinder  in  any  way  the  entrance  of  animals.  In  this  series  are 
included,  firstly,  a  few  species  of  Sarracenia,  viz.  Sarracenia  Drummondii  and 
S.  undvdata,  next,  the  Australian  Gephalotus  follicularis,  and  lastly,  the  numerous 
species  of  the  genus  Nepenthes,  which  are  designated  by  gardeners  by  the  name  of 
"pitcher-plants"  in  the  narrow  sense. 

The  leaves  in  both  the  Sarracenias  just  named  are  heteromorphic.  Some  of  them 
have  acute  linear-lanceolate  petioles  of  a  uniform  green  colour,  and  not  hollowed 
out;  and  it  is  only  in  the  case  of  from  three  to  five  leaves  in  each  individual  plant 
that  the  petioles  are  transformed  into  tubes  with  inf undibulif orm  enlargements  at  the 
top.  The  rim  round  the  mouth  of  the  funnel  is  somewhat  swollen  and  doubled  down 
externally;  but  above  the  orifice  the  lamina  is  arched  so  as  to  form  a  cover  to  the 
pitcher.  The  margin  of  the  leaf  of  Sarracenia  laciniata,  which  is  shown  in  fig.  21  3, 
is  crinkled  and  sinuously  folded.  The  cover  and  also  the  upper  funnel-shaped 
•enlargement  of  the  pitcher  are  very  conspicuous  on  account  of  the  contrast  of  the 
•colours  displayed  upon  them.  The  green  of  the  lower  part  of  the  pitcher  gets 
paler  and  paler  above,  and  merges  into  a  pure  white,  whilst  dark-red  veins  stand 
out  from  the  green  and  white  ground  tints,  having  the  effect  of  a  net- work  of  blood- 
vessels. At  the  mouth  of  the  pitcher,  and  on  the  under  side  of  the  lid,  honey  is 
secreted  in  such  abundance  that  little  drops  of  it  are  not  infrequently  to  be  seen 
on  the  swollen  rim,  and  some  oozes  down  into  the  infundibuliform  portion  of  the 
pitcher.  But  at  the  very  spots  where  the  honey  occurs  there  are  also  innumerable 
smooth  conical  cells  with  their  solid  apices  directed  downwards;  and  these  cells 
become  longer  the  lower  their  position  in  the  pitcher.  When  insects,  attracted  by 
the  gay-coloured  lid,  and  lured  on  by  the  honey,  come  to  the  mouth  of  the 
pitcher  and  tread  upon  the  parts  covered  with  the  sharp  slippery  papillae,  they  are 
drawn  into  the  depths  as  though  by  an  invisible  power.  After  they  have  once 
alighted  on  the  perilous  area,  every  movement  and  every  effort  to  climb  up  against 
the  points  causes  them  to  slide  further  and  further  down  towards  the  bottom  of 
the  pitcher,  where  they  are  hopelessly  lost,  being  killed  within  a  short  time  and 
ultimately  decomposed. 

An  instance  of  an  exactly  similar  kind  is  afforded  by  Cephalotus  follicularis, 
which  has  long  been  known  as  a  plant  native  on  moorlands  in  eastern  Australia. 
It  is  allied  to  saxifrages  and  currants,  and  is  represented  on  a  scale  of  half  the 


PLANTS  WITH  TRAPS  AND   PITFALLS  TO  ENSNARE   ANIMALS. 


131 


natural  size  in  fig.  22.     This  Cephalotus  also  has  two  kinds  of  leaves,  which  are 

closely  crowded  in  a  rosette  round  the  erect  flower-stalk.     Only  the  lower  leaves 

of  the  rosette  are  transformed  into  traps  for  animals,  and  these  are  pre-eminently 

adapted   for   wingless   creatures   creeping  upon   the   earth.     The   tankard-shaped 

traps  all  rest  on  the  damp  earth,  and  are  furnished  externally  with  borders  or 

winged  ridges,  which  facilitate  the  ascent 

of  crawling  animals  to  the  mouth  of  the 

tankard.     Flying  insects  are  of  course  not 

excluded,  and  here  again  they  are  made 

aware  from   afar  of   the   feast  of   honey 

provided  by  the  presence  of  bright  colours. 

The  half-open  lid  is  very  prettily  adorned 

with  white  patches  and   brilliant  purple 

veins,  and  at  a  distance  is  readily  mistaken 

for  a  flower. 

When  small  animals,  whether  with  or 
without  wings,  approach  to  take  the 
honey,  they  are  so  eager  in  their  search 
that  they  get  upon  the  inner  surface  of 
the  mouth  of  the  tankard-pitcher,  which, 
though  fluted,  is  also  very  smooth  and 
slippery,  and  thence  they  easily  slide  into 
the  interior  of  the  cavity.  The  pitchers 
being  half-full  of  liquid,  most  of  the  un- 
lucky creatures  die  there  in  a  short  time 
by  drowning.  But  even  if  this  were  not 
the  case,  they  would  never  succeed  in 
working  their  way  up  to  the  light  of 
day.  For  every  animal  that  wishes  to  save 
itself  from  a  Cephalotus  pitcher  has  three 
obstacles  to  overcome :  first,  a  circular 
ridge  projecting  inside  the  pitcher;  sec- 
ondly, a  bit  of  wall  thickly  covered  with 

little  papillae,  sharp,  ridged,  and  pointed  downward,  the  whole  being  comparable 
to  a  flax-comb;  and,  lastly,  on  the  involute  rim  round  the  mouth  of  the  pitcher, 
another  fringe  composed  of  hooked,  decurved  spines  which  bristle  like  an  im- 
penetrable row  of  bayonets  in  front  of  such  animals  as  may  have  surmounted 
the  other  difficulties.  The  abundance  of  the  booty  found  at  the  bottom  of  Cepha- 
lotus pitchers  shows  how  efficiently  these  contrivances  serve  to  prevent  escape. 
Ants,  for  instance,  sacrifice  themselves  recklessly  in  their  pursuit  of  honey,  and 
one  often  finds  great  numbers  of  them  drowned  in  the  liquid  in  the  pitchers.  The 
prey  is  not  in  this  case  converted  into  a  putrid  liquor,  but  is  partially  dissolved  by 
a  secretion  having  an  acid  reaction.  This  secretion  is  separated  out  by  special 


Fig.  22.— Cephalotus  follicularis. 


132  PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 

glandular  cells  situated  on  the  lining  of  the  pitcher;  and  the  whole  process,  wherein 
they  are  concerned,  corresponds  to  that  which  obtains  in  the  pitchers  of  Nepenthes, 
and  which  will  be  more  thoroughly  discussed  in  the  case  of  these  latter  plants. 

The  species  of  the  genus  Nepenthes,  of  which  we  know  at  the  present  time 
thirty-six,  are  all  confined  to  the  tropics.  Their  area  of  distribution  extends  from 
New  Caledonia  and  New  Guinea  over  tropical  Australia  to  the  Seychelles  Islands 
and  Madagascar,  and  over  the  Sunda  Islands,  the  Philippines,  Ceylon,  Bengal,  and 
Cochin-China.  They  only  nourish  on  marshy  ground  on  the  margin  of  small 
collections  of  water  in  damp  primeval  forests.  There  the  seeds  germinate  in 
shallow  water.  The  young  plants  (see  fig.  23),  which  spring  from  the  boggy 

ground,  have  their  leaves  ar- 
ranged in  rosettes  just  like 
those  of  Sarracenias  (see  fig. 
20).  They  are,  too,  so  nearly 
identical  in  form  with  the 
latter  that  anyone  seeing  a 
young  Nepenthes  plant  for 
the  first  time,  and  not  knowing 
the  history  of  its  development, 
would  take  it  for  a  Sarracenia. 
Fig.  2s.-Yowg  Nepenthes  plants.  The  leaves,  succeeding  the 

cotyledons  and  forming  a  circle 

above  them,  rest  their  lower  portions  upon  the  mud,  but  their  upper  parts  are 
curved  upwards,  and  each  carries  at  its  extremity  a  scale  resembling  a  cock's  comb, 
which  is,  strict  speaking,  the  lamina.  This  scale  roofs  over  a  slit-like  aperture,  the 
entrance  to  a  cavity  within  the  swollen  petiole.  In  addition  a  green  lobe  with  a  few 
coarse  projecting  points  is  to  be  seen  on  either  side  of  the  orifice. 

Altogether  different  from  the  rosettes  of  young  Nepenthes  plants  are  the  foliar 
structures  clothing  the  stems  which  subsequently  arise  from  the  rosettes  (see  fig.  24). 
In  these  leaves  the  lower  part  of  the  petiole  is  winged  and  flat,  has  a  linear  or 
lanceolate  outline,  and  resembles  the  leaf -blade  of  Draccena;  its  functions,  too,  are 
those  of  a  green  lamina.  This  expanded  section  of  the  leaf-stalk  passes  next  into 
a  part  which  is  terete  and  coiled  like  a  snake,  and  acts  as  a  tendril.  Every  stem  or 
branch  belonging  to  a  plant,  whether  living  or  dead,  with  which  this  part  of  the 
petiole  comes  into  contact,  is  seized  and  encircled  by  it;  and  the  third  portion  of 
the  petiole,  i.e.  the  pitcher,  being  situated  at  the  extremity  of  this  clasping  portion, 
is  thus  slung  upon  the  branch  of  some  other  plant  growing  at  the  edge  of  a  pool 
of  water.  Meanwhile  the  Nepenthes  plant  rises  higher  and  higher  above  the  wet 
soil  where  its  seeds  germinated  and  the  young  rosette  rested,  becomes  entangled 
with  the  ramifications  of  the  underwood  and  with  prostrate  branches  of  trees  of 
the  primeval  forest;  in  a  word,  with  everything  available  as  a  support,  and  so  not 
infrequently  climbs,  as  a  true  liane,  to  the  tops  of  trees  of  moderate  height. 

The  pitcher  must  be  looked  upon  as  an  excavated  portion  of  the  petiole,  and 


PLANTS   WITH  TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 


133 


Fig.  24.— Nepenthes  destillator 


134  PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 

what  appears  to  be  the  lid  of  the  pitcher  is  the  lamina,  as  it  is  in  Cephalotus  and 
the  Sarracenias.  In  this  case  also  the  lamina  seems  to  be  but  little  developed  in 
comparison  with  the  wonderfully  metamorphosed  petiole.  In  the  majority  of  the 
species  of  Nepenthes,  the  mature  pitchers  are  from  10  cm.  to  15  cm.  in  height.  In 
the  graceful  Nepenthes  ampullaria  they  are  only  from  4  cm.  to  6  cm.  high;  but, 
on  the  other  hand,  in  the  species  indigenous  to  the  primeval  forests  of  Borneo  they 
reach  a  height  of  30  cm.  or  even  more.  The  pitchers  of  Nepenthes  Rajah  have  a. 
height  of  50  cm.,  and  their  orifices  are  10  cm.  in  diameter,  whilst  below  the  orifice 
they  expand  to  16  cm.;  so  that  if  a  pigeon  were  to  fly  into  a  pitcher  of  this  kind 
it  would  be  completely  hidden  in  it.  Immature  pitchers  are  still  closed  by  their 
covers.  Often  they  are  hairy  outside;  and,  according  to  the  colour  and  lustre  of 
the  hairs,  they  may  be  rusty  in  tone  or  glittering  like  gold;  not  rarely  they  look  as 
if  they  were  powdered  with  flour  (e.g.  N.  albo-marginata),  and  sometimes  are  even 
snow-white.  Subsequently  the  lid  is  raised,  and  the  downy  coat  disappears  either 
partially  or  entirely.  Having  thus  become  glabrous,  the  pitchers  display  a  yellowish- 
green  ground  colour,  for  the  most  part  flecked  and  veined  with  purple;  and  many 
are  of  a  bluish,  violet,  or  rose  tint  near  the  orifice,  or  dark-red  as  though  saturated 
with  blood.  The  lid  is  similarly  gaily  coloured;  and  the  variety  of  the  tints  is 
increased  by  the  fact  that  a  pale-blue  zone  is  visible  in  the  interior,  beneath  the 
swollen  involute  rim  of  the  opening,  which  is  itself  brownish,  yellowish,  or  orange- 
red.  Gaily-coloured  pitchers  of  this  kind  look  at  a  distance  just  like  flowers, 
and  remind  one,  in  particular,  of  the  most  brilliant  floral  forms  of  the  liane-like 
Aristolochias  indigenous  to  tropical  forests.  This  fact  is  the  more  noteworthy, 
because  the  genus  Nepenthes  is  closely  allied  to  the  genus  Aristolochia  in  respect 
of  systematic  relations. 

The  bright  pitchers  of  Nepenthes,  visible  from  afar,  are  sought,  just  as  flowers 
are,  by  insects,  and  probably  by  other  winged  creatures  as  well;  and  this  occurs  all 
the  more  because  there  is  a  copious  secretion  of  honey  by  the  epidermal  cells  upon 
the  under  surface  of  the  lid,  and  on  the  rim  round  the  mouth  of  each  pitcher.  The 
swollen  and  often  delicately-fluted  rim,  in  particular,  drips  and  glitters  with  the 
sugary  juice;  and  it  would  be  permissible  in  this  connection  to  speak  of  a  honeyed 
mouth  and  sweet  lips  in  the  most  literal  sense  of  the  words.  Animals  which  suck 
honey  from  the  lips  of  Nepenthes  pitchers  wander,  as  they  do  so,  only  too  readily 
upon  the  interior  surface  of  the  orifice.  But  the  inner  face  is  smooth  and  precipitous, 
and  rendered  so  slippery  by  a  bluish  coating  of  wax  that  not  a  few  of  the  alighted 
guests  slip  down  to  the  bottom  of  the  pitcher  and  fall  into  the  liquid  there 
collected.  Many  of  them  perish  in  a  short  time;  others  try  to  save  themselves  by 
climbing  up  the  internal  face  of  the  pitcher,  but  they  always  slip  again  on  the 
polished,  wax-coated  zone,  and  tumble  back  once  more  to  the  bottom.  In  large 
pitchers  the  involute  rim  of  the  aperture  is  in  addition  armed  with  sharp 
teeth,  which  are  pointed  downwards  and  bristle  in  front  of  such  of  the  unlucky 
victims  in  the  pitfall  as  try  to  emerge  (see  fig.  19 3).  In  a  number  of  species 
(N.  Rafflesiana,  N.  echinostoma,  N.  Rajah,  N.  Edwardsiana,  and  N.  Veitchii,  all 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS.  135 

natives  of  Borneo)  this  fringe  of  sharp  teeth  looks  like  the  set  of  teeth  of  a  beast 
of  prey;  and  in  Nepenthes  villosa,  of  which  a  pitcher  is  represented  in  fig.  21*,  a 
double  row  of  bigger  and  smaller  teeth  directed  towards  the  bottom  of  the  pitcher 
is  developed,  and  renders  the  escape  of  prey,  once  caught  in  the  trap,  impossible. 

Most  of  the  creatures  that  fall  into  the  pitchers  are,  however,  speedily  drowned 
in  the  large  quantity  of  liquid  at  the  bottom.  For  a  third  part  or  even  a  half  of 
the  cavity  is  filled  with  liquid.  This  liquid  originates  from  special  gland-cells  on 
the  inner  surface  of  the  pitcher,  consists  mainly  of  water,  and  so  long  as  there  are 
no  animals  in  the  pitfall,  gives  only  a  very  weak  acid  reaction.  But  as  soon 
as  the  body  of  an  animal  reaches  the  bottom,  more  fluid  is  secreted.  This  has 
a  distinctly  acid  taste,  possesses  the  power  of  dissolving  albuminous  substances, 
such  as  flesh  and  coagulated  blood,  and  corresponds,  not  only  in  respect  of  this 
action  but  also  in  chemical  composition,  to  the  gastric  juice.  For,  in  addition  to 
organic  acids  (malic,  citric,  and  formic  acids),  an  organic  body  like  pepsin  has 
been  detected  in  it,  and  nitrogenous  organic  compounds  have  been  brought  into 
solution  in  it  artificially  as  well.  If  the  liquid  from  a  Nepenthes  pitcher,  which  has 
not  yet  captured  any  animal,  is  poured  into  a  glass  vessel  containing  a  small  piece 
of  meat,  the  flesh  is  at  first  but  little  affected;  but,  if  a  few  drops  of  formic  acid  are 
added,  the  flesh  is  dissolved  and  undergoes  the  very  same  changes  as  it  does  in  the 
stomach  of  a  mammal.  The  process  going  on  in  the  pitchers  of  Nepenthes  when 
animals  fall  into  them  is  therefore  not  only  analogous  to  digestion,  but  may  be 
properly  designated  digestion. 

The  digested  portions  of  the  bodies  are  afterwards  absorbed  by  special  cells  at 
the  bottom,  and  on  the  lower  parts  of  the  lining  wall  of  the  Nepenthes  pitchers. 

Another  series  of  plants  was  at  one  time  regarded  as  belonging  to  our  present 
section  of  carnivorous  plants.  These  include  forms  possessing  subterranean  stem 
structures,  bearing  hollow,  scale-like  leaves,  or  leaves  so  arranged  that  chink-like 
spaces  exist  between  them.  Into  these  chambers  or  spaces  it  was  supposed  that 
minute  animals,  Infusoria,  Rhizopods,  Aphides,  and  the  like  found  their  way,  and 
that  here  they  met  their  death,  their  bodies  being  digested  through  the  agency 
of  peculiar  glands  which  line  the  walls  of  these  chinks  and  spaces.  Though  this 
view  of  the  carnivorous  function  of  these  subterranean  organs  has  failed  to  become 
established  on  a  solid  basis  of  fact,  the  plants  in  question  are  of  considerable 
interest  and  may  be  conveniently  treated  here. 

One  of  the  most  remarkable  of  the  plants  belonging  to  this  group  is  the  Tooth- 
wort  (Lathrcea  Squamaria),  of  which  we  shall  repeatedly  have  occasion  to  speak. 
It  is  nearly  allied  to  the  Yellow- Rattle  and  Cow- wheat,  but  it  is  destitute  of 
chlorophyll,  and  lives  underground,  parasitic  on  the  roots  of  arborescent  Angio- 
sperms,  except  during  a  brief  period  annually  when  it  sends  up  above-ground  a  few 
short  shoots  covered  with  flowers.  The  subterranean  stems  are  white,  have  a 
fleshy,  solid,  and  elastic  appearance,  and  are  covered  throughout  their  entire  length 
with  thick  squamous  leaves  placed  closely  one  above  the  other  (see  fig.  25 l  and 
fig.  37).  In  colour  and  consistence  these  leaves  are  like  the  stem;  in  outline  they 


136  PLANTS   WITH   TRAPS   AND   PITFALLS   TO    ENSNARE   ANIMALS. 

are  broadly  cordate,  and  they  give  the  impression  of  being  mounted  fairly  and 
squarely  upon  the  stem  by  means  of  the  highly  swollen  and  notched  basal  portion. 
But  it  is  only  necessary  to  detach  one  of  the  scales  from  the  stem  to  convince 
one's  self  that  this  is  not  the  case,  and  that  the  part  taken  at  first  sight  to  be  the 
underside  or  back  of  the  leaf  is  only  a  portion  of  the  superior  surface.  In  reality 
each  of  these  thick  squamiforui  leaves  is  rolled  back,  and  in  it  the  following  parts 
may  be  distinguished:  first,  the  place  of  insertion  on  the  stem  (fig.  25  3),  which  is 
relatively  small;  secondly,  the  portion  taken  on  cursory  examination  to  be  the 
whole  upper  surface  of  the  leaf,  and  consisting  of  an  obliquely  ascending  blade 
limited  by  a  sharp  border;  next,  starting  from  this  sharp  border,  the  part  which* 
owing  to  its  being  suddenly  bent  down  at  an  acute  angle  and  falling  away  steeply, 
is  usually  taken  for  the  dorsal  or  inferior  surface  of  the  leaf,  but  which  belongs,  in 
point  of  fact,  to  the  front  of  the  lamina;  fourthly,  the  free  extremity  of  the  leaf  in 
the  form  of  an  involute  limb;  and  fifthly,  the  true  dorsal  part,  which  is  very  small 
relatively  and  is  not  visible  until  the  involute  tip  is  removed.  Owing  to  the 
involution  of  the  apex,  a  canal  or  rather  a  recess  is  formed  and  runs  across  beneath 
the  leaf,  close  under  the  place  where  the  latter  is  joined  to  the  stem  (see  fig.  25 2). 
From  five  to  thirteen  (usually  ten)  chambers  open  into  these  recesses  through  a 
series  of  little  holes.  They  are  excavations  in  the  thickness  of  the  scales  and  are 
probably,  in  this  form  at  any  rate,  unique  in  the  realm  of  plants.  These  extraordi- 
nary chambers  must  be  described  as  deep  excavations  in  the  foliar  substance 
proceeding  from  the  back  of  the  leaf.  With  a  view  to  elucidating  their  function 
in  relation  to  the  life  of  the  plant,  their  structure  must  be  more  particularly 
considered. 

The  chambers  radiate  as  it  were  from  the  orifice  at  the  base  of  the  leaf.  Though 
closely  adjoining  one  another,  they  are  not  in  lateral  connection  by  means  of  pass- 
ages or  canals.  Their  walls  are  irregular  and  undulating  (see  fig.  25 3),  and  are 
characterized  by  the  peculiar  structures  which  are  borne  on  the  lining — raised  up 
above  the  ordinary  epidermal  cells  and  projecting  into  the  cavity.  These  structures, 
of  two  sorts,  are  shown  in  fig.  254,  under  a  considerable  magnification.  One  sort, 
and  these  are  by  far  the  more  numerous,  are  of  the  nature  of  short  capitate  hairs. 
The  head  is  formed  of  a  pair  of  cells,  and  they  are  supported  on  a  short  cylindrical 
cell  which  serves  as  a  stalk.  The  other  sort  is  sparsely  scattered  amongst  these 
capitate  hairs.  They  are  oval  in  outline  and  but  slightly  raised  above  the  ordinary 
epidermal  cells.  Each  consists  of  a  tabular  cell  upon  which  rests  a  slightly  convex 
cushion  composed  of  not  more  than  four  cells  all  lying  in  the  same  plane.  One  such 
sessile  gland  is  shown  in  the  centre  of  fig.  25  4.  In  this  case  the  cushion  consists  of 
three  cells.  A  further  peculiarity  has  been  observed  in  these  sessile  glands.  The 
summit  of  each  is  marked  by  a  tiny  pore  (not  shown  in  the  figure),  an  actual  hole 
in  the  wall  at  the  geometrical  centre  of  the  convex  surface. 

In  the  wall  of  the  chamber,  just  below  the  lining  epidermis,  run  the  vascuL 
strands  (fig.  253).     The  vessels  of  which  they  are  composed  form  a  considerable 
plexus  or  net-work  in  this  region.     Now  it  is  known  that  the  ground  in  which 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 


137 


Lathrcea  passes  its  existence  is  often  drenched  through  with  moisture  in  the 
immediate  neighbourhood  of  the  plant.  A  water-excreting  function  has  long  been 
attributed  to  the  chambered  leaves  of  the  rhizomes.  That  such  is  the  case  has 
been  demonstrated  by  forcing  water  under  pressure  through  the  cut  ends  of  the 
rhizomes,  when  streams  gush  forth  from  the  basal  orifices  of  the  leaves.  In  this 


Fig.  25.— Glandular  structures  In  the  Tooth  wort,  Bartsia,  and  Butterwort. 

i  Piece  of  an  underground  leaf-shoot  of  the  Tooth  wort.  2  Longitudinal  section  through  the  same;  x2.  8  Longitudinal  section 
through  one  of  these  underground  leaves;  x60.  *  Piece  of  the  wall  of  a  cavity;  x200.  «  Subterranean  bud  of  Lartsia; 
natural  size.  6  Cross-section  through  part  of  this  bud;  x60.  *  The  margin  of  a  bud-scale  in  section;  x200.  8  piece  of  the 
epidermis  of  a  leaf  of  Butterwort;  xlSO.  »  Transverse  section  through  the  leaf  of  a  Butterwort  (Pinguicula  alpina);  x50. 
10  Transverse  section  through  Butterwort  leaf;  natural  size. 

instance  it  is  uncertain  whether  the  stalked  or  the  cushion  glands  assist  in  this 
excretion,  though  from  the  minute  details  of  their  structure  it  would  seem  probable 
that  it  is  the  latter.  On  any  other  hypothesis  it  is  difficult  to  understand  the 
meaning  of  the  pore  on  the  summit.  The  matter  has,  however,  been  placed  beyond 
doubt  by  experiments  on  other  allied  plants,  as,  for  instance,  the  Lousewort  (Pedicu- 
laris  palustris),  in  which  the  glands  are  more  easily  kept  under  observation.  We 
have  apparently  in  these  gland-bearing  chambers  of  Lathrcea  a  water-excreting 
mechanism  for  the  elimination  of  the  surplus  moisture,  which  in  most  plants  is 
transpired  or  evaporated  into  the  air.  Lathrcea  being  almost  wholly  subterranean 


138  PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 

is  unable  to  do  this,  as  the  air  in  the  chinks  and  crannies  of  its  matrix  of  soil  is 
generally  saturated.  The  water  is  therefore  excreted  in  liquid  form  by  a  special 
mechanism. 

This  view  of  the  function  of  the  scales  is  confirmed  by  reference  to  other  allied 
types  with  subterranean  scales.  An  instance  in  point  is  afforded  by  Bartsia  alpina. 
This  remarkable  plant  is  distributed  in  the  Arctic  region  and  amongst  the  high 
mountain  flora  throughout  almost  the  whole  of  Europe,  and  is  very  striking  owing 
to  the  colour  of  its  foliage  being  a  mixture  of  black,  violet,  and  green.  The  flower, 
too,  is  of  a  sombre  dark- violet  hue,  and  the  entire  plant,  by  reason  of  this  peculiar 
colouring,  gives  a  truly  funereal  impression.  We  may  remark  incidentally  that 
the  name  Bartsia  was  chosen  by  Linnaeus  for  this  sad-hued  plant  as  an  expression 
of  his  own  grief  at  the  death  of  the  zealous  naturalist  and  physician,  Bartsch,  who 
was  his  intimate  friend,  and  who  succumbed  at  a  comparatively  early  age  to  the 
climate  of  Guiana.  Damp  black  earth  in  the  neighbourhood  of  springs  constitutes 
the  favourite  habitat  of  these  plants.  Upon  digging  in  summer  time  down  to  their 
roots,  one  sees  that  a  few  suckers  proceed  from  them,  and  fasten  upon  the  sedges 
and  other  plants  growing  in  the  vicinity;  but  one  also  discovers  subterranean  shoots 
having  "root-hairs"  developed  near  the  nodes,  at  which  are  inserted  the  paired 
white  scales;  and  these  "root-hairs"  have  the  function  of  absorption-cells.  To- 
wards the  autumn,  oval  buds,  likewise  subterranean,  are  matured,  in  form  not 
unlike  horse-chestnut  buds  (see  fig.  25  5),  and  composed  of  etiolated  scales  arranged 
in  four  rows  and  overlapping  one  another  like  tiles,  so  that  only  the  back  of  the 
upper  part  of  each  scale  is  visible,  the  lower  part  being  covered  by  the  scale  next 
beneath  it. 

On  the  visible  part  of  each  scale's  convex  under  surface  three  sharply  projecting 
ribs  are  noticeable  near  the  middle,  whilst  the  two  margins  are  rolled  back  so  as 
to  form  a  recess  in  each  case.  But,  as  may  be  seen  in  the  cross-section  of  a  Bartsia 
bud  (see  fig.  25  6),  one  pair  of  scales  lies  over  the  next  higher  pair  in  such  a  way  as 
to  convert  the  recesses  into  ducts.  Owing  to  this  construction  the  interior  of  the 
bud  is  perforated  by  twice  as  many  ducts  as  there  are  covered  leaf-scales,  and  the 
orifices  of  each  pair  of  ducts  occur  at  the  spots  where  the  evolute  margins  of  one 
scale  begin  to  be  covered  by  the  middle  of  the  next  lower  scale.  On  one  wall  of 
the  ducts,  i.e.  in  the  recesses,  structures  like  those  which  occur  in  the  cavities  of 
Lathrcea  are  developed,  i.e.  stalked  glands,  each  composed  of  two  cells  borne  upon 
a  basal  cell;  secondly,  pairs  of  hemispherical  domed  cells;  and,  lastly,  ordinary  flat 
epidermal  cells  (see  fig.  25 7).  There  can  be  little  doubt  that  the  whole  apparatus 
acts  in  the  same  way  as  in  Lathrcea,  and  is  adapted  to  the  excretion  of  water.  The 
cavities  and  spaces  between  the  scales  of  the  buds  serve  the  same  purpose  as  the 
chambers  in  the  leaves  of  Lathrcea,  viz.,  that  of  affording  cover  to  the  delicate 
excretory  glands  and  of  protecting  them  from  immediate  contact  with  the  soil. 

Mechanisms  of  this  sort  are  not  restricted  to  subterranean  organs,  but  are  found 
likewise  on  the  aerial  leaves  of  many  plants.  Indeed  such  arrangements,  supple- 
menting ordinary  transpiration,  are  common,  especially  amongst  tropical  plants. 


PLANTS   WITH   TRAPS   AND   PITFALLS   TO   ENSNARE   ANIMALS. 


139 


Fig.  25A.  -Swarmspores,  Zygospores,  and  Chlorophyll-bodies. 

-d,  Development  of  Swarmspores  in  the  tubular  cells  of  Vaucheria  clavata.  e-h,  Swarmspores  and  Resting-cells  of  "red- 
snow  "  (Sphaerella  nivalis),  mixed  with  pollen-grains  of  Pines,  i— k,  Forms  of  Chlorophyll  in  cells  of  Desmidieae  (i,  Clos- 
terium  Leibleinii;  k,  Penium  interruptum).  1,  Formation  of  Zygospores  and  spiral  arrangement  of  Chlorophyll-bodies 
in  cells  of  Spirogyra  arcta.  m,  Star-shaped  Chlorophyll-bodies  in  cells  of  Zygnema  pectinatum.  n— o,  Gloeocapsa  san- 
guinea.  p,  Protonema  of  Schistostega  osmundacea.  q,  Transverse  section  of  the  foliage-leaf  of  Satureja  hortensis.  All 
figs,  enlarged. 

Restricting  ourselves  to  a  consideration  of  other  members  of  the  family  Scro- 
)hulariaceae  allied  to  Lathrcea  and  Bartsia,  we  find  in  the  Lousewort  (Pedicularis) 


140  PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 

a  similar  mechanism.  Here  the  glands  occur  on  the  under  surface  of  the  aerial  leaves, 
the  cushion  glands  being  by  far  the  most  numerous.  Shoots  of  this  plant  if  in- 
jected with  water  at  the  cut  end,  readily  pass  it  out  by  their  leaves,  and  in  particu- 
lar by  those  portions  which  abound  in  cushion  glands.  As  the  water  percolates 
through  these  areas  it  gushes  from  the  leaves  with  great  rapidity.  The  younger 
leaves  drip  with  moisture  and  water  drops  from  the  leaf -tips  and  wells  up  in  the 
leaf -axils,  running  in  cascades  down  the  stem. 

Somewhat  similar  is  the  Yellow-Rattle  (Rhinanthus  Christa-galli).  Here,  too, 
under  pressure,  water  is  forced  from  the  leaves,  but  less  rapidly  than  in  the  last 
instance.  It  is  thus  possible  to  observe  its  excretion  from  the  edges  of  the  under 
surfaces  of  the  leaves,  to  see  the  water  drawn  round  by  capillarity  on  to  the  upper 
surface,  whence  it  runs  down  the  vein  furrows,  as  in  irrigation  canals,  to  the  base 
of  the  leaf. 

In  these  and  other  cases  like  them  we  are  dealing  with  plants  which  live  in 
moist  or  even  marshy  situations.  When  this  is  understood,  it  is  not  surprising  that 
they  should  exhibit  supplementary  mechanisms  for  eliminating  their  excess  of  water. 


CAENIVOEOUS  PLANTS  WHICH  EXHIBIT  MOVEMENTS  IN  THE  CAPTUKE 

OF  PREY. 

We  have  taken  Nepenthes,  Sarracenia,  and  other  forms  as  types  of  that  section 
of  carnivorous  plants  which  manifest  no  external  visible  movement  in  the  pitfalls 
for  the  purpose  of  capture  or  digestion.  The  second  section,  now  to  be  discussed, 
includes  plants  in  which  movements  of  the  leaves,  or  parts  of  leaves,  modified  as 
organs  of  seizure  and  digestion,  take  place  as  a  result  of  the  contact  of  animal 
bodies — movements  which  have  the  common  object  of  bringing  about  the  digestion 
of  the  animals,  whilst  the  retention  of  the  latter  is  effected  in  very  various  ways. 

Whilst  in  the  forms  hitherto  considered  the  mechanism  of  capture  is  wholly 
passive,  the  plant  with  its  pitfall  attractively  coloured  or  cunningly  baited  with 
honey  merely  awaiting  the  moment  when  the  insect  slips  on  the  treacherous 
surface,  in  those  which  we  are  now  about  to  review,  a  series  of  movements  simple 
or  complex  is  set  up  by  the  stimulus  received  when  the  insect  alights.  In  some 
cases  the  whole  leaf  suddenly  changes  its  form,  going  off  like  a  rat-trap,  in  others 
it  is  merely  the  digestive  tentacles  which  change  their  position.  In  general,  when 
the  movement  is  slow  the  organ  is  sticky,  but  when  instantaneous,  adhesiveness 
is  not  met  with. 

The  first  group  of  carnivorous  plants  which  perform  movements  for  the  capture 
of  prey  is  composed  of  the  various  species  of  the  genus  Pinguicula  (Butterwort). 
Of  this  stock  nearly  forty  species  are  known;  and  they  are  all  much  alike.  Scarcely 
any  difference  would  be  detected  by  an  ordinary  person  between  Pinguicula 
calyptrata  from  the  mountains  of  New  Granada  and  Pinguicula  vulgaris  from 
our  own  hills.  In  respect  of  habitat,  too,  they  exhibit  close  conformity.  In  both 
the  Old  World  and  the  New  they  only  thrive  on  damp  spots,  the  neighbourhood  of 


PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY.  141 

springs,  banks  of  brooks,  moorlands,  and  black  peat-bogs.  In  the  equatorial  zone 
they  have  retired  into  the  cool  regions  of  the  higher  mountains.  The  mountain 
ranges  of  Mexico  are  particularly  rich  in  species  of  Pinguicula,  but  all  the  forms 
existing  there  occupy  a  circumscribed  area.  Southern  and  western  Europe  also 
harbour  a  few  native  species  whose  area  of  distribution  is  surprisingly  limited. 
The  species  occurring  in  the  arctic  and  sub-arctic  zones  are,  on  the  contrary,  exceed- 
ingly widely  distributed.  One  species  has  been  found  in  antarctic  regions  at  the 
Straits  of  Magellan. 

The  species  best  known  and  most  available  for  study  is  Pinguicula  vulgaris. 
The  area  of  its  distribution  extends  over  the  whole  of  the  arctic  and  sub-arctic 
regions,  over  the  part  of  North  America  which  lies  to  the  north  of  the  Mackenzie 
River,  over  Labrador,  Greenland,  Iceland,  and  Lapland,  throughout  Siberia  down 
to  the  Baikal  Mountains,  and  through  Europe  to  the  Balkans,  Southern  Alps,  and 
Pyrenees.  This  graceful  plant,  generally  referred  to  the  family  Lentibulariacese, 
is  nearly  allied  to  the  group  of  scrophulariaceous  genera  of  our  last  section. 
It  has  bilabiate  flowers  of  a  violet -blue  colour,  with  palates  covered  with 
velvety-white  hairs,  and  with  a  sharp  spur  at  the  back.  The  flowers  are  borne 
singly  on  slender  stalks  which  rear  themselves  in  an  elegant  curve  from  the  centre 
of  a  rosette  of  leaves  that  rests  upon  the  ground.  The  leaves  of  the  rosette  in 
Pinguicula  vulgaris,  as  in  all  other  species  of  Butterwort,  are  oblong-ovate  or 
ligulate  and  of  a  yellowish-green  colour,  and  rest  their  under-surfaces  upon  the  wet 
ground,  whilst  their  upper  faces  are  exposed  to  the  sky  and  rain.  Owing  to  the 
lateral  margins  being  somewhat  upturned,  each  leaf  is  converted  into  a  broad  flat- 
bottomed  trough  (cf.  the  section  taken  right  across  a  leaf  in  fig.  25 10  and  25  n). 
The  trough  is  covered  with  a  colourless  sticky  mucilage  which  is  secreted  by  glands 
distributed  in  large  numbers  over  the  entire  upper  surface  of  the  leaf. 

The  glands  are  of  two  kinds.  One  variety  is  distinguishable  by  the  naked  eye 
as  consisting  of  a  stalked  head,  and  looks  under  the  microscope  like  a  tiny  mush- 
room (see  fig.  25  9).  Its  parts  are  a  swollen  disc  composed  of  from  eight  to  sixteen 
cells  grouped  radially,  and  a  stalk,  consisting  of  an  erect  tubular  cell  supporting  this 
disc.  A  gland  of  the  other  sort  is  made  up  of  eight  cells  grouped  in  the  form  of 
a  wart  or  knob  supported  by  a  very  short  stalk-cell,  and  only  slightly  raised  above 
the  surface  of  the  leaf.  For  the  rest,  ordinary  flat  epidermal  cells  make  up  the 
epidermis,  with  here  and  there  interspersed  the  guard-cells  of  stomata. 

It  has  been  calculated  that  there  are  25,000  mucilage-secreting  glands  on  a 
square  centimeter  of  a  butterwort  leaf,  and  that  a  rosette  composed  of  from  six  to 
nine  leaves  bears  about  half  a  million  of  them.  Momentary  contact,  whether  due  to 
rapid  brushing  by  a  solid  body  or  to  the  incidence  of  drops  of  rain,  causes  no  kind 
of  movement  in  them.  The  long-continued  pressure  of  grains  of  sand  or  of  solid 
insoluble  bodies  in  general  stimulates  the  glandular  cells  to  an  inconsiderable 
augmentation  of  the  quantity  of  mucilage  discharged,  but  does  not  cause  secretion 
of  any  acid  digestive  fluid.  But  as  soon  as  a  nitrogenous  organic  body  is  brought 
into  continuous  contact  with  the  glands,  they  are  forthwith  stimulated  not  only  to 


142  PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 

a  more  profuse  elimination  of  mucilage,  but  also  to  the  secretion  of  an  acid  liquid, 
which  has  the  power  of  dissolving  all  bodies  of  the  kind,  namely,  such  as  clotted 
blood,  milk,  albumen,  and  even  cartilage.  It  has  been  experimentally  established 
(for  example)  that  small  solid  bits  of  cartilage  placed  on  a  leaf  of  Pinguicula 
vulgaris,  whose  mucilage  shows  no  sign  of  an  acid  reaction,  cause,  after  ten  or 
eleven  hours,  the  secretion  of  an  acid  liquid,  and  after  forty-eight  hours  are  almost 
entirely  dissolved  by  it.  At  the  end  of  eighty-two  hours  the  bits  of  cartilage  used 
in  the  experiment  were  completely  liquefied,  the  whole  secretion  was  reabsorbed, 
and  the  glands  had  become  dry.  When  small  insects  such  as  midges  alight  from 
flight  on  a  leaf  of  Pinguicula  they  remain  glued  by  the  mucilage,  and  their 
struggles  to  extricate  themselves  only  cause  them  to  sink  deeper  into  it.  Thus 
they  generally  perish  in  a  very  short  time,  are  digested  by  the  acid  juice  poured 
from  the  glands  in  response  to  the  stimulus,  and  are  absorbed  with  the  exception 
of  the  wings,  claws,  and  other  parts  of  the  skeleton. 

The  acid  liquid  secreted  by  the  glands  is  viscous,  and  when  a  number  of  glands 
are  irritated  it  may  exude  so  copiously  as  to  fill  the  whole  trough  of  the  leaf.  If 
the  margin  of  the  leaf  alone  is  stimulated,  as  when  a  small  creeping  insect,  or  a 
midge  alighting  from  above,  gets  upon  the  slightly  up-curved  margin  of  the  leaf, 
not  only  do  the  marginal  glands,  which  are  comparatively  infrequent,  discharge 
their  secretion,  but  in  addition  the  edge  curls  over;  the  object  of  this  movement 
being  to  cover,  if  possible,  the  prey  whilst  it  is  held  fast  by  the  sticky  mucilage,  or 
to  push  it  into  the  middle  of  the  flat  channel,  and  so,  in  one  way  or  another,  to 
bring  it  into  contact  with  as  many  glands  as  possible.  The  marginal  glands  alone 
could  not  produce  the  requisite  quantity  of  acid  liquid  to  effect  solution,  and,  on 
this  account,  the  glands  on  a  wider  area  are  summoned  to  assist  in  the  manner 
described.  The  involution  of  the  margin  takes  place  very  slowly;  it  is  usually 
some  hours  before  the  animal  sticking  to  the  edge  is  enfolded,  or,  in  the  case  of 
the  larger  specimens,  is  pushed  into  the  middle  of  the  leaf.  After  solution  and 
absorption  are  accomplished,  usually  by  the  end  of  twenty-four  hours,  the  leaf 
expands  again,  and  its  margins  assume  the  position  which  they  had  before  their 
involution. 

Besides  small  insects,  pieces  of  plants,  such  as  spores  and  pollen-grains  brought 
by  the  wind,  not  infrequently  fall  on  the  viscid  surfaces  of  Pinguicula  leaves. 
These  are  subjected  to  the  same  fate  as  animal  organisms,  their  protoplasts  being 
dissolved  and  absorbed  like  the  flesh  and  blood  of  insects. 

The  action  of  the  acid  juice  secreted  by  the  glands  of  butter  wort  leaves  upon 
albuminous  bodies  is  identical  with  that  of  the  gastric  juice  of  animals.  We  may 
presume  therefore  that  there  are  in  it,  as  in  the  gastric  juice,  two  kinds  of  sub- 
stance: firstly,  a  free  acid,  and,  secondly,  a  ferment  completely  analogous  to  pepsin 
in  its  action;  for,  as  is  well  known,  it  is  by  means  of  this  combination  that  the 
juice  of  the  animal  stomach  effects  the  solution  of  albuminoid  compounds.  Inas- 
much as  the  gland-cells  of  Pinguicula  absorb  all  the  soluble  part  of  the  prey,  and 
re-absorb  the  solvent  previously  discharged  by  them,  the  action  of  this  plant's  leaves 


PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY.  143 

is  exceedingly  like  that  of  the  animal  stomach,  and  the  process  may,  as  in  the  case 
of  Nepenthes,  be  fairly  regarded  as  digestion.  Whether,  in  carrying  out  this 
process,  the  different  forms  of  glands  have  also  different  functions,  whether  those 
of  one  kind  serve  principally  to  secrete  and  those  of  the  other  to  absorb,  or 
whether,  perhaps,  the  one  variety  only  discharges  viscid  mucilage  to  capture  the 
prey,  and  the  other  only  a  liquid  containing  acid  and  pepsin,  are  questions  not  yet 
determined  with  certainty,  although  such  a  division  of  labour  is  in  itself  highly 
probable. 

The  similarity  existing  between  the  leaf  of  Pinguicula  and  the  animal  stomach 
in  respect  of  their  action  on  albuminous  substances  was  turned  to  a  practical 
application  in  dairy-farming  long  before  the  discovery  of  the  relationship  by  men 
of  science.  The  very  same  changes  as  are  brought  about  in  milk  by  the  addition 
of  the  rennet  from  a  calf's  stomach  can  be  induced  by  means  of  butterwort  leaves. 
If  fresh  milk,  warm  from  the  cow,  is  poured  over  these  leaves,  a  peculiar  tough 
mass  of  close  consistence  is  formed,  the  "  Tatmiolk  "  or  "  Satmiolk  "  of  Laplanders, 
mentioned  by  Linnaeus  a  hundred  and  fifty  years  ago  as  constituting  a  very 
favourite  dish  in  northern  Scandinavia.  In  particular,  the  fact  that  by  means 
of  a  trifling  quantity  of  Tatmiolk,  produced  in  the  manner  described,  a  large 
amount  of  fresh  sweet  milk  may  be  also  converted  into  Tatmiolk  is  specially 
worthy  of  emphasis,  for  we  learn  from  it  that  the  substance  generated  by 
Pinguicula  behaves  in  this  respect  too,  like  other  ferments.  The  immemorial  use 
of  Pinguicula  leaves  by  shepherds  in  the  Alps  as  a  cure  for  sores  on  the  udders 
of  milch  cows  is  also  interesting,  inasmuch  as  the  curative  effect  on  the  sores  is  to 
be  explained  by  the  antiseptic  action  of  the  secretion  of  the  leaves  in  question; 
and  a  method  of  healing,  used  empirically  two  centuries  ago,  thus  finds  confirmation 
and  a  scientific  explanation  at  the  present  day. 

Since  the  curling  up  and  unrolling  of  the  leaf-margin  in  butterwort  is 
accomplished  but  slowly,  the  process  above  described  is  not  at  all  conspicuous. 
Moreover,  the  margin  of  a  young  leaf  is  always  incurved,  and  that  of  a  mature 
leaf  is  also  somewhat  turned  up  before  stimulation  has  taken  place;  so  that,  strictly 
speaking,  we  only  have  to  do  with  a  greater  or  smaller  degree  of  involution,  and  its 
nature  can  only  be  determined  by  careful  observation. 

In  the  plants  which  form  the  second  group  in  this  section  of  carnivorous 
plants,  and  of  which  the  best  known  representatives  are  the  various  species  of  the 
genus  Sun-dew  (Drosera),  the  movements,  whereby  the  capture  and  digestion  of 
small  animals  is  effected,  occur  much  more  rapidly  and  obviously.  These  species 
are  usually  rooted  in  the  damp  dark  soil  of  moors.  They  have  also  the  same 
habitats  as  Pinguiculse,  and  often  enough  sun-dew  and  butterwort  are  to  be  seen 
flourishing  close  together  on  a  patch  of  boggy  ground  no  larger  than  a  pocket- 
handkerchief.  Hunting  thus  in  couples  these  two  bloodthirsty  organisms,  quite 
unrelated  as  regards  family,  alike  only  in  their  common  object,  seem  to  thrive 
amazingly.  The  thing  that  strikes  one  most  at  sight  of  the  round-leaved  sun-dew 
-as  it  grows  in  its  natural  marshy  habitat,  and  in  general  of  all  the  forty  known 


144  PLANTS  WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 

species  of  Drosera,  is  the  presence  of  the  delicate  wine-red  filaments,  clavate 
at  their  free  ends  and  each  supporting  a  glistening  droplet  of  fluid,  which  stand 
out  from  the  leaves,  and  whose  function  is  essentially  the  same  as  that  of  the 
glands,  stalked  and  sessile,  on  the  leaf  of  Pinguicula.  These  filaments  only 
proceed  from  the  upper  surface  and  margin  of  the  sun-dew  leaf.  The  under  surface 
is  smooth  and  hairless,  and  in  many  species,  including  the  Drosera  rotundifolia 
of  our  indigenous  flora,  it  rests  upon  the  damp  mossy  ground.  In  this  particular, 
and  also  in  the  circumstance  that  all  the  leaves  of  each  individual  are  adpressed  to 
the  ground  and  grouped  in  a  rosette  or  radially  around  the  central  slender  flower- 
ing-stem, there  exists  a  very  obvious  analogy  between  Drosera,  and  not  Pinguicula 
alone,  but  many  other  carnivorous  plants,  such  as  Sarracenia,  Heliamphora,  Cepha- 
lotus,  and  Dioncea,  the  fly-trap  presently  to  be  described. 

The  filaments  or  tentacles  projecting  from  the  upper  surface  and  margin  of  the 
leaf  look  like  pins  inserted  in  a  flat  cushion  and  are  of  unequal  size.  Those  which 
stand  up  perpendicularly  from  the  middle  are  the  shortest,  and  those  which  radiate 
from  the  outermost  edge  are  the  longest  (see  fig.  26 4).  Between  these  extremes 
are  intermediate  lengths  gradually  leading  from  the  one  to  the  other.  There  are 
on  a  leaf,  in  round  numbers,  about  two  hundred  of  these  tentacles.  The  clavate 
head  at  the  free  extremity  of  each  tentacle  is  really  a  gland.  It  secrets  a  clear, 
thick,  sticky  matter  which  is  readily  drawn  out  into  threads,  and  which  shines 
and  glitters  in  the  sunlight  like  a  drop  of  dew,  whence  the  plant  has  derived 
its  name  of  sun-dew.  Shocks  occasioned  by  wind  or  the  dropping  of  rain  do  not 
excite  any  kind  of  movement  in  the  tentacles.  If  grains  of  sand  are  blown  upon 
them  by  the  wind,  or  if  little  bits  of  glass,  coal,  gum,  or  sugar,  or  minute  quantities 
of  paste,  wine,  tea,  or  any  other  non-nitrogenous  substance  are  brought  by  artificial 
means  into  contact  with  the  enlarged  extremities  of  the  tentacles,  the  exudation  of 
liquid  at  the  places  in  question  is  augmented,  and  the  secretion  also  becomes  acid, 
but  there  is  no  elimination  of  pepsin,  and  no  change  of  importance  ensues  in  the 
direction  of  the  tentacles,  or  the  attitude  of  the  leaf-margin.  But  the  moment 
a  small  insect,  mistaking  the  glittering  drops  on  the  tentacles  for  honey  as  it 
flies  by,  alights  on  the  leaf  and  so  touches  the  glands,  or  upon  the  artificial 
placing  of  particles  of  nitrogenous  organic  matter,  such  as  flesh  or  albumen,  on  the 
tentacle-heads,  there  ensues,  as  in  the  case  of  Pinguicula,  an  increase  in  the  dis- 
charge of  acid  juice,  as  well  as  the  addition  of  a  ferment  to  its  composition.  The 
action  of  this  ferment  on  albuminous  compounds  is  entirely  similar  to  that  of 
pepsin,  and  we  may  even  go  so  far  as  to  speak  of  it  as  pepsin. 

The  insects  that  fly  on  to  the  leaves  and  are  caught  by  the  sticky  juice  try  to- 
disencumber  themselves  by  stroking  the  viscous  matter  off  with  their  legs,  but  they 
only  besmear  themselves  still  more,  and  are  soon  plastered  all  over  the  body,  and 
have  their  movements  greatly  impeded  by  the  secretion.  Their  efforts  to  save 
themselves  soon  cease,  the  orifices  of  their  respiratory  organs  are  covered  with  the 
juice  and  choked,  and  after  a  brief  interval  they  die  from  suffocation.  All  these 
phenomena  correspond,  in  the  main,  to  those  occasioned  by  identical  causes  in  the- 


PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY.  145 

case  of  Pinguicula.  But  the  leaves  of  the  sun-dew  are  especially  characterized  by 
the  movements  performed  by  the  tentacles  in  response  to  stimulation  by  animal 
matter.  These  movements  are  exhibited  most  conspicuously  by  the  longest 
tentacles,  which  stand  out  radially  from  the  edge  of  a  leaf.  A  few  minutes  after 
the  gland  of  one  of  these  marginal  tentacles  has  been  excited  by  a  living  or  dead 
animal  becoming  glued  to  it,  a  systematic  disturbance  is  set  up  in  the  whole  fringe 
of  tentacles.  First,  the  tentacle  bearing  the  gland  originally  irritated  with  the 
animal's  body  attached  to  it,  bends  inwards,  performing  a  movement  similar  to  that 


Fig.  26.— Tentacles  on  leaf  of  Sun-dew. 

i  Glands  at  the  extremity  of  a  tentacle;  x30.    2  Leaf  with  all  its  tentacles  inflexed  towards  the  middle,    s  Leaf  with  half  the 
tentacles  inflected  over  a  captured  insect.    *  Leaf  with  all  the  tentacles  extended.    Figs.  2,  »,  and  *x4. 

of  the  hand  of  a  watch.  Under  peculiarly  favourable  circumstances  it  describes  an 
angle  of  45°  in  from  two  to  three  minutes,  and  an  angle  of  90°  in  ten  minutes.  A 
still  more  intelligible  comparison  than  that  of  the  hand  of  a  watch  is  afforded  by  the 
human  hand.  Supposing  that  the  foreign  body  is  glued  to  the  tip  of  a  finger  it 
would  be  moved  by  the  curvature  of  the  finger  to  the  palm  in  the  course  of  ten 
minutes.  About  ten  minutes  after  the  first  tentacle  has  been  set  in  motion,  those 
standing  near  it  begin  to  bend  also  (see  fig.  26 3).  After  another  ten  minutes, 
tentacles  situated  further  off  follow  suit;  and  in  the  course  of  from  one  to  three 
hours  all  the  tentacles  are  inflected  and  converge  upon  the  body  in  question. 

We  must  not  omit  to  mention  that  this  object  does  not  always  occupy  the  same 
place  on  the  surface  of  the  leaf.  Often,  no  doubt,  the  prey  is  exactly  in  the  middle, 
and  the  tentacles  then  swoop  down  one  after  the  other  to  that  spot;  but  often  also 
the  place  is  elsewhere  and  yet  the  movements  never  fail  in  their  aim.  It  may 
happen  that  a  median  tentacle,  on  repeated  excitation,  may  have  to  bend  now  to  the 
right,  now  to  the  left.  When  little  bits  of  meat  are  placed  simultaneously  on  the 
right  and  left  halves  of  the  same  sun-dew  leaf,  the  two  hundred  tentacles  divide 
into  two  groups,  and  each  one  of  the  groups  directs  its  aim  to  one  of  the  bits  of 
meat.  This  happens  also  if  two  small  insects  alight  at  the  same  moment  on  a  leaf, 


VOL.  I. 


146  PLANTS  WHICH   EXHIBIT  MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 

one  on  one  side  and  the  other  on  the  other.  The  movement  of  the  tentacles  is  often 
accompanied  by  an  inflection  of  the  whole  surface  of  the  leaf,  the  lamina  becoming 
concave  like  a  hollow  palm,  and  when,  under  these  circumstances,  the  tentacles  have 
converged  from  the  margin  on  to  the  concave  central  part,  the  leaf  resembles  a 
closed  fist  (see  fig.  26  2). 

All  these  movements  vary  from  one  case  to  another  and  supplement  one  another 
according  to  the  needs  of  the  moment  and  with  a  view  to  immediate  advantage. 
The  one  result  that  is  always  attained  by  the  combined  action  is  the  covering  of  the 
prey  with  a  copious  supply  of  the  secretion  poured  from  a  number  of  glands,  so  that 
it  is  dissolved  and  rendered  fit  for  absorption  and  for  the  purposes  of  nourishment. 
When  an  insect  is  caught  by  one  of  the  marginal  tentacles,  the  secretion  there 
discharged  would  not  suffice  for  these  purposes.  The  prey  is  accordingly  trans- 
ported as  far  as  possible  towards  the  middle  of  the  lamina,  where  it  comes  into 
contact  with  the  digestive  juice  exuded  from  a  maximum  number  of  glands.  It  is 
only  when  the  size  of  the  animal  is  rather  large  that  the  leaf  becomes  hollow  in  the 
middle  like  a  spoon,  with  the  juice  of  more  than  fifty  glands  concentrated  in  the 
depression.  In  a  case  of  this  kind  the  tentacles  remain  inflected  much  longer, 
because  the  solution  of  the  prey  requires  more  time.  If  the  captive  is  very  small, 
its  solution  and  absorption  are  completed  in  a  couple  of  days.  Afterwards,  the 
tentacles  lift,  straighten  themselves,  and  resume  their  original  positions.  The  jaws, 
wings,  compound  eyes,  leg-bones,  claws,  &c.,  of  the  captured  animals  are  left  behind 
undigested;  but  the  flesh  and  blood  are  totally  absorbed,  and  the  liquid  poured  out 
by  the  glands  to  effect  solution  is  also  re-imbibed  by  them.  The  undigested 
remnants  being  now  suspended  on  dry  tentacles  are  easily  blown  away  from  the 
sun-dew  leaves  by  the  wind.  After  an  interval  of  a  day  or  two  the  glands  at  the 
ends  of  the  tentacles,  now  occupying  their  original  positions,  again  separate  out  a 
viscid  fluid  in  the  form  of  tiny  dewdrops,  and  the  leaf  is  once  more  furnished 
with  the  means  of  securing  insects,  and  is  able  to  repeat  the  movements  above 
described. 

Amongst  the  animals  which  fall  victims  to  the  sun-dew  the  most  predominant 
are  little  midges;  but  rather  larger  flies,  too,  ants  both  with  and  without  wings, 
beetles,  small  butterflies,  and  even  dragon- flies,  as  they  run,  creep,  or  fly  past, 
adhere  to  the  extended  gland-bearing  tentacles  as  though  they  were  limed-twigs. 
The  larger  animals,  such  as  dragon-flies,  are  secured  by  the  co-operation  of  two 
or  three  adjacent  leaves.  Some  idea  of  the  large  number  of  captives  made  by  a 
sun-dew  is  given  by  the  fact  that  once  upon  a  single  leaf  were  found  the  remains  of 
thirteen  different  insects. 

In  order  to  place  in  a  true  light  the  vast  significance  of  the  movements  of  the 
tentacles  belonging  to  Drosera  leaves  in  relation,  not  only  to  the  nourishment  of 
that  plant,  but  to  plant-life  in  general,  it  is  necessary  to  direct  attention  to  the 
facts  that  these  movements  are  accomplished  not  in  the  cell  directly  excited,  but  in 
others,  i.e.  in  adjacent  cells  belonging  to  the  same  community;  that  a  propagation  of 
the  stimulus  takes  place  from  one  protoplast  to  a  second,  thence  to  a  third,  fourth, 


PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN  THE   CAPTURE   OF   PREY.  147 

tenth,  and  so  on,  to  a  hundredth,  and  that  the  speed  of  transmission  is  susceptible  of 
measurement.  The  movements  occasioned  in  protoplasts  situated  at  a  distance  from 
the  seat  of  irritation  by  the  stimulus  propagated  from  its  vicinity  are,  according  to 
the  position  of  the  stimulating  object,  sometimes  in  one  direction,  sometimes  in 
another,  but  in  every  case  they  are  purposeful  and  for  the  benefit  of  the  whole 
organism. 

Investigations  with  a  view  to  determining  the  degree  of  sensitiveness  of 
Drosera  leaves  yielded  the  following  results.  A  particle  of  a  woman's  hair,  0'2  mm. 
long  and  weighing  0'000822  mg.,  when  placed  upon  a  gland  of  Drosera  rotundifolia, 
caused  a  movement  of  the  tentacle  belonging  to  the  excited  gland,  which  manifested 
itself  externally  as  an  inflection.  If  so  minute  a  body  of  the  kind  is  placed  on  the 
human  tongue,  its  presence  is  not  perceived,  so  that  the  sensitiveness  of  the 
protoplasts  in  the  glands  of  the  sun-dew  is  greater  than  that  of  the  nerve  extremities 
in  the  tip  of  the  tongue,  though  the  latter  are  well  known  to  be  the  most  sensitive 
in  the  human  body.  A  four-thousandth  part  of  a  milligram  of  ammonium 
carbonate  sufficed  to  induce  motion,  as  also  did  37^^  mg.  of  ammonium  phosphate, 
It  would  lead  us  too  far  to  consider  all  the  experiments  in  detail,  but  they  point  to 
the  conclusion  that  liquid  substances  stimulate  more  strongly  than  solid  bodies,  and 
that  the  more  nutritious  to  the  plant  the  material  placed  upon  the  gland,  the  more 
quickly  does  the  inflection  of  the  tentacles  ensue. 

The  propagation  or  conduction  of  a  stimulus  from  cell  to  cell,  as  it  takes  place 
in  the  cell-community  constituting  a  sun-dew  leaf,  may  be  compared  to  the 
conduction  of  stimulus  by  nerves  from  a  sense-organ  to  the  central  organ,  and  of  the 
force  of  will  from  the  brain  to  the  muscles.  This  transmission  is  conceived  to  be  a 
progressive  movement  affecting  the  ultimate  particles  of  the  nerves,  and  comparable 
to  the  conduction  of  sound,  light,  and  electricity;  but  no  one  has  yet  succeeded  in 
making  these  movements  visible.  So  much  the  more  interesting  is  it  to  be  able  to 
see  and  follow  in  the  glands  and  tentacles,  by  the  aid  of  very  slight  magnifying 
power  or  even  with  the  naked  eye,  the  material  change  which  occurs  in  the 
protoplasts  of  the  sun-dew  leaf  when  they  are  receiving  or  transmitting  a  stimulus. 
The  pedicel  of  a  tentacle  is  penetrated  by  one  or  two  vessels  with  fine  spiral 
sculpturing  on  the  inner  surface,  and  around  these  are  parenchymatous  cells.  The 
gland  has  in  the  middle  a  group  of  oblong  cells  sculptured  internally  with  very 
delicate  spiral  thickenings  ("spiroids"),  and  the  vessel  or  pair  of  vessels  running 
down  the  middle  of  the  tentacle  (see  fig.  26 l)  merge  into  these  spiroids.  A 
parenchyma  composed  of  two  or  three  layers  surrounds  the  median  group  of 
spiroids.  In  each  parenchymatous  cell  the  protoplast  is  discerned  forming  a  thick 
lining  to  the  wall,  and  having  a  continuous  streaming  motion:  whilst  within 
the  vacuole  is  contained  a  homogeneous  liquid  of  a  purple  colour.  If  the  minutest 
fragment  of  animal  matter,  such  as  flesh  or  albumen,  be  placed  on  these  cells  it  acts 
as  a  stimulant  on  the  contents  of  the  cell-cavities,  and  the  impulse  manifests  itself 
in  a  division  of  the  hitherto  homogeneous  purple  liquid  into  dark,  roundish,  club- 
shaped  and  vermiform  lumps,  cloudy  spheres,  and  an  almost  colourless  liquid. 


148 


PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 


This  change,  known  as  "aggregation",  is  propagated  from  the  spot  irritated  down 
from  one  cell  to  another  through  the  tentacle,  across  the  leaf  surface  to  adjoining 
tentacles,  up  to  the  heads  of  these,  and  so  further  and  further  radiating,  so  to  speak, 
in  all  directions.  Accompanying  this  visible  sign  of  conduction,  we  have  the 
bending  of  all  tentacles  in  which  the  purple  fluid  is  altered  in  the  way  described. 
When  the  source  of  excitation,  the  piece  of  flesh,  is  dissolved  and  digested,  and  the 
tentacles  resume  their  original  position,  the  dark  lumps  and  spheres  in  the  cavities 


Fig.  27.— Venus's  Fly- trap  (Dioncea  muscipula). 

of  the  protoplasts  disappear,  and  the  homogeneous  purple  colour  is  restored  as  it 
existed  before  the  stimulation. 

The  various  species  of  the  Sun-dew  genus  are  distributed  over  all  parts  of  the 
world,  and  are  more  numerous  than  those  of  any  other  genus  of  the  family  of 
Droseraceae.  Most  of  the  other  genera  belonging  to  this  order  (Dioncea, 
Aldrovandia,  ByUis,  Roridula,  Drosophyllum)  are  by  no  means  rich  in  members. 
Each  is  represented  merely  by  a  single  or  few  species,  and  is  found  exclusively  in 
a  very  limited  district.  Like  Drosera,  they  are  all  insectivorous  plants,  and  all 
have  the  power  of  dissolving,  absorbing,  and  using  as  supplementary  nutriment, 
nitrogenous  compounds  from  dead  animals.  The  most  striking  of  them  are  Dioncea 
and  Aldrovandia.  They  form  the  very  small  third  group  of  animal-captors,  in 
which  movements  are  performed  for  the  purpose  of  prey,  and  their  apparatus  for 


PLANTS   WHICH   EXHIBIT   MOVEMENTS    IN   THE    CAPTURE   OF   PREY.  149 

seizure  and  digestion  is  one  of  the  most  curious  adaptations  displayed  by  the 
vegetable  world. 

The  Venus's  Fly-trap  (Dioncea  muscipula),  represented  opposite  (fig.  27),  in 
half  its  natural  size,  grows  wild  only  in  a  narrow  strip  of  country  in  the  east  of 
North  America  (from  Long  Island  to  Florida)  in  the  vicinity  of  peat-bogs.  The 
leaves,  like  those  of  many  other  carnivorous  plants,  are  grouped  in  rosettes  round 
the  flowering  axes,  and  for  the  most  part  rest  their  under  surfaces  either  entirely 
or  partially  upon  the  ground.  Each  leaf  consists,  first,  of  a  flat,  spatulate  petiole, 
which  is,  as  it  were,  truncated  in  front  and  suddenly  contracted  to  the  midrib,  and, 
secondly,  of  a  roundish  lamina.  The  latter  is  divided  by  the  midrib  into  two 
symmetrical  halves,  inclined  to  one  another  at  an  angle  of  from  60°  to  90°  like  the 
leaves  of  a  half-open  book.  Both  margins  of  the  lamina  run  out  into  from  twelve 
to  twenty  long,  sharp  teeth,  which,  however,  do  not  carry  either  glands  or  any 
other  special  structures  on  their  tips. 

On  the  central  part  of  each  half  of  the  leaf  there  are  three  very  stiff  and  sharp 
spines,  which  are  always  shorter  than  the  marginal  teeth,  and  which  stand  up 
obliquely.  They  are  composed  of  elongated  cells  whose  protoplasm  throughout 
life  is  in  very  active  circulation.  At  the  base  of  each  spinous  process  is  a  short 
cylindrical  pad  of  tissue  formed  of  small  parenchymatous  cells,  and  this  pad  allows 
the  spine  to  be  deflected.  The  spines  themselves  are  rigid  and  do  not  bend  in 
response  to  pressure;  they  are  forced  down  on  to  the  surface  of  the  leaf,  the  pad  of 
tissue  referred  to  acting  as  a  hinge.  In  addition  to  these  processes,  glands  are 
scattered  over  the  whole  upper  surface  of  the  lamina.  They  look  like  the 
shortly -stalked  glands  of  a  butterwort  leaf,  are  composed  of  some  twenty -eight 
small  cells,  are  purple  in  colour,  and  capable  of  secreting  a  mucilaginous  liquid. 
Little  trichomes,  stellate  hairs,  are  also  borne  on  the  edge  of  the  leaf  between  the 
sharp  teeth,  and  also  on  the  under-surface. 

No  visible  change  is  produced  by  a  blow  or  shock  or  by  pressure  affecting  the 
whole  plant  or  leaf,  as  might  be  caused  by  wind  or  falling  drops  of  rain,  nor  even 
by  injuries  to  the  petiole  or  back  of  the  lamina.  But  as  soon  as  the  upper  surface 
of  the  lamina  is  touched,  the  two  lobes,  hitherto  at  right  angles,  approach  one 
another  until  the  sharp  marginal  teeth  are  interlocked,  and  the  body  touching  the 
leaf  is  inclosed  within  two  walls  (fig.  28 2).  When  the  places  beset  with  purple 
glands  are  alone  excited  by  contact  with  the  object,  this  inflection  and  closing 
follows  very  slowly;  but  if  one  of  the  six  spines  projecting  in  trios  from  the  two 
foliar  lobes  is  ever  so  lightly  touched,  the  leaf  shuts  up  within  10-30  seconds,  i.e. 
quickly  and  steadily;  an  action  best  compared  to  the  closing  of  a  half -open  book. 
The  teeth  standing  at  the  edge  of  the  leaf  lock  into  one  another  on  these  occasions 
like  the  fingers  of  clasped  hands.  The  lobes,  however,  whose  surfaces  were  hitherto 
plane,  become  at  the  moment  of  closing  somewhat  concave,  so  that  when  contracted 
they  do  not  lie  flat  against  one  another  but  inclose  a  cavity,  the  contour  of  which 
nearly  corresponds  with  that  of  a  bean. 

The  further   changes   and   processes  now  ensuing  depend  upon  whether  the 


150 


PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 


sensitive  part  of  the  leaf  was  subjected  to  prolonged  or  only  momentary  contact, 
and  also  upon  the  nature  of  the  body  touching  it,  whether  inorganic  or  organic, 
non-nitrogenous  or  nitrogenous.  When  rapidly  touched  or  stroked,  the  leaf  folds 
together,  but  only  remains  closed  for  a  short  time.  The  lobes  soon  begin  to  re- 
open, and  can  be  stimulated  afresh  immediately  and  caused  to  shut  again.  This 
is  also  the  case  when  the  disturbance  was  due  to  the  impact  of  a  grain  of  sand  or 
any  other  inorganic  body,  and  likewise  when  the  stimulus  proceeded  from  an 
organic  but  non-nitrogenous  object.  But  if,  on  the  other  hand,  the  body  upon  the 
upper  surface  of  the  lamina  was  nitrogenous  and  the  contact  not  too  hasty,  the 
two  lobes  of  the  leaf  remain  closed  over  the  object  for  a  longer  period.  They  also 


Fig.  28.— Capturing  apparatus  of  the  leaves  of  Aldrovandia  and  Venus's  Fly-trap. 

»  Expanded  leaf  of  a  Venus's  Fly-trap.  2  Section  of  a  closed  leaf.  «  One  of  the  sensitive  bristles  on  the  surface  of  the  leaf. 
*  Expanded  leaf  of  Aldrovandia.  *  Section  of  a  closed  leaf.  «  Glands  on  the  surface  of  leaf  of  Aldrovandia.  1  Gland 
from  the  wall  of  a  Sarracenia  pitcher. 

become  flat  and  even  again,  and  are  pressed  together  so  tightly  that  intervening 
bodies,  if  soft,  are  squeezed  and  crushed  to  pieces.  In  addition,  the  glands,  dry 
till  then,  begin  to  secrete  a  slimy,  colourless,  highly  acid  juice;  and  this  is  true 
even  of  those  glands  which  are  not  at  all  in  contact  with  the  nitrogenous  bodies 
inclosed.  The  secretion  flows  so  copiously  that  it  can  be  seen  in  the  form  of  drops 
if  the  shut  lobes  be  forcibly  separated.  It  covers  the  imprisoned  body  and  gradu- 
ally dissolves  the  albuminous  compounds  therein  contained.  Afterwards,  the 
secretion  and  the  matter  dissolved  in  it  are  re-absorbed  by  the  same  glands  as 
previously  discharged  the  acid  liquid,  containing  pepsin,  in  response  to  the  stimulus; 
and  when  the  trap  reopens,  the  glands  are  dry.  The  soluble  part  of  the  prey  has 
now  vanished:  the  six  little  spinous  processes,  which  were  bent  in  the  closed  leaf 
like  the  blades  of  a  pocket-knife  and  lay  pressed  down  upon  the  surface,  stand  up; 
and  the  leaf  is  once  more  equipped  for  making  fresh  captures. 

The  time  requisite  for  the  digestion  of  a  nitrogenous  body  resting  upon  the 
surface  of  a  leaf  varies  according  to  the  size  of  the  body.  The  leaf  usually  remains 
closed  for  from  eight  to  fourteen  days,  but  often  even  for  twenty  days.  Although 


PLANTS  WHICH   EXHIBIT  MOVEMENTS   IN   THE   CAPTURE   OF   PREY.  151 

the  larger  live  articulated  animals — earwigs,  millipedes,  and  dragon-flies — caught 
upon  the  upper  surface  of  the  leaf,  cause  the  lobes  to  slam  together,  they  are  able 
to  slip  out  if  part  of  their  bodies  projects  beyond  the  toothed  margin,  for  the  teeth 
are  flexible  and  yield  to  strong  pressure.  But  small  creatures  are  hopelessly  lost 
when  the  lobes  have  closed  over  them.  They  are  at  once  suffocated  in  the  liquid 
which  is  poured  out  copiously  by  the  glands  and  are  then  dissolved  and  absorbed 
with  the  exception  of  their  claws,  leg-bones,  chitinous  rings,  &c.,  which  are  incapable 
of  being  digested. 

In  spite  of  the  identity  of  aim  and  of  result,  the  mechanism  of  a  Dioncea  leaf 
differs  very  materially  from  that  of  the  sun-dew  leaf  described  above.  Division  of 
labour  is  carried  much  further  in  the  Fly-trap.  The  pre-eminently  sensitive 
structures,  viz.,  the  six  filaments  situated  upon  the  upper  surface  of  the  leaf, 


Fig.  29.— Aldrovandia  vesiculosa. 

do  not  act  also  as  digestive  glands.  Again,  the  long  sharp  teeth  at  the  edge 
of  the  leaf,  which  correspond  in  position  to  the  marginal  tentacles  of  a  sun-dew  leaf, 
carry  no  glands,  and  only  serve  to  close  the  trap  securely  when  an  animal  has  been 
caught.  Accordingly  in  Dioncea  there  exist  special  structures  for  three  different 
functions,  namely,  stimulation,  seizure,  and  digestion,  whilst  in  the  case  of  Drosera 
all  these  functions  belong  to  the  gland-bearing  tentacles  alone.  The  stimulus  acting 
on  the  sensitive  filaments  on  the  leaf  of  the  Fly-trap  is  liberated  in  the  form  of  a 
rapid  motion  of  the  lobes  and  a  discharge  of  digestive  fluid  from  the  glands,  and 
this  discharge  of  secretion  ensues  therefore  through  the  mediation  of  cells  which 
have  not  themselves  been  directly  excited.  The  process  here  again  is  much  more 
striking  than  in  the  sun-dew  leaf.  The  transmission  of  stimulus,  though  as  a  fact 
identical  in  the  two  plants  we  are  comparing,  proceeds  at  any  rate  with  much 
greater  rapidity  in  Dioncea  than  in  Drosera. 

The  analogy  existing  between  these  processes,  especially  the  conduction  and 
liberation  of  stimulus,  and  similar  phenomena  of  the  muscles  and  nerves  in  an 
animal  organism,  has  already  been  brought  out  in  discussing  the  sun-dew.  It  is  a, 
noteworthy  fact  that,  in  the  fly-traps,  actual  electric  currents  have  been  observed, 
which  shows  that  the  greatest  resemblance  exists  to  muscles  and  nerves  as  regards 
electro-motor  action  also.  A  positive  current  runs  from  the  base  to  the  apex  of  the 
lamina;  another  current  running  in  the  opposite  direction  is  demonstrable  in  the 
petiole;  and  the  upper  layers  of  cells  in  the  lamina  and  the  midrib  are  ascertained 


152  PLANTS   WHICH   EXHIBIT   MOVEMENTS   IN   THE   CAPTURE   OF   PREY. 

to  be  the  seat  of  origin  of  this  phenomenon.  A  great  alteration  in  the  intensity  of 
the  current  ensues  upon  each  excitation  of  the  leaf;  and,  inasmuch  as  this  fluctuation 
of  the  electric  current  precedes  the  movement  of  the  leaf  caused  by  the  stimulus, 
it  is  natural  to  assume  that  it  depends  upon  the  conduction  and  liberation  of  the 
stimulus. 

Aldrovandia,  the  plant  nearest  allied  to  the  Fly-trap  in  the  structure  of  its  leaf, 
is  a  water-plant,  which  occurs  scattered  over  the  southern  and  central  parts  of 
Europe.  It  only  flourishes  in  shallow  ditches,  pools,  and  small  ponds  inclosed  by 
banks  of  reeds  and  rushes,  where  the  plants  are  immersed  in  clear,  so-called  soft 
water,  attaining  in  summer  to  a  temperature  of  30°  C.,  and  are  exempt  from  any 
incrustation  of  carbonate  of  lime,  whereby  the  tender  parts  of  the  leaves  might 
be  hindered  in  their  movements.  On  cursory  inspection,  one  might  take  Aldro- 
vandia vesiculosa,  which  is  represented  in  fig.  29  full  size  and  in  its  natural  position, 
for  a  Utricularia  (cf.  fig.  17).  It  lives,  like  the  latter,  floating  in  water;  is  destitute 
of  roots,  and  has  a  slender  filiform  stem  with  leaves  arranged  in  whorls  and  ter- 
minating in  bristles.  In  proportion  as  it  grows  at  the  apex,  the  hinder  part  dies 
away  and  decays.  The  development  of  hibernating  buds  takes  place  also  in 
precisely  the  same  manner  as  in  Utricularia.  Towards  autumn,  the  stem  ceases 
to  elongate,  and  the  two  hundred  small  and  young  leaves,  which  adorn  the  ex- 
tremity of  the  stem  and  whose  cells  are  quite  full  of  starch,  remain  lying  closely 
wrapped  one  upon  another  and  form  a  dark,  oval,  bristly  ball,  which  sinks  at  the 
•commencement  of  winter  to  the  bottom  of  the  pool  or  pond  and  hibernates  there 
lying  upon  the  mud. 

It  is  not  till  very  late  in  the  following  spring,  when  little  midge-larvae  and 
•other  animals  begin  to  move  about  in  the  water,  that  fresh  life  is  awakened  in 
these  structures.  The  starch-grains  in  the  leaves  are  brought  into  solution  and 
used  for  building-material;  the  axis  elongates,  and  lacunse  filled  with  air  are 
developed,  whereupon  the  plant  becomes  lighter,  ascends,  and  remains  throughout 
the  summer  and  autumn  floating  just  below  the  surface  of  the  water.  Although 
the  little  leaves  of  the  winter-buds  generally  admit  of  the  recognition  of  their 
future  form,  the  apparatus  adapted  to  the  capture  of  animals  is  but  little  developed 
•on  them.  But  when  once  the  leaves  are  mature,  they  bear  laminae,  which  are 
extremely  like  those  of  Dioncea  in  shape,  and  serve,  as  do  the  latter,  for  the  capture 
of  small  animals.  Each  leaf  is  differentiated,  as  in  Dioncea,  into  a  strong,  dark-green 
petiole  expanded  and  anteriorly  clavate,  and  into  a  roundish  lamina  with  a  delicate 
epidermis  and  with  two  lobes  connected  by  the  midrib  and  inclined  nearly  at  right 
angles  to  one  another  (see  fig.  28 4).  The  midrib  projects  beyond  the  apex  of  the 
delicate  lamina  in  the  form  of  a  bristle.  In  addition,  comparatively  long,  rigid 
bristles,  tipped  with  extremely  fine  spines,  proceed  from  the  petiole  close  to  where 
the  latter  is  joined  to  the  lamina;  and  these  bristles,  which  are  directed  forwards, 
give  the  whole  leaf -structure  a  spiky  appearance  and  prevent  the  approach  of  such 
animals  as  are  not  suitable  for  prey.  The  two  margins  of  the  lamina  are  bent 
inwards,  and  their  rims  are  studded  with  small  conical  points.  On  the  surface  of 


CARNIVOROUS   PLANTS  WITH   ADHESIVE   APPARATUS.  153 

the  lamina,  especially  along  the  midrib,  there  are  pointed  hairs,  whilst  a  great 
number  of  glands,  some  larger  and  some  smaller,  occur  from  the  midrib  to  nearly 
the  middle  of  each  lobe.  The  larger  glands  are  discoid,  and  not  unlike  the  sessile 
glands  on  the  leaves  of  Pinguicula.  They  consist  of  four  median  cells  with 
twelve  others  grouped  round  about  them,  and  are  borne  upon  a  very  short  stalk. 
The  small  glands  are  few-celled,  being  usually  composed  simply  of  a  capitate-cell 
resting  upon  a  short  foot-cell  (see  fig.  28  6).  Towards  the  incurved  margin  of  the 
lamina  are  displayed  scattered  stellate  hairs,  i.e.  groups  of  cells  so  arranged  as 
to  present  the  appearance  of  a  St.  Andrew's  cross  when  seen  from  above. 

If  minute  animals  or  Diatomaceae,  especially  species  of  Navicula,  whilst 
swimming  about  in  the  water,  touch  the  upper  surfaces  of  the  lobes  set  at  right- 
angles — in  particular,  if  the  hairs  in  the  middle  are  stroked  as  they  creep 
by — the  two  lobes  shut  together  quickly  in  the  same  way  as  those  of  Dioncea,  and 
the  animal  or  Navicula,  as  the  case  may  be,  is  then  enclosed  in  a  cage  between 
two  somewhat  inflated  walls.  The  possibility  of  an  attempt  on  the  part  of  the 
captive  to  escape  by  the  place  where  the  margins  of  the  lamina  meet  is  met  by  the 
circumstance  that  the  edges  of  the  incurved  margins  are  furnished  with  sharp 
indentations  turned  towards  the  interior  of  the  cavity  enclosed  between  the  lobes 
(see  fig.  28 5). 

Amongst  the  prisoners  we  find  the  same  company  as  in  the  traps  of  Utricularia, 
namely,  small  species  of  Cyclops,  Daphnia,  and  Cypris,  larvae  of  aquatic  insects,  and 
not  infrequently  also  species  of  Navicula  and  other  free  and  solitary  Diatomaceae. 

How  the  prey  is  killed  and  digested  has  not  yet  been  ascertained.  It  does  not 
in  any  case  take  place  so  quickly  as  in  Dioncea,  for  instances  have  been  seen  of 
animals  still  living  in  their  prison  six  days  after  being  caught.  But,  at  last,  move- 
ments and  vital  actions  cease,  and  if  after  a  couple  of  weeks  the  two  lobes  of  the 
lamina  are  pulled  apart,  the  only  contents  to  be  found  are  shells,  bristles,  rings,  and 
siliceous  skeletons,  whilst  everything  soluble  has  vanished,  having  evidently  been 
absorbed. 

Very  similar  to  the  species  distributed  through  Southern  and  Central  Europe 
are  Aldrovandia  australis,  a  native  of  Australia,  and  Aldrovandia  verticillata, 
inhabiting  tropical  India.  The  fact  that  the  remains  of  small  aquatic  beetles 
-and  other  creatures  have  been  found  within  their  closed  laminae,  leads  us  to  the 
•conclusion  that  they  act  as  entrappers  of  animals  in  the  same  way  as  Aldro- 
vandia vesiculosa. 

CARNIVOROUS  PLANTS  WITH   ADHESIVE  APPAEATUS. 

The  forms  constituting  the  third  section  of  carnivorous  plants  neither  have  pit- 
falls nor  move  in  response  to  the  contact  of  animal  matter,  but  the  leaves  act  as 
motionless  limed  twigs,  their  glands  having  the  power  of  pouring  out  sticky  sub- 
stances to  capture  prey  and  juices  to  digest  it,  being  able  besides  to  re-absorb  the 
albuminoid  compounds  dissolved.  The  most  striking  representative  of  this  section, 


154  CARNIVOROUS   PLANTS   WITH   ADHESIVE    APPARATUS. 

and  the  one  most  accurately  studied,  is  the  Fly-catcher  (Drosophyllum  lusitanicum), 
which  is  indigenous  to  Portugal  and  Morocco,  and  is  shown  in  the  illustration  on 
p.  155.  This  plant  differs  from  all  the  carnivorous  kinds  hitherto  discussed  in 
respect  of  habitat,  inasmuch  as  it  does  not  grow  under  water  or  even  in  swampy 
places  but  on  sandy  ground  and  dry  rocky  mountains.  The  stem  in  robust 
specimens  is  nearly  9  inches  high,  and  bears,  on  a  few  short  branches  at  the 
top,  flowers  from  2  to  3  cm.  in  diameter.  The  leaves  are  very  numerous  and 
particularly  crowded  round  the  base  of  the  stem.  Their  shape  is  linear  and 
much  attenuated  towards  the  filiform  tip,  whilst  the  upper  surface  is  somewhat 
hollowed  so  as  to  form  a  groove.  With  the  exception  of  these  grooves,  the  leaves  are 
entirely  covered  with  beads,  which  glisten  in  the  sunshine  like  dewdrops;  and  it  is 
to  this  circumstance  that  the  plant  owes  its  name  of  Drosophyllum,  i.e.  Dew-leaf. 
The  glittering  drops  are  the  secretion  of  glands,  which  in  form  remind  one  in  some 
respects  of  the  long-stalked  glands  of  the  butterwort,  and  in  others  of  those  of  the 
Sun-dew  (Drosera).  They  resemble  the  latter  in  their  red  coloration,  in  the  fact 
that  the  pedicel  bearing  the  gland  contains  vessels  whilst  the  glands  themselves 
have  oblong  cells  with  internal  walls  thickened  by  fine  spiral  ridges,  and  further, 
in  the  circumstance  that  the  secretion  covers  the  gland  with  a  colourless  film  in  the 
form  of  a  drop.  But  in  shape  they  especially  resemble  the  glands  of  the  butter- 
wort,  being  just  like  little  mushrooms. 

Besides  these  glands,  which  are  borne  on  stalks  of  unequal  lengths  and  are 
plainly  to  be  distinguished  with  the  naked  eye,  there  are  also  very  small  sessile 
glands.  These  latter  are  colourless,  and  in  particular  differ  from  the  stalked  variety 
in  the  fact  that  they  discharge  an  acid  liquid  only  when  they  come  into  contact 
with  nitrogenous  animal  matter,  whereas  the  production  of  drops  on  the  stalked 
glands  is  accomplished  without  any  such  contact.  This  secretion  is  acid  and  ex- 
tremely viscid.  It  has  the  property  of  adhering  immediately  to  foreign  bodies  coming 
into  contact  with  it,  though  it  is  readily  withdrawn  from  the  gland  itself.  When 
an  insect  alights  on  the  leaf,  its  legs,  abdomen,  and  wings  instantly  stick  to  the  drop 
touched  by  them.  The  insect,  however,  is  not  held  fast  by  the  gland  which  secreted 
that  drop,  but,  being  able  to  move,  drags  the  drop  off  the  gland.  Its  movements 
bring  it  into  contact  with  other  drops,  which  thereupon  are  similarly  detached 
from  the  glands;  and  so,  in  a  very  short  time,  the  insect  is  smeared  with  the 
secretion  from  a  number  of  glands.  Thus  clogged  and  overwhelmed,  it  is  no  longer 
able  to  crawl  along,  but,  suffocating,  sinks  down  to  the  sessile  glands  which  cover 
the  surface  of  the  leaf  at  a  lower  level.  All  the  soluble  parts  of  its  body  are  then 
dissolved  by  means  of  the  secretion  of  these  glands  and  are  afterwards  absorbed. 

The  glands  renew  the  drops  of  secretion  of  which  they  are  despoiled  with 
great  rapidity.  The  quantity  of  acid  liquid  secreted  is,  in  general,  very  great,  so- 
that  it  is  not  surprising  to  find  Drosophyllum  covered  at  the  same  time  with 
remains  of  besmeared  dead  bodies  drained  of  their  juices,  and  with  still  struggling 
insects  which  have  recently  alighted  and  become  clogged.  The  number  of  animals 
caught  by  the  leaves  of  a  single  plant  is  very  great;  and  even  people  who  are  not 


CARNIVOROUS   PLANTS   WITH   ADHESIVE   APPARATUS. 


155 


otherwise  interested  in  the  vegetable  world  are  impressed  by  the  sight  of  a  plant 
with  its  leaves  covered  with  a  number  of  insects  adhering  to  them  as  though  they 
were  limed  twigs.  In  the  neighbourhood  of  Oporto,  where  Drosophyllum  grows 
abundantly,  the  peasants  use  these  plants  instead  of  limed  twigs,  hanging  them  up 


Fig.  30.— The  Fly-catcher  (Drosophyllum  lusitanicum). 

in  their  rooms,  and  so  getting  rid  of  numbers  of  troublesome  flies  which  stick  to 
them  and  are  killed. 

A  number  of  other  plants  have  the  power,  though  in  a  less  conspicuous  degree 
than  Drosophyllum,  of  obtaining  additional  nitrogenous  food  out  of  adherent 
animals  by  means  of  secretory  and  absorptive  glands.  Such  are  many  species  of 
primulas,  saxifrages,  and  house-leeks,  which  bury  their  roots  in  cracks  and  crevices 
of  rock  (e.g.  Primula  viscosa,  P.  villosa,  P.  hirsuta,  Saxifraga  luteo-viridis,  S. 
bulbifera,  S.  tridactylites,  Sempervivum  montanum),  secondly,  caryophyllaceous 


156  CARNIVOROUS   PLANTS   WITH    ADHESIVE   APPARATUS. 

plants  and  species  of  the  caper  order  (e.g.  Saponaria  viscosa,  Silene  viscosa,  Cleome 
omithopodioides,  Bonchea  cohiteoides),  and  lastly,  a  series  of  plants  which  nourish 
in  peat-bogs  and  upon  deep  beds  of  humus,  such  as  Sedum  villosum,  Roridula 
dentata,  Byblis  gigantea,  and  many  others  besides. 

It  would,  however,  be  erroneous  to  suppose  that  in  all  cases  where  a  sticky 
coating  occurs  on  leaves  and  stem  a  solution  and  digestion  of  the  insects  adhering 
to  the  viscid  parts  is  necessarily  denoted.  In  many  instances  structures  of  this 
kind,  which  are  analogous  to  limed  twigs,  are  a  means  of  protecting  honey-bearing 
flowers  against  unwelcome  guests  belonging  to  the  world  of  insects,  as  will  be 
explained  in  greater  detail  later  on.  Glands  secreting  a  viscid  substance  may,  no 
doubt,  often  possess  two  kinds  of  function — they  may,  on  the  one  hand,  prevent 
unbidden  animals  from  approaching  the  honey,  and,  on  the  other,  by  dissolving  their 
flesh  and  blood  with  the  aid  of  the  secretion  and  then  absorbing  them,  turn  to  ad- 
vantage such  insects  as  are  tempted  by  immoderate  craving  to  step  upon  the  perilous 
path  leading  to  the  honey-receptacles  and  adhere  there  and  die. 

Many  plants  have  structures  on  the  epidermis  of  their  leaves  corresponding  in 
form  to  the  glands  of  insectivorous  plants,  but  which  do  not  discharge  secretions 
either  spontaneously  "or  when  irritated.  On  the  other  hand,  these  structures  have 
the  power  of  imbibing  water,  and  are,  in  this  relation,  of  the  greatest  importance 
to  the  plants  in  question.  Although  the  more  detailed  treatment  of  them  is  post- 
poned until  we  have  occasion  to  deal  with  the  absorption  of  water  by  aerial  organs, 
it  is  advisable  to  refer  now  to  the  fact  that  chemically  pure  water  only  very  rarely 
reaches  the  interior  of  a  plant  by  means  of  the  absorptive  organs  mentioned. 
Sulphuric  acid  is  almost  always  introduced  with  atmospheric  water,  and  in  some 
circumstances  ammonia  also.  However  trivial  the  amount  of  the  nitrogen  con- 
veyed to  plants  in  this  way,  it  must  not  be  undervalued,  at  all  events  in  the  case 
of  those  which  are  only  able  to  acquire  small  quantities  of  nitrogenous  compounds 
from  the  ground  by  means  of  their  roots.  Now,  it  is  very  probable  that  plants  of 
this  kind  do  not  reject  even  other  nitrogenous  compounds  which  are  brought 
with  the  water  from  the  atmosphere  to  their  aerial  leaves.  The  foliage-leaves  of 
many  plants  display  contrivances  whereby  rain-water  is  often  retained  for  a 
considerable  time  in  special  hollows.  In  these  depressions  there  is  invariably  a 
collection  of  dust-particles,  small  dead  animals,  pollen-grains,  &c.,  which  have  been 
blown  in  by  the  wind,  whilst  rain  trickling  down  the  stem  brings  very  various 
objects  with  it  from  higher  up  and  washes  them  into  these  reservoirs  in  the  leaves. 
Sometimes  too  a  few  animals  are  drowned  in  the  water-receptacles.  As  a  matter 
of  fact,  the  water  in  the  hollows  of  the  leaves  of  the  Peltate  Saxifrage  and  of 
Bromeliads,  in  the  inflated  vaginae  of  many  umbelliferous  plants,  and  in  the 
cups  formed  by  the  coalescence  of  opposite  leaves  in  many  Gentianeae,  Compositae, 
and  Dipsaceae,  is  always  brown-coloured,  and  contains  nitrogenous  compounds  in 
solution,  derived  from  the  decaying  bodies  of  dead  animals  which  have  fallen  into 
these  receptacles. 

If  absorbent  organs  are  present  in  the  reservoirs  in  question,  the  water,  together 


CARNIVOROUS   PLANTS   WITH   ADHESIVE   APPARATUS.  157 

with  the  nitrogenous  compounds  dissolved  therein,  is  absorbed  without  delay. 
Hollows  of  this  kind  occurring  in  foliage-leaves  only  differ  from  those  above 
described  as  developed  on  sarracenias  in  being  destitute  of  special  contrivances  for 
decoying  animals  into  the  traps,  and  for  rendering  their  escape  from  the  latter 
impossible.  It  cannot  be  denied  that  through  forms  of  this  kind  a  gradual 
transition  has  been  proved  to  exist  between  plants  which  absorb  nearly  pure  water 
by  means  of  their  foliage-leaves  and  those  which  capture  animals.  And,  further, 
amongst  the  latter  we  find  all  gradations  of  mechanism  from  Drosophyllwn,  and 
the  Primulas  with  their  epiphyllous  secretory  glands  up  to  the  Fly-trap  (Dioncea), 
which  exhibits  the  most  complex  apparatus  of  all  for  capturing  and  digesting  prey, 
and  in  which  division  of  labour  is  carried  to  its  highest  development  by  the  com- 
munities of  cells  constituting  the  foliage-leaves. 

It  is  not  surprising  that  the  first  apparatus  for  capturing  and  digesting  insects 
to  be  noticed,  to  have  its  functions  recognized  and  to  be  described,  was  that  of 
Dioncea.  But  it  strikes  one  as  all  the  more  strange  that  of  late  the  question  has 
repeatedly  been  mooted  in  the  very  case  of  Dioncea,  as  to  whether  the  capture  and 
digestion  of  insects  is  not  injurious  instead  of  beneficial  to  these  plants.  Gardeners, 
who  have  cultivated  Dioncea  in  greenhouses,  have  made  the  observation  that 
individuals  protected  from  the  visits  of  insects  thrived  at  least  as  well  as  those 
whose  leaves  were  covered  with  bits  of  meat,  &c.,  or,  to  employ  the  usual  phrase, 
were  fed  with  meat.  It  has  also  been  found  that  a  leaf  cannot  stand  more  than 
three  meals;  indeed,  it  often  happens  that  even  after  the  first  occasion  of  digesting 
a  bit  of  meat,  the  leaf  concerned  shows  signs  of  having  been  injured  by  the 
repast.  That  is  to  say,  a  long  time  elapses  before  leaves  which  have  digested 
a  largish  albuminoid  mass  regain  their  normal  irritability;  and  often  they  wither 
and  die.  If  cheese  is  placed  on  Dioncea,  it  is  true  the  leaf  closes  over  it,  and  there 
is  a  commencement  of  the  process  of  solution,  but  before  this  is  accomplished  the 
leaf  turns  brown  and  perishes.  Yet  if  Dioncea  were  obliged  to  lose  a  leaf  after 
every  meal,  the  result  would  be  very  disadvantageous. 

As  against  these  considerations,  we  have  first  of  all  to  remark  that  the 
absorption  of  nutriment  takes  place  in  nature  in  a  manner  differing  materially  from 
the  phenomenon  in  greenhouses.  A  leaf  of  Dioncea  in  the  wild  state  is  protected 
against  the  possibility  of  receiving  too  plentiful  a  dose  of  albuminoid  substances  at 
a  time.  Insects  so  large  as  not  to  allow  the  lobes  to  close  together  over  them  slip 
out  again,  and  only  small  ones  are  caught  and  retained.  When,  in  the  latter  case, 
one  deducts  the  chitinous  coat,  and  in  general  all  parts  not  susceptible  of  being 
digested,  such  a  small  quantity  of  albuminoid  compounds  is  left  that,  compared  with 
it,  the  little  cubes  of  meat  used  in  the  experiments  made  in  greenhouses  must  be 
looked  upon  as  an  exceedingly  sumptuous  repast.  But  that  so  small  an  amount  of 
nitrogenous  food  as  is  to  be  derived  from  a  tiny  captured  insect  does  not  act 
injuriously,  follows  from  the  fact  that  dionaeas  growing  wild  flourish  excellently, 
and  do  not  exhibit  the  brown  discoloration  of  the  leaves  which  is  caused  in  a 
greenhouse  by  placing  bits  of  cheese  upon  them.  If  the  absorption  of  nitrogenous 


158  CARNIVOROUS  PLANTS  WITH   ADHESIVE   APPARATUS. 

aliment  from  prey  were  injurious  to  Dioncea,  the  plant  would  certainly  have  died 
out  long  ago.  If,  therefore,  cultivated  specimens  of  Dioncea  have  suffered  from 
being  fed  with  meat,  fibrin,  cheese,  and  other  such  materials,  only  this  much  is 
proved,  that  the  nutriment  in  question  was  not  beneficial  to  them  owing  to  its 
quality  or  to  its  being  too  concentrated. 

As  regards  the  other  point,  that  Dioncea  thrives  well  under  cultivation,  even 
when  all  visits  from  insects  are  excluded,  we  must,  on  the  other  hand,  bear  in  mind 
that  the  successful  growth  of  Dioncea,  like  that  of  Drosera,  Pinguicula,  &c.,  is  not 
conceivable  unless  in  some  way  or  another  the  nitrogen  indispensable  for  the 
construction  of  the  protoplasm  is  conveyed  to  the  individuals  in  question.  The 
source  from  which  it  is  taken  varies  according  to  the  site.  If  the  roots  are  buried 
in  deep  sods  of  bog-moss  upon  a  flat  expanse  of  moorland,  the  supply  of  nitrogen 
from  the  ground,  and  also  from  the  air,  will  be  extremely  limited,  and  probably 
insufficient;  under  these  circumstances  the  nutriment  derived  from  the  dead  bodies 
of  captured  insects  would  be  not  only  useful  and  beneficial,  but  may  be  even 
essential.  If,  on  the  contrary,  the  place  where  the  plants  have  been  reared  or  have 
grown  up  spontaneously  is  such  that  they  can  obtain  the  requisite  nitrogen  from 
the  ground  or  air,  they  are  able  without  harm  to  dispense  with  the  available  source 
of  nitrogen  afforded  by  the  capture  of  insects.  It  is  worthy  of  notice  that  insect- 
ivorous plants  always  grow  wild  only  in  places  that  are  poorly  supplied  with  nitro- 
genous food.  The  majority  occur  in  pools  fed  by  subterranean  water,  whose  course 
lies  through  layers  of  peat,  or  in  the  spongy  peat  itself,  or  in  the  sods  of  Sphagnum. 
Others  are  rooted  in  deep  chinks  in  the  stone  on  the  declivities  of  rocky  mountains, 
whilst  yet  others  occur  in  the  sand  of  steppes.  The  water  available  in  such  situa- 
tions for  absorption  by  the  suction-cells  is,  to  say  the  least,  very  poorly  furnished 
with  nitrogenous  compounds;  and  the  quantity  of  these  compounds  passing  from 
the  ground  into  the  air  at  the  places  mentioned  is  extremely  minute  and  inconstant. 
Under  these  circumstances,  the  acquirement  of  nitrogen  from  the  albuminoid 
compounds  of  dead  animals  is  certainly  of  benefit,  and  all  the  various  pitfalls, 
traps,  and  limed  twigs  are  explained  as  contrivances  by  means  of  which  this 
advantage  is  secured. 


CLASSIFICATION   OF   PARASITES.  159 


4.  ABSORPTION   OF  NUTRIMENT   BY   PARASITIC   PLANTS. 

Classification  of  parasites.— Bacteria.— Fungi.— Twining  parasites.— Green-leaved  parasites.— Tooth- 
wort.— Broom-rapes,  Balanophoreae  and  Bafflesiaceae.— Mistletoe  and  Loran thus.— Grafting  and 
budding. 

CLASSIFICATION   OF  PARASITES. 

The  ancients  understood  by  parasites  people  who  intruded  uninvited  into  the 
houses  of  the  rich  in  order  to  obtain  a  free  meal.  The  designation  was  first  applied 
to  plants  by  an  eighteenth  -  century  botanist,  named  Micheli,  in  his  work  "De 
Orobanche"  (1720)  wherein  are  described  amongst  others,  many  kinds  of  "plantse 
secundariae  aut  parasiticaa".  Micheli  included  under  this  term  plants  which  with- 
draw organic  compounds  from  living  plants  or  animals,  thus  sparing  themselves  the 
labour  of  forming  those  compounds  out  of  water,  salts,  and  constituents  of  the  air. 
For  a  long  time  all  epiphytes,  including  mosses  and  lichens  growing  on  the  bark  of 
trees,  and  indeed  even  many  climbing  plants,  were  held  to  be  parasites.  Thus,  it  is  not 
long  ago  that  Clusia  rosea,  which  occurs  in  the  Antilles,  was  described  as  a  regular 
vampire,  in  whose  embraces  other  plants  met  their  death;  and  it  has  been  asserted 
respecting  a  whole  series  of  other  plants  of  the  tropical  zone,  including,  for  instance, 
several  species  of  fig,  that  they  attach  their  stems  and  branches  to  other  trees, 
divest  themselves  of  their  bark,  and  cause  the  death  of  that  of  the  neighbour 
attacked  as  a  consequence  of  the  pressure  which  they  exert.  The  young  wood  of 
the  invader  would  then  come  into  direct  connection  with  the  young  wood  of  the 
plant  assailed,  and  the  possibility  would  thus  be  afforded  of  draining  the  latter  of 
all  its  juices. 

These  assumptions,  at  least  as  regards  the  exhaustion  of  juices,  have  not  been 
confirmed.  When  individuals  of  species  of  Clusia  or  Ficus,  which  have  roots  buried 
in  the  earth,  and  are  themselves  already  grown  up  into  stately  leaf -bearing  plants, 
attach  their  flattened  stems  and  branches  to  other  plants,  investing  them  so 
completely  as  to  interfere  with  the  process  of  respiration,  this  constitutes,  at  all 
events,  an  invasion  of  one  of  the  most  important  of  the  vital  functions  of  the  plant 
attacked,  and  may  ultimately  cause  its  death;  but  the  killing  is  not  under  these 
circumstances  due  to  drainage  of  juices,  but  is  brought  about  by  suffocation. 
Lichens,  too,  when  they  cover  the  bark  of  trees  with  a  close-fitting  mantle,  may 
possibly  restrict  the  process  of  respiration  through  particular  parts  of  the  cortex, 
and  thereby  injure  the  development  of  the  tree  in  question;  but  they  are  not  on 
that  account  to  be  looked  upon  as  parasites  any  more  than  the  fructifications  of  the 
species  of  Telephora,  and  other  Basidiomycetes,  which  grow  up  rapidly  from  the 
ground,  and,  spreading  out  like  plastic  doughy  masses,  envelop  all  objects  which 
come  in  their  way,  and  ultimately  stifle  such  as  are  living,  namely,  grass  haulms, 
bilberry  bushes,  &c.  Even  creepers,  which  impose  woody  stems  upon  the  trunks  of 
young  trees,  winding  round  them  like  serpents,  and  restricting  their  circumferential 
growth  at  the  parts  in  contact  with  the  coils,  so  that  ultimately  the  latter  lie 


160  CLASSIFICATION   OF   PARASITES. 

imbedded  in  regular  grooves  in  the  cortex,  ought  not  to  be  considered  as  parasites. 
The  Lonicera  ciliosa  of  North  America,  represented  in  fig.  31,  may  be  taken  as  an 
example  of  creepers  of  this  kind.  They  only  interfere  with  the  conduction  of  the 
constructive  materials  generated  in  the  green  foliage,  preventing,  in  particular,  the 


Fig.  31.— Lonicera  ciliosa  in  South  Carolina. 

part  of  the  axis  below  the  strangulating  coils  from  being  supplied  with  those 
materials;  and  so  at  last  they  cause  the  whole  trunk,  which  serves  as  their  support, 
to  dry  up.  The  assertion  may  then  be  made  that  the  young  tree  assailed  has  been 
strangled  or  throttled  by  the  creeper,  but  not  that  the  latter  has  drained  it  of  juices 
and  adapted  them  to  its  own  use.  Still  less  would  the  statement  be  applicable  to 
the  numerous  brown  and  red  sea- weeds,  which  settle  upon  the  ramifications  of  the 
great  species  of  Sargassum,  or  of  the  innumerable  Diatomacese,  which  often  entirely 


BACTERIA.      FUNGI. 

cover  both  fresh  and  salt-water  plants.  In  still  inlets  of  the  sea  it  is  not  rare  to  see 
the  larger  sea- wracks  with  smaller  specimens  clinging  to  them,  whilst  Floridese  are 
fastened  to  the  latter,  and  minute  siliceous-coated  diatoms  to  the  Florideas.  Even 
in  fresh  water,  e.g.  in  cold  and  rapid  mountain  streams,  we  find  little  tufts  of 
Chantransia  or  Batrachospermwm  developed  as  epiphytes  upon  the  black-green 
filaments  of  Lemanea,  and  on  the  former,  again,  Diatomacese.  One  of  these 
Diatomaceae,  which,  from  its  resemblance  to  a  scale  insect,  has  received  the  name  of 
Cocconeis  Pediculus,  is  especially  conspicuous,  and  is  often  found  by  the  score  upon 
the  green  filaments  of  Algae.  Such  a  connection  does,  no  doubt,  suggest  the  idea 
that  the  Cocconeis  drains  the  green  algal  cells  of  nutriment;  nevertheless,  such  an 
assumption  is  not  well  founded,  and  if  algae,  beset  by  Cocconeis,  derive  injury  at  all 
from  their  presence,  it  is  chiefly  owing  to  a  restriction  of  their  absorption  of 
nutrient  substances  from  the  surrounding  water  and  to  interference  with  their 
respiration. 

The  distinctive  property  of  true  parasites  does  not  lie,  therefore,  in  the  habit  of 
growing  upon  other  plants  and  animals,  or  even  in  the  fact  of  killing  their  living 
supports,  but  resides  exclusively  in  the  withdrawal  of  nutrient  substances  from  the 
living  vegetable  or  animal  bodies  which  they  invest. 

The  plants  and  animals  attacked  and  drained  of  their  juices  by  parasites  are 
called  hosts. 

From  the  point  of  view  of  food  absorption,  true  parasites  may  be  classified  in 
three  groups.  The  first  group  includes  generally  all  microscopic  forms  which  live 
in  the  interior  of  human  beings  and  animals,  chiefly  in  the  blood;  the  second 
comprehends  fungi  possessing  mycelia,  which  have  the  power  of  withdrawing  by 
the  entire  surface  of  their  filamentous  cells,  or  by  clavate  outgrowths  of  the  same,, 
nutritive  material  from  the  tissues  of  the  host  invaded  by  them;  and  the 
third  group  comprises  flowering  plants  wherein  the  seedling,  upon  emerging  from 
the  seed,  penetrates  into  the  host,  by  means  of  suction-roots  or  some  other  part 
which  subserves  the  function  of  a  suction-root,  so  as  to  absorb  juices  from  the 
host. 

BACTERIA      FUNGL 

In  treating  of  parasites  of  the  first  group,  we  must,  in  the  first  place,  refer  to 
several  of  the  unwelcome  visitors  known  by  the  name  of  Bacteria.  They  appear 
to  be  invariably  unicellular,  sometimes  spherical,  sometimes  shortly  cylindrical  or 
rod-shaped;  some  are  straight,  and  others  curved  in  arcs  or  spirals;  a  few  are  non- 
motile,  whilst  some  are  actively  motile.  The  largest  forms  have  a  diameter  of 
•5-5^  mm.;  the  smallest  do  not  measure  more  than  ^nnr  mm.,  and  are  reckoned 
amongst  the  minutest  organisms  hitherto  revealed  by  the  aid  of  the  best  micro- 
scopes. In  liquids  of  suitable  chemical  composition  and  temperature,  they  multiply 
with  extraordinary  rapidity,  reproduction  being  effected  by  division.  The  rod- 
shaped  cells  elongate  somewhat  and  divide  into  two  equal  halves,  each  half,  when 

grown  to  a  certain  size,  divides  once  more  into  two,  and  so  on  without  limit 
VOL.  I.  11 


162  BACTERIA.      FUNGI. 

The  process  is  of  the  nature  of  a  repeated  splitting  of  the  cells,  and  this  is  the 
origin  of  the  name  of  Fission-fungi  (Schizomycetes)  used  to  designate  these 
organisms.  It  has  been  observed  that  within  20  minutes  a  bacterium-cell  grows 
enough  to  be  able  to  divide  or  split  into  two,  and  hence  it  has  been  calculated  that 
from  a  single  cell,  under  favourable  external  conditions,  upwards  of  16  millions  of 
similar  cells  are  produced  in  8  hours;  and  in  24  hours  many  millions  of  millions. 

It  is  this  very  capacity  for  rapid  multiplication  that  gives  so  great  an  impor- 
tance to  Bacteria  as  parasites.  For  multiplication  can  only  take  place  at  the 
expense  of  the  juices  and  nutrient  substratum  in  which  they  live.  If  this  nutrient 
substratum  is  to  afford  materials  for  constructing  the  millions  of  millions  of  cells 
produced  within  two  periods  of  24  hours,  a  far-reaching  transformation  is  inevitable. 
Now,  for  certain  bacteria,  the  blood,  with  its  albuminoid  compounds  and  carbo- 
hydrates, is  an  extremely  favourable  medium  of  nutrition;  moreover,  the  tempera- 
ture of  the  blood  of  men  and  other  mammals  (35°-37°C.)  could  not  be  more 
suitable  for  the  development  of  bacteria.  Hence,  it  is  readily  intelligible  that  if  a 
single  parasitic  bacterium-cell  gets  into  the  blood,  it  may  be  the  origin  of  innumer- 
able other  cells,  and  that  these  are  in  a  position,  in  a  comparatively  short  time,  to 
alter  and  decompose  the  whole  mass  of  the  blood.  Owing  to  their  extraordinary 
minuteness,  bacteria  are  able  to  penetrate  from  outside  into  the  channels  of  the 
blood  by  a  number  of  spots;  every  abrasion,  pin-prick,  and  sore  place,  may  become 
an  entrance-door;  so,  too,  through  all  the  external  orifices  of  the  various  canals  in 
the  bodies  of  men  and  animals,  the  bacteria  can  enter,  especially  through  the  pas- 
sages to  the  respiratory  organs — and  it  becomes  more  and  more  probable  that  bacteria, 
diffused  in  the  air,  are  in  the  main  introduced  into  the  respiratory  organs  by  the 
process  of  breathing,  thence  penetrating  into  the  finest  blood-vessels,  the  so-called 
capillaries,  and  so  pass  into  the  current  of  the  blood. 

As  regards  the  parasitic  action  of  bacteria  when  they  have  penetrated  into  the 
bodies  of  men  and  animals,  the  supposition  is  that  the  protoplasm  of  each  bacterium 
works  as  a  ferment  upon  the  environment,  splitting  up  the  chemical  compounds  in 
immediate  proximity  to  it,  and  attracting  and  incorporating  such  products  of  the 
decomposition  as  are  necessary  for  its  own  growth.  Parasites  with  this  method  of 
operation  act,  at  all  events,  much  more  destructively  than  those  which,  although 
they  too  absorb  part  of  the  host's  juices,  yet  do  not  enter  upon  the  necessary 
decompositions  until  the  juices  have  passed  into  the  cavities  of  their  own  bodies, 
and,  therefore,  do  not  alter  the  constitution  of  the  unabsorbed  residue.  When  the 
component  parts  of  the  blood  are  split  up  and  resolved  by  bacteria,  the  nutrition  of 
the  host  must  be  especially  disturbed,  and  so  must  all  the  functions  of  the  organs 
through  which  the  blood  perpetually  circulates.  Ultimately  it  may  culminate  in 
the  organs  ceasing  to  exercise  their  functions,  and  in  the  death  of  the  host.  When 
one  remembers  how  fast  the  blood  is  pumped  by  the  heart's  action  into  every  part 
of  the  body,  it  becomes  intelligible  how  bacteria,  possessing  the  power  of  decom- 
posing the  blood,  may  also  cause  the  death  of  the  host  at  very  short  notice,  as  we 
have  occasion  to  observe  whenever  there  is  an  epidemic  of  cholera. 


BACTERIA.      FUNGI.  163 

That  numerous  diseases  affecting  men  and  animals  are  caused  by  bacteria  is 
established  beyond  question.  Indeed,  the  conviction  is  gradually  gaining  ground 
that  all  infectious  illnesses  are  occasioned  by  bacteria,  and  that  the  contagious 
matter  which  used  to  be  called  virus  or  miasma,  but  as  to  the  nature  of  which 
people  formerly  had  only  very  confused  notions,  consists  of  parasitic  bacteria. 
Different  phenomena  in  organisms  in  which  illness  has  been  induced  by  infection 
point  to  differences  in  the  decompositions  effected  by  the  bacteria.  But  a  par. 
ticular  kind  of  parasitic  cell  can  only  set  up  the  same  decomposition  in  any 
given  liquid.  If,  therefore,  the  products  of  separation  or  decomposition  vary  in  one 
and  the  same  liquid,  this  can  only  be  attributed  to  a  difference  in  the  impetus 
causing  decomposition,  and  therefore  to  a  difference  in  the  parasitic  cells;  in  other 
words,  we  are  justified  in  assuming  that  every  distinct  infectious  disease  is  due  to 
a  special  kind  of  parasitic  bacterium.  This  assumption  is  believed  to  be  warranted 
even  when  no  difference  in  the  form  of  the  bacteria  is  to  be  discovered  which  is 
discernible  to  sight  or  demonstrable  by  the  expedients  of  research. 

Most  of  the  parasitic  bacteria  regarded  as  causes  of  diseases  in  man  and  beast 
are  moreover  capable  of  being  very  clearly  distinguished  from  one  another  by  the 
shape  of  their  cells.  The  bacterium  supposed  to  be  the  cause  of  diphtheria  (Micro- 
coccus  diphthericus)  presents  itself  in  the  form  of  minute  spherical  cells  crowded 
together  in  close  masses.  The  bacterium  which  causes  anthrax  in  cattle  (Bacteriwn 
Anthracis)  has  straight  rod-like  stationary  cells.  In  the  blood  of  people  suffering 
from  relapsing  typhus,  infinitesimally  fine  spiral  filaments  (Spirochaete  Obermeieri) 
are  found  during  the  fever,  whilst  in  the  intestines  of  cholera  patients,  the  comma- 
bacilli,  so  frequently  described,  occur;  and  in  these  cases,  likewise,  the  organisms 
are  brought  into  causal  connection  with  the  illnesses  mentioned  respectively.  The 
answer  to  the  question  as  to  whether  parasitic  bacteria  are  developed  and  propa- 
gated in  dead  bodies  also,  thus  becoming  saprophytic,  and,  in  general,  the  detailed 
description  of  the  organisms,  which  are  so  important  a  factor  for  the  weal  or 
woe  of  humanity,  are  reserved  for  another  section. 

The  second  group  of  parasitic  plants,  according  to  the  classification  above  given, 
includes  several  thousands  of  different  kinds  of  moulds,  toad-stools,  and  Dis- 
comycetes,  which,  notwithstanding  great  diversity  in  the  conditions  of  life,  dis- 
similarity in  the  history  of  their  development,  and  endless  variety  in  the  form  of 
their  fructifications,  yet  exhibit  great  uniformity  in  respect  of  food-absorption  and 
in  their  methods  of  attacking  and  draining  their  hosts.  Spores,  conveyed  by 
currents  of  air  or  carried  by  animals,  germinate  under  the  influence  of  atmospheric 
moisture  wherever  they  happen  to  come  to  rest.  Tubular  thin- walled  cells,  called 
hyphse,  emerge  from  them  and  endeavour  to  grow  into  the  stems,  branches,  leaves, 
or  fruits  of  the  host,  sometimes  horizontally,  sometimes  from  above  downward, 
sometimes  up  in  the  opposite  direction.  Many  select  spots  where  the  resistance 
offered  is  nil  or  only  very  weak:  they  grope  about  on  the  surface  of  the  host  until 
they  find  a  stoma,  and  then  use  it  as  an  entrance,  and  so  enter  the  passages  and 
lacunae,  of  which  the  stomata  are  the  orifices.  Others  seek  out  places  where  the 


164  BACTERIA.      FUNGI. 

surface  of  the  plant  serving  as  host  has  become  broken — wounds  occasioned  by 
animals,  violent  wind,  hailstones,  or  the  weight  of  superincumbent  snow — and  use 
these  as  means  of  ingress.  Yet  others  adopt  the  shortest  route  by  breaking  through 
the  wall  and  so  effecting  an  entrance  for  themselves.  The  tips  of  the  hyphge  and 
also  of  the  outgrowths  developed  by  them  have  the  power  of  decomposing  and 
destroying  the  membrane  of  cells  in  the  living  plant  serving  as  their  host.  At  the 
spots  to  which  they  apply  themselves,  little  gaps  are  shortly  produced  in  the  cell- 
membranes,  and  through  them  the  hyphse  penetrate,  either  in  their  entirety 
or  by  means  of  special  processes,  into  the  interior  of  the  cells  attacked.  In 
this  operation  it  does  not  matter  whether  the  hypha  concerned  has  just  emerged 
from  a  germinating  spore  or  is  a  ramification  of  a  mycelium  several  years  old, 
which  has  been  quiescent  for  a  time  and  then  begun  to  germinate  again  vigorously; 
the  power  of  perforating  cell-walls  is  a  property  possessed  by  the  one  as  much  as 
the  other. 

The  aspect  of  the  host's  epidermal  cells  at  the  places  where  the  hypha  comes 
into  contact  with  its  victim  is,  on  the  other  hand,  not  quite  such  a  matter  of 
indifference.  For  plants  liable  to  become  hosts  are  'not  without  contrivances  for 
protecting  themselves  against  intruders.  Thus  their  epidermal  cells  have  their 
external  walls  greatly  thickened  and  invested  with  cuticle.  Although  the  main 
object  of  this  is  merely  to  afford  protection  against  excessive  transpiration  and 
desiccation  of  cells  filled  with  sap,  a  thickening  of  the  kind  constitutes  also  a  coat 
of  armour  which  is  not  liable  to  be  broken  through  by  every  hypha.  Still  greater 
security  is  afforded  by  a  double  or  triple  layer  of  thick- walled  cells  destitute  of 
sap,  such  as  a  solid  corky  bark.  Coats  of  this  kind  are  not  penetrated  even  by  the 
most  vigorous  hyphae.  In  order  to  gain  admittance  notwithstanding,  many  force 
their  conical  tips  into  the  fissures  and  crannies  of  the  bark,  push  the  peeling  scales 
apart  or  even  burst  them,  and  so  succeed  ultimately  in  reaching  parts  which  are 
susceptible  of  being  pierced  and  allow  the  hyphse  to  conduct  their  mining  operations 
with  effect.  In  the  majority  of  cases  the  parasite  is  not  content  with  perforating 
and  exhausting  the  superficial  cells  alone  of  the  host;  its  hyphse  grow  faster  as 
they  penetrate  deeper,  a  process  generally  accomplished  irrespective  of  the  number 
or  direction  of  the  partition  walls  in  their  way.  Thus  the  hyphse  of  Polyporese, 
which  are  parasitic  in  the  wood  of  living  trees,  penetrate  whole  series  of  cells,  now 
growing  through  a  bordered-pit,  now  piercing  the  uniformly  thickened  part  of  the 
wall  of  a  wood-cell  (see  fig.  32  3).  Others,  as,  for  instance,  the  Peronosporeas,  prefer 
to  bury  themselves  in  the  passages  between  individual  cells,  i.e.  in  the  so-called 
intercellular  spaces.  The  hyphse  imbedded  in  this  way  then  develop  lateral  out- 
growths which  perforate  the  walls  of  the  cells  adjoining  the  intercellular  space,  and 
upon  entering  the  interior  of  the  cells  swell  up  to  the  shape  of  a  club  (see  fig.  32  *). 
By  means  of  these  clavate  or  almost  spherical  excrescences,  which  are  named 
haustoria,  the  parasite  sucks  the  substances  required  for  its  own  nourishment  from 
the  living  substance  of  the  penetrated  cells. 

The  hyphse  of  the  above-mentioned  parasitic  fungi  have  the  peculiarity  that  in 


BACTERIA.      FUNGI. 


165 


proportion  as  the  one  end  elongates  the  other  dies  away.  Hence  the  same  effect  is 
produced  as  if  the  progressive  motion  of  these  hyphse  were  like  that  of  ship-worms. 
This  impression  is  particularly  strong  in  cases  wherein  one  part  of  the  mass  of 
wood  attacked  exhibits  hyphoa  occupied  with  their  mining  operations  and  growing 
through  partition  walls,  whilst  the  other  part  has  been  the  scene  of  past  activity, 
and  exhibits  numbers  of  drilled  holes,  but  no  longer  any  trace  of  hyphse.  The  fact 
that  a  plant  is  thus  invaded  internally  by  the  parasitic  mycelia  of  fungi  is  not 
always  betrayed  by  its  external  appearance.  Sometimes  the  hosts  remain  somewhat 
backward  in  development,  but  this  circumstance  might  be  just  as  well  due  to  other 
causes,  perhaps  to  unsuitability  of  situation.  It  is  not  till  the  mycelia  need  once 


Fig.  32.— Hyphse  of  Parasitic  Fungi, 
i  Of  one  of  the  Peronosporese.    «  Of  a  Mildew.    »  Of  one  of  the  Polyporese. 

more  to  multiply  and  distribute  their  kind  that  they  emerge  partially  from  the 
host;  they  then  lift  their  spore-forming  hyphge  above  the  surface,  leaving  it  to  the 
wind  to  distribute  the  spores  as  they  are  detached. 

This  process  vividly  recalls  the  similar  behaviour  of  those  water-plants  which, 
in  a  similar  manner,  vegetate  submerged  for  months,  and  only  come  to  the  surface 
at  the  flowering  and  fruiting  seasons,  in  order  to  expose  their  flowers  to  insects, 
and  their  seeds  to  the  breeze.  We  are  also  reminded  of  the  saprophytic  orchids 
already  described,  which  nourish  themselves  and  grow  for  years  imbedded  in  the 
humus  of  woods,  and  then  seize  the  opportunity  afforded  by  a  favourable  summer 
to  raise  up  in  a  few  weeks  flowering  stems  above  the  bed  of  the  forest.  As  a  rule 
the  spore-bearing  hyphse,  emerging  from  the  hosts  of  parasitic  fungi,  are  highly 
conspicuous  both  in  form  and  colour.  As  well-known  instances  we  may  here 
mention  the  powdery,  rust-coloured,  chocolate-brown,  or  coal-black  masses  of  spores, 
known  by  the  names  of  rust  and  smut;  the  mealy,  orange-coloured  masses  which 
make  their  appearance  on  the  green  stems  and  fruits  of  roses  (^Ecidium  stage  of 
Phragmidium  subcorticum),  and  the  discomycetous  Peziza  Willkommii,  which 
is  parasitic  in  the  branches  of  green  larches,  and  exposes  its  fructifications  beyond 


166  BACTERIA.      FUNGI. 

the  bark  in  the  form  of  small  scarlet  shields.  Again,  we  have  the  yellow  Poly- 
porus  sulfureus  with  its  immense  yolk-coloured,  bracket-like  fructifications,  which 
in  the  space  of  a  week  grow  out  from  the  trunks  of  larches,  although  the  outward 
appearance  of  the  host  gives  no  indication  of  its  being  completely  occupied 
internally  by  a  mycelium.  Polyporus  betulinus  and  P.  fomentarius  likewise 
grow  to  a  considerable  size,  and  in  both  cases  it  is  specially  deserving  of  notice 
that  the  colour  and  structure  of  the  surface  of  the  fructification  is  surprisingly 
like  the  bark  of  the  trees  upon  which  they  are  respectively  parasitic;  that  is  to  say, 
the  fructification  of  Polyporus  betulinus  strongly  resembles  the  whitish  bark  of 
the  birch,  and  that  of  Polyporus  fomentarius,  parasitic  on  old  beech-trees,  exhibits 
the  same  pale  gray  as  does  the  trunk  of  a  beech. 

Mildews  form  in  some  respects  a  contrast  to  these  parasites  whose  hyphae  pene- 
trate into  the  interior  of  their  hosts.  They  attack  tender  green  leaves,  stems,  and 
young  fruits,  and  accomplish  their  entire  development  upon  the  epidermal  cells  of 
the  hosts.  At  first  sight  the  parts  assailed  appear  to  be  strewn  with  flour  or  dust 
from  the  road.  But  on  closer  inspection  a  delicate  weft  is  to  be  distinguished, 
composed  of  filaments  ramifying  extensively  upon  the  green  substratum,  intersect- 
ing one  another,  uniting  to  form  reticula,  and  in  parts  a  regular  felt- work  covered 
at  certain  spots  with  the  small  dark  spheres  of  the  sporocarps.  Individual  hyphae 
of  this  weft  adhere  closely  to  the  epidermal  cells  of  the  host,  dissolve  the  outer 
walls  of  these  cells  at  the  points  of  contact,  so  as  to  make  little  apertures,  and  then 
develop  processes  which  grow  into  the  interior  of  the  epidermal  cells  in  question, 
assume  a  club-like  form,  and  exhaust  the  cell-contents.  The  mycelia  of  mildews 
do  not  penetrate  into  the  host  beyond  the  epidermal  cells.  Fig.  32  2  shows  a  piece 
of  a  leaf  of  Acanthus  mollis  attacked  by  mildew,  with  hyphal  suckers  penetrating 
into  the  epidermal  cells  of  the  leaf.  One  of  the  best-known  mildew  fungi  is 
the  Vine-mildew  (Erysiphe  Tuckeri),  which  weaves  itself  over  the  epidermis  of 
still  green  and  unripe  grapes,  and  has  frequently  manifested  itself  through  the 
districts  where  the  vine  is  cultivated  in  southern  and  central  Europe  in  the  form  of 
a  ravaging  disease. 

The  protuberances  sent  by  the  hyphae,  in  the  form  of  clavate  swellings,  or  more 
rarely  winding  tubes,  into  the  cells  of  the  host-plants,  correspond  to  the  absorption- 
cells  of  land  plants,  and  the  conditions  under  which  suction  takes  place  are 
essentially  analogous  in  the  two  cases.  The  absorption-cells  on  the  roots  of  land 
plants  do  not  take  in  all  the  substances  in  their  nutrient  substratum,  and  similarly 
the  hyphae  only  appropriate  by  means  of  their  organs  of  suction  a  portion  of  the 
contents  of  the  cells  invaded.  They  begin  by  dissolving  and  breaking  up  for  this 
purpose  the  substances  in  the  infested  cells  of  the  host.  What  compounds  they 
then  select  from  among  the  products  of  decomposition,  and  what  they  leave  behind, 
cannot  certainly  be  specified  in  detail.  It  is  believed  that,  in  many  cases,  tannin 
is  appropriated  first  of  all  by  parasites.  The  wood  of  a  healthy  oak,  for  instance, 
has  a  characteristic  smell  due  to  the  abundance  of  tannin  it  contains,  whereas  this 
odour  is  not  emitted  by  wood  attacked  by  the  mycelia  of  fungi,  and  this  decayed 


BACTERIA.      FUNGI.  167 

wood  is  destitute  of  tannin.  It  is  natural  to  suppose,  therefore,  that  the  mycelium 
takes  away  and  uses  up  the  tannin.  It  has  also  been  observed  that  wherever  the 
hyphae  of  the  Pine-blister  (Peridermiwm,  Pini)  ensconce  themselves,  the  nitrogenous 
parts  of  the  protoplasm  and  the  starch  vanish,  whilst  turpentine  remains  behind, 
clinging  in  drops  to  the  inner  walls  of  the  cells.  These  are,  to  be  sure,  very  sparse 
data ;  but  they  show  that  the  entire  cell-contents  are  not  absorbed  by  the  parasite 
unaltered,  or  used  in  that  condition  as  material  for  the  building  up  of  its  own  body. 

Not  only  the  contents  of  the  cells  preyed  upon,  but  the  walls  as  well,  are  partially 
used  as  food  by  the  hyphae  which  penetrate  into  the  woody  axes  of  arborescent 
angiosperms  and  gymnosperms.  The  mycelium  of  several  species  of  Polyporus  and 
Trametes  begins  by  bringing  the  lignin  in  the  cell-walls  into  solution,  leaving 
nothing  but  a  pale-coloured  cellulose  wall.  Soon  afterwards,  the  so-called  middle 
lamella,  which  connects  adjoining  wood-cells,  is  also  dissolved,  and  the  colourless 
wood-cells,  now  almost  like  asbestos-fibres  in  appearance,  fall  apart  at  the  slightest 
touch.  When  the  wood  of  a  larch  has  been  infested  by  the  mycelium  of  Polyporus 
sulfureus,  there  are  always  deep  furrows  running  obliquely  on  the  internal  walls  of 
the  wood -cells;  this  loss  of  substance,  too,  can  only  arise  from  the  solution,  and 
absorption  as  nutriment,  of  parts  of  the  walls  by  the  action  of  the  hyphae. 

All  decompositions  and  alterations  of  structure  of  the  above  kind  within  the 
precincts  of  the  host's  cells  are  naturally  followed  by  a  disturbance  of  function,  and 
ultimately  by  death.  The  entire  plant  is,  however,  but  rarely  killed  by  parasites 
belonging  to  this  group.  The  decomposition  by  bacteria  of  a  mammal's  blood, 
though  at  first  confined  to  a  particular  part  of  the  body,  spreads  in  a  moment 
throughout  the  whole  organism,  owing  to  the  heart's  action  and  the  circulation  of 
the  blood.  But  the  decomposition  taking  place  in  the  manner  just  described, 
through  the  intervention  of  hyphae,  propagates  itself;  on  the  contrary,  only  very 
gradually  from  the  cells  immediately  attacked  to  their  neighbours,  and  it  gets 
weaker  and  weaker  as  the  distance  from  the  site  of  the  invasion  increases,  a 
circumstance  to  which  we  shall  recur  later  on  when  discussing  the  phenomena  of 
fermentation  and  decay.  The  nature  of  the  parasite  and  the  power  of  resistance  of 
the  host  have  an  undoubted  influence  on  the  rate  of  distribution.  In  many  cases 
alteration  is  limited  to  the  cells  attacked  and  those  immediately  adjoining,  so  that 
the  area  destroyed  is  circumscribed.  It  is  manifested  on  fresh  green  leaves,  often 
merely  in  the  form  of  small,  isolated,  yellow,  brown,  or  black  spots  and  patches, 
which  only  slightly  interfere  with  the  activity  of  the  leaf,  and  do  not  cause  it  to 
change  colour,  wither,  or  fall  off  any  earlier.  In  other  instances,  however,  the 
entire  leaves  and  stem  do  undoubtedly  become  flaccid  and  shrivelled  and  dried  up 
into  a  black  mass,  looking  as  though  they  had  been  carbonized;  or  else  putrefaction, 
such  as  that  which  is  excited  by  bacteria,  invades  the  whole  mass. 

As  above  stated,  when  the  wood  in  the  trunks  of  trees  is  perforated  and 
consumed  by  hyphse  it  is  resolved  into  fragments.  It  becomes  rotten,  takes  the 
form  of  an  asbestos-like  or  crumbling  and  pulverulent  mass,  and  is  then  obviously 
no  longer  capable  of  fulfilling  its  various  functions  in  the  living  plant.  If  the 


168  BACTERIA.      FUNGI. 

invasion  is  limited  in  extent,  and  the  host  succeeds  in  surrounding  the  area  of 
infection  with  a  rampart  of  cells  capable  of  resistance,  and  not  liable  to  be  pierced 
by  the  hyphse,  then  the  tree  may  live  for  years  although  its  trunk  is  infested,  and 
in  parts  rotten.  Such  is  also  the  case  when  particular  branches  of  a  tree  are  alone 
attacked  by  the  mycelium  of  a  fungus.  When,  for  example,  the  branch  of  a  larch 
is  assailed  by  the  mycelium  of  the  Discomycete,  Peziza  Willkommii,  the  fact  is  first 
manifested  externally  by  the  fascicles  of  needles  on  the  branch  in  question  becoming 
discoloured  in  the  summer,  and  acquiring,  prematurely,  an  autumnal  appearance,  so 
that,  among  the  fresh  green  shoots,  individual  branches  are  to  be  seen  bearing  golden - 
yellow  needles.  Towards  autumn,  scarlet  cup-shaped  fructifications  make  their 
appearance  upon  the  surface  of  the  bark  on  the  branch;  in  the  course  of  the  next 
few  years  the  whole  branch  as  a  rule  dries  up,  withers,  and  dies.  It  is  then 
broken  by  the  first  violent  shock  of  wind  and  falls  to  the  ground;  but  the  tree, 
disembarrassed  of  the  dead  bough,  continues  to  grow  unharmed,  and  to  put  forth 
green  shoots.  It  is  only  when  almost  all  the  branches  of  the  larch  are  infested  by 
the  mycelium  of  this  fungus  that  the  whole  tree  perishes  as  a  result  of  the 
invasion. 

Certain  groups  of  plants  are  specially  liable  to  be  attacked  by  parasitic  fungi, 
and  there  are  some  conifers  and  angiospermous  trees  in  which  the  same  stem  is 
colonized  by  three,  four,  or  five  kinds  of  parasite.  The  green  foliage  leaves  of  large 
numbers  of  flowering  plants  are  also  apt  to  be  selected  by  parasites,  as  also  are  their 
roots,  tubers,  and  bulbous  structures.  Many  parasites  only  attack  the  anthers  in 
flowers;  others,  as  for  instance  the  ergot,  only  the  young  ovaries.  Parasitic  fungi 
are  rarely  found  on  mosses  or  ferns;  whereas  a  considerable  number  of  parasites 
settle  upon  lichens  and  even  on  the  fructifications  of  fungi,  moulds  even  being 
infested  by  other  fungi;  for  example,  a  fungus  named  Piptocephalis  Freseniana  is 
parasitic  upon  the  very  common  mould,  Mucor  Mucedo. 

A  fungus  known  by  the  name  of  Cordyceps  militaris  is  parasitic  in  the  cater- 
pillars and  pupae  of  butterflies  and  other  insects,  and  its  relatively  very  large 
fructification  at  length  bursts  out  of  the  body  infested  by  the  mycelium  in  the  form 
of  a  club  nearly  6  cm.  long.  This  clavate  structure,  built  up  at  the  expense  of  the 
insect's  flesh  and  blood,  produces  tubular  cells  in  special  receptacles,  and,  inside 
these,  little  rod-like  spores,  which  afterwards  fall  out  and  infect  other  caterpillars, 
developing  within  the  bodies  of  these  animals  into  a  hoary  mycelium  and  ultimately 
causing  their  death.  The  disease  of  silk- worms,  known  as  muscardine,  is  likewise 
occasioned  by  a  species  of  Cordyceps.  We  must  also  refer  here  to  the  widely- 
distributed  Empusa  Muscce,  a  mould  which  attacks  flies  and  causes  every  autumn 
a  regular  epidemic  amongst  house-flies.  The  flies  so  often  seen  at  that  season 
adhering  stiff  and  dead  to  window-panes  are  surrounded  by  a  whitish  halo,  and  this 
is  composed  of  a  conglomerate  of  spores  thrown  off  by  the  mould  which  is  parasitic 
upon  the  flies  and  causes  their  death.  Parasitic  fungi  have  also  been  observed  in 
the  human  skin,  and  recognized  as  the  causes  of  skin-diseases.  For  instance,  to  the 
mould  Achorion  Schoenleinii  is  due  the  disease  of  the  skin  popularly  known  as 


BACTERIA.      FUNGI. 


169 


"honey-combed  ringworm",  and  named  Favus  by  doctors;  dandruff  (Pityriasis 
versicolor)  is  produced  by  Microsporon  furfur,  and  Herpes  tonsurans  by  Tricho- 
phyton  tonsurans.  The  latter  has  a  remarkable  effect  on  the  hair,  causing  it  to  fall 
out  and  leave  the  part  of  the  skin  affected  bald. 

Water-plants  are  attacked  by  parasitic  fungi  comparatively  rarely,  which  is  the 
more  noteworthy  because  such  large  numbers  of  non-parasitic  epiphytes  settle  upon 
the  filaments  of  green  algae,  and  on  the  brown  Fucoidese,  and  red  Floridese.  Minute 


Fig.  33— Parasites  on  Hydrophytes. 
*,  2,  and  s  Lagenidium  Rabenhorstii.    •*,  «  Polyphagus  Euglence.    •  Rhizidiomyces  apophysatu*. 

forms  of  fungi,  invisible  to  the  naked  eye,  and  belonging  to  the  Chytrideae  and 
Saprolegniae,  are  parasitic  upon  green  algal  filaments,  especially  on  the  fresh-water 
species  of  the  genera  (Edogonium,  Spirogyra,  and  Mesocarpus.  One  of  these 
microscopic  parasites  is  represented  in  fig.  33  *• 2>  3,  and  bears  the  name  Lagenidium 
Rabenhorstii.  It  develops  non-ciliated,  spherical  swarm-spores,  which  lay  them- 
selves upon  the  walls  of  Spirogyra-cells,  perforate  them,  and  insert  a  club-like 
process.  The  protuberance  forthwith  becomes  a  tube,  which  increases  rapidly  in 
size  in  the  interior  of  the  cell,  ramifying  and  completely  destroying  the  bands  of 
chlorophyll.  The  branched  tubes  of  Lagenidium  reproduce  themselves  in  two 
ways  at  the  expense  of  the  host's  cells  infested  by  them:  they  form  on  the  one 
hand  so-called  oospores  by  means  of  fertilization,  and  on  the  other  sporangia.  The 
latter  process  is  clearly  shown  in  fig.  331>2>3.  In  this  case,  one  of  the  tubular 


170  BACTERIA.      FUNGI. 

processes  of  the  parasite  fungus  pushes  out  of  the  cell-cavity  of  the  invaded 
Spirogyra  into  the  surrounding  water  again  and  there  swells  up  into  a  spherical 
vesicle,  within  which  the  protoplasm  divides  into  eight  spores.  These  spores  are 
then  set  free  as  swarm-spores  and  attack  new  healthy  Spirogyra-cells. 

Materially  different  is  the  behaviour  of  the  parasite  Chytridium  Ola,  which 
attacks  the  green  cells  of  fresh-water  (Edogonise.  Its  roundish  swarm-spores  are 
furnished  each  with  one  long  cilium,  and  swim,  searching  about  in  the  water  until 
they  meet  with  an  (Edogonium-cell  to  their  taste  just  occupied  in  the  formation  of 
oospores.  When  they  find  one,  they  fasten  upon  it  and  send  infinitesimally  fine 
hair-like  tubes  (which  have  been  called  rhizoids)  into  the  interior.  By  means  of 
these  tubes  they  derive  their  nutriment  from  the  host.  The  body  of  the  parasite, 
which  remains  outside  the  invaded  cell,  increases  in  size,  and  at  length  grows  out 
into  a  sporangium;  the  latter  opens  at  the  top  by  a  lid  and  once  more  sets  free 
swarm-spores  into  the  surrounding  water. 

Polyphagus  Euglence,  a  member  of  the  Chytrideae,  is  parasitic  on  the  green 
cells  of  Euglense  living  in  water.  The  swarm  spores  of  this  microscopic  fungus 
(see  fig.  33 4)  are  oval  and  furnished,  like  those  of  Chytridium  Ola,  with  a  long 
cilium.  They  swim  about  the  water  with  the  non-ciliate  extremity  leading,  so  that 
the  cilium  appears  to  be  a  tail  at  the  posterior  end.  As  soon  as  these  swarm-spores 
have  come  to  rest,  they  assume  a  spherical  form  and  send  out  in  all  directions  thin, 
hair-like  tubes,  which  search  for  a  host.  When  a  tube  reaches  an  Euglena-cell,  it 
penetrates  into  the  body  of  the  latter,  drains  it,  and,  continuing  to  grow,  produces 
fresh  hair-like  tubes,  which  attack  other  green  Euglenaa,  often  linking  together 
dozens  of  them  (see  fig.  33 5).  In  this  way  the  Polyphagus  grows  apace  and 
becomes  a  comparative  large  oblong  vesicle,  whilst  the  protoplasm  within  it  divides 
into  a  number  of  parts.  These,  again,  turn  into  swarm  spores,  with  long  ciliary 
filaments,  and  they  slip  out  of  the  vesicle  and  may  attack  fresh  Euglense. 

Curiously  enough,  even  saprophytic  water-plants  destitute  of  chlorophyll  are 
sometimes  attacked  by  parasites,  and  that,  indeed,  by  species  belonging  to  the  same 
group.  Thus,  for  instance,  the  species  of  Achlya  growing  on  the  dead  bodies  of 
fishes  and  other  animals  which  have  perished  in  the  water,  are  themselves  infested 
by  small  parasitic  Saprolegniacese  and  Chytrideae.  The  example  of  these  minute 
parasites  represented  in  fig.  33 6  is  named  Rhizidiomyces  apophysatus,  and  its 
host  is  Achlya  racemosa.  The  swarming  spores  of  the  parasite  lay  themselves, 
in  the  manner  described  in  previous  instances,  upon  the  spherical  oogonia  of  A  My  a, 
and  insert  extremely  fine  hair-like  tubes  into  the  interior  of  the  cells  attacked. 
These  ramify  like  roots  in  the  Achlya-cells,  exhaust  them  of  nutriment,  grow 
perceptibly,  and  at  length  form  spherical  swellings,  which,  after  reaching  a  certain 
size,  break  through  the  walls  of  the  host-cells,  project  from  the  opening,  and, 
lastly,  push  out  in  each  case  a  sporangium.  The  latter  produces  a  number  of 
swarm-spores,  which  escape  into  the  water  and  are  able  to  seek  fresh  prey. 

We  cannot  here  enter  into  details  respecting  the  other  kinds  of  reproduction 
occurring  in  the  minute  fungi  parasitic  upon  hydrophytes.  This  is  the  right  place, 


CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT.  171 

however,  to  mention  the  fact  that  the  various  species  of  Chytridese  and  Sapro- 
legniaceae  do  not  content  themselves  with  plants  that  are  second-rate  hosts,  but 
exercise  a  selection  amongst  the  different  green  algse  living  in  the  water.  It  is 
astonishing  to  find  that  the  swarm -spores  invariably  swim  to  cells  whose  protoplasm 
affords  the  most  suitable  nutrient  basis  for  them,  and  attach  themselves  to  those 
cells  only,  and  never  on  other  species  unadapted  to  their  requirements. 

CLIMBING  PAKASITES.    GREEN-LEAVED  PARASITES.    TOOTHWORT. 

The  third  group  into  which  parasites  were  divided  at  the  beginning  of  this 
chapter  is  composed  of  flowering  plants  throughout.  According  to  their  method  of 
attacking  the  host  for  the  purpose  of  absorbing  nutriment  from  it,  they  range 
themselves  in  six  series.  In  the  following  pages  we  shall  discuss  the  charac- 
teristics of  each  series  as  manifested  in  the  most  remarkable  forms  belonging 
to  it. 

The  first  series  includes  plants  destitute  of  green  leaves  and  of  chlorophyll  in 
general,  whose  seeds  germinate  on  the  ground  and  send  forth  each  a  filiform  stem, 
which  brings  itself,  by  means  of  peculiar  movements,  into  contact  with  the  host- 
plant,  coils  round  it,  and  develops  organs  of  suction  whereby  it  takes  nutriment 
from  the  plant  assailed. 

To  this  series  belong  the  genera  Cassytha  and  Cuscuta.  The  former  includes 
some  thirty  species,  all  of  which  appertain  to  warm  climates.  Most  of  the  Cassy- 
thse  inhabit  Australia,  where  they  attack,  in  particular,  the  copses  of  Casuarinse 
and  Melaleucse,  fastening  their  wart-shaped,  or,  in  many  cases,  shield-like  or  discoid 
suckers  upon  the  young  green  shoots  of  those  plants.  Several  species  also  are 
indigenous  to  New  Zealand,  others  to  Borneo,  Java,  Ceylon,  the  Philippines,  and 
the  Moluccas.  South  Africa,  too,  is  the  home  of  a  few  Cassythse,  and  one  species 
(C.  Americana)  is  distributed  over  the  West  Indies,  Mexico,  and  Brazil.  A 
European,  seeing  these  parasites  with  their  twining,  thread-like,  leafless  stems,  and 
their  flowers  aggregated  in  capitula,  umbels,  or  spikes,  takes  them  at  first  to  be 
species  of  the  genus  Cuscuta,  popularly  called  Dodder.  That  these  plants  should 
be  most  nearly  related  to  laurel -trees  is  the  last  thing  one  would  expect.  Ex- 
amination of  the  flowers  and  fruit  reveals,  it  is  true,  a  close  resemblance  to  those  of 
laurel  and  cinnamon  trees,  and,  therefore,  these  Cassythae  are  rightly  placed  by 
systematic  botanists  among  the  Lauracese.  But  in  respect  of  food-absorption,  as  in 
general  aspect,  they  are  entirely  analogous  to  the  various  species  of  the  genus 
Cuscuta,  which  belong  to  the  family  of  Bindweeds  (Convolvulaceae).  The  last- 
named  genus  is  even  more  variously  differentiated  than  the  genus  Cassytha,  and 
includes  about  fifty  species  dispersed  pretty  evenly  over  the  whole  world.  Every 
part  of  the  world  has  its  own  characteristic  forms.  One  group  occurs  in  California, 
Carolina,  Indiana,  Missouri,  and  Mexico,  another  in  the  West  Indies,  Brazil,  Peru,  and 
Chili,  a  third  at  the  Cape  of  Good  Hope.  Other  species  are  natives  of  China,  the 
East  Indies,  the  steppes  of  Central  Asia,  Persia,  Syria,  the  Caucasus,  and  Egypt. 


172  CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT. 

A  comparatively  large  number  of  species,  i.e.  twenty-five,  are  distributed  through 
central  and  southern  Europe.  A  few  have  been  introduced  recently  for  the  first 
time  with  seeds  from  the  New  World,  as,  for  instance,  C.  corymbosa,  which  was 
accidentally  conveyed  with  lucerne  seeds  from  South  America  to  Belgium,  and  has 
latterly  begun  to  range  over  central  Europe. 

The  various  species  of  Cuscuta  attack  chiefly  small  herbaceous,  suffruticose,  and 
shrubby  plants ;  but  a  few  American  species  coil  themselves  round  branches  growing 
at  the  top  of  the  highest  trees.  Notice  has  been  especially  drawn  to  certain 
European  species  on  account  of  their  disastrous  effects  upon  cultivated  plants.  The 
most  famous  is  Cuscuta  Trifolii,  known  as  the  Clover-Dodder,  the  appearance  of 
which  in  clover-fields  causes  so  much  anxiety  to  farmers,  and  which  is  so  difficult 
to  exterminate.  Another  unwelcome  visitor  is  Cuscuta  Epilinum,  which  coils 
round  flax  stems  and  hinders  their  growth,  and  a  third  species,  Cuscuta  Europcea, 
sometimes  ravages  hop-plantations.  This  last  is,  indeed,  the  most  widely  dis- 
tributed of  all  the  Cuscutas,  and  extends  from  England  over  central  Europe  and 
Asia  to  Japan,  and  southwards  as  far  as  Algiers.  It  is  parasitic  not  only  on  hops, 
but  also  on  elder,  ash,  and  various  other  shrubs  and  herbs;  in  particular  it  exhibits 
a  preference  for  nettles. 

The  seeds  of  this  species,  and  of  Dodders  in  general,  germinate  on  damp  earth, 
on  wet  foliage  undergoing  putrefaction,  or  on  the  weathered  bark  of  old  trunks. 
The  seedling,  which  in  the  seed  lies  imbedded  in  a  cellular  mass  full  of  reserve- 
food,  is  filiform  and  spirally  coiled.  It  is  twisted  once,  or  once  and  a  half,  and 
is  thickened  at  one  end  like  a  club.  In  true  Cuscutas,  no  trace  of  cotyledons 
is  to  be  perceived,  nor  does  one  find  vessels  in  the  interior  of  the  seedling;  but 
chains  of  cells  arranged  with  great  regularity  are  noticed  in  the  axis  of  the  filiform 
body,  and  are  easily  distinguished  from  the  surrounding  cells.  In  nature,  the 
seeds,  after  falling  to  the  ground  and  lying  there  through  the  winter,  do  not 
germinate  till  very  late  in  the  following  year,  i.e.  at  least  a  month  later  than  the 
majority  of  the  other  seeds  reaching  the  same  ground  simultaneously  with  them. 
Perennial  herbs,  also,  have,  by  the  time  that  germination  takes  place,  already 
developed  shoots  from  their  subterranean  roots  or  rhizomes  above  the  surface  of 
the  ground,  later  a  circumstance  of  great  importance  to  the  parasites.  If  a 
Cuscuta  were  to  germinate  early  in  the  spring,  it  would  not  readily  find  close  by  a 
support  up  which  to  twine;  whereas  later,  there  is  seldom  any  lack  of  annual  stems 
or  shoots  of  perennial  plants  in  the  immediate  neighbourhood. 

When  the  twisted  embryo  germinates,  it  stretches  and  at  the  same  time  revolves 
from  right  to  left,  assuming  the  shape  of  a  screw  and  pushing  its  lower  clavate 
extremity  out  beyond  the  coat  of  the  seed  (see  fig.  34 1-2'3'4'5'6).  This  extremity  forth- 
with grows  into  the  earth  and  fastens  tightly  on  to  particles  of  the  soil,  withered 
foliage,  and  other  objects  of  the  sort.  The  other,  attenuated  extremity  of  the 
filiform  seedling,  which  is  still  wrapped  in  the  seed-coat  and  the  mass  of  reserve- 
food,  lifts  itself  up  in  the  opposite  direction,  avoiding  such  solid  bodies  as  it  may 
happen  to  encounter,  and  grows  in  a  curve  round  them.  Further  growth  does 


CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT.  173 

not  take  place  at  either  extremity,  but  always  in  the  median  part  of  the  filament. 
It  is  so  rapid  that  by  the  fifth  day  after  the  commencement  of  germination  the 
entire  seedling  has  increased  fourfold  in  length.  As  early  as  the  third  day 
after  the  emergence  of  the  tip  that  fastens  itself  in  the  earth,  the  integument  of 
the  seed,  which  until  then  continues  to  envelop  the  opposite  extremity,  is  thrown 
off  and  the  seedling's  apex  is  exposed.  The  reserve-food,  given  by  the  parent- 
plant  to  the  seedling  as  provision  for  the  journey,  has  meanwhile  been  absorbed 
and  consumed,  so  that  the  seedling  is  now  thrown  entirely  upon  its  own  resources, 
and  depends  for  sustenance  upon  the  earth,  to  which  it  is  firmly  attached,  and  upon 
the  surrounding  air.  Having  no  chlorophyll,  it  is  not  in  a  position  to  take  up 


Fig.  34.— Seedlings  of  Parasitic  Plants. 

i.  a.  s,  i  a. «  The  Great  Dodder  (Cuscuta  Europcea).    *.«.«.  i*.  »«•  «  A  Broom-rape  (Orobanche  Epithymum). 
is.  14.  ia  Wood  Cow-wheat  (Melampyrum  sylvaticum). 

materials  from  the  air;  nor  can  it  derive  sufficient  nutriment  from  the  earth,  even 
supposing  that  water  is  imbibed  by  the  cells  of  the  clavate  extremity.  There  is  no 
doubt  that  it  now  grows  at  the  expense  of  the  substances  contained  in  the  cells  of 
this  club-shaped  end.  The  latter  at  once  begins  to  shrivel  and  soon  dies,  whilst 
the  upper  part  of  the  filament  elongates  conspicuously.  Should  this  portion  of  the 
seedling  meantime  come  into  contact  with  a  neighbouring  plant,  a  rigid  haulm, 
or  anything  else  that  will  serve  as  a  support,  it  straightway  coils  itself  round  the 
object  in  question,  and  its  future  is  then,  as  a  rule,  assured. 

Failing  such  a  support,  the  seedling,  after  the  death  of  the  clavate  extremity, 
falls  down  and  sinks  to  the  ground.  In  doing  so,  it  almost  invariably  touches 
an  adjoining  object,  whereupon  it  immediately  winds  tendril-like  round  the 
support  thus  afforded.  But  if  there  is  nothing  anywhere  around  to  serve  as 
a  prop,  and  the  young  seedling,  by  this  time  from  1  to  2  centimeters  long, 
comes  to  rest  upon  the  bare  earth,  all  further  growth  is  stopped.  It  preserves 
its  vitality,  however,  for  a  surprisingly  long  time,  and  may  remain  almost  unal- 


174  CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT. 

tered,  lying  on  the  damp  earth  for  four  or  five  weeks  waiting  for  something  to 
turn  up.  Not  infrequently  something  of  the  sort  happens,  for  another  plant  may 
germinate  close  by  or  extend  a  growing  shoot  from  the  vicinity  and  touch  the 
Cuscuta  seedling.  In  this  event,  the  latter  at  once  seizes  the  anchor  thus  thrown 
out,  and  winds  round  it.  But  if  no  support  of  the  kind  is  to  be  had,  the  seedling 
must  ultimately  perish.  It  is,  to  say  the  least,  a  very  remarkable  thing  that  a 
filament,  capable  of  developing  suckers  when  adherent  to  a  living  plant,  is  not  able 
in  damp  earth  to  produce  any  absorbent  organs  whatsoever. 

If  the  thread-like  Dodder  plantlet  succeeds  in  seizing  a  support  of  any  kind, 
either  during  the  existence  of  the  swollen  extremity,  or  later,  after  it  has  been 
absorbed,  it  makes  a  single,  or  from  two  to  three,  coils  round  the  prop,  raises  its 
growing  point  from  the  substratum,  and  moves  it  round  in  a  circle  like  the  hand 
of  a  watch.  By  means  of  these  manoeuvres,  which  look  exactly  like  a  process 
of  feeling  or  seeking,  the  filament  is  brought  into  contact  with  fresh  haulms, 
twigs,  and  petioles  belonging  to  other  plants.  To  these  it  adheres,  making  once 
more  two  or  three  tight  coils  round  them.  Throughout,  it  is  obvious  that  the 
growing  point  of  the  young  Dodder  rejects  dead  props,  as  far  as  is  practicable, 
and  shows  a  striking  preference  for  living  parts  of  plants. 

At  each  place  where  the  Dodder  is  pressed  in  a  coil  against  the  support,  the 
filament  becomes  somewhat  swollen,  and  wart-like  suckers  are  developed,  which  are 
usually  situated  close  together  in  rows  of  three,  four,  or  five  (see  fig.  35  1 ). 

A  piece  of  stem  thus  furnished  with  suckers  or  haustoria  resembles  a  small 
caterpillar  creeping  up  the  supporting  stem.  These  haustoria,  arranged  close 
together  in  rows,  and  corresponding  in  origin  entirely  to  rudimentary  roots,  are  at 
first  smooth,  but  acquire  soon  a  finely-granulated  aspect  owing  to  the  walls  of  the 
epidermal  cells  projecting  outwards.  With  the  help  of  the  papillae  thus  formed, 
and  especially  through  the  action  of  a  juice  secreted  by  them,  the  suckers  fasten 
themselves  to  the  host.  If  the  plant  has  been  obliged  to  clasp  a  dead  object  for 
support,  the  wart-like  processes  flatten  themselves  against  it  and  assume  the  form 
of  a  kind  of  disc,  which  exhibits  no  further  development,  and  only  serves  as  an 
organ  of  attachment;  but,  if  the  substratum  is  a  living  plant,  a  bundle  of  cells 
forces  its  way  out  from  the  middle  of  the  haustorium  and  grows  into  the  sub- 
stratum direct.  The  phenomenon  here  manifested  is  altogether  characteristic. 
Each  sucker  from  the  time  of  its  production  exhibits  a  kind  of  core  composed  of 
cells  arranged  in  regular  rows,  which,  together  with  a  few  spirally-thickened 
vessels,  constitute  a  bundle  standing  at  right  angles  to  the  axis  of  the  Dodder's 
stem.  This  bundle  now  breaks  through  the  coat  formed  by  the  rest  of  the  cells 
of  the  sucker  and  penetrates  into  the  living  tissue  of  the  plant  attacked  (see  fig. 
35  2).  Great  force  is  exerted  in  the  penetrating  process.  The  closely-joined  cells 
of  the  epidermis,  and  not  infrequently  a  cortex  of  considerable  density  are  pierced, 
and  the  bundle  of  cells  often  penetrates  right  into  the  body  of  the  wood.  Having 
once  reached  the  interior  of  the  host,  the  cells,  till  then  bound  together  in  a 
bundle,  diverge  a  little,  insert  themselves  singly  between  the  cells  of  the  host. 


CLIMBING   PARASITES.      GREEN-LEAVED    PARASITES.      TOOTHWORT. 


175 


and  energetically  absorb  food-materials.  They  withdraw  organic  compounds  from 
the  host  and  convey  them  by  a  short  route  to  the  strands  developed  meantime 
in  the  axis  of  the  Cuscuta-stem.  When  once  a  union  of  this  kind  between 
the  parasite  and  the  host  has  been  established,  the  portion  of  the  Cuscuta 
situated  below  the  first  haustorium  gradually  dies.  The  lowest  extremity,  i.e.  the 
clavate  tip,  has  already  perished,  so  that  the  Cuscuta-plant  is  now  no  longer 
in  any  connection  with  the  ground  whereon  it  germinated,  but  only  remains 
rooted  to  its  living  host  by  means  of  the  suckers.  If  it  has  had  the  good  fortune 
to  cling  to  a  host  with  green  foliage,  which  generates  an  abundance  of  organic 
compounds,  such  as  the  luxuriant  juicy  stems  of  the  Hop,  or  the  Nettle,  with  its 


Fig.  35.—  Cuscuta  Europcea  parasitic  on  a  Hop-stem, 
i  Natural  size.        a  Section ;  x  40. 

plentiful  dark  green  leaves,  which  are  shunned  and  spared  by  grazing  animals  on 
account  of  their  unpalatable  stinging  hairs,  the  parasite  continues  to  grow  with 
extraordinary  rapidity,  and  puts  forth  a  number  of  branches  immediately  above 
the  lowest  group  of  haustoria.  All  these  again  feel  around  with  their  tips,  develop 
tendrils  and  suckers,  sometimes  intertwining  and  becoming  entangled  together, 
•cover  an  ever-increasing  area  of  the  host  with  their  network,  and  in  this  condition 
fully  deserve  the  name  of  "  Hell-bind  ",  sometimes  popularly  applied  to  this  plant. 
Little  spheres  of  rose-coloured  flowers  are  then  formed  on  individual  threads  of 
this  tangle,  and  from  them  balls  of  small  capsular  fruits,  which  dehisce  by  means  of 
lids  and  have  their  seeds  shaken  out  by  the  wind. 

The  European  species  of  Cuscuta  are  all  annuals.  Even  when  their  haustoria 
are  attached  to  perennial  plants,  as,  for  instance,  on  young  branches  of  woody 
plants,  they  wither  after  the  seeds  have  ripened,  and  nothing  is  to  be  seen  of  them 
in  the  following  spring  except  a  few  dried  tendrils  coiled  round  branches  of  ash 
or  willow.  But  under  a  tropical  sun,  perennial  species  flourish  as  well.  The 
suckers  of  Cuscuta  verrucosa,  for  example,  continue  to  exercise  their  function 


176  CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT. 

throughout  the  year  wherever  they  have  once  attacked  the  host.  If  the  woody 
branches  of  the  host,  with  haustoria  fastened  in  them,  grow  in  thickness  and 
superimpose  new  wood-cells  upon  the  wood,  down  to  which  the  absorbent  cells  of 
the  haustoria  have  penetrated,  these  suction-cells  of  the  Dodder  are  likewise  inclosed 
by  the  wood-cells,  and,  in  proportion  to  the  augmentation  of  the  circumference  of 
the  wood  in  the  branch  in  question,  they  also  lengthen  out  so  that  the  bundle  of 
absorption-cells  proceeding  from  a  sucker  may,  in  such  cases,  be  seen  imbedded  in 
the  wood  to  a  depth  of  several  annual  rings. 

The  Cassythae,  referred  to  above,  behave  exactly  like  the  Dodders.  In  them 
also  the  seedling  which  issues  from  the  seed  is  filiform,  and  lives  originally  at  the 
expense  of  reserve-food  stored  up  within  the  coat  of  the  seed.  So,  too,  it  grows 
upward,  ramifies,  and  endeavours,  by  means  of  revolving  movements  of  the  apex, 
to  reach  a  living  support,  coils  round  the  latter  when  found,  and  uses  it  as  a 
nutrient  substratum.  Here,  again,  at  the  parts  where  the  tendrils  of  the  filiform 
stem  are  firmly  appressed  to  the  living  support,  rows  of  wart-like  suckers  are 
developed,  and  a  bundle  of  absorption-cells  grows  from  each  into  the  host.  As  in 
the  Dodder,  the  lower  extremity  of  the  filiform  stem  then  dries  up  at  once,  and 
connection  with  the  earth  is  thus  cut  off.  The  parasite,  once  attached  by  its 
haustoria  to  the  host,  is  able  to  branch  repeatedly,  to  weave  its  thread-like  stems 
over  all  the  branches  and  to  climb  to  the  top  of  the  host,  even  should  the  latter  be 
a  tall  bush.  At  some  spots  everything  is  entangled  to  such  an  extent  that  one 
would  think  there  were  birds'  nests  amongst  the  boughs. 

The  second  series  of  parasitic  Phanerogams  consists  of  herbs  bearing  green 
foliage-leaves,  whilst  the  seed  contains  an  embryo  furnished  with  seed-leaves 
(cotyledons)  and  root.  The  seeds  germinate  in  the  earth  and  there  develop  seed- 
lings without  the  support  of  a  host;  it  is  branches  of  the  root  that  first  attach 
themselves  by  means  of  suckers  upon  the  roots  of  other  plants.  To  this  series 
belong  about  a  hundred  Santalacese,  mainly  of  the  genus  Thesium,  and  many  more 
than  two  hundred  Rhinanthacese  besides.  The  chief  examples  of  this  latter  family 
are  the  various  species  of  the  Eyebright  (Euphrasia),  the  Yellow-rattle  (Rhinan- 
thus),  Cow-wheat  (Melampyrum)  and  Lousewort  (Pedicularis),  and  also  Bartsia, 
Tozzia,  Triocago,  and  Odontites.  The  most  extensive  genera  are  Euphrasia  and 
Pedicularis,  the  species  of  which,  with  few  exceptions,  are  found  in  the  northern 
hemisphere,  adorning  grassy  meadows  with  their  pretty  flowers,  especially  in  the 
arctic  zone,  and  the  high  mountain  regions  of  the  Himalaya,  the  Altai  and  Caucasus, 
the  Alps  and  the  Pyrenees. 

Little  suggestion  of  parasitic  habit  is  given  in  the  first  stages  of  development 
of  any  of  these  plants.  A  seedling  of  the  Cow- wheat  within  a  week  puts  forth  a 
primary  root  4  cm.  long,  from  which  half  a  dozen  lateral  roots  ramify  at  right 
angles  without  there  being  any  attachment  to  a  host  to  be  noted  (see  fig.  34  13-14.15). 
Suckers  are  never  developed  until  the  secondary  roots  have  attained  a  length  of 
from  12  to  24  mm.,  and  then  only  if  the  latter  come  into  contact  with  other  living 
plants  to  their  taste,  a  circumstance  which  doubtless  is  almost  certain  to  happen, 


CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT.  177 

seeing  that  the  lateral  roots  are  numerous  and  are  sent  out  in  all  directions  from  the 
main  root,  and  therefore  must  inevitably  come  across  the  root-systems  of  other  plants. 
The  seedling  in  perennial  species  of  Thesium  develops  comparatively  slowly. 
It  reaches  a  length  of  from  3  to  4  cm.  in  the  first  year,  sends  a  tap-root  into  the 
earth,  and  puts  forth  a  few  branchlets,  which  do  not  fasten  upon  the  roots  of  other 
plants  by  means  of  suckers  until  several  weeks  after  germination.  These  suckers 
are  relatively  large  in  all  species  of  Thesium,  and  they  catch  one's  eye  the  moment 
the  roots  of  a  plant  are  carefully  divested  of  earth.  They  are  then  recognized, 
as  may  be  seen  in  fig.  36  \  as  little  white  knobs,  which  stand  out  clearly  from  the 
dark  earth  and  are  always  inserted  laterally  upon  the  secondary  roots.  They  are 


Fig.  36.— Bastard  Toad-flax  (Thesium  alpinum). 
i  Root  with  suckers;  natural  size.    2  piece  of  a  root  with  sucker  in  section;  x35. 

constricted  near  their  insertion,  and  the  strangulated  portion  often  gives  the 
impression  of  being  a  pedicel  upon  which  the  knob  is  seated.  This  knob  is 
differentiated  into  a  central  core  and  a  multicellular,  cortical  coat  enveloping  it. 
The  cellular  coat  rests  upon  the  root  of  the  host  attacked,  and  does  not  merely 
adhere  to  one  limited  spot,  but  spreads  itself  out  over  the  root  like  a  plastic 
mass,  and  forms  a  cushion  surrounding  about  a  fourth  or  fifth  part  of  the  circum- 
ference (see  fig.  36 2)  without,  however,  penetrating  into  the  substance  of  the  root. 
There  are  in  the  core  two  strands  or  bundles  of  vessels,  and  between  them  small 
cells  arranged  in  rows,  from  which  absorption-cells  arise  at  the  spot  where  the 
sucker  first  applied  itself  to  the  nutrient  root.  These  absorption-cells  grow  out 
beyond  the  rind-like  envelope  round  the  core,  perforate  the  cortex  of  the  host, 
penetrate  into  the  wood  at  the  centre  of  the  invaded  root,  and  there  diverge  like 
the  hairs  of  a  dry  paint-brush. 

The  suckers  of  the  green-leaved  Khinanthaceae  are  on  the  whole  similarly 
constructed;  only  they  are  relatively  smaller  and  more  delicate,  being  sometimes 
almost  translucent,  and  they  are  either  not  at  all  or  only  slightly  constricted  at  the 

VOL.  I.  12 


178  CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTH  WORT. 

base.  Whereas  in  Thesium  they  never  issue  otherwise  than  laterally  from  the 
ramifications  of  the  roots,  in  Rhinanthaceae  they  are  often  terminal.  A  differentia- 
tion into  core  and  rind-like  envelope  is  never  clearly  marked;  a  vascular  bundle 
runs  through  the  middle  of  the  sucker  and  is  surrounded  by  thick-walled  cells. 
The  absorbent  cells  are,  moreover,  shorter  than  in  the  Santalaceae.  The  individual 
genera  of  the  Rhinanthacese  exhibit  amongst  themselves  only  very  slight  differences 
in  respect  of  their  suckers.  On  the  roots  of  Eyebright  (Euphrasia),  the  haustoria 
are  tiny  roundish  nodules  which  rest  upon  the  host's  root  without  encompassing  it. 
The  absorption-cells  are  very  short,  and  only  just  penetrate  into  the  host.  The 
vascular  bundle  is  either  entirely  wanting  within  the  sucker,  or  its  place  is  taken 
by  a  single,  comparatively  large  vessel.  On  the  roots  of  the  Yellow-rattle 
(Rhinanthus)  the  suckers  are  spherical  and  of  considerable  size  (up  to  3  mm.  in 
diameter);  their  margins  are  swollen  and  often  encompass  more  than  half  the 
circumference  of  the  roots  attacked.  The  absorbent  cells  are  short  but  very 
numerous.  In  the  Cow- wheat  (Melampyrum)  the  suckers  resemble  those  of  the 
Yellow-rattle  in  size  and  shape  and  in  the  shortness  of  the  absorption-cells;  but  in 
the  former  the  margins  of  the  suckers  not  only  embrace  the  roots  of  the  host,  but 
cling  to  them  in  such  a  way  as  to  penetrate  their  substance  and  form  circular 
grooves  upon  them. 

All  the  Rhinanthaceae  mentioned  are  herbaceous  annuals.  Their  suckers  are 
few  in  number,  and  therefore  easily  escape  observation.  By  the  time  these  plants 
ripen  their  seeds  any  piece  of  a  root  that  has  been  attacked  has  for  the  most  part 
already  turned  brown  and  been  killed,  and  is  in  a  state  of  decay.  But  shortly 
afterwards  the  parasite  itself  withers.  The  comparatively  large  seeds,  well- 
furnished  with  reserve-material  for  the  nourishment  of  the  embryo,  fall  out  of  the 
dry  capsules,  and  generally  reach  the  ground  at  no  great  distance  from  the  mother- 
plant  and  germinate  there.  In  the  autumn,  close  to  Cow-wheat  plants,  which  are 
still  green  but  have  already  let  fall  the  seeds  from  their  lowest  capsules,  individual 
examples  of  those  seeds  may  be  seen  already  sprouting  in  the  damp  moss  and  mould 
on  the  ground  of  woods.  If  they  fall  to  earth  not  very  far  from  the  parent-plant, 
the  seedlings  may  happen  to  attack  the  host  which  has  already  had  one  of  the 
branches  of  its  root  sucked  and  killed  by  the  latter  in  the  previous  summer. 

Nearly  all  these  annual  green-leaved  parasites  make  their  appearance  in  num- 
bers close  together.  If,  for  instance,  a  species  of  Cow-wheat  has  taken  up  its 
quarters  in  a  particular  part  of  a  wood,  there  are  always  collections  of  hundreds 
and  thousands  of  specimens  to  be  found  together.  The  small-flowered  Yellow- 
rattle  often  grows  so  abundantly  in  damp  meadows  that  one  might  suppose  it  to 
have  been  sown  by  the  bushel.  The  large-flowered,  hairy  Yellow-rattle  is 
similarly  exuberant  in  ploughed  fields,  and  the  Eyebright,  with  its  large  number  of 
species,  is  produced  in  such  abundance  in  mountainous  districts  that,  at  the  season 
when  its  little  milk-white  flowers  are  open,  regular  milky  ways  seem  to  stretch 
across  the  green  meadows.  Millions  of  them  are  situated  together  rooted  in  the 
grass-covered  ground,  and  one  would  suppose  that  in  course  of  time  the  growth  of 


CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT.  179 

grass  at  such  places  would  be  injured.  This  conclusion  appears  to  be  supported 
by  the  assertion  of  the  country  folk  that  after  the  season  when  the  Eyebright  is  in 
full  bloom,  the  cows  yield  less  milk,  a  fact  which  explains  the  German  name  of 
" Milchdieb "  (milk-thief)  popularly  given  to  the  plant.  The  diminution  in  the 
quantity  of  milk  yielded  is,  however,  certainly  connected  with  other  circumstances. 
It  depends  especially  upon  the  universal  abatement  of  the  growth  of  grasses  in 
early  autumn  and  the  consequent  curtailment  of  the  food  afforded  by  the  pastures. 
The  injury  done  by  the  Eyebright  to  its  hosts  by  the  withdrawal  of  nutriment 
and  destruction  of  rootlets  cannot  be  very  considerable,  for  the  appearance  of  the 
grasses  and  other  host-plants,  which  are  affected,  is  not  noticeably  different  from 
that  of  the  plants  of  the  same  kind  which  escape  invasion. 

The  same  statement  is  true  in  the  case  of  the  various  species  of  Lousewort 
(Pedicularis),  almost  all  of  which  are  meadow-plants;  that  is  to  say,  they  are 
present  in  great  abundance  in  upland  and  alpine  pastures  without  apparently 
injuring  the  species  growing  in  their  company  and  used  by  them  as  hosts.  Unlike 
the  species  of  Cow-wheat,  Yellow-rattle,  and  Eyebright,  however,  nearly  all  the 
Louseworts  are  perennial,  and  accordingly  differ  from  them  also  in  the  construction 
of  their  suckers.  There  is,  it  is  true,  no  difference  in  shape  between  the  suckers  of 
the  Cow-wheat  and  those  of  Pedicularis,  but  they  are  dissimilar  in  respect  of  size 
and  place  of  origin.  The  suckers  of  the  perennial  Louseworts  are  barely  more 
than  half  the  size,  and  are  only  developed  near  the  attenuated  extremity  of  a 
rootlet.  They  are  very  few  in  number;  each  of  the  long,  thick,  fleshy  rootlets, 
proceeding  from  the  base  of  the  stem  usually  produces  a  single  sucker  only  which 
settles  upon  the  root  of  a  suitable  host-plant  in  the  same  way  as  the  suckers  of 
Cow-wheat.  By  the  time  that  the  parasite's  fruit  ripens,  the  piece  of  root  which 
has  been  invaded  has  usually  already  turned  brown  and  fallen  into  decay.  Now  in 
the  case  of  Cow- wheat  it  may  undoubtedly  be  immaterial  whether  the  piece  of  root 
attacked  by  it  is  living  or  not  when  its  fruit  is  ripening,  inasmuch  as  its  own 
annual  root  rots  as  soon  as  the  seeds  have  been  produced  from  the  flowers  above 
ground.  But  with  Pedicularis  it  is  different.  The  perennial  roots  of  this  plant 
require  a  host  to  nourish  them  next  year,  and  when  the  piece  of  a  host's  root  which 
has  been  attacked  and  sucked  as  a  nutrient  substratum  one  year  dies,  the  sucker 
belonging  to  the  root  parasitic  upon  it  is  no  longer  in  a  position  to  fulfil  its  function 
by  continuing  to  absorb  fresh  juices.  Suckers  thus  reduced  to  a  state  of  quiescence 
soon  perish,  and  only  leave  little  scars  to  indicate  the  places  where  they  existed. 
The  perennial  root  of  the  Pedicularis  has  now  to  seek  a  new  source  of  nutriment, 
and  this  is  effected  by  the  elongation  of  its  tip,  which  continues  to  grow  until  it 
reaches  the  living  root  of  another  plant  suitable  as  host,  whereupon  it  develops  a 
fresh  sucker  upon  that  root.  This  elongation  doubtless  requires  a  large  quantity  of 
plastic  materials;  but  these  are  found  stored  in  abundance  in  the  older  parts  of  the 
parasitic  root. 

These  circumstances  explain,  at  anyrate  in  part,  the  characteristic  structure  and 
disproportionate  length  of  the  roots  of  Pedicularis.  From  all  round  the  short  erect 


180  CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT. 

root-stock,  which  is  generally  only  from  J  cm.  to  2  cm.  long,  issue  fleshy  rootlets  of 
the  thickness  of  a  quill,  but,  in  many  species,  as  long  and  thick  as  a  little  finger. 
These  rootlets  are  abundantly  supplied  with  starch,  and,  in  course  of  time,  elongate 
till  they  measure  20  cm.  They  radiate  in  all  directions  in  the  black  soil  of  the 
meadow,  wherein  are  buried  the  root-systems  of  grasses,  sedges,  and  various  other 
plants,  and  fasten  on  to  suitable  hosts  by  means  of  one  or  two  suckers  yearly,  and 
repeating  this  process  until  at  length  their  tips  travel  into  earth  devoid  of  roots, 
where  no  more  prey  is  to  be  found,  and  there  growth  ceases.  This  explains  also 
why  these  long  Pedicularis-roots  never  descend  vertically  in  the  earth,  but  remain 
only  in  the  upper  strata  of  soil  on  a  meadow,  where  a  number  of  other  roots  are 
interwoven  together,  and  where  it  is  most  likely  that  the  tapering  growing-point 
will  meet  with  the  root  of  some  new  host  or  other. 

The  Alpine  Bartsia  (Bartsia  alpind),  one  of  the  perennial  Rhinanthacese 
prevailing  in  the  arctic  regions  as  well  as  in  mountainous  parts  of  Europe  on  damp, 
marshy,  grass-covered  spots,  is  distinguished  by  the  sombre  dusky  violet  colouring 
of  its  leaves,  and  has  already  been  noticed  amongst  carnivorous  plants.  On  the 
secondary  roots  are  suckers  exactly  like  those  of  the  Yellow-rattle  (Rhinanthus), 
and  by  means  of  these  organs  it  clings  to  the  fibrous  roots  of  sedges  and  grasses,  and 
sucks  their  juices.  The  long,  subterranean,  runner-like  stems,  which  are  covered 
with  small,  whitish  scales,  also  bear,  however,  elongated  absorption-cells  (root-hairs), 
which  are  distinctly  differentiated,  and  take  up  nutriment  from  the  vegetable  mould 
around.  This  Bartsia  is,  therefore,  half -parasitic  and  half-saprophytic,  and  it  is  not 
improbable  that  many  other  perennial  Rhinanthacese  behave  in  the  same  way. 

The  species  of  Pedicularis  which  constitute  the  most  extensive  group  of 
perennial  green-leaved  and  parasitic  Rhinanthacese  are,  it  is  true,  destitute  of 
tubular  absorption-cells  (root-hairs)  whether  on  the  subterranean  stem -structures 
or  on  the  root-tip,  with  the  exception  of  those  which  develop  in  the  middle  of  the 
suckers.  But  the  construction  of  the  epidermal  cells  on  the  roots,  and  the  circum- 
stance that  these  epidermal  cells  are  always  in  intimate  connection  with  dark 
particles  of  humus,  would  favour  the  idea  that  these  plants  are  capable  of  taking  up 
organic  compounds  from  the  mould  of  meadows  in  addition  to  the  food  acquired  by 
means  of  suckers  from  their  hosts.  This  supposition  is  further  supported  by  the 
fact  that  I  succeeded  in  rearing  a  species  belonging  to  the  Rhinanthacese,  namely, 
Odontites  lutea,  from  a  soil  composed  of  a  mixture  of  sand  and  humus,  in  which  no 
other  plants  were  rooted,  so  that  the  possibility  of  a  withdrawal  of  nutritive  matter 
from  hosts  was  excluded.  It  is  true  that  the  plants  thus  reared  remained 
comparatively  small  and  poor,  and  only  developed  few  flowers  and  fruits.  But  at 
anyrate  they  may  be  considered  to  prove  that  plants  exist,  which,  though  normally 
parasitic,  are  yet  on  occasion  able  to  subsist  in  vegetable  mould  without  the 
assistance  of  hosts. 

The  third  series  of  parasitic  flowering-plants  is  very  restricted,  contrasting  in 
this  respect  with  the  second  series,  composed  of  the  numerous  green-leaved 
Santalaceae  and  Rhinanthacese.  The  species  belonging  to  it  differ  from  those  of  the 


CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT.  181 

second  series  chiefly  in  their  lack  of  chlorophyll.  They  all  live  underground  on  the 
roots  of  trees  and  shrubs,  develop  deep  down  in  the  earth  a  number  of  flowerless 
perennial  shoots  thickly  covered  with  scales,  and,  in  addition,  push  up  annually  into 
the  light  temporary  axes  bearing  flowers,  which  ripen  their  fruits  and  die  after  the 
fall  of  the  seed. 

As  the  best  known  representative  of  this  series,  we  may  take  the  Toothwort 
(Lathrcea   Squamaria),  which   is   represented   in   fig.   37,  and   has  been   already 


Fig.  37. — Toothwort  (Lathrcea  Squamaria,)  with  suckers  upon  the  roots  of  a  Poplar. 

described  on  a  previous  occasion  as  an  instance  of  a  plant  possessing  in  the 
seclusion  of  its  curious  hollowed  scale-leaves  a  special  mechanism  for  the  elimi- 
nation of  water  from  its  system  quite  supplementary  to  the  normal  method  of 
surface  transpiration.  Formerly,  the  Toothwort  used  to  be  included  in  the 
family  of  Broom-rapes  (Orobancheae)  on  account  of  the  structure  of  its  capsules, 
but  it  is  entirely  different  as  regards  the  form  of  its  seedling.  For,  whereas  the 
seedling  of  a  Broom-rape  is  a  thread  without  any  trace  of  cotyledons,  as  will  be  seen 
when  we  study  its  development  and  mode  of  attachment  to  the  host  in  the  next  few 
pages,  that  of  the  Toothwort  is  clearly  differentiated  into  radicle,  cotyledons,  and 
rudimentary  stem,  corresponding  in  this  respect  entirely  with  the  Ehinanthaceae. 
Moreover,  the  Toothwort  resembles  Rinnan thacese  much  more  than  Broom-rapes  in 
the  manner  in  which  it  attacks  its  hosts  and  withdraws  nutriment  from  them. 


182  CLIMBING   PARASITES.      GREEN-LEAVED   PARASITES.      TOOTHWORT. 

The  seed  of  Lathrcea  germinates  on  damp  earth.    The  young  root  of  the  seedling 
grows  at  first  at  the  expense  of  reserve  material  stored  in  the  seed,  penetrates 
vertically  into  the  earth  and  sends  out  lateral  branches,  which,  like  the  main  root, 
follow  a  serpentine  course  and  search  in  the  loose  damp  earth  for  a  suitable  nutrient 
substratum.     If  one  of  these  meets  with  a  living  root  belonging  to  an  ash,  poplar, 
hornbeam,  hazel,  or   other   angiospermous   tree,  it   fastens  on  to  it  at  once  and 
develops  suckers  at  the  points  of  contact;  these  suckers  are  at  first  shaped  like 
spherical   buttons,  but   soon   acquire,   as   their   size   increases,  the   form   of   discs 
adherent  to  the  host's  root  by  the  flattened  side  and  with  the  convex  hemispherical 
side  turned  towards  the  rootlet  of  the  parasite.     These  discoid  suckers  cling  to  the 
root  attacked  by  means  of  a  viscid  substance  produced  by  the  outermost  layer  of 
cells.     As  in  the  case  of  the  parasites  already  described,  a  bundle  of  absorption-cells 
grows  out  of  the  core  of  each  sucker  into  the  root  of  the  plant  serving  as  host,  and 
the  tips  of  the  absorbent  cells  reach  to  the  wood  of  the  root.     The  shoot  extremity 
of  the  seedling,  thus  nourished  by  the  juices  of  the  host,  now  develops  very  quickly, 
elongating  and  producing  thick,  white,  fleshy,  scale-like  leaves  which  overlap  one 
another  closely,  the  whole  thus  acquiring  the  appearance  of  an  open  fir-cone.     The 
scaly  stems  also  branch  underground,  and  thus  a  curious  structure  is  gradually 
produced,  consisting  of  crossed  and  entangled  cone-like  shoots  covered  with  white 
scales,  and  this  structure  fills  entirely  the  nooks  and  corners  between  the  woody 
roots  on  which  it  preys.     Individual  plants  extending  over  a  square  meter  and 
weighing  5  kilograms  are  by  no  means  rare.     Later  on,  inflorescences  raise  them- 
selves above  the  surface  from  the  extremities  of  the  scaly  subterranean  shoots. 
Their  axes  are  at  first  curved  like  crooks,  but  straighten  themselves  out  by  the 
time  the  fruit  ripens.     Whereas  the  subterranean  portions  are  white  as  ivory,  the 
flowers  and  bracts  pushed  up  above  the  earth  are  of  a  purplish  tinge.     The  roots, 
which  issued  originally  from  the  seedling,  and  their  suckers  have  long  since  ceased 
to  meet  the  requirements  in  respect  of  nourishment  of  so  greatly  augmented  a 
structure,  and   therefore   additional   adventitious   roots  are  produced  every  year, 
springing   from   the   stem   and  growing  towards   living  woody  branches   of   the 
thickness  of  a  finger,  belonging  to  the  root  of  the  tree  or  shrub  that  serves  as  host. 
When  there,  they  bifurcate,  forming  numerous  thickish  filiform  arms,  which  lay 
themselves  upon  the  bark  of  the  nutrient  root  and  weave  a  regular  web  over  it. 
Sometimes  two  or  three  of  these  root-filaments  of  the  parasite  coalesce,  forming 
tendrils,   and   the   resemblance   to   a   lace- work   or   braid    is   then   all   the   more 
pronounced.     Suckers  such  as  have  been  described  are  developed  by  these  root- 
filaments  laterally,  and  more  especially  on  the  ends  of  the  branches. 

Lathrcea  is  interesting  in  so  many  different  connections  that  we  shall  again 
return  to  this  plant  later  on.  As  has  been  stated  before,  it  affords  a  type  of  a  series 
of  parasites  which  resembles  the  species  of  Cassytha  and  Cuscuta  in  the  absence  of 
chlorophyll,  Rhinanthaceae  in  the  shape  and  development  of  the  seedling  and  the 
form  of  the  suckers,  and  the  Balanophorese,  presently  to  be  described,  in  being 
parasitic  upon  the  roots  of  woody  plants.  Lathrcea  Squamaria,  the  species  repre- 


BROOM-RAPES,    BALANOPHORE^,    RAFFLESIACE^.  183 

sented  in  fig.  37,  is  indigenous  to  Europe  and  Asia,  its  area  of  distribution  extending 
from  England  eastwards  to  the  Himalayas,  and  from  Sweden  southwards  to  Sicily. 
Two  species  are  confined  to  the  East,  the  Crimea  and  the  Balkans,  and  another 
Toothwort  (Lathrcea  clandestina),  distinguished  by  large  flowers,  but  slightly 
raised  above  the  earth,  extends  in  western  and  southern  Europe  from  Flanders  over 
France  to  Spain  and  Italy.  This  last  has  the  distinctive  feature  that  the  discoid 
suckers  developed  on  its  yellow  roots,  which  latter  are  of  the  thickness  of  a  quill, 
are  as  large  as  lentils  and  the  biggest  hitherto  discovered  on  any  plant. 

BROOM-RAPES,    BALANOPHORE^E,    RAFFLESIACE^E. 

The  fourth  series  of  parasitic  Phanerogamia  is  composed  of  plants  destitute  of 
chlorophyll,  whose  seed  contains  an  amorphous  embryo  without  cotyledons  or 
radicle.  The  seed  germinates  on  the  earth,  and  the  embryo  grows  as  a  filiform  body 
into  the  ground  and  there  fastens  upon  the  root  of  a  host-plant,  penetrates  into 
and  coalesces  with  it  in  growth,  forming  a  tuberous  stock,  from  which,  later  on, 
flowering  stems  are  projected  above  the  earth. 

To  this  series  belong  the  Broom-rapes  or  Orobanchese  and  the  Balanophorese. 
Of  the  genus  Orobanche  about  180  species  are  recognized,  which,  exhibiting  great 
uniformity  in  floral  structure  and  in  their  general  development,  can  only  be 
distinguished  by  minute  characteristics.  The  flowering  stem  growing  up  from  the 
subterranean  tuber  is,  in  all  the  species,  rigid,  erect,  thick,  fleshy,  and  covered  at  the 
top  with  dry  scales.  The  open  flowers,  ringent  in  shape,  are  crowded  together  in  a 
terminal  spike,  and  often  emit  a  strong  scent  like  that  of  pinks  or  sometimes  of 
violets.  The  colour  of  the  flowers  is  in  one  group  (Phelypcea)  mostly  blue  or  violet; 
in  the  rest  it  is  waxen  yellow,  yellowish-brown,  dark-brown,  rose-red,  flesh-tint,  or 
whitish.  Orobanche  violacea  and  0.  lutea,  both  natives  of  Northern  Africa,  have 
stems  which  grov/  to  a  height  of  half  a  meter  and  become  almost  as  thick  as  an  arm. 
The  best-known  species  is  the  Branched  Broom -rape  (Orobanche  ramosa),  which  is 
parasitic  on  the  roots  of  hemp  and  tobacco  plants,  and  is  very  widely  distributed. 
The  greatest  number  of  species  belong  to  the  East  and  to  Southern  Europe.  The 
extreme  north  of  America  harbours  one  species  which  bears  a  single  flower  at  the 
end  of  its  stem.  In  all  the  species  the  stem  projects  only  partially  above  the  earth. 
The  subterranean  portion,  adherent  to  the  root  of  a  host,  is  often  greatly  swollen 
and  thickened  above  the  place  of  attachment;  in  the  case  of  Striga  orobanchoides, 
which  is  prevalent  in  the  Nile  basin,  it  is  irregularly  lobed  above  the  host's  root. 
The  root  of  the  nutrient  plant  also  is  usually  somewhat  swollen  wherever  a 
parasitic  Orobanche  has  settled  upon  it,  and  sometimes  it  exhibits  an  irregular 
outgrowth  inclosing  the  spot  whereto  the  Orobanche  is  adnate  like  a  cup.  Beyond 
the  place  of  attachment  of  the  parasite  the  root  has  often  the  appearance  of  having 
been  bitten  off,  and  this  is  owing  to  the  fact  that  the  particular  piece  of  root  has 
been  killed  and  demolished  by  the  attack  of  the  parasite.  From  the  base  of  the 
stem,  near  the  point  of  adhesion  to  the  host,  spring  short,  thick,  fleshy  fibres,  and 


184  BROOM-RAPES,   BALANOPHORE^,   RAFFLESIACE^. 

one  or  other  of  these  bends  its  tip  towards  the  root  of  the  foster-plant  and  clings 
to  it.  These  fibres  are,  in  many  species,  very  numerous,  and  are  interlaced  and 
entangled  so  as  to  form  a  reticulate  mass,  which  vividly  recalls  that  of  the  Bird's- 
nest,  and  is  an  instance  of  the  general  resemblance  existing  between  Orobancheae 
and 'the  Orchidese  destitute  of  green  leaves  (Neottia,  Corallorhiza,  Epipogum, 
Limodorum),  which  have  already  been  discussed. 

The  establishment  of  parasitic  Orobanchese  upon  the  roots  of  host-plants  takes 
place  in  the  following  manner.  The  embryo  imbedded  in  the  small  seed  shows  no 
trace  of  differentiation  into  root  ,and  stem,  possesses  no  cotyledons,  and  indeed 
consists  only  of  a  group  of  cells;  it  is  surrounded  by  other  cells  filled  with  reserve- 
nutriment.  When  this  embryo  grows  forth  from  the  seed,  during  which  process  it 
consumes  the  reserve-food,  it  exhibits  no  distinction  between  root,  stem,  and  leaf, 
but  is  a  spiral  filament  consisting  of  delicate  cells.  One  extremity,  the  shoot  end, 
of  this  filiform  seedling,  remains  covered  by  the  seed-coat,  which  looks  like  a  dark 
cap  (fig.  34  8);  the  opposite  extremity  is  the  root. 

The  seedling  Broom-rape  stretches  downwards  just  as  the  Dodder  (Cuscuta) 
extends  upwards.  In  so  doing  the  descending  tip  traces  a  spiral  line,  and  so, 
as  it  were,  seeks  in  the  earth  for  the  root  of  a  plant  suitable  as  host.  If  the 
search  is  fruitless,  and  if  the  reserve-material  in  the  seed  has  meantime  been 
altogether  consumed,  the  seedling  begins  to  wither  and  gradually  shrivels,  turns 
brown,  and  dries  up.  It  lacks  the  power  of  nourishing  itself  by  means  of  the 
surrounding  earth.  But,  if  the  lower,  foraging  extremity  of  the  seedling  succeeds 
in  finding  a  live  root  belonging  to  a  plant  able  to  serve  as  host,  it  not  only  adheres 
closely  to  it,  but  swells  in  such  a  way  as  to  give  the  young  plantlet  a  flask-shaped 
appearance  (fig.  34 9  and  fig.  34 10).  The  upper  end  is  still  inclosed  by  the  seed- 
coat,  but  in  proportion  as  the  lower  part  thickens,  the  upper  shrivels  till  no  trace 
of  it  is  left.  The  thickened  part,  on  the  other  hand,  which  has  become  attached 
to  the  root  of  the  host,  becomes  nodulated  and  papillose.  Some  of  the  papillae 
•develop  into  elongated  conical  pegs,  and  the  young  Broom-rape  now  rests  upon  the 
nutrient  root  in  the  shape  of  the  head  of  a  fighting-club  (see  fig.  34 12).  At  the 
place  of  attachment  one  of  the  conical  pegs  has  meanwhile  penetrated  the  cortex  of 
the  root,  and  there  it  continues  to  grow  energetically,  forcing  the  cortical  tissue 
apart,  until  it  reaches  the  wood.  Vessels  now  arise  in  the  body  of  the  young  club- 
like  plant,  and,  passing  through  the  middle  of  the  plug,  wedged  in  the  nutrient 
root,  are  brought  into  connection  with  the  vessels  of  the  latter.  At  the  point  of 
union  between  host  and  parasite,  a  bud  is  formed,  clothed  with  abundant  scales, 
which  may  best  be  likened  to  the  bulb  of  the  Martagon  Lily.  Lastly,  out  of 
this  bud  grows  a  strong,  thick  stem,  which  breaks  through  the  earth  and  lifts  a 
spike  of  flowers  into  the  sunlight. 

That  portion  of  the  Broom-rape  which  is  buried  in  the  root  of  the  host-plant  is  so 
intimately  associated  with  the  separate  parts  of  that  root  in  the  development  of  a 
tuber  that  it  is  usually  difficult  to  determine  which  cells  belong  to  the  parasite  and 
which  to  the  host.  The  degree  of  union  is  such  that  one  cannot  even  state  with 


BROOM-RAPES,   BALANOPHORE^,    RAFFLESIACE^.  185 

certainty  where  the  epidermis  of  the  nutrient  root  ceases,  and  that  of  the  Broom- 
rape  begins.  The  latter  looks  as  if  it  were  a  branch  growing  out  of  the  root  it  preys 
upon,  and  this  apparent  fusion  gave  some  colour  to  the  view  of  the  earlier  botanists, 
who,  ignorant  of  the  life-history  of  these  parasites,  believed  that  they  did  not  arise 
from  seeds,  but  were  pathological  outgrowths  of  the  roots,  produced  from  their 
tainted  juices;  in  other  words,  that  they  were  "pseudomorphs"  sprouting  from 
diseased  roots  in  the  place  of  leafy  branches. 

It  is  also  deserving  of  mention  that  some  of  the  thick,  fleshy  fibres  issuing 
laterally  from  the  nodulated  seedlings  curve  towards  the  host's  root,  bury  their  tips 
in  the  cortex,  and  thenceforth  behave  exactly  like  the  peg  which  was  inserted  at  the 
point  where  the  seedling  first  became  attached.  We  must  leave  undecided  the 
questions  as  to  whether  the  other  fibres,  which  terminate  freely  in  the  earth,  are 
capable  of  taking  up  food-materials  from  that  source,  whether  these  fibres  are  only 
present  in  perennial  species  and  become  the  starting-points  of  new  individuals,  and 
lastly,  whether  they  should  be  looked  upon  as  root-structures  or  as  stem-structures. 

In  addition,  it  is  noteworthy  that  in  many  Orobanchese  only  those  embryos 
continue  to  develop  which  meet  with  a  plant  suitable  to  be  their  host.  Although  it 
is  not  the  case  that  every  species  of  Orobanche  adopts  one  particular  species  of 
plant  as  foster-parent,  yet  thus  much  is  certain,  that  most  of  them  only  thrive  on 
members  of  a  limited  circle  of  species;  one  lives  exclusively  on  kinds  of  Wormwood, 
a  second  on  species  of  Butter-bur,  and  a  third  on  those  of  Germander.  For 
example,  Orobanche  Teucrii  prevails  on  Teucrium  Chamcedrys,  Teucrium  mon- 
tanum, &c.,  the  hosts  being  invariably  species  of  the  genus  Teucrium.  Suppose  a 
hill  thickly  covered  with  plants  comprising  Teucrium  montanum  growing  in 
company  with  thyme,  rock-roses,  globe-flowers,  sedges,  and  grasses,  but  no  great 
abundance  of  the  Teucrium,  a  plant  belonging  to  the  species  named  occurring  only 
here  and  there,  and  let  Orobanche  Teucrii  have  established  itself  at  one  particular 
spot,  have  attained  to  flowering  and  developed  fruits,  the  tiny  seeds  of  which  have 
been  shaken  by  the  wind  out  of  the  ripe  capsules.  Owing  to  the  exceptional 
minuteness  and  lightness  of  its  seeds,  every  gust  of  wind  will  scatter  them  in 
innumerable  quantities  over  the  entire  hillside  and  beyond  it.  The  next  step  is 
germination.  Filiform  embryos  emerge  from  the  seeds,  in  the  manner  described 
above,  and  penetrate  into  the  earth.  Teucrium  montanum  being  only  sparsely 
present  on  the  hill  in  question,  comparatively  few  seedlings  will  meet  with  the  roots 
of  that  plant,  whereas  thousands  will  fall  in  with  the  roots  of  the  thymes,  rock- 
roses,  globe-flowers,  sedges,  and  grasses.  But,  curious  to  relate,  only  those  seedlings 
of  Orobanche  Teucrii  which  come  into  contact  with  the  roots  of  Teucrium 
montanum  establish  themselves  firmly,  penetrate  into  them,  and  continue  their 
development;  whilst  the  numerous  individuals  which  touch  the  roots  of  the  thyme 
and  other  plants  perish.  This  phenomenon  can  scarcely  be  explained  in  any  other 
way  than  by  the  supposition  that  the  roots  of  Teucrium  montanum  alone,  by 
virtue  of  their  special  structure  and  quality,  afford  a  suitable  nutrient  substratum, 
and  therefore  constitute  centres  of  attraction  for  seedlings  of  Orobanche  Teucrii; 


BROOM-RAPES,    BALANOPHOREJi,    RAFFLESIACE^. 

and  that  the  roots  of  the  thyme,  rock-roses,  and  other  plants  growing  upon  the  hill 
side  by  side  with  Teucrium  montanum  do  not  share  this  property. 

Whereas  the  Broom-rapes  constitute  a  family  of  plants,  the  species  of  which, 
though  very  numerous,  are  so  similar  in  the  structure  of  flowers  and  fruit,  in  the 
history  of  their  development  and  in  the  general  impression  they  convey,  that  it  is 
necessary  to  discover  minute  distinctive  marks  in  order  to  be  able  to  classify  them 
with  tolerable  completeness,  the  Balanophorece,  which,  together  with  these  Oro- 
banchese,  belong  to  the  fourth  series  of  parasitic  Phanerogams,  are  related  to  one 
another  in  a  manner  quite  the  reverse.  Only  forty  species  of  them  are  known,  but 
they  are  so  various  that,  on  the  basis  of  the  obvious  differences,  no  less  than 
fourteen  genera  have  been  distinguished,  among  which  the  forty  species  are  fairly 
equally  divided.  In  respect  of  distribution  and  occurrence  they  also  contrast 
strikingly  with  both  Broom-rapes  and  Rhinanthaceae.  The  Orobancheae  belong  in 
particular  to  the  Mediterranean  flora,  and  to  the  East,  and  the  Rhinanthacese,  as  has 
been  already  stated,  adorn  chiefly  sunny  pastures  in  arctic  regions  and  in  moun- 
tain districts  of  the  northern  hemisphere.  Balanophorese,  on  the  other  hand, 
are  only  found  within  a  belt  encircling  the  Old  and  New  Worlds,  which  stretches 
little  beyond  the  equatorial  zone  to  the  north  or  south,  and  they  almost  all  inhabit 
the  dark  bed  of  primeval  forests,  where  they  are  parasitic  on  the  roots  of  woody 
plants,  beneath  a  covering  of  vegetable  mould. 

The  genus  of  Balanophorese  named  Langsdorffia  is  confined  exclusively  to- 
tropical  America.  One  of  its  species  (Langsdorffia  Moritziana)  is  found  native 
in  the  damp  forests  of  Venezuela  and  New  Granada,  where  it  is  parasitic  on  the 
roots  of  palms  and  fig-trees;  a  second  species  (Langsdorffia  rubiginosa)  occurs  in 
Guiana  and  Brazil  in  the  region  of  the  sources  of  the  Orinoco,  and  a  third,  the 
most  common  of  all  (Langsdorffia  hypogced)  represented  in  fig.  38,  has  an  area  of 
distribution  extending  from  Mexico  to  the  south  of  Brazil.  They  all  avoid  the 
hottest  districts,  remaining  rather  in  cool  regions;  indeed  the  species  first  named 
has  been  found  at  an  elevation  of  from  2000  to  3000  meters.  Unlike  all  the 
rest  of  the  Balanophoreas,  Langsdorffia  exhibits  a  branched,  cylindrical  stock 
ascending  from  the  place  of  attachment  to  the  nutrient  root,  more  or  less  felted 
externally,  and  before  putting  forth  any  flowers  has  a  remote  resemblance  to  a 
doe's  antlers  with  their  winter  covering  of  downy  skin.  These  stems  are  almost  as 
thick  as  a  little  finger,  have  a  fleshy  consistence,  and  exhibit  a  clavate  expansion 
at  the  base  where  they  rest  upon  the  root  of  the  host.  Many  of  those  stems  which 
bear  the  male  flowers  are  30  cm.  long;  those  which  bear  the  female  flowers  are 
usually  somewhat  shorter.  They  are  all  of  a  pale-yellowish  colour;  the  thickly 
tomentose  Langsdorffia  rubiginosa  looks  as  if  it  were  covered  with  a  yellowish 
velvet.  At  the  extremity  of  each  of  the  ramifications  of  the  stem,  which  are  often 
extremely  short,  having  then  the  form  of  lobes  or  knobs,  a  bud  is  developed  sooner 
or  later  in  the  lower  cortical  layer.  This  bud  swells,  bursts  the  outer  layer  of 
cortex,  uplifts  itself  and  grows  out  as  an  inflorescence  between  the  four  lobes 
formed  by  the  cruciform  rupture  of  the  bark.  The  inflorescence  is  surrounded,  like 


BROOM-RAPES,   BALANOPHORE.E,    RAFFLESIACE^. 


187 


the  capitulum  of  a  composite,  by  a  whorl  of  imbricating  scales,  of  which  the  lower 
are  shorter  and  broader,  and  the  upper  longer,  narrower,  and  pointed  at  the  apex. 
These  scales  being  stiff,  somewhat  shiny,  and  varying  in  colour  from  a  waxen 
yellow  to  orange  or  red— in  the  case  of  Langsdorffia  Moritziana  brown-red,— the 
whole  inflorescence  has  a  vivid  resemblance  to  certain  immortelles,  namely,  the 
large  species  of  Helichrysum  occurring  at  the  Cape.  The  inflorescences  bearing 
male  flowers  are  elongated  and  egg-shaped,  those  possessing  only  female  flowers  are 
shorter  and  capitulate.  The  seeds  dropped  from  the  nut -like  fruits,  which  are 
pulpy  internally,  have  no  special  integument.  The  embryo  exhibits  no  trace  of 


Fie.  SS.—Langsdorffla  hypogcea,  from  Central  America. 

cotyledons  or  radicle,  but  consists  of  an  undifferentiated  group  of  cells  which  may 
be  likened  to  a  tiny  bulbil. 

Seeds  of  this  kind  germinate  like  those  of  Lathrcea,  and  upon  meeting  with 
the  root  of  a  tree  or  shrub  suitable  for  prey,  develop  into  larger  tubercles  and  have 
a  remarkable  effect  upon  the  substratum.  The  cortex  of  the  host-root  is  destroyed 
at  the  place  of  adhesion  of  the  tubercle,  and  its  wood  is  laid  open,  lacerated,  and 
unravelled.  The  woody  bundles  are  diverted  from  their  previous  direction,  ascend 
towards  the  parasitic  tubercle,  which  meantime  has  grown  into  a  full-sized  tuber, 
and  spread  out  like  fans.  The  cells  and  vessels  of  the  parasite  penetrate  between 
the  ascending  wood-fibres,  and  this  results  in  the  formation  of  a  zone  at  the  place 
of  union  of  the  parasite  and  root,  where  cells  and  vessels  belonging  to  both  inter- 
lace, traverse,  and  join  one  another,  coalescing  completely  in  exactly  the  same  way 
as  happens  in  the  case  of  the  species  of  Toothwort.  A  similar  phenomenon  occurs 
also  when  one  of  the  wavy  stems  of  Langsdorffia  comes  into  contact  with  a  root 
adapted  to  the  purpose.  The  cortex  of  the  root  is  demolished  at  the  place  of 


188  BROOM-RAPES,   BALANOPHORE^,    RAFFLESIACE^. 

contact;  the  wood  is  exposed,  split  open,  and  unravelled,  whilst  the  tissue  of  the 
parasitic  stem  fills  up  all  the  interspaces  in  the  upcurved  and  sundered  woody 
bundles  and  fibres,  and  so  intimate  is  the  union  thus  effected  that  the  stem  of  the 
Langsdorffia  might  be  taken  to  be  a  branch  of  the  root  of  the  host-plant  which 
sustains  it.  At  the  point  of  connection  of  an  already  adult  Langsdorffia  stem,  the 
hypertrophy  of  the  tissue  is  not  very  striking;  but  the  base  of  each  stem  of  an  indi- 
vidual produced  from  a  seed  presents  a  highly  swollen  and  clavate  appearance.  At 
first  the  parasite  is  only  fastened  by  one  side  of  this  thickened  base  to  the  nutrient 
root,  but  later  on  it  wraps  both  sides  round  the  root,  and  rests  upon  the  latter  like 
a  saddle  on  the  back  of  a  horse. 

Between  the  bundles  of  a  Langsdorffia  stem  there  are  passages  filled  with  a 
peculiar  wax-like  matter  named  balanophorin.  The  quantity  of  this  substance  is 
so  great  that  if  one  end  of  a  stem  of  Langsdorffia  is  lighted,  it  burns  like  a  wax- 
taper,  and  in  the  region  of  the  Bogota  these  Langsdorffias  are  collected  and  sold 
under  the  name  of  "siejos",  and  are  used  for  illuminating  purposes  on  festive 
occasions.  In  New  Granada  they  have  also  been  employed  in  the  making  of 
candles;  and,  although  this  source  of  wax  is  not  sufficiently  abundant  for  us  to  be 
able  to  believe  in  its  consumption  and  conversion  on  a  large  scale,  the  fact  of  its 
application  in  this  manner  shows  that  the  parasite  we  are  discussing  must  occur  in 
great  exuberance  in  many  tracts  of  country  in  Central  America. 

Much  rarer  than  the  parasitic  Langsdorffias  are  the  species  belonging  to  the 
genus  Scybalium.  Like  the  former  these  are  confined  to  the  equatorial  zone  of 
America.  Two  species,  viz.  Scybalium  Glaziovii  and  S.  depressum,  flourish  in 
mountainous  districts,  one  of  them  indeed  occurring  only  on  the  mountains  of  New 
Granada;  two  other  species  (Scybalium  jamaicense  and  S.  fungiforme)  live  in  the 
woods  and  savannahs  of  lower-lying  regions.  The  aspect  of  the  last-named  species 
when  seen  growing  on  the  ground  of  a  primeval  forest,  tempts  one  to  suppose  it  to 
be  a  fungus,  and  it  is  easily  understood  why  the  first  discoverer  selected  the  term 
fungiforme  to  apply  to  it.  Figure  39  \  representing  this  rare  and  marvellous  plant, 
is  taken  from  the  original  specimens  discovered  in  the  year  1820  by  Schott  in  the 
Sierra  d'Estrella  of  Brazil,  and  brought  thence  by  him  to  Vienna.  We  see  that,  in 
this  case,  instead  of  the  elongated,  wavy,  branched  stem  characteristic  of  Langs- 
dorffias,  a  lumpy,  tuberous  mass  rests  upon  the  root  of  the  host-plant.  This  tuber 
is  sometimes  rounded  and  sometimes  compressed  and  discoid;  it  is  nodulated  and 
often  irregularly  lobed  also,  and  grows  to  the  size  of  a  fist.  It  is  developed  from 
a  seed  which,  as  is  the  case  in  all  Balanophorese,  is  a  cellular  structure  without 
integument  containing  an  embryo  destitute  of  cotyledons  and  radicle,  and  is  best 
described  as  a  minute  tubercle.  The  embryo,  after  emerging  from  the  seed  and 
finding  the  living  root  of  a  woody  plant,  increases  in  volume,  and,  in  the  form  of 
a  little  knob  the  size  of  a  pea,  exercises  the  same  influence  on  the  plant  preyed 
upon  as  has  been  noted  in  the  case  of  Langsdorffia.  The  root  attacked  is  stripped 
of  bark  at  the  place  where  the  tubercle  is  attached;  the  wood  is  then  resolved  into 
a  fringe  of  fibres  which  stand  straight  up,  and,  diverging  like  the  spokes  of  a  fan, 


BROOM-RAPES,   BALANOPHORE.E,   RAFFLESIACE.E.  189 

distribute  themselves  in  the  tissue  of  the  parasite,  the  latter  having  in  the  mean- 
time developed  into  a  tuberous  stock  as  large  as  a  nut.  These  radiating  bundles, 
issuing  from  the  wood  of  the  nutrient  root,  come  then  into  such  intimate  connection 
with  the  vessels  formed  in  the  tuber  of  the  parasite,  that  the  one  appears  to  be  a 
continuation  of  the  other.  They  are,  besides,  entangled  together,  and  between  them 
is  intercalated  a  mass  of  small  parenchymatous  cells  which  also  adheres  to  the  yet 
unfrayed  portion  of  the  foster-root's  wood,  and  coalesces  with  it.  The  tuberous 
body  of  the  parasite,  which  in  the  first  instance  is  only  adnate  to  the  host  on  one 


Fig.  39.— Parasitic  Balanophoreaj. 
i  Scybaliumfungiforme,  from  Brazil.  a  Balanophora  Hildenbrandtii,  from  the  Comoro  Islands. 

side,  gradually  encompasses  it  entirely,  and  the  nutritive  root  then  appears  to 
perforate  this  irregular  tuber.  The  inflorescences  are  produced  direct  from  buds, 
which  are  formed  under  the  bark  at  projecting  spots  of  the  brown  tuberous  stem, 
the  cortex  bursting  open  and  allowing  a  thick  flesh-coloured  shoot,  closely  beset  by 
ovoid  pointed  scales,  to  emerge  and  grow  up  into  a  form  resembling  a  mortar-pestle. 
At  the  summit  this  shoot  expands  into  a  disc,  and  upon  this  are  borne  little  capitu- 
late groups  of  flowers,  which  are  inserted  amongst  innumerable  quantities  of  scales 
and  hairs.  The  pistillate  and  staminate  flowers  are  separated  in  different  inflo- 
rescences, whilst  the  entire  structure  has  an  undeniable  resemblance  when  in  bloom 
to  the  inflorescence  of  an  artichoke  gone  to  seed,  and  later  on  to  a  toad-stool. 

In  the  eastern  hemisphere  we  find  the  various  species  of  the  genus  Balanophora 
replacing  the  Langsdorffias  and  Scybalia.     One  of  these,  Balanophora  Hilden- 


190  BROOM-RAPES,    BALANOPHORE^,    RAFFLESIACE^. 

brandtii,  which  is  represented  on  the  left  side  of  the  figure  39,  occurs  in  the 
Comoro  Islands  off  the  east  coast  of  Africa;  seven  species  inhabit  the  islands  of 
Java,  Ceylon,  Borneo,  Hong-Kong,  and  the  Philippines,  and  three  species  the  East 
Indies.  Balanophora  fungosa,  first  discovered  by  Forster,  is  parasitic  on  the  roots 
of  Eucalyptus  and  Ficus,  and  is  indigenous  to  Australia  and  the  New  Hebrides. 
The  more  elevated  regions  of  Java  and  the  Himalaya  abound  especially  in 
these  singular  organisms.  Balanophora  elongata  is  so  prevalent  in  Java  on 
mountains  of  between  2000  and  3000  metres,  that  it  is  collected  in  quantities  for 
the  sake  of  the  wax-like  matter  obtained  from  it.  In  that  island  candles  are  made 
from  Balanophoras  as  they  are  from  Langsdorfnas  in  New  Granada,  or  else  rods  of 
bamboo  are  smeared  with  the  viscid  substance,  as  they  are  then  found  to  burn  quite 
quietly  and  slowly.  In  the  Himalaya,  Balanophora  dioica  or  B.  polyandra  are 
the  commonest  and  most  widely  distributed  species,  and  Balanophora  involucrata 
is  there  met  with  upon  the  roots  of  oaks,  maples,  and  araliads  even  at  a  height  of 
from  2300  to  2500  metres  above  the  sea-level.  They  possess  in  almost  all  cases 
very  vivid  and  conspicuous  colouring — deep-yellow,  purple,  red-brown  or  flesh-tint, 
thus  resembling  the  Gastromycetes,  Clavariese,  and  Toad-stools,  in  whose  company 
they  grow,  and  with  which  they  manifest  an  additional  uniformity  in  being  all  of 
fleshy  consistence  and  containing  no  trace  of  chlorophyll.  At  a  certain  distance, 
moreover,  the  inflorescences  rising  from  the  dark  ground  in  a  wood,  have  the 
appearance  of  fungi,  and  all  the  early  observers  describe  these  Balanophorese  with 
one  accord  as  truly  abnormal  growths,  viz.  as  fungi  which  by  some  marvellous 
accident  bear  flowers.  They  were  also  the  object  of  the  boldest  speculations  and 
most  exuberant  imagery  on  the  part  of  the  botanists  belonging  to  the  school  of  the 
"  nature  philosophers  "  of  the  first  decades  of  this  century.  Even  as  late  as  the  forties 
a  famous  German  botanist  says  of  them:  "They  are  in  the  position  of  a  hiero- 
glyphic key  between  two  worlds,  which  intercept  and  evade  one  another  in  an 
infinite  variety  of  ways,  like  dreaming  and  waking  moments",  and  the  worthy 
Junghuhn,  who  discovered  several  of  these  plants  in  Java,  writes:  "Those  are 
words  which  we  may  hope  will  be  rightly  interpreted  thousands  of  years  hence. 
Their  sublime  truth  affected  me  deeply.  There,  flowerless  and  leafless,  stood  the 
mysterious  plants  which  afford  an  instance  of  the  combination  of  special  vessels 
in  a  stalk  like  that  of  Balanophorese  with  the  fructification  of  imperfect  Hypho- 
mycetes!" 

A  young  Balanophora  not  in  flower  is  not  unlike  a  Scybalium  in  appearance 
at  the  corresponding  stage  of  its  development.  It  consists  of  an  irregular  tuberous 
stem,  which  rests  upon  the  creeping  root  of  a  tree  or  shrub.  The  exterior  of  this 
structure,  which  sometimes  attains  to  the  size  of  a  man's  head,  is  uneven,  and  in 
some  cases  convoluted  like  the  human  brain,  or  it  may  project  in  humps  and  knobs, 
or  be  divided  into  lobes  or  short  branches  like  a  coral-stem.  The  resemblance  to 
the  latter  is  heightened  by  the  fact  that  the  surface  is  covered  by  little  papillae 
shaped  like  stars  or  forget-me-nots,  which  distinguish  the  genus  Balanophora 
from  all  allied  genera. 


BROOM-RAPES,    BALANOPHORE.E,    RAFFLESIACE^E. 


191 


The  seeds  settle  upon  the  roots  of  trees,  develop  into  tuberous  axes,  and  unite 
with  the  nutrient  root  in  the  same  manner  as  the  Balanophorese  already  described. 
Also  the  inception  of  the  rudimentary  inflorescence  beneath  the  cortex  of  the  tuber 
and  its  eruption  are  similarly  accomplished.  In  this  genus  the  cortical  layer  thus 
broken  through  and  forced  outward  always  forms  a  large  cup-shaped  or  crateriform 
sheath  with  an  irregularly-lobed  margin  surrounding  the  base  of  the  inflorescence. 


Fig.  40.— Parasitic  Balanophorese. 
»  Rhopalocnemis  phalloides,  from  Java.  2  Helosis  gujanensis,  from  Mexico. 

The  inflorescence  itself  is  spadiciform,  and  is  borne  by  a  thick  shaft  beset  with 
large  squamous  leaves.  The  spadices  growing  from  a  tuber-stock  are,  for  the  most 
part,  only  as  long  as  a  little  finger,  but  occasionally  they  reach  a  height  of  30  cm., 
as,  for  example,  is  the  case  in  the  Balanophora  elongata  of  Java,  which  is  parasitic 
on  the  roots  of  Thibaudia. 

The  species  of  the  American  genus  Helosis,  whereof  the  most  common  (Helosis 
gujanensis)  is  represented  above,  resemble  those  of  the  genus  Balanophora  in  the 
shape  of  the  inflorescence.  There  is,  however,  considerable  difference  in  the  method 
adopted  by  these  Helosis  species  of  settling  upon  the  roots  of  host-plants  and  in 


192  BROOM-RAPES,   BALANOPHORE.E,   RAFFLESIACE^E. 

the  whole  mode  of  growth.  The  phenomena  of  the  swelling  of  the  embryo  into  a 
tubercle  after  it  has  chanced  upon  a  nutritive  root,  the  destruction  of  the  cortex, 
the  exposure  of  the  wood  at  that  part  of  the  root  where  the  tubercle  is  adnate,  and 
the  derangement  of  the  course  of  the  woody  bundles  ensue,  it  is  true,  in  the  same 
manner  as  in  the  other  Balanophoreae;  but  the  frayed  wood-bundles  of  the  foster- 
root  only  form  quite  short  lobules  which  penetrate  but  a  short  distance  into  the 
parasitic  tuber-stock,  whilst  the  vascular  bundles,  formed  meantime  in  the  latter, 
adhere  to  them  in  such  a  manner  that  they  might  be  mistaken  for  direct  continua- 
tions of  them. 

When  once  the  parasitic  tubers  have  thus  become  adnate  to  a  root,  and  by 
means  of  this  union  are  provided  with  food,  they  grow  round  the  nutrient  roots  in 
such  a  way  that  the  latter  appear  to  perforate  or  actually  to  issue  from  the 
tubers.  They  are  always  roundish,  brown  outside,  and  warty,  but  without 
scales,  and  they  never  produce  inflorescences  directly,  but  put  forth  in  the  first 
place  several  whitish  or  yellowish  runners  varying  in  thickness  from  a  quill  to  a 
finger,  which  creep  along  horizontally  under  the  ground,  bifurcating,  and  becoming 
interlaced  with  other  ramifications.  At  the  places  of  contact  they  coalesce,  and  so 
occasionally  form  a  net- work  which  is  almost  inextricably  entangled  with  the  root- 
system  of  the  plant  preyed  upon.  Whenever  a  runner  of  this  kind  comes  into 
contact  with  a  living  root  belonging  to  the  host -plant,  the  surface  of  contact  at  once 
swells  up.  The  part  affected  is  converted  into  a  tuberous  mass  and  becomes  adnate 
to  the  root,  the  process  being  the  same  as  occurs  in  the  case  of  the  tubercle  pro- 
duced from  seed.  A  net-work  of  runners  thus  connected  with  the  root-system  of 
the  nutrient  plant  at  several  spots  by  means  of  tubers  as  large  as  peas  might  be 
compared  to  the  reticulum  woven  by  Laihrcea  round  the  roots  of  its  hosts;  but, 
apart  from  the  size,  there  is  the  essential  difference  that  inflorescences  are  never  pro- 
duced from  the  white  threads  of  the  ramifying  and  sucker-bearing  roots  of  Lathrcea, 
whereas  the  runners  of  Helosis  afford  points  of  origin  for  new  inflorescences.  Warts 
are  produced  on  the  surfaces  of  the  thicker  cylindrical  runners,  and  within  these 
are  developed  the  buds  of  the  inflorescences.  The  outer  coat  of  the  warts  is  then 
rent  open  at  the  top  and  constitutes  a  little  cup,  out  of  which  grows  a  naked,  scale- 
less  shaft  terminated  by  an  oval  spadix.  Seeing  that  the  runners  rest  horizontally 
under  the  earth  whilst  the  shafts  ascend  bolt  upright  from  the  ground,  the  latter 
are  always  at  right  angles  to  the  runners,  of  which  they  are  to  be  regarded  as 
branches. 

The  flowers  are  grouped  in  capitula,  presenting  in  the  spadix  a  dense  mass. 
They  are  protected  by  peculiar  bract-scales,  each  of  which  by  itself  is  like  a  nail 
with  a  facetted  head.  These  heads  are  in  close  contact  with  one  another,  so  that 
the  young  inflorescence  seems  to  be  inclosed  in  a  panelled  coat  of  mail,  and 
resembles  to  a  certain  extent  a  closed  fir-cone.  By  degrees,  however,  these  bract- 
scales  detach  themselves  and  fall  off,  and  thus  the  flowers,  till  then  roofed  over  by 
them,  become  visible.  When  the  seeds  are  mature,  the  whole  runner  concerned  in 
the  production  of  the  inflorescence,  and  usually  also  the  tuber  which  served  as  the 


BROOM-RAPES,   BALANOPHORE^,   RAFFLESIACE^.  193 

starting-point  of  that  runner,  perishes,  and  another  tuber  belonging  to  the  net- work 
above  described,  or  rather  the  system  of  runners  proceeding  from  it,  becomes  the 
basis  for  the  development  of  new  inflorescences.  To  this  extent  we  may  regard 
these  Helosis  species  as  perennial  plants,  whereas  the  majority  of  the  other 
Balanophoreae  can  lay  no  claim  to  this  distinction,  inasmuch  as  in  their  case  the 
whole  plant  dies  after  it  has  flowered  and  ripened  its  seeds.  The  floral  spadices  in 
Helosis  have  a  purple  or  blood-red  colour,  and  in  Brazil  are  called  "Espigo  de  sangue". 
Only  three  species  of  Helosis  have  been  discovered  up  to  the  present  time,  and 
those  are  distributed  over  equatorial  America,  in  the  Antilles,  and  from  Mexico  to 
Brazil. 

Nearly  allied  to  Helosis  is  the  genus  Coryncea,  which  resembles  it  in  having 
facetted  bract-scales  like  nails  and  a  cone-like  inflorescence,  but  differs  entirely  in 
other  respects  in  its  mode  of  growth,  especially  in  being  without  runners.  Four 
species  of  this  genus  have  been  discovered  in  the  Andes  of  South  America,  in  Peru, 
Ecuador,  and  New  Granada,  where  they  are  parasitic,  like  the  rest  of  the  Balano- 
phorese,  upon  the  roots  of  trees.  One  of  them,  Coryncea  Turdiei,  is  worthy  of 
notice  as  living  on  the  roots  of  Peruvian-bark  trees,  and  is  rendered  conspicuous  by 
its  purple  spadix,  borne  on  a  white  shaft.  Ehopalocnemis  phalloides  (see  fig.  40 1 ) 
is  another  root-parasite  related  to  Helosis,  and  the  single  representative  in  Asia  of 
these  pre-eminently  American  groups.  It  is  found  preying  upon  the  roots  of 
fig-trees,  oaks,  and  various  lianes,  in  mountainous  parts  of  Java  and  the  eastern 
Himalayas,  and  is  one  of  the  biggest  of  all  the  BalanophoreaB.  The  fleshy, 
yellowish  or  reddish-brown  tuber-stock  attains  to  the  size  of  a  man's  head;  the 
inflorescences,  which  burst  from  the  protuberances  of  this  lumpy  mass  and  are 
from  two  to  six  in  number,  are  over  30  cm.  long  and  from  4  to  6  cm.  thick.  The 
protuberances  are  light-brown  in  colour,  and  resemble  in  form  a  cycad-cone. 
Ehopalocnemis,  a  drawing  of  which  is  given  in  fig.  40 l  on  a  scale  of  one-half  the 
natural  size,  is  distinguished,  like  Coryncea,  from  Helosis  by  having  no  runners 
issuing  from  the  tuberous  axes. 

The  Lophophytese  are  set  apart  as  a  further  group  of  parasitic  Balanophoreas, 
and  differ  from  all  the  groups  hitherto  described  in  having  their  flowers  arranged  in 
separate  roundish  capitula  upon  a  fleshy  rachis  springing  from  the  tuberous-stock. 
They,  again,  belong  to  Central  America,  and  are  divided  into  three  genera 
(Lophophytwm,  Ombrophytum,  and  Lathrophytum)  into  particulars  of  which  we 
cannot  enter  without  exceeding  our  limits.  Only  the  genus  Lophophytwn,  which 
is  in  many  respects  different  from  other  Balanophoreae,  and  in  particular  has  been 
more  thoroughly  studied  with  reference  to  its  peculiar  mode  of  connection  with  the 
host-plant,  demands  special  consideration.  The  Lophophytum  mirabile  (see 
fig.  41 a )  found  in  the  primeval  forests  of  Brazil  adhering  to  the  roots  of  Mimoseae, 
to  those  of  Inga-trees  especially,  occurs  at  some  places  in  such  profusion  that  areas 
of  ground,  occupied  by  Inga-roots,  from  twenty  to  thirty  paces  in  circumference 
appear  to  be  entirely  overgrown  by  the  parasite.  Hundreds  of  tubers,  some  large, 
some  small,  rest  upon  the  roots  of  the  trees,  covered  by  fallen  leaves  and  a  light 

VOL.  I.  13 


BROOM-RAPES,   BALANOPHORE.E,    RAFFLESIACE^. 

stratum  of  vegetable  mould.  Most  of  them  are  the  size  of  a  fist,  but  a  few  are  as 
big  as  a  head,  and  then  weigh  15  kilogr.  and  more.  The  tubercles  formed  directly 
by  the  germinating  seeds  which  chance  upon  the  roots  are,  by  the  time  they  attain 
to  about  the  size  of  a  pea,  already  in  connection  with  the  wood  of  the  attacked 
root.  The  cortex  and  a  portion  of  the  wood  at  the  place  where  the  parasite  is 
adnate  are  absorbed  by  this  root.  The  tissue  of  the  small  tuber-stock  is  squarely 
and  firmly  inserted  into  the  superficial  notch  thus  made  in  the  root,  and  short,  peg- 
shaped  bundles,  isolated  by  the  loosening  of  the  wood  of  the  nutrient  root,  appear 
to  grow  into  the  substance  of  the  parasite.  As  the  tuber  increases  in  size  vascular 
bundles  are  developed  in  it  also,  and  these  grow  towards  the  said  bundles  of  the 
host  and  unite  with  them. 

No  boundary  can  then  any  longer  be  certainly  recognized  between  host  and 
parasite,  and  the  strangest  fact  of  all  is  that  we  find,  in  these  bundles,  cells 
concerning  which  we  are  not  able  to  decide,  even  by  reference  to  their  shape, 
whether  they  belong  to  the  one  or  to  the  other.  The  cells  which  belong 
undoubtedly  to  the  wood  of  the  nutrient  root  have  dotted  walls;  the  bundles 
unquestionably  developed  in  the  parasitic  tuber  exhibit,  on  the  other  hand,  cells 
with  reticulate  thickening,  which,  when  slightly  magnified,  look  as  if  they  were 
transversely  striated.  Wherever  these  pitted  and  reticulate  cells  meet,  cells  are 
intercalated  which  do  not  altogether  correspond  either  to  the  pitted  variety 
belonging  to  the  host  or  to  the  reticulate  cells  of  the  parasite,  but  display  a  form 
intermediate  between  the  two.  Here  and  there,  too,  cell-groups  belonging  to  the 
parasite  are  entirely  buried  in  the  wood  of  the  foster-root  in  its  growth,  and  in 
the  older  tubers  the  cellular  elements  of  the  two  plants  there  bound  together  are  so 
involved  that  it  is,  as  has  been  stated,  impossible  to  establish  any  line  of  demarca- 
tion between  the  two. 

By  the  time  the  tubers  have  reached  the  size  of  a  fist  their  cortical  layer  is 
always  solid,  corky,  and  areolated;  each  of  the  areas  being  more  or  less  uniformly 
angled,  as  is  shown  in  the  illustration  below.  Some  of  the  more  protuberant  portions 
elongate  and  grow  out  into  short,  thick  stumps  bearing  scales  all  round,  each  of 
the  little  areas  having  a  triangular-pointed  scale  situated  in  the  middle  of  it.  At 
this  stage  of  development  the  entire  Lophophytum  plant  has  an  extraordinary 
resemblance  to  the  squamigerous  rhizome  of  a  fern,  or  to  a  dwarf  cycad-tree, 
stripped  of  its  green  leaves;  and  this  likeness  is  enhanced  by  the  fact  that  the  bark 
and  scales  of  Lophophytum  are  dark-brown  in  colour.  From  the  centre  of  each  of 
these  thick  stumps,  which  often  reach  a  height  of  15  cm.,  there  now  arises  a 
spadiciform  inflorescence.  At  first  it  is  so  thickly  covered  with  ovate  lanceolate 
scales  possessing  dark-brown,  quasi-horny  tips,  overlapping  one  another  like  tiles, 
that  the  spadix  as  a  whole  looks  extremely  like  an  erect  cycad-cone.  Imagine  the 
surprise  of  a  traveller,  who  chances  upon  a  spot  in  the  depths  of  a  primeval  forest 
where  the  ground  is  occupied  by  Lophophytum,  upon  seeing  hundreds  of  these 
brown,  scaly  cones  grow  up  suddenly,  in  the  course  of  a  night  following  some  days 
of  rain,  from  the  subterranean  roots  of  the  trees.  A  day  or  two  later,  this  garden 


BROOM-RAPES,   BALANOPHOREJS,   RAFFLESIACE^.  195 

of  Lophophyta  presents  an  altogether  different  picture.  The  brown  scales  have 
detached  themselves  from  the  rachis,  first  those  at  the  base  of  the  cone,  then  also 
those  on  the  upper  parts.  They  fall  off  almost  simultaneously,  and  with  them  the 
envelope  which  up  to  that  time  has  concealed  the  flowers.  The  erect,  fleshy,  white 
or  reddish  rachis  bearing  the  flowers  then  becomes  visible.  The  female  flowers  are 


Fig.  41.— Parasitic  Balanophorese. 
i  Lophophytum  mirabile,  from  Brazil.  2  Sarcophyte  sanguined,  from  the  Cape  of  Good  Hope. 

on  the  lower  part,  and  arranged  in  spherical,  deep  yellow  or  orange-coloured 
•capitula  which  are  packed  close  together;  the  male  flowers  are  situated  above  the 
lowermost  third  of  the  spadix,  and  are  arranged  in  looser  and  less  crowded  capitula 
of  a  pale  yellow  colour. 

However  striking  the  phenomenon  presented  by  these  flowering  cones  of 
Lophophytum  mirabile,  it  is  surpassed  by  another  native  of  Brazilian  forests,  the 
Lophophytum  Leandri.  The  colouring  of  the  inflorescence  in  this  species  cannot 


BROOM-RAPES,   BALANOPHORE.E,   RAFFLESIACE^. 

be  exceeded  in  variety,  its  rachis  being  pale  reddish-violet,  the  bract-scales 
gamboge,  the  ovaries  yellowish,  the  styles  red,  and  the  stigmas  white.  It  is  not 
surprising  that  even  in  Brazil,  where  there  is  certainly  no  lack  of  curious  plant- 
forms,  they  have  attracted  attention,  and  that  they  are  used  there,  as  is  the  case 
with  all  rare  plants,  for  purposes  of  healing  and  magic.  The  tubers  of  Lopho- 
phytum  mirabile,  which  have  a  disagreeable,  bitter,  resinous  taste,  and  bear  the 
popular  name  of  "Fel  de  terra",  or  earth-gall,  are  employed  by  quacks  against 
jaundice,  and  a  belief  also  prevails  that  by  secretly  eating  the  blossoms  youths  are 
enabled  to  win  the  affection  of  the  maidens  they  admire.  The  same  may  be  said 
of  Lophophytum  Leandri,  and,  in  addition,  there  is  a  tradition  that  the  eating  of 
it  brings  luck  and  agility  in  hunting,  fishing,  fighting,  and  dancing,  and  for  this 
reason  the  Indian  youth  collect  the  plants  secretly  and  eat  them  on  particular  days. 

Of  the  other  parasitic  Balanophoreae  most  nearly  allied  to  Lophophytum  we  will 
here  only  mention  in  passing  the  species  of  Ombrophytum,  known  in  Peru  by  the 
name  of  "Mays  del  monte",  which  has  a  yellowish  inflorescence  over  30  cm.  high, 
and  from  6  to  7  cm  thick,  somewhat  resembling  a  spike  of  maize,  and  lastly,  the 
Lathrophytum  Peckoltii  of  Brazil,  to  which  a  special  interest  attaches  inasmuch 
as  it  is  the  sole  instance  of  a  flowering  plant  entirely  destitute  of  all  structures  of 
the  nature  of  leaves,  with  the  exception  of  the  stamens  and  ovaries.  Langsdorffia, 
Scybalium,  Lophophytum,  and  even  Balanophora,  Helosis,  and  Rhopalocnemis 
exhibit  scales,  which,  though  transformed  in  various  ways,  are  yet  always  in  point 
of  position  and  form  recognizable  as  leaves;  but  neither  on  the  tuber,  shaft,  nor 
spadix  of  this  Lathrophytum  is  any  trace  of  a  scale  to  be  seen,  nor  even  a  swelling 
or  rim  that  might  be  looked  upon  as  a  degenerate  leaf. 

In  comparison  with  equatorial  America  with  its  wealth  of  parasitic  Balano- 
phoreaa  the  corresponding  zone  of  Africa  must  be  called  poor  so  far  as  these  plants 
are  concerned.  Possibly  further  explorations  may  bring  to  light  a  few  more  of 
these  wonderful  vegetable  parasites,  but  it  is  hardly  to  be  expected  that  such  a 
variety  as  is  presented  in  Brazil,  the  Peruvian  Andes,  New  Granada,  and  Bolivia 
will  be  found  Only  three  Balanophoreae  have  been  discovered  in  the  Cape  regions, 
where  the  flora  is  well  known.  One  of  these,  which  is  represented  on  the  right- 
hand  side  of  fig.  41,  bears  the  name  of  Sarcophyte  sanguined  (i.e.  blood-red  flesh- 
plant),  whilst  the  name  of  Icthyosoma  (i.e.  fish-carcase)  has  also  been  applied  to  it 
because  it  smells  of  rotten  fish.  These  names  imply  that  the  plant  resembles  an 
animal  rather  than  a  vegetable  organism.  The  host -plants  adapted  to  this 
Sarcophyte  are  various  Mimosese,  especially  Acacia  ca/ra,  Acacia  capensis,  &c. 
In  the  first  place,  as  is  the  case  with  all  Balanophorese,  small  tubers  are  formed  on 
the  roots  of  the  above-mentioned  woody  hosts,  and  enter  into  connection  with  the 
wood  of  the  nutrient  roots  in  the  manner  already  described  more  than  once.  An 
inflorescence  then  emerges  from  a  bud  originating  beneath  the  cortex  of  the  tuber, 
and  rapidly  grows  up  from  out  of  the  cortex,  which  is  rent  and  pushed  up  in  the 
process.  The  axis  of  this  inflorescence  resolves  itself  into  a  number  of  thick, 
repeatedly  ramifying,  fleshy  branches,  differing  in  this  respect  from  every  other 


Fig.  42. — Cytinus  Hypocistus  on  the  left;  Cynomorium  cocdneum  on  the  right 

197 


BROOM-RAPES,   BALANOPHORE^,   RAFFLESIACE^. 

example  of  the  Balanophorese.  The  flowers  are  arranged  side  by  side  on  the 
branches,  staminate  flowers  on  one  plant,  and  pistillate  flowers  on  another,  the 
latter  always  grouped  in  spherical  capitula,  as  is  shown  in  fig.  41  2.  Reddish-brown 
scale-like  leaves  are  situated  at  the  points  of  origin  of  the  branches,  and  also  at  the 
base  of  the  entire  inflorescence.  The  general  aspect  is  that  of  a  bunch  of  verrucose 
grapes  ascending  from  the  root,  or  of  the  fruiting  axis  of  Ricinus,  and  is  very 
striking  owing  to  the  blood-red  colouring  of  all  the  parts. 

As  a  final  instance  of  the  Balanophorese  we  may  take  the  genus  Cynomorium, 
which  was  so  highly  valued  in  olden  times,  and  is  the  sole  species  belonging  to  this 
family  of  plants  indigenous  in  the  south  of  Europe.  A  drawing  of  it  is  given  on 
the  right-hand  side  of  fig.  42. 

Whilst  other  Balanophorese  are  parasitic  on  the  roots  of  trees  and  lianes  in  the 
shade  of  lofty  woods,  this  Cynomorium  thrives  most  luxuriantly  upon  plants  near 
the  sea -coast,  on  the  roots  of  Pistacias  and  Myrtles,  and  even  on  actual  salt-loving 
maritime  plants,  the  various  Tamarisks,  Salicorniae,  Salsolaceao,  and  Oraches,  which 
are  sprinkled  with  foam  whenever  the  breakers  are  high.  The  seed  is  like  that  of 
other  Balanophoreae  and  those  of  the  Orobanche  species,  and  germinates  in  the 
same  way  as  they  do.  From  the  group  of  cells  in  the  seed  which  represent  the 
embryo,  a  filiform  body  emerges,  and  then  grows  downwards,  its  upper  part 
remaining  for  some  time  in  connection  with  the  other  cells  in  the  seed,  which  are 
richly  furnished  with  food -materials.  The  filiform  embryo  continues  to  grow 
deeper  and  deeper  at  the  expense  of  this  nutritive  store,  and  as  soon  as  it  reaches  a 
living  root,  swells  into  an  oval  or  irregularly-lobed  tubercle,  which  unites  with 
the  wood  of  the  nutrient  root  in  the  manner  already  described.  These  tubercles 
swell,  and  from  the  summit  of  each  a  spadix  is  produced,  as  in  Lophophytum, 
which  is  raised  above  the  surface  of  the  earth.  The  spadix  is  clothed  with 
pointed  scales,  and  is  clearly  differentiated  into  a  lower  stalk-like  support,  and 
a  fleshy  inflorescence  resembling  a  cone.  The  small  scales  are  separated  from  one 
another  by  the  process  of  elongation  of  the  spadix,  and  some  fall  off.  Others 
of  them,  situated  about  the  middle  of  the  inflorescence,  persist,  however,  until 
the  time  when  the  entire  spadix  dries  up.  The  whole  of  the  structure  standing 
above  the  ground  has  a  blood-red  colour,  and  when  it  is  injured  a  red  fluid  exudes, 
which  was  at  one  time  supposed  to  be  blood.  At  an  age  when  the  peculiar  pro- 
perties of  extraordinary  plants  were  looked  upon  as  an  indication  given  by  higher 
powers  that  they  were  to  be  used  for  curative  purposes,  it  was  believed  that  the 
spactices  of  Cynomorium,  being  blood-red  in  colour,  and  bleeding  when  wounded, 
had  styptic  properties.  In  those  days  they  were  even  collected  for  the  sake  of  this 
property,  and  sold  in  apothecaries'  shops  under  the  name  of  the  Maltese  fungus 
(Fungus  melitensis).  Various  miraculous  virtues  were  also  attributed  to  this  plant, 
and  the  demand  for  it  was  so  great  that  it  became  a  regular  article  of  commerce, 
its  main  source  being  the  Island  of  Malta,  whence  is  derived  the  name  above 
referred  to. 

Of  the  Hydnorese,  which  are  most  properly  included  in  the  same  series  as 


BROOM-RAPES,    BALANOPHORE.E,    RAFFLESIACE^.  199 

Balanophoreae  in  consideration  of  their  coalescence  with  the  roots  of  their  hosts, 
only  three  species  are  known.  Two  of  them  (Hydnora  Africana  and  H.  triceps) 
belong  to  South  Africa,  the  third  (Hydnora  Americana  =  Prosopanche  Burmeisteri) 
to  South  Brazil.  The  tuber  is  represented  by  a  prismatic  body  with  from  four  to 
six  angles  furnished  with  papillae  along  the  edges.  The  flower -buds  which  burst 
from  it  have  at  first  the  form  of  spherical  Gasteromycetes,  but  gradually  elongate 
and  assume  the  form  of  a  large  fig  or  upright  club.  This  structure  opens  at  the 
thickened  upper  extremity  by  three  stout  fleshy  valves  representing  petals.  Afc 
the  base  of  this  curious  flower  no  appendage  is  to  be  seen  that  could  be  interpreted 
as  a  bract  or  leaf.  The  fleshy  mass  of  flowers  evolves  a  disagreeable  putrid  odour, 
and  in  this  property  the  Hydnorese  resemble  the  Rafflesise,  which  belong  to  the 
next  group  of  parasitic  Phanerogams. 

The  fifth  series  of  flowering  parasites  is  composed  of  the  Rafflesiacese,  plants 
connected  with  Balanophorese  and  Hydnoreae  by  their  general  aspect,  the  absence 
of  chlorophyll,  and  the  undifferentiated  embryo  which  consists  merely  of  a  group  of 
cells.  They  used  all  to  be  classed  together  under  the  name  of  Rhizanthese;  but  the 
Rafflesiacese  are  now  treated  as  a  separate  family  on  account  of  the  characteristic 
structure  of  their  flowers  and  fruit.  The  formation  of  these  organs  will  again  come 
up  for  discussion  later  on  when  we  treat  of  the  wonderful  structure  of  the  famous 
giant-flower  Rafflesia;  at  present  we  are  only  concerned  with  the  relationship  of 
the  parasite  to  the  food-providing  host-plant.  This  is,  if  possible,  even  more 
remarkable  than  in  the  case  of  Balanophorese  and  Hydnorese.  In  the  latter  the 
union  is  effected  within  a  structure  like  a  tuber  or  a  rhizome,  the  vessels  and  cells 
of  the  parasite  coalescing  with  the  exfoliated  and  disordered  wood-cells  belonging 
to  the  root  or  stem  of  the  host-plant;  whereas  in  Rafflesiaceae  the  embryo,  having 
penetrated  beneath  the  cortex  of  the  host,  produces  a  more  or  less  definite  hollow 
cylinder  which  surrounds  the  wood  of  the  host's  root  or  stem  (as  the  case  may  be), 
and  constitutes  a  sort  of  vestment  intercalated  between  the  wood  and  the  cortex  of 
the  host.  There  is  no  production  of  tuberous  enlargements  as  in  the  Balanophorese. 
The  stem  or  root  attacked  by  the  parasite  only  exhibits  a  moderate  thickening  at 
the  place  where  the  parasite  dwells  beneath  the  cortex,  and  the  cortex  itself  is  only 
destroyed  at  the  spot  where  the  embryo  pierces  through  it,  and  where  subsequently 
the  flowers  emerge.  When  roots  constitute  the  substratum  whereupon  the  parasite 
has  established  itself,  they  are  always  of  a  kind  that  run  throughout  upon  the 
surface  of  the  ground;  when  stems  are  chosen  for  attack,  they  are  either  the 
branches  of  trees  or  shrubs,  shoots  clothed  with  dead  foliage  belonging  to  dwarf 
suffruticose  bushes,  or  else  woody  lianes  of  tropical  forests.  The  seeds  are  con- 
veyed to  the  host-plants  through  the  intervention  of  animals. 

Rafflesias  are  found  in  the  haunts  of  elephants  and  along  the  tracks  followed  by 
those  beasts.  The  Rafflesia-fruits  are  accordingly  no  doubt  trampled  upon  and 
crushed,  and  the  little  seeds  imbedded  in  the  pulpy  mass  of  the  fruit  thus  have  an 
opportunity  of  adhering  to  the  elephants'  feet.  The  seeds  are  afterwards  rubbed  oft 
by  projecting  roots  at  places  more  or  less  remote  from  the  original  locality,  and  if 


200  BROOM-RAPES,   BALANOPHORE.E,   RAFFLESIACEJL 

the  root  upon  which  they  are  detained  belongs  to  a  Cissus  plant,  they  germinate. 
On  the  other  hand,  such  Rafflesiacese  as  occur  on  the  woody  branches  of  trees, 
shrubs,  and  undergrowths,  or  on  lianes,  develop  succulent  fruits,  which  are  eaten 
by  animals.  Their  seeds  are  protected  by  a  horny  coat,  and  preserve  their  power  of 
germination  unimpaired  as  they  pass  through  the  animals'  alimentary  canals  and  are 
deposited  with  the  excrements  on  the  stems  of  fresh  host-plants;  or  the  seeds  may 
stick  to  some  part  of  an  animal  that  happens  to  rub  against  them,  and  be  brushed 
off  later  on  as  being  an  uncomfortable  appendage,  and  in  this  way  also  they  may 
fall  upon  the  stem  of  a  host-plant.  Those  Rafflesiacese  which  occur  in  Venezuela  on 
the  woody  lianes  (Caulotretus),  known  by  the  name  of  "monkey -ladders",  owe  their 
dispersion  for  the  most  part  probably  to  monkeys. 

Now,  if  a  seed  has  been  deposited  in  one  way  or  another  upon  a  woody  root, 
creeping  along  the  surface  of  the  ground,  or  upon  the  stem  of  a  woody  plant,  the 
filiform  embryo  emerging  from  the  seed  finds  a  suitable  nutrient  substratum  present 
and  it  pierces  the  cortex  of  the  root,  and  develops  beneath  it  a  tissue,  which  incloses 
the  wood  like  a  sheath.  In  Rafflesia  and  in  the  Pilostyles  parasitic  on  the 
suffruticose  shrubs  of  Tragacanth  (P.  Haussknechtii,  see  fig.  43 1 ),  this  tissue  consists 
of  rows  of  cells,  which  to  the  naked  eye  look  like  threads.  Some  are  simple  and 
greatly  elongated,  others  branched,  and  they  are  united  together  to  form  a  net- work, 
so  closely  resembling  the  mycelium  of  a  fungus  as  to  be  readily  mistaken  for  one. 
The  most  complete  similarity  to  these  vegetative  bodies  living  beneath  the  cortex  of 
a  host-plant  is  exhibited  by  the  mycelia  of  the  toad-stools  which  spread  themselves 
in  the  form  of  nets  and  webs  between  the  wood  and  the  cortex  of  old  trunks  of 
trees.  The  vegetative  bodies  of  the  other  species  of  Pilostyles  consist,  in  each  case, 
of  a  tissue  composed  of  many  layers  of  cells  forming  a  parenchyma  imbedded 
between  wood  and  cortex  in  the  host-plant  and  including  some  vessels  and  rows  of 
cells  capable  of  being  interpreted  as  vascular  bundles.  Only  in  rare  instances  does 
this  tissue  of  the  parasite  form  an  unbroken  hollow  cylinder  encompassing  the 
wood  of  the  host;  usually  the  elements  of  the  host's  tissues  penetrate  into  it  and 
permeate  and  split  up  the  cylindrical  soma  (vegetative  body)  in  the  form  of  bands, 
ribs,  and  fibres.  Many  elements  of  the  tissues,  which  the  imbedded  parasite  has 
displaced  from  the  living  wood,  and  carries,  as  it  were,  on  its  back,  perish;  but 
sometimes  these  discarded  layers  remain  in  connection  with  other  living  tissues  and 
so  preserve  their  own  vitality  and  power  of  expansion,  and  develop  layers  of  wood- 
cells  covering  the  parasite.  There  is  then  a  general  confusion  and  entanglement, 
and  it  is  difficult  to  say  what  part  belongs  to  the  parasite  and  what  to  the  host. 

When  the  somatic  tissue  of  the  parasite  has  accomplished  its  connections  with 
the  host-plant  in  the  manner  just  described,  the  latter  is  unable  to  rid  itself  of 
its  occupant.  A  portion  of  the  juices  of  the  host-plant  passes  into  the  parasite's 
cells  and  the  unwelcome  guest  augments  in  volume,  and  endeavours  forthwith  to 
reproduce  and  distribute  its  kind  by  the  formation  of  fruit  and  seeds.  For  this 
purpose  buds  are  developed  at  suitable  spots  in  the  reticular  body  of  the  parasite, 
each  of  which  is  manifested  as  a  parenchyma  of  pulvinate  appearance,  and  is 


BROOM-RAPES,   BALANOPHORE^,    RAFFLESIACE^.  201 

termed  a  floral  cushion.  The  cells  in  this  cushion,  however,  now  group  them- 
selves  in  a  definite  way;  ducts  and  vessels  are  produced,  and,  at  the  same  time, 
a  differentiation  into  axis  and  flowers  is  exhibited.  These  members  continue  their 
development,  increase  in  size,  and  finally  the  enlarged  bud  breaks  through  the 
cortex  of  the  host-plant  under  shelter  of  which  it  has  been  evolved. 

In  the  genus  Cytinus  alone  do  we  find  a  stem  richly  furnished  with  leaves  and 
bearing  at  the  top  a  flattened  symmetrical  tuft  of  flowers  (see  fig.  42,  left-hand 
side)  developed  from  this  bud;  in  the  rest  of  the  Rafflesiacese,  the  bud,  which  has 


Fig.  43. — llafflesiacese  parasitic  on  trunks  and  branches. 
»  Pilostyles  Haussknechtii.  a  Apodanthes  Flacourtiana.  «  Pilostyles  Caulotret*. 

emerged  from  beneath  the  cortex  of  the  host,  is  the  flower-bud  itself.  The  axis 
supporting  the  bud  is  extremely  abbreviated  and  clothed  merely  by  a  few  scales, 
and  the  flowers  are  sessile  directly  upon  the  root  or  stem  of  the  host  (see  fig.  43). 
In  the  case  of  roots  creeping  upon  the  ground,  the  buds  always  emerge  only  on  the 
side  turned  towards  the  light;  on  lianes,  also,  they  are  only  formed  on  the  side  more 
exposed  to  light  where  subsequently  the  opened  flowers  are  easily  accessible  to 
flying  insects  (see  fig.  43 3);  on  upright  shrubs  and  under-shrubs,  on  the  other  hand, 
they  burst  forth  on  all  sides  upon  the  branches.  Branches  of  this  kind  bearing 
ubiquitously  extruded  flowers  of  a  parasite  such  as  Apodanthes  Flacourtiana  (see 
fig.  43  2)  look  delusively  like  the  Mezereon  (Daphne  Mezereum)  when  the  latter  is 
in  bloom  in  the  early  spring  before  the  development  of  foliage-leaves,  its  woody 
branches  being  similarly  studded  all  round  with  flowers,  which  stand  out  horizontally 


202  BROOM-RAPES,   BALANOPHORE^,    EAFFLESIACEA 

from  them-  but,  in  the  one  case  the  flowers  belong  to  a  foreign  parasite  living 
under  the  cortex  and  have  broken  through  it,  whereas  in  Mezereon  it  is  the 
flowers  of  the  plant  itself  that  have  unfolded.  In  the  case  of  Pilostyles 
Haussknechtii,  which  is  parasitic  on  the  low  bushy  tragacanth  shrubs  of  the 
Persian  plateaus,  the  buds  are  formed  regularly  on  both  sides  of  the  leaf-bases  of 
the  host,  so  that  at  the  insertion  of  every  one  of  the  older  foliage-leaves,  one  finds 
a  pair  of  buds,  which  subsequently  expand  into  flowers  (see  fig.  43 1). 


Fig.  44. — Parasitic  Rafflesiacea  (Brugmansia  Zipellii)  upon  a  Cissus-root. 

Throughout  the  species  of  Apodanthes  and  Pilostyles  the  flowers  are  small— 
about  the  size  of  elder,  jasmine,  or  winter-green  blossoms — and  by  no  means 
conspicuous.  But  this  is  not  the  case  in  the  genera  Brugmansia  and  Rafflesia. 
The  Brugmansias,  indigenous  to  Borneo  and  Java,  have  very  handsome  flowers, 
as  may  be  seen  in  the  above  drawing,  which  represents  on  the  natural  scale 
Brugmansia  Zipellii  parasitic  upon  the  root  of  a  Cissus.  But  in  magnitude  they 
are  far  surpassed  by  the  flowers  of  the  Rafflesiae,  one  of  which,  viz.:  Rafflesia 
Arnoldii,  may  be  described  as  actually  the  largest  flower  in  the  world.  When 
open  it  has  a  diameter  of  1  meter,  a  dimension  exceeding  even  that  of  the  gigantic 
blooms  of  South  American  aristolochias.  At  the  period  of  emergence  of  the  buds 
of  Rafflesia  Arnoldii  from  the  roots  of  the  vines  which  serve  them  as  hosts,  they 


BROOM-RAPES,    BALANOPHORE^,    RAFFLESIACE^. 


203 


are  only  as  large  as  a  walnut  and  give  scarcely  any  indication  of  their  future 
magnitude;  but  they  gradually  increase  in  size,  and  before  opening  are  curiously 
like  a  cabbage.  Up  to  this  time  the  bracts  still  inclose  the  flower  proper,  and 
to  them  is  due  the  above-mentioned  resemblance.  They  now  open  back,  and 
the  flower,  which,  to  the  last,  grows  rapidly,  unfolds  and  displays  five  immense 
lobes  around  a  central  bowl  or  cup-shaped  portion.  The  form  of  the  giant-flower 
when  open  is  best  likened  to  that  of  a  forget-me-not  blossom.  The  semicircular 
outline  of  the  lobes,  at  least,  is  similar,  and  the  very  short  throat  of  the  flower  also 
exhibits  a  distant  resemblance.  At  the  part  where  the  bowl-shaped  centre,  which 


Fig.  45. — Raffles-fa  Padma,  parasitic  on  roots  upon  the  surface  of  the  ground. 

has  the  stamens  and  styles  inserted  in  it,  passes  into  the  lobes  there  is  a  thick, 
fleshy  ring  like  a  corona.  The  upper  surface  of  the  lobes  is  covered  with  numbers 
of  papillae.  The  lobes  themselves,  the  hollow  central  bowl,  and  the  ring,  are  all 
fleshy,  and  the  flower,  as  a  whole,  emits  an  unpleasant  putrescent  smell.  This 
floral  prodigy  was  first  discovered  in  the  year  1818  in  the  interior  of  Sumatra  at 
Pulo  Lebbas  on  the  river  Manna,  where  it  occurs  parasitic  on  the  roots  of  wild 
vines  in  places  where  the  ground  is  strewn  with  the  dung  of  elephants.  It  has 
never  yet  been  seen  anywhere  outside  Sumatra.  Four  other  Raiflesise  have, 
however,  been  discovered,  but  all  in  the  islands  of  the  Indian  Ocean — Java,  Borneo, 
and  the  Philippines.  In  mode  of  growth,  as  also  in  the  form  of  the  flowers,  they 
resemble  the  species  above  described,  but  their  flowers  are  rather  smaller. 
Rafflesia  Padma,  which  occurs  in  Java,  and  is  represented  in  fig.  45,  possesses 
flowers  with  a  diameter  of  half  a  meter.  The  hollow,  somewhat  ventricose  centre 
and  the  ring  bordering  the  floral  receptacle  are  in  this  Rafflesia  of  a  dirty 


204  MISTLETOES   AND   LORANTHUSES. 

blood-red,  whilst  the  verrucose  lobes  have  almost  the  colour  of  the  human  skin. 
The  flowers  are  sessile  upon  roots  which  wind  about  upon  the  dark  forest  ground, 
and  a  cadaverous  smell,  anything  but  pleasant,  issues  from  them.  All  these 
peculiarities  explain  the  uncanny  impression  made  by  the  organisms  in  question 
upon  their  original  discoverers  and  upon  all  subsequent  observers. 

Whilst  the  Rafflesise,  as  well  as  the  genera  Brugmansia  and  Sapria,  belong 
to  the  tropical  and  sub- tropical  regions  of  Asia,  and  to  the  world  of  islands  adjacent 
thereto  on  the  south  side,  the  genus  Apodanthes  is  confined  to  tropical  America. 
Most  of  the  species  of  Pilostyles  also  appertain  to  tropical  America,  especially  to 
Brazil,  Chili,  Venezuela,  and  New  Granada.  One  species  alone — Pilostyles  ^Eikio- 

pica has  been  observed  in  the  mountains  of  Angola,  and  another,  as  has  been 

mentioned  before,  in  Persia. 

The  only  European  representative  of  the  remarkable  group  of  Rafflesiaceas  is 
Gytinus  Hypocistus,  represented  on  the  left  side  of  fig.  42,  but  its  distribution  is 
coincident  with  the  entire  range  of  the  Mediterranean  flora.  The  roots  of  cistus 
shrubs,  plants  which  are  characteristic  of  the  vegetation  belonging  to  the  basin  of 
the  Mediterranean,  constitute  the  nutrient  substratum  in  the  case  of  Gytinus.  It  is 
especially  where  the  layer  of  earth-mould  is  not  deep,  and  consequently  the  roots  of 
the  shrubs  in  question  are  exposed,  that  Gytinus  is  met  with  growing  in  abundance 
amongst  the  under- wood  of  the  cistus  plants.  The  squamous  leaves  clothing  the 
stem  of  this  parasite  being  scarlet,  and  the  plants  not  solitary  but  in  large  numbers, 
one  sees  here  and  there  a  flaming  red  colour  glowing  in  the  gaps  in  the  cistus- 
groves,  and  one  is  thus  from  far  off  made  aware  of  the  presence  of  the  parasite. 
The  flowers  themselves,  which  open  between  the  red  scale-like  bracts,  are  yellow. 
The  combination  of  colour  thus  afforded  is  a  rare  phenomenon  in  the  vegetable 
world,  and  gives  a  very  strange  appearance  to  the  plant.  Besides  the  species  of 
Gytinus  distributed  over  the  area  of  the  Mediterranean  flora,  there  are  two  other 
species  in  Mexico,  and  one  also  at  the  Cape,  which,  although  not  parasitic  on  Cistus 
shrubs  but  on  other  woody  plants,  especially  Eriocephalus,  yet  do  not  differ  from 
Gytinus  Hypocistus  in  floral  structure  or  in  mode  of  connection  with  their  host. 

MISTLETOES  AND  LORANTHUSES. 

The  sixth  and  last  series  of  parasitic  phanerogams  includes  epiphytes  of  bushy 
appearance  with  much  bifurcated  branches,  green  cortex,  green  leaves,  and  berries 
containing  large  seeds,  which  germinate  whilst  resting  immediately  upon  the 
branches  of  such  trees  as  are  adapted  to  act  as  host-plants,  and  will  surrender  to  the 
invader  a  portion  of  their  nutriment.  To  this  series  belong  a  dozen  different  species 
of  the  genus  Henslowia,  belonging  to  the  family  of  Santalacese,  and  indigenous 
to  the  South  of  Asia— chiefly  the  East  Indian  Archipelago— and,  in  addition, 
upwards  of  300  species  included  in  the  family  Loranthacese.  Amongst  the 
latter,  the  plant  that  is  best  known  and  most  widely  distributed  is  the  Euro- 
pean Mistletoe  (Viscum  album)  represented  in  fig.  46,  and  as  it  is  also  fitted,  in 


MISTLETOES   AND   LORANTHUSES.  205 

respect  of  its  life-history,  to  serve  as  type  of  the  entire  series,  we  will  describe  it 
first  of  all. 

As  is  well  known,  the  Mistletoe  is  parasitic  upon  trees,  and  these  may  be  either 
Angiosperms  or  Gymnosperms.  Most  frequently  it  establishes  itself  upon  trees  the 
branches  of  which  are  coated  by  a  soft  sappy  cortex— an  extremely  delicate  and 
tender  cork-tissue  in  particular  —  as  is  the  case  with  silver-firs,  apple-trees,  and 
poplars.  The  Mistletoe's  favourite  tree  is  certainly  the  Black  Poplar  (Populus 
nigra).  It  flourishes  with  astonishing  luxuriance  on  the  branches  of  that  tree,  and 
wherever  there  is  a  small  plantation  of  Black  Poplars,  the  Mistletoe  takes  up  its 
abode. 

Along  the  shores  of  the  Baltic  and  by  the  Danube  near  Vienna — especially 
in  the  celebrated  Prater  from  which  fig.  47  is  taken,  one  finds,  on  many  of 
the  Black  Poplars,  tufts  of  Mistletoe  measuring  4  meters  in  circumference,  and 
with  axes  of  a  thickness  of  5  cm.  Birds  use  their  most  crowded  branches,  by 
preference,  to  nest  in.  In  the  forests  of  Karst,  in  Carniola,  and  in  the  Black  Forest, 
where  poplar  trees  play  merely  a  subordinate  part,  whilst  on  the  other  hand, 
quantities  of  silver  firs  shade  the  ground,  large  numbers  of  these  conifers  have 
their  tops  covered  with  Mistletoe;  and  in  the  Rhine  districts  and  the  valley  of  the 
Inn  in  Tyrol,  the  same  parasite  occurs  as  a  troublesome  visitor  upon  apple-trees 
in  the  neighbourhood  of  the  peasants'  farms.  In  localities  destitute  of  these  three 
kinds  of  trees,  which  are  pre-eminently  the  Mistletoe's  favourite  host-plants,  it  puts 
up  with  other  trees,  and  is  then  usually  found  on  whatever  species  happens  to  be 
the  most  common  in  each  particular  country.  Thus,  in  the  Black  Pine  district  of 
the  Wiener  Wald,  it  occurs  upon  the  Corsican  Pine,  whilst  on  the  heaths  of  the 
sandy  lowlands  of  the  March,  it  settles  upon  the  Scotch  Pine.  Much  less  frequently 
it  has  been  observed  on  walnut-trees,  limes,  elms,  robinias,  willows,  ashes,  white- 
thorns, pear-trees,  medlars,  damsons,  almond-trees,  and  on  the  various  species  of 
Sorbus.  Mistletoe  has  also  been  found  by  way  of  exception  upon  the  oak  and  the 
maple,  and  upon  old  vines.  On  one  occasion,  in  the  district  of  Verona,  it  has  been 
seen  established  upon  the  parasitic  shrubs  of  Loranthus  Europceus,  that  is  to  say, 
one  member  of  the  Loranthacese  was  found  parasitic  upon  another.  The  birch,  the 
beech,  and  the  plane,  are  avoided  by  the  Mistletoe,  a  fact  which  no  doubt  depends 
upon  the  special  structure  of  the  cortex  in  those  trees. 

The  dissemination  of  the  European  Mistletoe  is  effected,  as  in  all  the  other 
Loranthacese,  through  the  agency  of  birds — thrushes  in  particular — which  feed 
upon  the  berries  and  deposit  the  undigested  seeds  with  their  excrement  upon  the 
branches  of  trees.  That  a  preliminary  passage  through  the  alimentary  canal  of 
birds  is  essential  to  the  germination  of  these  seeds  is  no  doubt  a  delusion,  this 
assumption  of  former  times  being  easily  refuted  by  the  fact  that  one  can  readily 
induce  the  seeds  of  berries,  taken  fresh  from  a  tree,  and  stuck  into  fissures  in  the 
bark  of  moderately  suitable  trees,  to  germinate;  it  is,  however,  true,  that  in  nature, 
mistletoe-seeds  are  dispersed  exclusively  by  birds  in  the  manner  above  mentioned. 
To  this  method  of  dissemination  must  be  attributed  the  phenomenon,  which,  at  first 


206 


MISTLETOES   AND   LORANTHUSES. 


sight  is  surprising,  that  Mistletoe-plants  are  rarely  seated  upon  the  upper  surface 
of  branches,  but  very  frequently  on  the  sides.  For  the  dung  of  thrushes,  which 
live  upon  Mistletoe-berries,  is  in  the  form  of  a  semi-fluid,  highly  viscid  mass,  ductile 
like  bird-lime;  and,  even  when  it  is  deposited  upon  the  upper  surface  of  slanting 
branches,  it  immediately  runs  down  the  sides,  sometimes  extending  in  ropes 
20  or  30  centimeters  in  length.  Owing  to  the  viscous  mass  thus  following  the 
law  of  gravity,  the  Mistletoe-seeds  imbedded  in  it  are  conveyed  to  the  sides,  and 
even  to  the  under  surface  of  the  bark,  and  there  remain  cemented. 


Fig.  46.— The  European  Mistletoe  (Viseum  album). 

It  may  be  a  long  time  before  a  seed  of  the  kind  germinates,  especially  if  it  does 
not  become  attached  until  the  autumn.  The  embryo  is  completely  surrounded  in 
the  seed  by  reserve  food.  It  is  club-shaped  and  comparatively  large,  and  is  dis- 
tinguished by  the  fact  that  the  two  oblong  cotyledons,  which  are  closely  pressed 
together,  but  often  somewhat  wavy  at  the  margins,  are  coloured  dark  green  by 
chlorophyll,  like  the  environing  cellular  mass  filled  with  reserve  materials.  In  the 
process  of  germination  the  axis  of  the  embryo,  especially  the  part  lying  beneath 
the  cotyledons,  and  passing  into  the  hemispherical  radicle,  lengthens  out;  the  white 
seed-coat  is  pierced,  and  the  radicle  makes  its  appearance  through  the  breach. 
Under  all  circumstances  the  emergent  radicle  is  directed  towards  the  bark  of  the 
branch  to  which  the  seed  is  adherent.  This  is  the  case  even  when  the  seed  chances 


MISTLETOES   AND   LORANTHUSES. 


207 


to  stick  with  the  radicle  of  the  seedling  pointing  away  trom  the  branch;  the 
whole  axis  of  the  embryo  curving  towards  the  surface  of  the  bark  in  a  very  striking 
manner.  Thus  the  radicle  always  reaches  the  bark,  and  having  done  so  it  becomes 
adpressed  and  cemented  to  its  surface,  spreads  itself  out  in  the  form  of  a  doughy 
mass,  and  so  develops  into  a  regular  attachment-disc.  From  its  centre  a  slender  pro- 
cess now  grows  into  the  bark  of  the  host-plant,  piercing  the  latter  and  penetrating 
as  far  as  the  wood,  but  not  growing  into  that  tissue.  This  penetrating  process  has 
been  termed  a  "sinker",  and  must  be  looked  upon  as  a  specially  modified  root. 


~ --  • 


Fig.  47.— Bushes  of  Mistletoe  upon  the  Black  Poplar  in  winter. 

The  development  of  the  first  year  ends  with  the  formation  of  this  sinker. 
When  the  winter  is  over,  the  branch,  into  which  the  sinker  is  inserted  so  as  just 
to  reach  the  wood  with  its  point,  grows  in  thickness,  a  new  layer  of  wood-cells — a 
so-called  annual  ring — being  superimposed  upon  the  wood  of  the  previous  year. 
The  increasing  mass  of  wood  first  surrounds  the  tip  of  the  sinker  with  wood-cells, 
then  forms  a  rampart  all  round  it,  pushing  the  cortical  tissue,  wherein  that  organ 
has  hitherto  been  wedged,  in  front  of  it  in  an  outward  direction,  and  in  this  way 
the  sinker  is  at  length  fixed  deep  within  the  woody  cylinder.  The  process  of 
inclosure  by  the  wood-layers,  as  they  are  built  up,  may  be  compared  to  the  gradual 
surrounding  of  a  stake  on  the  sea-shore  by  the  rising  tide;  the  lowermost  extremity 
is  first  immersed  and  then  higher  and  higher  parts  until  the  whole  is  enveloped.  The 


208  MISTLETOES  AND  LORANTHUSES. 

sinker  itself  remains,  strictly  speaking,  stationary;  it  does  not  grow  into  the  wood, 
but  the  wood  overgrows  it.  But  what  happens  in  the  following  season  when  a 
fresh  annual  ring  is  once  more  added  to  the  wood?  If  the  sinker  had  entirely 
ceased  growing  it  would  of  necessity  be  ultimately  completely  closed  by  the  layers 
of  wood,  as  they  develop  with  ever-increasing  energy  and  add  to  the  thickness  of 
the  branch,  and  at  last  it  would  be  quite  buried.  To  prevent  this  result,  which 
would  be  fatal  to  the  Mistletoe,  a  zone  of  cells  is  provided  near  the  base  of  the 
sinker,  which  zone,  at  the  time  when  the  rampart  of  wood  is  being  raised,  adds  in 
an  equal  degree  to  its  own  height,  and  causes,  of  course,  an  elongation  of  the  sinker 
in  a  peripheral  direction.  The  length  of  the  piece  thus  intercalated  in  the  haus- 
torium  is  exactly  equal  to  the  thickness  of  the  corresponding  annual  ring  in  the 
surrounding  wood  of  the  branch.  Thus  at  length  the  Mistletoe-sinker  is  found 
imbedded  in  a  number  of  annual  rings,  although  it  has  not  grown  into  the  latter, 
but  has  been  banked  up  by  them  year  by  year. 

That  zone  of  the  sinker  which  possesses  the  capacity  for  growth,  and  which  is 
always  to  be  sought,  in  accordance  with  what  has  been  said  above,  at  the  outside 
limit  of  the  wood  of  the  branch,  in  the  so-called  "  bast "  layer  situated  on  the  inner 
face  of  the  cortex,  produces,  in  the  second  year  after  the  adhesion  of  the  Mistletoe- 
embryo,  lateral  ramifications  which  are  called  cortical  roots.  They  are  thick, 
cylindrical,  or  somewhat  compressed  filaments,  and  all  run  close  together  under  the 
cortex  in  the  bast  layer  of  the  invaded  branch.  These  rootlets  issuing  from  the 
sinkers  pursue  a  course  parallel  to  the  longitudinal  axis  of  the  branch,  whilst  the 
sinkers  themselves  are  at  right  angles  to  the  axis  (see  fig.  48  3).  If  a  rootlet  springs 
from  the  sinker  in  a  direction  transverse  to  the  longitudinal  axis  it  bends  imme- 
diately afterwards  so  as  to  be  parallel  to  the  long  axis,  and  adopts  the  same 
direction  as  the  rest,  or  else  it  bifurcates  just  above  its  place  of  origin  into  two 
branches  which  separate  suddenly,  and  in  their  further  course  follow  the  axis  of 
the  branch.  Thus  it  comes  to  pass  that  all  the  rootlets  of  a  Mistletoe  run  up  and 
down  in  the  infested  branch  of  the  host-plant  in  the  form  of  thick  green  parallel 
strands,  but  that  none  of  them  ever  encircle  the  branch  in  the  form  of  an  annulaj 
coil.  Each  of  these  cortical  roots  may  now  develop  from  behind  the  growing-point 
new  sinkers,  which  are  formed  in  the  same  way  as  the  first  one  above  described  as 
proceeding  from  the  actual  seedling.  They,  too,  penetrate  into  the  branch  per- 
pendicularly to  the  axis,  and  as  far  as  the  solid  wood  are  then  encompassed  by  the 
growing  mass  of  wood,  but  maintain  the  power  of  growth  in  the  part  close  to  theii 
insertions,  and  in  their  growth  keep  pace  with  the  thickening  of  the  wood  of  the 
branch.  The  fact  of  the  yearly  recurrence  of  this  formation  of  sinkers  explains  how 
it  is  that  those  situated  nearest  the  growing-points  of  the  cortical  roots  are  the 
shortest,  they  being  the  youngest,  whilst  those  which  arise  near  the  first  sinker  are 
the  longest  and  oldest.  It  also  accounts  for  the  former  being  only  inclosed  by  one 
annual  ring  of  the  host's  wood,  and  the  others  being  surrounded  by  an  increasing 
number  of  rings  the  nearer  they  are  to  the  spot  where  the  Mistletoe-plant  first 
struck  root. 


MISTLETOES   AND   LORANTHUSES. 


209 


The  root-system  of  the  Mistletoe  taken  as  a  whole  may  be  described  as  like  a 
jaw-bone  in  shape,  or,  still  better,  a  rake.  The  cross-beam  of  the  rake  corresponds 
to  the  cortical  root,  whilst  the  teeth  are  analogous  to  the  sinkers;  the  cross-piece 
must  be  supposed  to  be  parallel  to  the  axis  of  the  branch  and  lying  under  the  bark, 
and  the  spokes  must  be  thought  of  as  perpendicular  to  the  axis  and  driven  into  the 
wood. 

Whilst  the  roots  of  the  Mistletoe-plant  are  spreading  in  the  interior  of  the  branch 
in  the  manner  described,  the  stem  is  developed  outside.  At  the  time  when  the 
process,  subsequently  to  be  the  first  sinker,  emerges  from  the  attachment-disc  of 


Fig.  48.— i  Loranthus  Europceus,  and  s  Mistletoe  (  Viscum  album}—  both  parasitic  oil  branches  of  trees,  and  seen  in  section. 
»  A  piece  of  the  wood  of  a  Fir-tree  perforated  by  the  sinkers  of  a  Mistletoe. 

the  embryo  and  pierces  through  the  bark,  the  cotyledons  are  still  covered  by  the 
white  seed-coat,  which  rests  upon  them  like  a  cap.  But  when  once  this  first  sinker 
is  firmly  fixed  and  in  a  position  to  take  up  nutritive  juices  from  the  wood  of  the 
host,  the  seed-coat  is  thrown  off;  the  apex  of  the  stem,  which  is  still  very  short,  is 
raised;  the  cotyledons  are  detached,  whilst  close  above  them  is  produced  a  pair  of 
green  leaves.  Thenceforward  the  development  of  the  visible  portion  of  the  Mistletoe- 
plant  outside  the  bark  keeps  pace  with  that  of  the  roots  underneath  the  cortex,  and 
is  moreover  dependent  upon  the  quantity  of  food  taken  up  by  the  sinkers  from  the 
wood.  Where  there  is  an  abundant  supply  of  nutriment,  as  in  the  case  of  poplars, 
the  growth  of  the  Mistletoe  is  correspondingly  exuberant;  where  the  flow  of  juices 
is  scarce,  the  parasite  is  stunted  in  its  growth,  and  often  develops  only  small 
yellowish  sickly-looking:  tufts.  If  the  foster- plant  is  of  a  lavish  nature,  adven- 

VOL.  I.  14 


210  MISTLETOES   AND   LORANTHUSES. 

titious  buds  are  produced  regularly  by  the  cortical  roots  to  which  the  absorbed 
nutriment  is  first  of  all  conveyed  from  the  sinkers.  These  buds  occur  on  the  side 
of  the  rootlets  nearest  the  exterior  of  the  bark,  and  later  they  burst  through  the 
rind,  and  develop  into  new  Mistletoe-plants. 

These  outgrowths  are  analogous  to  the  adventitious  shoots  produced  from  the 
subterranean  roots  of  the  Aspen,  and  this  comparison  is  rendered  all  the  more 
appropriate  by  the  fact  that  the  removal  of  the  tuft  of  Mistletoe  encourages  the 
sprouting  of  adventitious  root-buds  just  as  in  the  case  of  the  Aspen,  the  growth  of 
shoots  from  the  roots  is  promoted  by  the  felling  of  the  trees  to  which  those  roots 
belong.  If  a  large  Mistletoe-bush,  growing  in  solitude  on  a  Black  Poplar,  is  removed 
from  the  tree  with  the  intention  of  freeing  the  latter  from  its  parasite,  the  hopes 
entertained  by  the  operator  are  disappointed;  for,  an  outgrowth  of  shoots  from  the 
cortical  roots  ensues  at  a  number  of  different  spots,  and  in  a  few  years'  time  the 
poplar  in  question  is  the  prey  of  a  dozen  Mistletoe-bushes  instead  of  one.  Inasmuch 
as  these  bushes,  produced  from  offshoots,  are  able,  under  favourable  conditions,  to 
send  out  fresh  roots,  and  these  again  may  develop  shoots,  a  good  host  of  the  kind 
will  at  last  have  all  its  boughs  from  top  to  bottom  overgrown  by  Mistletoes.  In 
the  Prater  at  Vienna  there  are  poplars  beset  by  at  least  thirty  large  Mistletoe- 
shrubs,  arid  double  that  number  of  small  ones,  and  if  one  catches  sight  of  such  a  tree 
at  some  distance  in  winter-time  when  the  branches  have  lost  their  leaves,  one  takes 
it  to  be  a  Mistletoe-tree,  for  almost  the  entire  system  of  branches  is  mantled  in  a 
continuous  tangle  of  evergreen  bushes  of  Mistletoe,  which  are  in  a  state  of  parasitism 
upon  it. 

Sinkers  of  the  Mistletoe,  10  cm.  in  length,  and  inclosed  in  forty  annual  rings, 
have  been  found  in  the  wood  of  the  Silver  Fir,  whence  we  may  conclude  that  the 
Mistletoe  may  live  for  forty  years.  A  greater  age  could  scarcely  be  attained 
by  one  and  the  same  bush  of  the  parasite.  If  the  Mistletoe  dies,  the  rootlets  and 
haustoria  survive  for  a  time,  but  at  length  moulder  and  fall  to  pieces,  whilst  the 
wood  in  which  they  were  imbedded  remains  unaltered.  The  affected  parts  of  the 
wood  exhibit  in  that  case  numerous  perforations,  and  look  just  like  the  wood  of  a 
target  which  has  been  fired  at  and  struck  by  shot  or  small  bullets  (see  fig.  48  2). 

A  small  plant  belonging  to  the  Loranthaceae  and  named  Juniper-Mistletoe  (  Vis- 
cum  Oxycedri  or  Arceuthobium  Oxycedri)  occurs  on  the  red-berried  juniper  bushes 
(Juniperus  Oxycedrus)  of  the  Mediterranean  flora.  It  is  very  different  from 
the  common  European  Mistletoe,  as  is  obvious  at  first  sight,  its  foliage-leaves  being 
reduced  to  little  scales,  which  gives  a  characteristic  jointed  appearance  to  the  rami- 
fications. A  whole  series  of  leafless  forms  allied  to  this  species  is  found  to  exist  in 
India,  Japan,  Java,  Bourbon,  Mexico,  Brazil,  and  at  the  Cape.  They  are  nearly  all 
small  bushes  which  project  from  the  boughs  of  host-plants  and  sometimes  clothe 
the  latter  so  thickly  that  the  boughs  in  question  serving  as  nutrient  substratum  are 
entirely  enshrouded  by  the  parasitic  growth.  The  Juniper-Mistletoe  is  only  from 
3  cm.  to  5  cm.  tall,  and  the  branchlets  are  not  woody,  but  soft  and  herbaceous; 
the  fruits  are  blue  oblong  berries,  almost  destitute  of  succulence.  The  latter  are 


MISTLETOES    AND    LORANTHUSES.  211 

dispersed  by  birds  like  the  berries  of  the  common  Mistletoe,  and  the  way  in  which  the 
parasite  settles  upon  and  clings  to  branches  of  the  host-plant  is  the  same  as  in  that 
species.  It  also  develops  sinkers  and  cortical  roots,  but  these  root-structures  are 
not  by  any  means  so  regularly  arranged  as  in  Viscum  album,  but  form  an  inex- 
tricable web  of  strands  and  filaments  pervading  the  internal  layers  of  cortex,  and 
resolving  itself  into  finer  and  finer  groups  of  cells,  which  end  by  looking  not  unlike 
a  mycelium,  and  also  remind  one  of  the  suction-apparatus  possessed  by  Rafflesiaceae. 
Such  of  these  strands  and  cellular  filaments  as  are  imbedded  in  the  wood  of  the 
juniper  do  undoubtedly  play  the  part  of  suction-organs.  They  are  present  in  large 
numbers,  and  some  of  them  are  occasionally  encompassed  by  several  annual  rings. 
They  possess  no  special  zone  of  growth.  The  elongation  necessary  to  prevent  their 
being  enveloped  and  overwhelmed  by  the  wood,  as  it  adds  to  its  thickness,  is  effected 
by  the  division  of  individual  cells  and  groups  of  cells.  The  outgrowth  of  shoots 
from  the  root  is  much  more  exuberant  than  in  the  common  Mistletoe;  but  the 
death  of  the  original  plant  takes  place  much  earlier,  and  close  to  yellowish-green 
bushes  of  various  degrees  of  smallness,  one  finds  very  regularly  dead  or  dying 
shrublets  already  turned  brown,  all  growing  promiscuously  over  the  somewhat 
swollen  branches  of  the  red-berried  Jumper. 

The  behaviour  of  Loranthus  Europceus,  which  is  parasitic  on  oaks  and  chestnuts 
in  the  east  and  south  of  Europe,  is  altogether  unique.  The  mode  of  its  attack  upon 
the  branches  of  oaks  is,  it  is  true,  similar  to  that  of  the  two  other  Loranthacese  just 
described.  The  yellow  berries,  which  are  grouped  in  graceful  biseriate  racemes,  are 
eaten  with  avidity  by  thrushes  in  the  autumn  and  winter,  and  the  undigested  seeds 
are  deposited  with  the  dung  of  those  birds  upon  the  branches  of  trees.  The  embryo, 
on  emerging  from  the  seed,  bends  towards  the  bark  and  sticks  to  it,  at  the  bottom 
of  little  rifts  and  crevices,  for  the  most  part,  by  means  of  the  radicle,  which  becomes 
an  attachment-disc.  A  process  now  arises  from  the  centre  of  the  attachment-disc, 
and  pierces  through  all  the  cortical  layers  of  the  oak -branch  as  far  as  to  the  zone 
of  young  wood,  just  as  if  it  were  a  small  nail  driven  in.  This  process  increases 
in  thickness  at  the  expense  of  the  nutriment  it  withdraws  from  the  young 
wood,  and  from  it  are  developed  one,  two,  or  three  branches,  which,  however, 
invariably  run  downwards  beneath  the  bark,  that  is  to  say,  in  the  direction  opposed 
to  that  of  the  stream  of  sap  ascending  in  the  oak-wood,  and  never  produce  the 
sinkers  so  characteristic  of  the  Mistletoe.  Each  of  these  roots  is  shaped  like  a 
wedge,  even  from  the  rudimentary  stage,  and  acts,  too,  in  the  manner  of  a  wedge, 
penetrating  between  the  yet  soft  and  delicate  cells  of  the  cambium,  which  were 
formed  in  the  spring  at  the  periphery  of  the  solid  older  wood  of  the  previous  year, 
and  were  destined  to  constitute  a  new  annual  ring,  splitting  and  tearing  in  the 
process  that  cell-tissue.  Such  of  these  tender  cells  as  lie  outside  the  wedge  die,  those 
situated  within  become  lignified  and  altered  into  solid  wood,  to  which  the  wedge- 
shaped  root  firmly  adheres.  Beneath  the  apex  of  the  wedge,  the  lignification  of 
cambium  cells  naturally  extends  much  further  towards  the  exterior,  because  there 
it  is  not  at  all  broken  or  dead.  In  front  of  the  apex  of  the  wedge,  therefore,  there 


212  MISTLETOES   AND   LORANTHUSES. 

is,  presently,  solid  resisting  wood.  The  root  being  no  longer  able  to  split  the  tissue 
with  its  point,  is  stopped  in  its  growth  at  this  spot.  But  there  is  nothing  to  pre- 
vent its  continuing  to  grow  along  a  course  somewhat  nearer  the  periphery,  and 
outside  the  limit  of  the  new  annual  ring  of  solid  wood,  where  a  fresh  development 
of  soft  and  tender  cells  has  taken  place  in  the  cambium,  and  this  indeed  actually 

happens. 

Thus,  every  addition  to  the  length  of  the  Loranthus-root,  as  it  grows  onward 
between  the  wood  and  the  cortex  of  the  oak-branch,  is  further  removed  from  the 
axis  of  the  branch;  or,  in  other  words,  the  surface  of  contact  between  root  and 
wood  has  the  conformation  of  a  flight  of  stairs,  of  which  the  lowest  step  constitutes 
the  base,  and  the  uppermost  the  apex  of  the  root  (see  fig.  48 1).  These  steps  are 
very  small,  their  height  varying  from  about  5  mm.  to  7  mm.,  but  they  may  be 
distinguished  quite  clearly  in  longitudinal  sections,  on  account  of  the  darker  colour 
of  these  roots  contrasting  with  the  lighter  oak-wood.  Nutritive  fluids  are  imbibed 
by  the  Loranthus-root  from  the  wood  of  the  oak  at  the  surface  of  contact,  and  it 
is  probable  that  this  absorption  takes  place  especially  at  the  notches  forming  the 
steps.  The  root  can  only  elongate,  naturally,  during  the  period  when  there  is  a 
young  and  fragile  cell-layer  superimposed  upon  the  solid  wood,  whence  it  follows 
that  in  Loranthus  the  continuation  of  the  root's  growth  is  more  dependent  upon  a 
particular  season  and  upon  the  annual  progress  of  development  of  the  host  than  is 
the  case  with  the  Mistletoe.  There  may  be  some  connection  between  this  circum- 
stance and  the  fact  that  the  Mistletoe  possesses  evergreen  leaves,  whilst  Loranthus 
is  green  only  in  summer,  acquiring  fresh  green  foliage  in  the  spring  in  the  very 
same  week  as  the  oak  does,  and  casting  its  leaves  in  the  autumn  simultaneously 
with  the  tree  it  infests. 

The  stem  which  issues  from  the  embryo  of  a  Loranthus-seed  grows  away  from 
the  oak-branch  into  the  open  air,  and  develops  with  great  rapidity  at  the  expense 
of  the  nutriment  absorbed  from  the  host's  wood,  and  conveyed  to  it  by  the  root 
above  described,  into  a  dense,  dichotomously-branched  bush.  In  summer  it  is  not 
unlike  a  Mistletoe-bush,  but  in  autumn,  when  it  has  cast  its  leaves,  it  acquires  a, 
totally  different  aspect  owing  to  the  dark-brown  branches  and  the  conspicuous 
yellow  clusters  of  berries. 

Bushes  of  Loranthus  grow  to  a  greater  size  even  than  those  of  the  Mistletoe; 
their  stems  attain  not  infrequently  a  thickness  of  4  cm.,  and  clothe  themselves  with 
a  blackish,  rugged  bark,  the  older  stems  of  this  kind  being  then  usually  studded  by 
an  abundance  of  lichens.  At  the  spots  where  stems  of  Loranthus  spring  from  an 
oak-branch  they  are  always  surrounded  by  a  great  rampart  of  wood  belonging  to 
the  oak,  and  the  base  of  the  stem  is  often  fixed  in  a  deep  symmetrically-rounded 
bowl  reminding  one  vividly  of  the  similar  structures  out  of  which  the  stems  of 
Balanophorese  arise.  But  whereas  in  Balanophoreae  this  bowl-shaped  rampart 
appertains  to  the  parasite,  in  Loranthus  it  is  formed  from  the  wood  of  the  host- 
plant,  i.e.  the  oak.  It  must,  in  the  case  we  are  considering,  be  interpreted  as  an 
exuberant  growth  of  wood-cells  and  compared  to  the  hypertrophies  called  galls, 


GRAFTING  AND   BUDDING.  213 

which  will  be  treated  of  in  detail  in  a  subsequent  part  of  this  book.  On  old  oaks 
in  the  east  of  Europe  these  growths  round  the  bases  of  Loranthus-plants  sometimes 
reach  the  size  of  a  man's  head.  In  the  case  of  a  bush  of  Loranihus  nearly  100  years 
old,  from  the  Ernstbrunner  Wald,  in  Lower  Austria,  which  had  reached  a  height  of 
1-2  m.  and  a  circumference  of  5*5  m.,  the  hypertrophy  in  question  measured  70  cm. 
round.  It  is  not  only  the  base  of  a  bush  that  is  overgrown  by  wood-cells,  but  the 
older  portions  of  the  roots  described  above  are  frequently  walled  in  and  partially 
inclosed  by  the  wood  of  the  branch  as  it  becomes  thicker.  They  may  often  be 
seen  fixed  deep  in  the  wood,  yet  still  preserving  their  freshness  and  vitality,  and 
this  is  to  be  explained  by  the  fact  that  they  retain  connection  with  other  parts  of 
the  roots  by  means  of  isolated  ledges  and  bridges.  Indeed  an  adventitious  shoot 
may  develop  from  a  piece  of  a  root  thus  deeply  wedged  in  the  wood  of  the  oak, 
and  this  shoot  then  grows  so  outwards  and  breaks  through  all  the  layers  lying  above 
it  and  originates  a  young  bush,  which  pushes  roots  under  the  host's  bark  and 
afterwards  behaves  in  exactly  the  same  manner  as  a  plant  produced  from  a  seed 
cemented  to  the  oak-branch. 

The  Loranthus  chosen  here  for  description  (L.  Europceus)  has  only  small 
inconspicuous  yellowish  flowers;  on  the  other  hand,  under  the  tropical  sun  of 
Africa,  Asia,  and,  above  all,  Central  America,  the  parasitic  species  of  this  genus  are 
amongst  the  most  splendid-flowered  of  plants.  There  are  species  in  the  tropics — 
e.g.  Loranthus  formosus,  L.  grandiflorus,  and  L.  Mutisii — whose  flowers  attain  a 
diameter  of  10,  15,  or  even  20  centimeters,  and  are  besides  clothed  in  the  most 
gorgeous  purple  and  orange  colours.  Many  Loranthi  are  like  small  trees  grafted 
upon  other  trees.  The  host-plants  of  these  Loranthi  are  principally  angiospermous 
trees;  members  of  the  genus  have  also  repeatedly  been  met  with  parasitic  upon  one 
another — as,  for  instance,  Loranthus  buxifolius  upon  L.  tetrandrus  in  Chili.  The 
fact  has  been  already  mentioned  that  the  European  Mistletoe  has  been  observed  near 
Verona  parasitic  upon  Loranthus.  It  is  also  worth  noticing,  in  order  to  complete 
the  account  of  the  complex  relationships  between  parasites,  that  one  species  of 
Viscum  has  been  found  in  India  parasitic  upon  another,  viz.: — Viscum  moniliforme 
on  V.  orientate. 

GKAFTING  AND  BUDDING. 

Parasitism  of  one  woody  plant  upon  another,  such  as  occurs  in  the  case  of 
Loranthacese,  calls  to  mind  certain  modes  of  organic  union  between  woody  plants 
that  are  artificially  effected  by  gardeners.  From  ancient  times  gardeners  have 
performed  special  operations  which  are  known  as  processes  of  "ennobling",  and 
consist  in  the  transference  of  the  branch  or  bud  of  one  plant  on  to  another  plant  as 
substratum,  and  the  inducement  of  organic  union  between  the  two.  The  plant  from 
which  the  branch  or  bud  is  taken  is  perhaps  a  valuable  variety  of  fruit-tree,  or  a 
handsome  specimen  of  an  ornamental  shrub,  whilst  for  the  purpose  of  a  substratum 
a  robustly-growing  individual  belonging  to  a  wild  species  of  shrub  or  tree  is  selected 


214,  GRAFTING   AND   BUDDING. 

as  a  rule,  and  constitutes  the  so-called  wild  "stock".  The  branch  which  yields  the 
bud  for  the  operation  or  which  is  itself  transferred  in  its  entirety  to  the  wild  stock 
is  named,  in  the  terminology  of  horticulture,  the  noble  "scion". 

The  process  of  ennobling  is  effected  either  by  grafting  or  by  budding.  In 
grafting  the  stem  of  the  stock  is  cut  off  transversely,  an  excision  is  made  at  the 
periphery  of  the  surface  of  the  section  and  the  scion  is  inserted  in  this  opening. 
The  scion  must  be  previously  trimmed  to  fit;  in  preparing  it  care  must  be  taken 
that  it  bears  a  pair  of  healthy  buds,  and  that  the  end  to  be  inserted  is  cut  so  as  to 
correspond  to  the  form  of  the  fissure  made  in  the  stock.  In  inserting  it  one  must 
see  that,  as  far  as  possible,  the  bark,  bast,  and  wood  of  the  one  come  into  contact 
with  the  corresponding  parts  of  the  other.  The  wounds  of  the  stock  caused  by  the 
operation  are  then  covered  by  a  mass  of  putty,  wax,  or  some  other  protective 
medium,  and  the  chances  are  that  the  branch  thus  introduced  will  contract  an 
organic  union  with  the  substratum,  that  nutritive  matter  will  be  supplied  it  by  the 
substratum,  and  that  new  branches  will  sprout  from  its  buds.  In  this  case  there- 
fore the  nutriment  taken  from  the  ground  by  the  stock  passes  into  the  grafted 
scion,  and  the  scion,  which  develops  branches  from  its  buds,  and  ultimately  may 
become  a  densely  ramifying  tree-top,  behaves  as  a  parasite,  whilst  the  stock  plays 
the  part  of  host. 

It  not  infrequently  happens  that  a  substratum  supporting  at  its  summit  the 
branches  of  a  grafted  scion  develops  subsequently  branches  of  its  own  lower  down 
as  well,  and  the  curious  sight  is  then  afforded  of  a  tree  or  shrub  bearing  different 
foliage,  flowers,  and  fruit  on  its  inferior  parts  from  those  of  its  upper  regions.  If, 
for  example,  the  stem  of  a  Quince  is  used  as  substratum,  and  Medlar  branches  are 
grafted  upon  it,  the  result  may  be  a  bush  or  tree  which  exhibits  below  branches 
with  the  round  leaves,  rose-coloured  flowers,  and  golden  "  pomes "  of  the  Quince, 
and  above  branches  with  the  oblong  leaves,  white  flowers,  and  brown  fruit  of  the 
Medlar.  Gardeners,  of  course,  do  not  willingly  allow  this  to  happen,  but  carefully 
remove  the  branches  belonging  to  the  stock  in  order  that  all  the  food  materials 
may  fall  to  the  lot  of  the  grafted  plant,  and  the  latter  thrive  as  vigorously  and 
luxuriantly  as  possible. 

The  same  result  is  obtained  by  budding  as  by  grafting;  but  here  a  single  bud  of 
the  scion,  instead  of  an  entire  branch,  is  transferred  to  the  stock.  This  is  accom- 
plished in  the  following  manner: — Two  incisions  at  right  angles  forming  a  T, 
are  made  in  a  branch  of  not  too  great  age  belonging  to  the  plant  employed  as 
substratum.  These  cuts  are  carried  through  the  bark  as  far  as  the  wood.  The  two 
lobes  of  bark,  formed  by  the  T-shaped  incision,  are  then  carefully  raised  from  the 
wood,  and  the  bud  to  be  transplanted  is  pushed  in  under  them.  The  bud  which  has 
previously  been  taken  away  from  the  scion  must  have  retained  in  that  process  a 
portion  of  bark,  and  usually  the  bit  of  bark  peeled  off  is  given  the  shape  of  a  little 
shield.  This  shield,  carrying  the  bud  that  is  to  be  transferred  upon  it,  is  now 
introduced  between  the  two  lobes  above  mentioned,  and  the  lobes  are  folded  over 
it  in  such  a  manner  as  to  allow  the  bud  to  project  freely  from  the  slit  between  the 


GRAFTING  AND   BUDDING.  215 

lobes.  Besides  this,  the  whole  is  held  together  by  a  bandage,  the  shield  in  particular 
with  its  bud  being  pressed  firmly  on  to  the  new  substratum,  and  thereupon,  as  a 
rule,  coalescence  takes  place  at  once,  and  the  inserted  bud  grows  out  into  a  branch 
which  stands  in  exactly  the  same  relation  to  the  stock  as  a  Loranthus  to  the  oak 
whereon  it  is  parasitic.  All  the  branches  belonging  to  the  substratum,  that  is  to 
say,  to  the  wild  stock,  may  then  be  removed,  leaving  only  the  one  branch  that  has 
sprung  from  the  stranger-bud,  the  result  being  that  all  the  juices  absorbed  from  the 
ground  by  the  substratum  are  concentrated  in  this  branch  and  cause  it  to  grow 
with  the  greatest  exuberance. 

There  is  between  this  process  of  budding  and  the  settling  of  a  parasite  a  further 
resemblance  in  that  shrubs  and  trees  cannot  all  be  made  to  unite  at  pleasure  one 
with  the  other.  A  successful  result  of  grafting  or  budding  can  only  be  counted 
upon  when  nearly  allied  species,  belonging  to  the  same  genus  or  family,  are 
employed  for  the  purpose.  Almonds,  peaches,  apricots,  and  plums  can  be  grafted 
the  one  upon  the  other;  so  also  can  quinces,  apples,  pears,  medlars,  and  white- 
thorns. But  we  must  relegate  to  the  realms  of  fiction  such  assertions  as  that 
peaches  might  be  successfully  grafted  upon  willow  stocks,  or  that  the  Siberian  Crab 
(Pyrus  salicifolia)  has  sprung  from  the  grafting  of  branches  of  the  Pear  upon  the 
Willow  and  other  tales  of  the  sort.  Whether  it  is  possible  by  grafting  or  budding 
to  produce  new  forms,  or  at  least  hybrids,  is  a  question  which  will  claim  our 
attention  in  connection  with  the  problem  of  the  origin  of  new  species.  The  only 
additional  remark  to  be  made  here  is  that  notwithstanding  the  undeniable  simi- 
larity between  grafted  or  budded  plants  and  the  parasitic  Loranthaceae,  a  very 
essential  difference  exists  in  the  circumstance  that  the  latter  develops  roots  which 
continue  to  grow  year  by  year,  and  are  always  penetrating  into  new  layers  of  the 
host's  tissues,  whereas  this  is  never  observed  in  the  case  of  grafted  or  budded 
plants.  When  the  branch  of  a  Peach  is  grafted  on  an  Almond-tree,  there  is,  it  is 
true,  an  organic  union  of  the  two  at  the  place  of  contact,  and  the  juices  from  the 
wood  of  the  Almond  stock  are  conducted  direct  into  the  grafted  Peach-branch;  but 
neither  roots  nor  sinkers  ever  arise  from  the  base  of  the  adnate  branch  or  penetrate 
into  the  stem  of  the  Almond-tree. 


IMPORTANCE   OF   WATER  TO   THE   LIFE   OF   A   PLANT. 


5.  ABSORPTION   OF  WATER 

Importance  of  water  to  the  life  of  a  plant- Absorption  of  water  by  Lichens  and  Mosses,  and  by 
Epiphytes  furnished  with  aerial  roots— Absorption  of  rain  and  dew  by  foliage-leaves— Develop- 
ment of  absorptive  cells  in  special  cavities  and  grooves  in  the  leaves. 

IMPORTANCE  OF  WATER  TO  THE  LIFE  OF  A  PLANT. 

In  the  building  up  of  the  molecules  of  sugar,  starch,  cellulose,  fats,  and  acids,  of 
proteids,  and,  in  short,  of  all  the  important  substances  of  which  a  plant  is  composed, 
atoms  of  water  have  to  be  incorporated  as  constructive  material,  and  without  water 
no  growth  or  addition  to  the  mass  of  a  plant  whatsoever  could  take  place.  From 
this  point  of  view  water  must  be  considered  just  as  indispensable  an  item  in  the  food 
of  plants  as  the  carbon-dioxide  of  the  air.  But  water  plays,  in  addition,  another 
important  part  in  plant-life.  The  mineral  salts  which  serve  to  nourish  hydro- 
phytes, land-plants,  and  lithophytes,  as  also  the  organic  compounds  which  are  the 
food  of  saprophytes  and  parasites,  can  only  reach  the  interior  of  plants  in  the  form 
of  aqueous  solutions.  They  can  only  pass  through  a  cell- wall  when  it  is  saturated 
with  water,  and,  having  reached  the  interior  of  a  plant,  they  can  only  be  conveyed 
to  the  places  where  they  are  worked  up  through  the  medium  of  water.  In  con- 
nection with  the  discharge  of  these  functions  in  a  living  plant,  water  must  be 
regarded  as  a  dynamic  agent.  Just  as  a  mill  on  a  stream  only  works  so  long  as  its 
wheels  are  kept  in  motion  by  the  water,  and  stops  at  once  if  the  latter  fails,  or  flows 
by  in  insufficient  quantity,  so  the  living  plant,  as  it  nourishes  itself,  grows  and 
multiplies,  needs  a  continuous  and  abundant  supply  of  available  water  to  render 
possible  the  performance  of  the  complicated  vital  processes  within  it.  This  avail- 
able or  organizing  water  is  not  in  chemical  combination  like  that  which  is  present 
as  food-material,  and  is,  in  general,  not  permanently  retained.  On  the  contrary,  we 
must  conceive  it  as  perpetually  streaming  through  the  living  plant.  In  the  course 
of  a  summer,  quantities  of  water,  weighing  many  times  as  much  as  the  plant  itself, 
pass  through  it.  The  total  amount  of  water  in  chemical  combination  in  the  organic 
compounds  of  a  plant  is  very  trifling  compared  with  this,  though  it  often  happens 
that  the  weight  of  the  latter  in  a  particular  plant  is  greater  than  that  of  all  the 
other  substances  put  together. 

Inasmuch  as  this  water  evaporates  from  plants  in  dry  air,  and  that  it  may  also 
easily  be  withdrawn  by  alcohol  or  other  means,  very  simple  experiments  suffice  to 
give  an  idea  of  the  great  bulk  of  free  water  in  any  plant.  Berries,  fleshy  fungi, 
succulent  leaves,  and  things  of  that  kind,  if  left  in  alcohol,  are  reduced  in  a  short 
time  to  barely  half  their  size  in  the  fresh  state.  The  Nostocinese,  which  are  gela- 
tinous when  alive,  and  many  fungi  (e.g.  Guepinia,  Phallus,  Spathularia,  Dacryo- 
myces)  shrivel  up  so  stringently  in  drying,  that  a  piece  possessing  an  area  of 
1  square  centimeter  when  fresh  leaves  only  a  dry  crumbling  mass  covering  scarcely 
3  square  millimeters.  A  Nostoc,  which  weighed  2'224  grms.  in  the  fresh  state  only 


ABSORPTION   OF   WATER   BY   LICHENS   AND   MOSSES.  217 

weighed  0126  grm.  after  desiccation,  so  that  when  alive  it  must  have  contained 
94  per  cent,  of  water.  Bog-moss,  weighing  25'067  grms.  before  the  abstraction  of 
the  water  was  reduced  to  2'535  grms.  afterwards,  showing  that  the  percentage  of 
water  was  90.  Similar  results  are  obtained  in  the  cases  of  succulent  leaves  and 
stems  of  flowering  plants,  Cucurbita,  and  other  fruits.  The  least  proportion  of 
water  is  contained  by  mature  seeds,  solid  stony  seed -coats,  wood,  and  bark;  but  even 
in  these  an  average  proportion  of  10  per  cent  of  water  has  been  detected.  We  shall 
not  go  wrong  in  assuming,  on  the  evidence  of  the  weights  determined,  that  most  parts 
of  plants,  when  fresh,  consist  of  dry  substance  only  as  regards  a  third,  and  as 
regards  two-thirds,  of  water  of  imbibition,  which  passes  over  into  the  surrounding 
air  in  the  form  of  vapour  when  desiccation  takes  place. 

From  all  this  it  follows  that  water  is  absolutely  necessary  to  plants  as  food- 
material,  that  it  is  indispensable  as  a  medium  of  transport  of  other  substances,  and 
that  the  demand  for  water  on  the  part  of  all  plants  is  very  great.  Further,  we  may 
infer  that  the  importation  and  exportation  of  water  must  be  regulated  with  exacti- 
tude if  the  nutrition  is  not  to  be  disturbed  and  development  hindered. 

Water-absorption  is  at  its  simplest  in  hydrophytes.  In  this  case  it  coincides 
with  the  absorption  of  the  rest  of  the  food-materials,  and  there  is  therefore  nothing 
material  to  add  to  the  statements  already  made  on  that  subject. 

As  regards  land-plants,  lithophytes,  and  epiphytes,  we  may  likewise  refer  to 
what  has  been  already  said  in  so  far  as  these  plants  suck  up  water  at  the  same  time 
as  food-salts,  by  means  of  absorption-cells,  from  the  substratum  to  which  they  are 
attached,  or  the  earth  in  which  they  are  rooted;  but  to  the  extent  that  they  take 
also  water  direct  from  the  atmosphere,  and  have  the  power  of  absorbing  that  water 
immediately  they  require  it,  must  be  discussed  in  the  following  pages. 

ABSOKPTION  OF  WATER  BY  LICHENS  AND  MOSSES,  AND  BY 
EPIPHYTES  FURNISHED  WITH  AERIAL  ROOTS. 

The  plants  which  absorb  water  direct  from  the  atmosphere  may  be  classified  in 
several  groups  with  reference  to  the  contrivances  adapted  to  the  purpose.  Of  all 
plants  lichens  are  most  dependent  on  atmospheric  moisture.  Many  of  them, 
•especially  the  Old  Man's  Beard  Lichens,  which  hang  down  from  dried  branches  of 
trees,  and  the  gelatinous,  crustaceous,  and  fruticose  lichens,  which  cling  to  dead 
wood,  and  on  the  surface  of  rocks  and  blocks  of  stone,  do  in  fact  derive  their 
necessary  supply  of  water  entirely  from  the  atmosphere,  and  that  by  absorbing  it, 
not  in  a  liquid  but  in  a  gaseous  form.  The  latter  circumstance  is  of  the  greatest 
importance  to  those  species  in  particular  which  occur  on  receding  rocks,  or  on  the 
under  face  of  overhanging  slabs  of  stone.  Rain  and  dew  cannot  reach  such  places 
directly,  but  only  by  some  of  the  water  trickling  down  from  the  wet  top  and  sides 
of  the  rocks  on  to  the  receding  wall,  and  this  happens  but  seldom.  Accordingly, 
lichens  occurring  in  situations  of  the  kind  are  entirely  dependent  upon  the  water 
-contained  in  the  air  in  the  form  of  vapour.  Lichens,  however,  are  also,  of  all  plants, 


218  ABSORPTION   OF   WATER   BY   LICHENS   AND   MOSSES. 

the  best  adapted  for  the  absorption  of  aqueous  vapour  from  the  air.  If  living 
lichens,  which  have  become  dry  in  the  air,  are  left  in  a  place  saturated  with  mois- 
ture, they  take  up  35  per  cent  of  water  in  two  days,  and  as  much  as  56  per  cent 
in  six  days.  Water  in  the  liquid  form  is  naturally  absorbed  much  more  rapidly 
still.  When  Gyrophoras,  which  project  in  the  form  of  cups  after  a  long  continuance 
of  dry  weather,  are  moistened  by  a  fall  of  rain,  they  swell  up  completely  within 
ten  minutes,  and  spread  themselves  flat  upon  the  rocks,  having  in  that  short 
space  of  time  absorbed  50  per  cent  of  water.  The  saying,  "  Light  come,  light  go," 
is  no  doubt  true  in  these  cases.  When  dry  weather  sets  in,  evaporation  from  the 
masses  of  lichens  goes  on  at  a  pace  corresponding  to  the  previous  absorption.  In 
the  Tundra,  the  lichens,  which  form  a  soft  tumid  carpet  when  moistened  by  rain, 
are  liable  to  be  so  powerfully  desiccated  in  the  course  of  a  few  hours  of  sunshine, 
that  they  split  and  crackle  under  one's  feet,  so  that  every  step  is  accompanied  by  a 
crunching  noise. 

In  the  power  of  condensing  and  absorbing  the  aqueous  vapour  of  the  atmos- 
phere, lichens  are  most  analogous  to  mosses  and  liverworts,  and  to  those  pre- 
eminently which  live  on  the  bark  of  dry  branches  of  trees  or  on  surfaces  of  rock, 
covering  places  of  the  kind  with  a  carpet  which  is  often  enough  interspersed  and 
interwoven  with  lichens.  Like  the  latter  these  mosses  and  liverworts  are  able  to 
remain  as  though  dead  in  a  state  of  desiccation  for  weeks  together,  but  as  soon  as 
rain  or  dew  falls  upon  them  they  resume  their  vitality;  and  similarly  if  the  air  is  so 
damp  as  to  enable  them  to  derive  sufficient  water  of  imbibition  from  that  source. 
A  specimen  of  Hypnum  molluscum,  a  moss  which  covers  blocks  of  limestone  in  the 
form  of  soft  sods,  was  after  a  few  rainless  days  detached  from  the  dry  rock  and 
placed  in  a  chamber  saturated  with  vapour,  and  it  was  found  that  after  two  days 
it  had  absorbed  water  from  the  air  to  the  extent  of  20  per  cent,  after  six  days  38 
per  cent,  and  after  ten  days  44  per  cent.  Many  mosses  condense  and  absorb  water 
with  the  whole  surfaces  of  their  leaflets,  others — as,  for  example,  the  gray  rock- 
mosses  clinging  to  slate  formations  (Rhacomitrias  and  Grimmiae) — do  so  especially 
with  the  long  hair-like  cells  at  the  apices  of  the  leaflets,  whilst  others  again  only 
use  the  cells  situated  on  the  upper  saucer-shaped  or  canaliculate  leaf -surface. 

In  some  bearded  mosses  (Barbula  aloides,  B.  rigida,  and  B.  ambigua)  chains  of 
barrel-shaped  cells  occur  closely  packed  together  upon  the  upper  surface  of  the  leaf 
and  at  right  angles  to  it,  which  to  the  naked  eye  have  the  appearance  of  a  spongy 
dark-green  pad.  The  terminal  cells  of  these  short  moniliform  chains  have  their 
upturned  walls  strongly  thickened,  but  the  other  cells  have  very  thin  walls  and 
take  up  water  rapidly.  It  is  the  same  with  the  various  species  of  Polytrichum, 
which  are  provided  on  their  upper  leaf-surfaces  with  parallel  longitudinal  ridges 
likewise  composed  of  thin-walled,  highly-absorbent  cells.  The  rhizoids  also  play 
an  important  part  in  these  processes.  These  brown,  elongated,  thin-walled  cells 
entirely  clothe  the  moss  stems,  usually  in  the  form  of  a  dense  felt,  and  often  pro- 
ject from  the  under  surface  of  the  leaves,  whilst  in  a  few  tropical  species  they  make 
their  appearance,  strangely  enough,  in  the  form  of  little  tufts  at  the  apices  of  the 


ABSORPTION    OF   WATER   BY    LICHENS   AND   MOSSES. 


219 


leaflets.  In  many  instances  this  felt  of  rhizoids  does  not  come  into  contact  at  all 
with  the  soil,  rock,  or  bark  (as  the  case  may  be),  but  is  surrounded  by  air  alone, 
and  is  able  to  condense  or  attract,  to  use  a  common  expression,  the  aqueous  vapour 
of  the  air  like  a  piece  of  cloth  or  blotting-paper.  In  dry  weather,  it  is  true,  mosses, 
like  lichens,  lose  their  water,  but  they  part  with  it  much  more  slowly  than  the 
latter.  This  is  chiefly  due  to  the  fact  that  the  moss-leaflets  at  the  commencement 
of  a  drought  wrinkle,  curl  up,  become  concave,  and  lay  themselves  one  above  the 
other,  so  that  the  water  is  retained  at  the  bottom  for  a  longer  period. 

A  very  remarkable  contrivance  for  the  absorption  of  water  from  the  atmos- 
phere is  also  exhibited  by  the  white-leaved  Fork-mosses  (Leucobryum)  and  Bog- 
mosses  (Sphagnaceae).  Although  they  possess  chlorophyll,  and  assimilate  under  the 


Fig.  49.— Porous  Cells. 

i  Of  the  white-leaved  Fork -moss  (Leucobryum) ;  x  550.    2  Of  the  Bog-moss  (Sphagnum);  x  230.    »  Of  the  root  of  an  Orchid 

(LceUagracilis);  x310. 

influence  of  sunlight,  yet  they  look  like  parasitic  and  saprophytic  plants  destitute 
of  chlorophyll.  They  are  of  a  whitish  colour  and  always  grow  in  great  cushion- 
like  sods,  so  that  the  spots  where  they  grow  are  deficient  in  verdure,  and  stand 
out  conspicuously  from  their  surroundings  in  consequence  of  their  pale  tint. 
Microscopic  investigation  at  once  explains  this  appearance.  The  cells  containing 
chlorophyll  and  living  active  protoplasts  are  relatively  small,  and,  as  it  were, 
wedged  and  hidden  between  other  cells  many  times  as  great,  which  have  entirely 
lost  their  protoplasm  by  the  time  they  are  mature,  and  then  cause  the  paleness  of 
colour  appertaining  to  the  plant  as  a  whole.  The  walls  of  these  large  colourless  cells 
are  very  thin,  and  in  the  Bog-mosses  have  spiral  thickening-bands  running  round 
them,  being  thus  secured  against  collapse.  After  remaining  for  a  time  in  a  dry 
environment  they  are  full  of  air  only;  but  the  moment  they  are  moistened  they 
fill  with  water.  If  there  were  an  actively  absorbent  protoplast  at  work  in  the 
interior,  the  water  would  be  able  to  pass  into  the  cell-cavity  through  this  easily 
moistened  wall,  as  in  the  case  of  other  mosses,  owing  to  the  delicacy  of  the  cell- 
membrane.  But  the  air  which  fills  the  cells  is  not  absorptive,  and  in  the  case  of 
Leucobryum  and  Bog-mosses  the  water  reaches  the  interior,  not  in  consequence  of 


22Q  ABSORPTION   OF   WATER   BY   LICHENS   AND   MOSSES. 

a  chemical  affinity  on  the  part  of  the  cell-contents,  but  solely  by  capillary  action. 
All  the  cell-walls  are  perforated  and  furnished  with  pores,  and  through  these  the 
water  rushes  into  the  interior  with  lightning  rapidity. 

This  extremely  rapid  influx  of  water  into  an  air-filled  cavity  leads  us  necessarily 
to  the  conclusion  that  each  cell  has  a  number  of  pores  in  its  walls,  and  that  in 
proportion  as  water  enters  through  one  of  the  small  apertures  the  air  can  escape 
equally  fast  through  another.  This  is  in  fact  the  case.  The  large  cells  not  only 
have  pores  on  their  external  walls,  but  communicate  one  with  another  by  similar 
holes,  and  the  water  soaks  in  from  the  one  side  as  it  does  into  a  bath-sponge,  whilst 
the  air  is  at  the  same  time  forced  out  on  the  other.  This  absorptive  apparatus  is 
exceptionally  elegant  in  Leucobryum,  which  grows  abundantly  in  many  woods. 
In  it,  as  is  shown  in  the  illustration  above  (fig.  49 1),  the  adjacent  prismatic  cells 
communicate  by  highly  symmetrical,  circular  gaps  made  in  the  middle  of  the 
partition-walls,  whilst  in  the  Bog-mosses  (the  various  species  of  Sphagnum),  they 
are  to  be  seen  scattered  here  and  there  between  the  thickening  bands  on  the  cell- 
walls  (see  fig.  49 2).  Now  these  porous  groups  of  cells  possess  not  only  the  power 
of  taking  up  water  in  the  liquid  state,  but  also  that  of  condensing  it  when  in  the 
form  of  vapour.  There  is  no  need  of  any  more  proximate  proof  of  the  fact  that 
the  cells  previously  mentioned  as  containing  chlorophyll,  and  lying  imbedded 
between  the  large  perforated  cells,  take  up  water  supplied  by  the  latter,  or 
perhaps  it  is  better  to  say  that  the  large  perforated  cells  suck  in  the  water  for 
the  living  green  cells.  We  have  only  to  ask  why  it  is,  then,  that  these  small  green 
cells  do  not  absorb  water  themselves  direct  from  the  environment,  as  is  done  in 
the  case  of  so  many  other  mosses  and  liverworts.  It  is  difficult  to  answer  this 
quite  satisfactorily,  but  thus  much  seems  certain,  that  the  large  porous  cells,  when 
full  of  air,  afford  a  means  of  protecting  the  small  living  cells  from  too  excessive 
desiccation,  and  that  they  are  in  addition  preservative  of  the  chlorophyll  in  the 
small  cells,  a  matter  to  which  we  shall  return  presently. 

A  certain  resemblance  to  these  Leucobryums  and  Sphagnums,  in  respect  of  water- 
absorption,  is  exhibited  by  a  few  Aroidese,  and  more  especially  by  a  whole  host  of 
Orchidacese.  Of  the  8000  different  orchids  hitherto  discovered,  a  good  proportion, 
it  is  true,  are  rooted  in  the  earth.  But  more  than  half  these  wonderful  plants 
flourish  only  on  the  bark  of  old  trees,  and  most  of  them  would  quickly  perish  if 
they  were  detached  from  that  substratum  and  planted  with  their  roots  buried  in 
earth.  A  double  function  appertains  to  the  roots  of  these  Orchideae  which  inhabit 
trees.  On  the  one  hand  they  have  to  fix  the  entire  orchid-plant  to  the  bark,  and, 
on  the  other,  to  supply  it  with  nutriment.  When  the  growing  tip  of  an  orchid's 
root  comes  into  contact  with  a  solid  body,  it  adheres  closely  to  it,  flattens  out  more 
or  less,  sometimes  even  becoming  strap-shaped  (see  fig.  15),  and  develops  papilli- 
form  or  tubular  cells,  which  grow  into  organic  union  with  the  substratum,  and 
might  conveniently  be  termed  clamp-cells.  In  many  cases  these  cells  creep  over 
the  bark,  divide,  interlace,  and  form  regular  wefts.  The  organic  connection  with 
the  substratum  is  so  intimate  that  an  attempt  to  separate  the  two  usually  results 


ABSORPTION   OF   WATER   BY   EPIPHYTES. 


221 


in  a  detachment  of  the  most  superficial  parts  of  the  bark,  but  not  of  the  tubular 
cells.  Now,  if  a  root,  after  having  sent  out  cells  of  this  kind  which  contract  an 
organic  union  with  the  substratum,  reaches  into  the  open,  beyond  the  limit  of  the 


Fig.  50.— Aerial  Roots  of  an  Orchid  epiphytic  upon  the  bark  of  the  branch  of  a  tree. 


substratum,  it  immediately  ceases  to  develop  clamp-cells,  loses  its  ligulate  shape, 
and  hangs  down  from  the  tree  in  the  form  of  a  sinuous  white  filament.  A  few 
root-fibres  are  as  a  rule  sufficient  to  fix  the  plant  to  its  substratum,  the  bark  of  the 
tree,  and  the  rest  of  the  roots  put  forth  by  the  orchid  grow  from  beginning  to  end 


292  ABSORPTION   OF   WATER   BY   EPIPHYTES. 

freely  in  the  air.  They  are  not  infrequently  to  be  seen  crowded  together  in  great 
numbers  at  the  base  of  the  plant,  forming  regular  tassels  suspended  from  the  dark 
bark  of  the  branches  as  may  be  seen  in  fig.  50,  where  an  Oncidium  is  represented. 

Each  of  these  aerial  roots  is  invested  externally  by  a  white  membranous  or 
papery  envelope,  and  it  is  the  cells  of  this  covering  that  own  the  resemblance,  above 
referred  to,  to  the  cells  of  Leucobryum  and  Bog-mosses.  Their  walls  are  furnished 
with  narrow,  projecting  spiral  thickenings  and  therefore  do  not  collapse,  notwith- 
standing their  delicacy  or  the  circumstance  of  their  inclosing  at  times  an  air-filled 
cavity;  they  are  further  abundantly  perforated,  two  kinds  of  apertures  indeed 
being  found.  The  one  variety  arises  in  consequence  of  the  tearing  of  the  portions 
of  the  cell- wall  situated  between  the  rib-like  projections  and  consisting  of  extremely 
thin  and  delicate  membranes  (see  fig.  49 3);  the  existence  of  the  other  variety  is  due 
to  the  detachment  of  the  cells  which  protrude  in  the  form  of  papillae,  the  result 
being,  in  this  latter  case,  the  formation  of  circular  holes  very  similar  to  those 
already  described  as  occurring  in  Leucobryum.  The  cells  resembling  papillae  have 
the  peculiarity  that  they  roll  off  when  they  get  old  in  the  form  of  spiral  bands. 
The  holes,  of  course,  can  only  occur  on  the  external  walls  of  the  outermost  cells 
which  border  upon  the  open  air,  whilst  in  the  interior  the  communication  between 
the  cells  themselves  is  established  by  means  of  the  rents  previously  referred  to. 
The  entire  covering  thus  composed  of  perforated  cells  may  be  compared  to  an 
ordinary  sponge,  and,  indeed,  acts  after  the  manner  of  a  sponge.  When  it  comes 
into  contact  with  water  in  the  liquid  state,  or  more  especially  when  it  is  moistened 
by  atmospheric  deposits,  it  imbibes  instantaneously  its  fill  of  water.  The  deeper- 
lying  living  green  cells  of  the  root  are  then  surrounded  by  a  fluid  envelope  and  are 
able  to  obtain  from  it  as  much  water  as  they  require. 

But  these  roots  also  possess  the  power  of  condensing  the  aqueous  vapour 
contained  in  the  air.  They  act  upon  the  moist  air  in  which  they  are  immersed  in 
exactly  the  same  way  as  spongy  platinum  or  any  other  porous  body.  If  the  aerial 
roots  of  Oncidium  sphacelatum  are  transferred  from  a  chamber  full  of  dry  air  to 
one  full  of  moist  air,  they  take  up  in  24  hours  somewhat  more  than  8  per  cent  of 
their  weight  of  water,  those  of  Epidendron  elongatum  absorb  11  per  cent,  whilst  in 
the  case  of  many  other  tropical  orchids  the  amount  thus  imbibed  is  doubtless  much 
more  considerable  still. 

The  power  of  condensing  aqueous  vapour,  and  other  gases  as  well,  is  of  the 
greatest  importance  to  these  plants.  The  tree-bark  serving  as  their  substratum,  to 
which  they  are  fastened  merely  by  a  few  fibres,  is  anything  but  a  permanent 
source  of  water.  Such  water  as  the  bark  does  contain  reaches  it,  not  from  the 
interior  of  the  trunk  and  indirectly  from  the  soil  in  which  the  trunk  has  its  roots, 
but  from  the  atmosphere;  that  is  to  say,  from  the  very  source  whence  the 
epiphytes  upon  the  bark  must  also  derive  their  supply.  Now,  when  on  the  occa- 
sion of  a  long-enduring  uniform  aerial  temperature,  there  is  a  failure  of  atmos- 
pheric deposits,  which  is  a  regularly  recurring  circumstance  in  the  habitat  of  the 
orchids  in  question,  the  sole  source  of  water  left  is  the  vapour  in  the  air,  and  the 


ABSORPTION   OF   WATER   BY    EPIPHYTES.  223 

only  possible  method  of  acquiring  that  vapour  is  the  condensation  of  it  by  the 
porous  tissue  investing  the  roots.  In  the  event  of  the  air  around  the  orchid-plant 
containing  temporarily  but  very  little  moisture,  the  porous  tissue  dries  up,  it  is  true, 
very  quickly;  its  cells  fill  with  air  and  their  function  as  condensers  is  interrupted. 
But  these  air-filled  cellular  layers  then  form  a  medium  of  protection  against 
excessive  evaporation  from  the  deeper  strata  of  the  root's  tissues,  which  might  be 
very  dangerous  in  the  case  of  this  kind  of  epiphyte.  There  is  a  wide-spread 
impression  that  the  tropical  orchids  grow  in  a  perpetually  moist  atmosphere  in  the 
dark  shade  of  primeval  forests,  and  this  preconception  is  fostered  by  pictures  of 
tropical  orchids  representing  these  plants  as  living  in  the  most  obscure  depths  of 
woods.  In  reality,  however,  the  orchids  of  the  tropics  are  children  of  light.  They 
thrive  best  in  sunny  spots  in  open  country.  Those  species  in  particular  which  have 
their  aerial  roots  invested  each  by  a  thick,  white,  papery,  porous  covering  belong  to 
regions  where  a  long  period  of  drought  occurs  regularly  every  year,  and  where,  in 
consequence,  vegetative  activity  is  subject  to  periodical  interruption,  as  it  is  in  the 
cold  winter  season  of  the  more  inclement  zones. 

For  epiphytes  inhabiting  these  regions  of  the  tropics  a  more  expedient  structure 
of  root  cannot  easily  be  imagined.  In  the  dry  season  the  papery  covering  reinforces 
the  safeguards  against  too  profuse  transpiration  on  the  part  of  the  living  cells  in  the 
interior  of  the  root,  and  in  the  wet  season  it  provides  for  the  continuous  supply  of 
the  requisite  quantity  of  water.  In  this  sense  the  porous  layer  is  to  a  certain 
extent  a  substitute  for  wet  soil,  or,  in  other  words,  the  concealment  of  the  living 
part  of  an  aerial  root  in  the  saturated  envelope  is  analogous  to  that  of  the  root- 
fibres  of  land-plants  in  the  damp  earth.  The  manner  in  which  the  water  reaches 
the  inner  cells  of  an  aerial  root  from  the  saturated  envelope  is  also  quite 
characteristic.  Under  the  porous  tissue  lies  a  layer  composed  of  two  kinds  of  cells 
of  different  sizes.  The  larger  cells  are  elongated  and  have  their  external  walls, 
which  are  adjacent  to  the  porous  tissue,  thickened  and  hardly  permeable  by  water. 
Between  these  lie  smaller,  thin- walled,  succulent  cells,  which  admit  the  water  from 
the  porous  envelope,  and  should  therefore  be  regarded  as  absorption-cells.  It  is  also 
noteworthy  that  the  porous,  paper-like  covering  is  discarded  as  soon  as  an  aerial 
root  is  placed  in  earth.  Most  orchids  with  aerial  roots  perish,  it  is  true,  when  they 
are  treated  like  land-plants  and  planted  in  soil;  but  a  few  species,  on  occasion,  bury 
their  aerial  roots  spontaneously  in  the  earth  and  push  off  their  envelopes,  and  then 
the  imbedded  parts  exercise  the  same  functions  as  in  the  case  of  land- plants. 

We  have  already  mentioned  that,  in  addition  to  thousands  of  orchids,  several 
Aroidese  exhibit  the  porous,  papery  covering  on  their  aerial  roots.  But  still  more 
frequently  the  air-roots  of  Aroids,  which  live  as  epiphytes  upon  trees,  are  furnished 
with  a  dense  fringe  of  so-called  root-hairs  in  a  broad  zone  behind  the  growing- 
point.  The  hairs  project  on  all  sides  from  the  roots,  which  are  surrounded  by  air; 
they  are  crowded  very  closely  together  and  give  the  parts  affected  a  velvety 
appearance.  Besides  several  Aroidese,  one  of  which  (Philodendron  Lindeni)  is 
drawn  on  the  left  side  of  fig.  51,  many  other  epiphytes,  such  as  the  South 


224 


ABSORPTION   OF   WATER   BY   EPIPHYTES. 


Fig.  51.— Aerial  Roots  with  root-hairs ;  on  the  left  Philodendron  Lindeni,  on  the  right  Campelia  Zanonia. 

coating  on  their  aerial  roots.  The  roots  of  the  tree-ferns  are  short,  but  spring  in 
thousands  from  the  thick  stem,  and  are  so  closely  packed  that  the  whole  surface  is 
clothed  as  it  were  by  a  woven  mantle  of  rootlets.  After  some  time  these  aerial 
roots  turn  deep  brown,  whilst  the  hairs  collapse  and  die,  and  both  are  converted 


ABSORPTION   OF   RAIN    AND   DEW   BY   THE   FOLIAGE-LEAVES.  225 

into  a  mouldering  mass.  But  as  soon  as  they  perish  other  new  air-roots,  covered 
with  golden-brown  velvet,  make  their  appearance  and  take  their  place.  These  aerial 
roots  never  reach  the  ground  or  adhere  to  any  substratum,  so  that  their  hairs 
cannot  contract  an  organic  connection  with  a  solid  body.  It  is  consequently  also 
impossible  in  this  case  for  the  root-hairs  to  draw  moisture  from  the  soil  in  the 
capacity  of  absorption-cells. 

These  root-hairs,  however,  are  scarcely  ever  in  a  position  to  take  up  even  the 
atmospheric  deposits.  The  various  species  of  Philodendron  and  the  other  epiphytes 
referred  to,  have  large  leaves  which  cover  the  air-roots  hanging  from  the  stem  like 
umbrellas,  and  every  tree-fern  also  bears  at  the  top  of  its  stem  a  tuft  of  great 
fronds,  which  prevents  falling  rain  from  wetting  the  aerial  roots.  Moreover,  the 
very  plants  whose  air-roots  exhibit  a  velvety  coating  occur  in  woods  where  the 
tops  of  the  trees  arch  over  the  ground  in  lofty  domes,  and  form  a  sheltering  roof 
against  deposits  from  the  atmosphere.  On  the  other  hand,  the  air  within  these 
forests  is  saturated  with  aqueous  vapour,  and  it  is  certain  that  the  velvety  roots 
have  the  power  of  condensing  vapour,  and  that  the  root-hairs  instantly  suck  up  the 
condensed  water  and  convey  it  to  the  deeper-lying  layers  of  cells.  The  truth  of 
this  has  been  established  by  the  results  of  repeated  experiments.  Thus,  air-roots  of 
the  tree-fern  Todea  barbata,  after  being  transferred  from  moderately  damp  air 
into  a  chamber  full  of  vapour,  condensed  and  absorbed  in  the  space  of  twenty-four 
hours  water  amounting  to  6 '4  per  cent  of  their  weight.  There  is,  therefore,  no  doubt 
that  water  may  be  acquired  in  this  way  also  by  plants,  even  though  the  instances- 
may  not  be  very  numerous.  All  plants  in  which  this  kind  of  water-absorption  has 
been  hitherto  observed  grow  in  places  where  the  air  is  very  moist  the  whole  year 
round,  and  where  there  is  also  no  risk  of  the  temperature  falling  below  freezing- 
point.  Under  other  conditions,  especially  in  places  where  the  air  is  periodically 
very  dry,  these  plants  would  not  be  able  to  survive;  for,  although  they  possess- 
organs  for  the  condensation  and  absorption  of  water,  they  have  no  means  of  protec- 
tion against  the  desiccation  of  these  organs. 

ABSOKPTION  OF  EAIN  AND  DEW  BY  THE  FOLIAGE-LEAVES. 

The  idea  that  plants  absorb  with  their  roots  such  water  as  they  require  is  so 
intimately  associated  with  our  whole  conception  of  plant-life,  that  this  process  is 
commonly  adduced  for  the  purpose  of  analogies  of  the  most  various  kinds,  and  one 
looks  upon  the  water-absorption  effected  by  aerial  roots  in  the  manner  just  described 
really  as  a  thing  to  be  expected,  notwithstanding  the  fact  that  in  this  case,  as  the 
above  account  shows,  the  phenomenon  is  not  so  simple  as  is  usually  supposed.  We 
now  turn  to  the  consideration  of  land-plants.  If  the  leaves  of  plants  cultivated  in 
pots  become  flaccid,  water  is  poured  as  quickly  as  possible  upon  the  dry  soil  with  a 
view  of  supplying  the  roots  which  ramify  in  it  with  moisture.'  Nor  does  the  result 
fail  to  be  produced.  In  a  short  time  the  foliage  becomes  fresh  and  elastic  again, 
the  roots  having  discharged  their  function.  Even  in  the  open  air,  it  is  especially 

VOL.  I.  15 


226  ABSORPTION   OF   RAIN   AND   DEW   BY   THE   FOLIAGE-LEAVES 

the  soil  in  which  the  roots  are  imbedded  that  a  gardener  waters  on  dry  days, 
although  incidentally  he  may  pour  the  water  over  the  aerial  parts  of  the  plants. 
He  sees,  however,  that  the  water  which  falls  in  the  form  of  rain  or  dew  upon  the 
foliage  and  stems  normally  runs  off  them  at  once,  or  else  collects  in  drops,  which 
trickle  down  whenever  the  plant  is  shaken  by  the  wind,  and  are  sucked  up  by  the 
thirsty  ground.  This  phenomenon  must  be  due  to  the  possession  by  the  leaves  of 
special  contrivances  to  prevent  their  being  wetted.  It  does  not  in  any  case  support 
the  idea  that  foliage  is  as  well  adapted  for  the  absorption  of  water  as  experience 
has  proved  subterranean  roots  to  be.  This  train  of  thought,  which  forces  itself 
upon  every  unbiassed  observer  of  the  processes  as  they  take  place  in  nature,  is 
certainly  warranted  in  the  majority  of  cases.  Each  absorption-cell  on  the  roots 
buried  in  the  earth  has  an  easily  permeable  membrane,  and,  as  is  well  known,  water 
passes  from  damp  earth  through  the  cell-membranes  into  the  interior  of  a  plant 
with  great  rapidity.  The  water  in  the  interior  of  the  plant  would  be  equally  easily 
withdrawn  through  these  cell-membranes  by  dry  surroundings,  but,  as  it  is,  this 
scarcely  ever  happens,  in  consequence  of  the  roots  being  situated  underground.  In 
the  case  of  aerial  parts,  especially  the  foliage-leaves,  the  circumstances  are  quite 
different.  The  leaves  have  to  yield  up  to  the  air  a  portion  at  least  of  the  water 
conducted  from  the  roots,  because,  as  will  be  more  thoroughly  explained  later  on,  it 
is  only  by  means  of  this  evaporation  that  the  entire  machinery  in  the  interior  of  the 
plant  can  be  kept  in  motion.  But  this  evaporation  must  not  go  too  far;  it  must  be 
in  proper  relation  to  the  absorption  of  water  by  the  subterranean  roots,  and  be 
regulated  to  that  end  if  the  plant  is  not  to  run  the  risk  of  drying  up  altogether  at 
times — an  occurrence  which  flowering  plants  are  unable  to  survive,  although  the 
mosses  described  in  former  pages  have  that  power.  Accordingly,  in  the  case  of  the 
foliage-leaves  of  flowering  plants,  evaporation  is  confined  to  certain  cells  and  groups 
of  cells,  and  these,  in  addition,  have  contrivances  by  means  of  which  evaporation 
<jan  be  entirely  stopped  on  occasion  of  great  drought.  It  stands  to  reason  that  all 
contrivances  which  make  it  impossible  for  water  to  pass  from  the  interior  of  the 
leaves  through  the  walls  of  the  superficial  cells  into  the  surrounding  air  also  hinder 
the  entrance  of  water  into  the  leaves  from  the  atmosphere. 

It  would  be  altogether  inconsistent  with  the  system  of  arrangement  of  the  sub- 
ject adopted  in  this  book  if  we  were  to  discuss  here  all  the  contrivances  serving  to 
regulate  the  exhalation  of  water  by  leaves,  and  we  must,  therefore,  confine  ourselves 
to  referring,  by  way  of  introduction,  quite  briefly,  to  the  following  facts,  namely, 
that  those  pores  on  the  surface  of  leaves  which  are  known  by  the  name  of  stomata, 
and  are  used  as  doors  of  egress  by  the  exhaled  water,  do  not  admit  rain  or  dew,  or  in 
general,  any  water  in  the  liquid  state;  that  the  so-called  cuticle  covering  the  exter- 
nal walls  of  the  epidermal  cells  in  leaves  is  an  additional  barrier  to  both  egress  and 
ingress  of  water;  that  when,  in  particular,  this  cuticle  is  furnished  with  a  wax-like 
coating,  water  does  not  adhere  to  the  surface  of  cells  so  protected;  and,  lastly,  that 
atmospheric  moisture  can  only  penetrate  into  the  interior  of  the  plant  at  parts  of 
the  leaves  where  the  waxen  incrustations  are  absent,  where  water  remains  adherent 


ABSORPTION   OF   RAIN    AND    DEW   BY  THE   FOLIAGE-LEAVES.  227 

to  the  leaf -surfaces,  and  they  are  distinctly  wetted.  But  even  cells  and  groups  of 
cells  of  this  kind  usually  act  but  for  a  short  time  as  absorption-cells,  and  only  when 
the  necessity  and  craving  for  water  is  very  great,  or  when  there  is  an  opportunity 
of  acquiring  nitrogenous  compounds  at  the  same  time  as  the  water;  and  here, 
again,  special  contrivances  are  always  present  which  regulate  this  kind  of  water- 
absorption,  and  render  it  impossible  whenever  it  is  not  truly  advantageous. 

At  first  one  would  suppose  that  amongst  the  cells  composing  the  epidermis  of 
foliage-leaves,  those  are  best  adapted  to  the  absorption  of  water  from  the  atmosphere 
which  take  the  form  of  hairs.  The  superficial  area  being  as  great  as  possible,  and 
the  contained  matter  relatively  little,  one  can  scarcely  in  fact  conceive  a  conforma- 
tion better  suited  to  the  purpose  of  water-absorption.  As,  moreover,  the  area  of 
contact  between  the  cells  of  the  leaf  and  of  a  hair  is  small,  there  would  afterwards 
be  but  very  little  evaporation  through  the  surface  of  the  hair  of  the  water  once 
sucked  up  by  it  and  conducted  into  the  interior  of  the  leaf.  In  a  word,  these  hairs 
on  the  surface  of  a  leaf  appear  to  be  peculiarly  adapted  to  the  taking  up  of  water,  and 
not  at  all  favourable  to  its  exhalation.  The  hypothesis  based  on  these  observations 
is  indeed  entirely  applicable  to  the  case  of  hairs  occurring  on  the  leaflets  of  mosses, 
as  has  been  already  stated.  But  it  does  not  hold  in  the  case  of  the  hair-like  struc- 
tures which  spring  from  the  leaf-surfaces  of  flowering  plants.  These  are  frequently 
not  wetted  at  all  by  water;  rain  and  dew  roll  off  them  in  drops,  and  cannot,  there- 
fore, be  absorbed  by  them.  This  is  true  even  of  many  soft  trichomes  (hair-structures) 
which  form  investments  upon  leaves,  and  which  seem  to  be  more  than  any  fitted 
for  the  absorption  of  water.  For  instance,  experiments  upon  the  woolly  leaves  of 
the  Great  Mullein  (Verbascum  Thapsus)  have  shown  that  they  neither  condense 
aqueous  vapour  nor  take  up  water  in  liquid  drops.  Small  importance  must  be 
attributed  to  the  thickness  of  the  cuticle,  for  sometimes  it  is  the  very  cells  which 
are  equipped  with  a  cuticle  of  considerable  stoutness  that  are  adapted  to  admit 
water,  under  certain  circumstances,  through  their  walls.  On  the  other  hand,  much 
depends  upon  the  presence  of  wax  in  the  cuticle  and  upon  the  contents  of  the  cells; 
that  is  to  say,  upon  whether  those  contents  in  particular  have  a  strong  or  weak 
affinity  for  water.  If  the  cells  of  the  hairs  are  full  of  air  they  are  not  adapted  to 
the  absorption  of  water. 

If  a  hair  is  septate,  i.e.  consists  of  a  simple  series  of  cells,  only  the  undermost  or 
else  only  the  uppermost  cells  of  the  series  absorb  water.  Instances  wherein  it  has 
been  observed  that  the  lowest  cells  alone  in  hairs  of  the  kind  become  absorption- 
cells  are  afforded  by  the  Alfredia,  represented  in  fig.  14,  by  Salvia  argentea,  and 
several  other  steppe-plants.  The  same  statement  is  made  concerning  the  widely- 
distributed  Stellaria  media,  the  common  Chickweed.  This  last  has  hairs  on  the 
internodes  of  the  stem,  running  down  in  ridges  from  node  to  node.  Usually  only 
one  side  of  the  stem  exhibits  a  ridge  of  hairs  of  the  kind,  and  the  ridge  always 
terminates  at  the  thickened  node,  whence  springs  a  pair  of  opposite  leaves.  The 
stalks  of  these  leaves  are  somewhat  hollowed  out  and  have  their  edges  beset  with 
hairs  like  lashes.  The  hairy  ridges  on  the  segments  of  the  stem  are  readily  wetted 


228  ABSORPTION   OF   RAIN   AND   DEW   BY   THE   FOLIAGE-LEAVES. 

by  rain  and  retain  a  considerable  quantity  of  water.  The  water  that  they  cannot 
hold  they  conduct  downwards  to  the  ciliate  axils  of  the  next  lower  pair  of  leaves, 
where  it  is  drawn  through  the  lash-like  hairs  in  due  course  and  collected  into  a 
ring  of  water  surrounding  the  node  (see  fig.  52  3).  If  this  accumulation  of  water 
becomes  so  voluminous  and  heavy  that  it  cannot  any  longer  be  retained  by  the 
fringe  of  lashes,  the  surplus  glides  on  to  the  unilateral  ridge  of  hairs  on  the  adjacent 
internode  down  to  the  pair  of  leaves  below.  Accordingly,  after  a  shower  every 
node  from  which  leaves  arise  is  seen  to  be  inclosed  in  a  water-bath,  and  the  hairy 


Fig.  52.— Hairs  and  Leaves  which  retain  Dew  and  Rain. 
i  Dwarf  Gentian  (Gentiana  acaulis).          a  Lady's  Mantle  (Alchetnilla  vulgaris).         »  Chickweed  (Stellaria  media). 

ridges  also  are  so  soaked  with  water  that  they  look  like  edgings  of  glass.  All  the 
individual  cells  in  each  of  the  hairs  are  full  of  protoplasm  and  cell-sap,  but  only  the 
lowest,  which  are  very  short,  really  act  as  absorption-cells.  When  these  cells 
become  at  all  relaxed  in  dry  air,  the  fact  is  indicated  by  the  appearance  on  the 
external  cell- wall  of  fine  striae  (see  fig.  53 l  and  53 2).  The  protoplasts  inhabiting  them 
attract  water,  and  after  being  relaxed  in  the  manner  referred  to  the  cells  regain 
their  turgidity  on  being  wetted,  whilst  the  fine  wrinkles  on  the  outer  membrane  are 
in  consequence  immediately  smoothed  out.  Although  the  upper  cells  of  the  hair 
possess  a  less  thick  cuticle,  they,  on  the  other  hand,  seem  not  to  absorb  any  water, 
but  to  serve  rather  to  conduct  it  by  their  surfaces. 

This  case  is,  as  we  have  said,  comparatively  rare,  and  the  corresponding  absorp- 


ABSORPTION   OF   RAIN   AND  DEW  BY  THE   FOLIAGE-LEAVES.  229 

tion  of  water  is  not  very  considerable.  But  it  often  happens  that  the  uppermost 
cells  of  a  septate  hair  are  developed  into  absorption-cells.  The  terminal  cell  is  then 
usually  spherical  or  ellipsoidal  and  larger  than  the  rest,  or  else  this  cell  is  divided 
into  two,  four,  or  a  greater  number  of  cells,  which  together  form  a  little  head,  whilst 
the  lower  cells  constitute  a  stalk  supporting  it  (see  fig.  53  3  and  53 4).  In  botanical 
terminology  structures  of  this  kind  are  named  capitate  or  glandular  hairs.  The 
protoplasm  in  the  cells  of  the  head  is,  for  the  most  part,  of  a  dark  colour,  and  the 


Fig.  53. — i  Hairs  from  stem  of  Stellaria  media ;  x  110.      *  Lowest  cells  of  the  same  hairs ;  x  200.      «  Capitate  hairs  of 
Centaurea  Balsamita ;  X150.    *  Capitate  hairs  of  Pelargonium  lividum ;  x  150. 

cell-membranes  are  readily  permeable  by  water,  which  is  attracted  with  great 
energy  by  the  cell-contents.  The  cell-membrane  is  often  very  thick,  it  is  true,  but 
as  soon  as  water  comes  into  contact  with  it  the  outer  layer  is  discarded,  the  inner 
layers  swell  up  and  the  water  passes  through  these  swollen  layers  into  the  interior 
of  the  cell.  This  happens,  for  instance,  in  many  pelargoniums  and  geraniums, 
wherein  the  capitate  cells  go  through  a  process  of  excoriation  on  every  occasion  of 
the  imbibition  of  water  (see  fig.  53  4).  In  other  plants  the  walls  of  the  capitate 
cells  are  everywhere  thin,  and  not  only  do  the  cell-contents  consist  of  a  viscid  gum- 
like  mass,  but  the  external  surface  of  the  wall  is  also  covered  by  a  layer  of  viscid 
excretion.  In  many  cases  the  viscid  matter  excreted  by  the  glands  spreads  over  the 
entire  surface  of  the  leaf,  so  that  the  latter  feels  sticky  and  looks  as  if  it  were 


230  ABSORPTION-CELLS   ON   LEAVES. 

coated  with  varnish.  Many  plants  which  have  their  roots  buried  in  crevices  of 
rock  and  no  small  number  of  herbaceous  steppe-plants  are  quite  thickly  covered 
with  glandular  hairs  of  the  kind.  Centaurea  Balsamita  (see  fig.  53 3),  a  plant 
occurring  on  the  elevated  steppes  of  Persia,  may  be  selected  as  an  example  of  the. 
latter  group.  The  advantage  of  the  structure  of  capitate  hairs  is  not  far  to  seek. 
In  dry  weather  the  thick  cuticle  (Pelargonium)  or  the  varnish  coating  (Centaur ea 
Balsamita),  as  the  case  may  be,  prevents  desiccation  of  the  cells  and  groups  of  cells 
in  question.  But  as  soon  as  rain  or  dew  falls,  the  cuticle  and  the  coat  of  varnish 
take  up  water,  and  it  is  by  their  instrumentality  that  water  reaches  the  interior  of 
the  cells.  Thus,  whilst  the  exhalation  of  water  is  hindered,  its  absorption  is  not. 

Other  epidermal  cells  of  foliage-leaves  besides  trichomes  are  capable  of  acting  as 
absorption-cells,  although  this  action,  for  reasons  already  given,  is  very  restricted, 
and  is  only  had  recourse  to  when  the  turgidity  of  the  cells  of  the  foliage-leaves  has 
diminished,  and  the  water  exhaled  by  those  cells  is  not  being  restored  by  the 
ordinary  apparatus  of  conduction  from  the  roots.  If  branches  are  cut  from  plants 
which  bear  no  glandular  or  other  form  of  hair  on  their  leaves  or  stems — as,  for 
instance,  the  leafy  stem  of  Thesium  alpinum — and  the  cut  ends  are  closed  with 
sealing-wax,  and  the  branches  left  to  wither,  and,  when  quite  withered,  are 
immersed  in  water,  they  freshen  up  speedily  and  the  leaves  become  tense  again,  the 
cells  having  recovered  their  turgidity.  Here,  then,  decidedly  absorption  has  taken 
place  through  the  ordinary  cuticularized  epidermal  cells.  Certainly  these  epidermal 
cells  in  Thesium  are  not  protected  against  wetting.  Wherever  the  epidermal  cells 
are  not  susceptible  of  being  wetted  owing  to  a  coating  of  wax  or  any  other 
contrivance  there  could  naturally  be  no  question  of  water  being  absorbed.  This 
very  circumstance,  however,  leads  to  the  supposition  that  an  important  part  in 
water  absorption  is  to  be  attributed  to  the  alternation  of  wettable  and  non-wettable 
parts  on  one  and  the  same  leaf.  In  the  case  of  many  foliage-leaves  one  can  see  that 
only  those  cells  of  the  epidermis  which  lie  above  the  veins  of  the  leaf  retain  the 
water  which  comes  upon  them,  that  is  to  say,  are  wetted  by  it,  whilst  the  water  rolls 
off  the  intervening  areas  of  the  lamina.  Indeed,  there  are  in  many  instances 
contrivances  obviously  designed  for  the  purpose  of  conducting  water  from  parts  of 
the  epidermis  not  liable  to  be  wetted  to  parts  that  can  be  moistened. 


DEVELOPMENT  OF  ABSOKPTION-CELLS  IN  SPECIAL  CAVITIES  AND 
GROOVES  IN  THE  LEAVES. 

The  contrivances  last  described  are  all  only  adapted  to  rather  a  casual  appropri- 
ation of  water  from  the  atmosphere.  But  besides  these  we  find  a  number  of  other 
contrivances,  which  render  it  possible  for  every  rolling  dewdrop  and  every 
passing  shower  to  be  made  of  use  to  the  utmost  extent.  These  contrivances 
consist  of  a  variety  of  depressions  and  excavations,  in  which  rain  and  dew  are 
collected  and  protected  against  rapid  evaporation.  Some  species  have  deep  hollows 
or  channels,  others  little  pits,  whilst  others  again  have  basins,  vesicular  or  bowl- 


ABSORPTION-CELLS   ON   LEAVES.  231 

shaped  structures,  to  collect  and  absorb  the  water;  and  the  construction  of  the 
protective  apparatus,  which  prevents  too  rapid  evaporation  into  the  air  of  water 
that  has  once  flowed  into  the  depressions,  is  as  various  as  the  form  of  the  depressions 
themselves.  A  short  account  of  the  most  striking  of  these  structures  will  now  be 
given. 

Such  water-collecting  grooves  as  are  closed,  so  as  to  form  ducts,  occur  principally 
in  petioles  and  in  the  rachises  of  compound  leaves.  For  instance,  in  the  Ash  the 
leaf  rachis,  from  which  the  leaflets  arise,  is  furnished  with  a  groove  on  its  upper 
surface.  Owing  to  the  fact  that  the  edges  of  this  groove,  which  are  strengthened 
by  a  so-called  collenchymatous  tissue,  are  bent  up  and  curved  over  the  groove,  a 
duct  or  conduit  pipe  is  produced,  and  this  duct  only  gapes  open  at  the  places  where 
the  leaflets  are  inserted  upon  the  rachis,  and  where,  therefore,  the  drops  of  rain  to 
which  the  leaflets  are  exposed  flow  off  into  the  groove  (see  fig.  54 l ).  The  simple 
hairs  and  peltate  groups  of  cells  developed  in  the  grooves  and  ducts  (fig.  54 2  and 
54 3)  are  not  merely  transiently  moistened,  but  inasmuch  as  the  water  is  retained 
there  for  several  days  after  a  fall  of  rain,  they  are  during  that  time  immersed  in  a 
regular  bath  of  water,  and  are  able  to  absorb  the  moisture  very  gradually. 

In  many  Gentianese — most  conspicuously  in  the  large-flowered  Dwarf  Gentian 
(Gentiana  acaulis) — the  decussate  pairs  of  radical  leaves  form  a  loose  rosette  (see 
fig.  52  T).  The  larger  dark-green  blade  of  each  leaf  is  flat  and  even,  and  only  the 
pale-coloured  base  is  fashioned  into  a  groove.  This  groove  is  made  deeper  by  the 
tissue  of  the  leaf  being  puffed  up  round  it,  and  as  all  the  leaves  of  the  rosette 
arise  close  together,  the  groove  of  each  leaf  is  covered  by  the  lamina  above  it. 
The  rain  or  dew  accumulated  from  the  blade  remains  standing  in  this  concealed 
nook  for  some  time  without  evaporating,  so  that  absorptive  apparatus  with  the 
power  of  taking  up  water  has  plenty  of  time  for  the  purpose.  In  this  case  the 
absorptive  apparatus  is  in  the  hindmost  extremity  of  the  groove,  and  consists  of 
long,  club-shaped  structures  composed  of  extremely  thin- walled  cells  (see  fig.  54  4), 
and  these  act  so  energetically  that  if  leaves  are  cut  off  and  left  to  fade,  and  if  the 
cut  surfaces  are  stopped  with  sealing-wax,  and  the  whole  then  bathed  with  rain- 
water, they  take  up  in  twenty-four  hours  about  40  per  cent  of  their  weight  of 
water.  A  similar  phenomenon  occurs  in  the  case  of  a  number  of  Bromeliacese 
which  adhere  by  a  few  roots  to  the  bark  of  trees  in  the  tropics,  and  have  grooved 
rosetted  leaves,  the  latter  covering  one  another,  and  being  arranged  in  such  a 
manner  as  to  form  a  regular  system  of  cisterns.  At  the  bottom  of  each  cistern 
there  are  special  groups  of  thin- walled  ceUs  which  suck  up  any  water  that  flows  in 
when  rain  falls. 

On  the  under  surface  of  the  leaves  of  the  Cow-berry  (Vaccinium  Vitis-Idcea) 
little  depressions  are  formed,  and  in  the  middle  of  each  depression  there  is  a  club- 
shaped  structure  composed  of  small  thin-walled  cells,  which  contain  slimy,  viscid 
substances  and  act  as  absorbent  organs.  The  rain  which  falls  upon  the  upper 
surface  of  the  leaf  gets  drawn  over  the  edges  on  to  the  under  surface,  fills  the  small 
depressions  occurring  there,  and  is  taken  up  by  the  absorptive  apparatus.  A 


232 


ABSORPTION-CELLS   ON   LEAVES. 


similar  contrivance  is  also  exhibited  by  the  leaves  of  alpine  roses  and  those  o  the 
American  Boxharis.  For  instance,  on  the  under  surface  of  the  leaves  of  the  Alpine 
Rose  (Rhododendron  hirsutum)  there  is  a  large  number  of  discoid  glands  fig.  54  ), 
each  of  which  is  supported  on  a  short  stalk  and  sunk  in  a  little  hollow  (fig.  54  ) 
The  cells  composing  the  gland  are  arranged  radially,  and  contain  slimy,  resmous 
matters  capable  of  swelling  up.  These  contents  are  also  excreted,  and  then  cover 
the  entire  glandular  disc,  and  often  even  the  whole  surface  of  the  leaf  m  t 


Fig.  54.— Absorption  of  Water  by  Foliage-leaves. 

i  Grooved  rachis  of  the  ash-leaf.  2  Section  through  the  same ;  x30.  »  Peltate  group  of  cells  from  the  groove.  «  Section 
through  the  base  of  a  leaf  of  the  Dwarf  Gentian ;  x20.  *  Under  side  of  a  leaf  of  Rhododendron  hirsutum;  x30.  6  Section 
through  a  leaf  of  Rhododendron  hirsutum. 

of  a  light-brown  crumbly  crust.  When  drops  of  rain  fall  upon  Alpine  Rose  leaves, 
the  whole  of  the  upper  surfaces,  in  each  case,  is  in  the  first  place  moistened;  but 
without  delay,  and  partly  through  the  action  of  the  hairs  fringing  the  margin,  the 
water  soaks  on  to  the  under  side  of  the  leaf.  As  soon  as  it  reaches  the  glands  it  is 
taken  up  by  the  crumbly  incrustation  mentioned  above,  which  swells  up  in  con- 
sequence. The  little  cavities  in  which  the  glands  are  situated  also  fill  with  water, 
and  each  gland  is  then  immersed,  as  it  were,  in  a  bath,  and  able  to  absorb  as  much 
moisture  as  is  required.  Owing  to  the  glands  being  invariably  developed  above 
the  vascular  bundles  of  the  leaf  (see  fig.  54 6),  the  water  that  is  absorbed  can  be 
conducted  without  delay  by  them  to  the  places  where  it  is  required.  As  soon  as 
the  leaves  of  alpine  roses  become  dry  again,  the  mass  of  resinous  mucilage  again 


ABSORPTION-CELLS   ON   LEAVES. 


233 


forms  a  dry  crust  over  the  glands  and  protects  their  tender-walled  cells  from  too 
great  evaporation. 

Very  remarkable  also  are  the  structures  adapted  to  absorption  on  the  leaves  of 
saxifrages  belonging  to  the  group  Aizoon,  and  on  those  of  a  large  proportion  of  the 
Plumbagineae.  The  saxifrages  in  question  have  little  depressions  visible  to  the 
naked  eye  upon  the  upper  surface  of  the  leaves  behind  the  apex,  and  along  the 
margins.  When  the  margin  is  dentate  or  crenate,  as,  for  instance,  in  Saxifraga 


Fig.  55. —Absorptive  Cavities  and  Cups  on  Foliage-leaves. 

i  Leaf  from  a  shoot  of  the  Aspen.  *  The  base  of  this  leaf ;  x3.  « Section  through  an  absorption-cup;  x25.  «  Leaf  of 
Acantholimon  Senganense.  «  Section  through  part  of  this  leaf;  xllO.  6Leaf  of  the  Evergreen  Saxifrage  (Saxifraga 
Aizoon).  t  Two  teeth  from  the  margin  of  this  leaf.  The  absorptive  cavity  in  the  upper  tooth  incrusted  with  lime ;  the 
lower  one  with  the  incrustation  removed.  •  Section  through  a  tooth  from  the  leaf  and  its  absorptive  cavity ;  xllO. 

Aizoon  (see  fig.  55 6),  one  of  these  cavities  occurs  in  the  middle  of  each  tooth. 
The  cells  forming  the  outer  edge  of  the  tooth  or  scallop  are  always  much 
thickened,  firm,  and  rigid;  but  the  median  portion  of  the  leaf  as  a  whole  is  fleshy, 
and  composed  of  a  bulky  large-celled  parenchyma.  The  vascular  bundle,  after 
entering  the  leaf  at  its  base,  divides  into  a  number  of  lateral  bundles  which  either 
run  towards  the  margin  without  further  ramification  (as  in  Saxiccesia),  or  else 
form  a  net-work  by  uniting  one  with  another  in  their  course  (as  in  Saxifraga 
Aizoon).  These  lateral  bundles  terminate  in  the  marginal  teeth  of  the  leaf  and 
immediately  beneath  the  little  cavities  which  occur  there,  whilst  the  extremity  of 
each  bundle  swells  into  a  knob  or  pear-shaped  enlargement  strongly  resembling 
the  roundish  groups  of  spirally-thickened  cells  in  the  tentacles  of  the  Sun-dew 


234  ABSORPTION-CELLS   ON   LEAVES. 

(cf.  fig.  26 l).  The  bottom  of  each  depression  is  made  up  of  cells  with  very  thin 
external  walls,  and  the  function  of  these  cells  is  to  suck  up  the  water  that  flows 
into  the  cavity.  It  is  obvious  that  the  absorbed  water  passes  thence  into  the 
enlarged  extremities  of  the  branches  of  the  vascular  bundles,  and  may  then  be 
conducted  to  other  parts  of  the  leaf.  Seeing  that  all  these  saxifrages  have  their 
habitat  in  crevices  of  rocks  on  sunny  declivities,  they  are  much  exposed  to 
desiccation  in  times  of  drought.  The  epidermal  cells  of  the  medial  area  and  those 
of  the  extreme  edge  are  no  doubt  protected  by  a  very  thick  cuticle  (see  fig.  55  8); 
but  in  the  case  of  the  thin- walled  cells  at  the  bottom  of  the  depression  there  is  the 
danger  of  as  much  or  even  more  water  escaping  through  them,  in  the  form  of  vapour, 
than  has  been  previously  taken  in  during  the  prevalence  of  rain. 

In  order  to  prevent  this  loss  of  moisture  recourse  is  had  to  a  very  remarkable 
contrivance  for  closing  the  cavity,  viz.,  an  incrustation  of  carbonate  of  lime.  In 
many  saxifrages  this  crust  covers  the  whole  face  of  the  leaf,  in  others  only  the 
margin,  or  the  spot  where  the  depression  occurs.  In  the  latter  case  it  looks  like  a  lid 
over  the  cavity.  At  that  spot  the  crust  is  always  thickened,  and  sometimes  it  forms 
a  regular  stopper  which  fills  up  the  entire  cavity.  It  rests  upon  the  epidermis  of 
the  leaf,  but  is  not  adnate  thereto,  and  may  be  removed  with  a  needle.  When  a 
leaf  is  bent  the  crust  is  ruptured  and  breaks  up  into  irregular  plates  and  scales, 
and  a  strong  gust  of  wind  would  then  easily  strip  off  the  fragments  and  blow  them 
away.  In  species  subject  to  this  danger,  as,  for  instance,  Saxifraga  Aizoon,  in 
which  the  resetted  leaves  curl  strongly  upwards  and  inwards  in  dry  weather,  the 
crust  of  lime  is  held  fast  by  peculiar  plugs  which  arise  from  individual  epidermal 
cells  projecting  above  the  rest  in  the  form  of  papillae  (see  fig.  55  8).  These  plugs 
are  found  principally  on  the  side  walls  of  the  cavities,  but  are  also  scattered  every- 
where on  the  epidermis  of  the  margin  of  the  leaf.  They  are  so  incrusted  with  the 
lime  that  the  latter  cannot  easily  fall  off,  and  a  comparatively  strong  pressure  must 
be  applied  with  the  needle  to  detach  it  from  the  substratum.  The  calcium  carbonate 
of  which  these  crusts  consist  is  excreted  in  solution  by  the  plant  from  pores  occur- 
ring at  the  bottom  of  the  depressions.  The  pores  are  constructed  like  ordinary 
stomata,  but  are,  as  a  rule,  somewhat  bigger,  and  it  is  not  improbable  that,  when 
once  the  lime  crust  has  formed  from  the  excreted  solution,  they  take  part  in  the 
function  of  transpiration. 

There  is  scarcely  any  need  for  further  explanation  of  the  manner  in  which  the 
apparatus  here  described  acts.  When  rain  or  dew  falls  on  a  saxifrage  leaf  the 
whole  upper  surface  is  moistened  directly,  whilst  the  water  soaks  under  the  crust 
of  lime,  and,  diffusing  itself  there,  fills  in  a  moment  the  depressions,  and  is  taken 
up  by  the  absorption-cells  situated  at  the  bottom  of  the  latter.  The  calcareous 
stopper  imbedded  in  each  cavity  is  only  upheaved  by  this  process  to  a  trifling 
extent.  In  dry  weather  the  crust  is  appressed  closely  to  the  epidermal  cells,  and 
the  stopper  descends  again  and  impedes  the  evaporation  of  water  from  the  thin- 
walled  cells  within  the  cavities. 

The  absorptive  organs  on  the  leaves  of  Acantholimon,  Goniolimon,  and  a  few 


ABSORPTION-CELLS   ON    LEAVES.  235 

other  Plumbagineae,  resemble  in  an  extraordinary  degree  those  pertaining  to  saxi- 
frages. The  depressions  are  here  found  uniformly  distributed  over  the  entire  sur- 
face of  a  leaf,  and  when  they  are  closed  by  a  crust  or  scale  composed  of  calcium 
carbonate,  the  leaves  are  dotted  with  white  spots,  as  may  be  seen  in  the  drawing 
of  a  leaf  of  Acantholimon  Senganense  given  in  fig.  55  4.  Upon  the  calcareous  scale 
being  removed,  a  little  cavity  is  revealed  beneath,  and  one  observes  that  the  floor  of 
this  cavity  is  composed  of  from  four  to  eight  cells,  separated  by  radial  partition- 
walls,  and  with  exceedingly  thin  and  delicate  outer  walls.  The  other  epidermal 
cells  adjoining  the  cavity  are,  on  the  contrary,  always  furnished  with  a  thick  cuticle 
(see  fig.  55 5).  Whenever  water  is  being  copiously  supplied  to  the  roots,  and  the 
turgidity  of  the  cells  in  the  leaves  is  great,  the  cells  forming  the  floor  of  the  cavity 
excrete  bicarbonate  of  lime  in  solution.  Part  of  the  carbonic  acid  escapes  into  the 
air,  and  the  insoluble  mono-carbonate  of  lime  in  the  water  then  forms  a  crust,  which 
fills  and  covers  the  cavity,  and  often  even  spreads  over  the  whole  leaf,  constituting  a 
coherent  calcareous  coat. 

All  Plumbagineaa  which  exhibit  this  contrivance — that  is  to  say,  the  various 
species  of  Acantholimon,  Goniolimon,  and  Statice — inhabit  steppes  and  deserts, 
where  in  summer  no  rain  falls  for  months  together,  and  the  soil  becomes  dry  to  a  con- 
siderable depth,  so  that  extremely  little  water  is  available  for  the  roots.  Although 
the  rigid  leaves  are  protected  by  a  thick  cuticle,  and  by  crusts  and  scales  of  lime 
against  excessive  evaporation  of  their  aqueous  contents,  still  it  is  difficult  to  avoid 
some  slight  loss  of  water,  especially  when  the  noon-day  sun  beats  down  upon  the 
steppe,  and,  owing  to  the  extremely  arid  nature  of  the  soil,  it  is  scarcely  possible  to 
replace  this  loss,  however  small  it  may  be,  by  absorption  from  the  earth  on  the  part 
of  the  suction-cells  on  the  roots.  All  the  more  welcome  to  plants  of  the  kind  is  the 
dew  which  sometimes  falls  copiously  on  steppes  and  in  deserts  in  the  course  of  the 
night;  it  wets  the  rigid  leaves,  and,  soaking  immediately  underneath  the  crusts  and 
scales  of  lime  to  the  thin-walled  cells  at  the  bottom  of  the  cavities,  is  absorbed  with 
avidity  by  them.  When  drought  returns  with  the  day,  the  scales  of  lime  close 
tightly  down  like  lids  on  the  epidermis  beneath,  and,  so  far  as  possible,  prevent 
evaporation.  In  particular,  they  impede  the  exhalation  of  water  from  the  thin- 
walled  cells  at  the  bottom  of  the  cavities — a  loss  which  would  otherwise  be  quite 
inevitable,  and  would  be  followed  by  a  rapid  desiccation  of  the  entire  plant.  To 
prevent  the  calcareous  lids  from  dropping  off,  there  are  either,  as  in  Saxifraga 
Aizoon,  papilliform  or  conical  projections  from  cells  in  the  immediate  vicinity  of 
the  cavities,  which  projections  often  have  hooked  ends  and  confine  the  crust  of 
lime,  or  else  each  cavity  is  somewhat  contracted  at  the  top  and  enlarged  below,  so 
that  the  lime  stopper,  being  shaped  according  to  the  contour  of  the  cavity,  cannot 
fall  out. 

A  significance  similar  to  that  attributed  to  calcium  carbonate  excretions  belongs 
also  to  the  saline  crusts  which  are  found  covering  the  leaves  of  a  few  plants  grow- 
ing on  the  arid  ground  of  steppes  and  deserts  in  the  neighbourhood  of  salt  lakes 
and  on  the  dry  tracts  of  land  near  the  seashore.  Owing  to  the  fact  that  in  these 


236  ABSORPTION-CELLS   OX    LEAVES. 

situations  crystals  of  salt  are  sometimes  to  be  seen  separated  out  from  the  soil,  and 
lying  as  a  white  efflorescence  upon  the  ground,  it  used  formerly  to  be  believed  that 
the  salt  incrusting  leaves  and  stems  was  derived,  not  from  the  plants  in  question, 
but  from  the  soil  around,  and  had  only  spread  from  there  over  the  various  plant- 
members.  But  this  is  not  the  case.  As  a  matter  of  fact,  the  salt  observed  on  the 
leaves  and  stems  of  Frankenia,  Reaumuria,  Hypericopsis  persica,  and  a  few  species 
of  Tamarix  and  Statice,  is  produced  from  the  substance  of  the  leaves.  It  is  excreted 
in  just  the  same  way  as  the  crust  of  lime,  above  described,  is  from  the  leaves  of 
saxifrages.  To  the  naked  eye  the  surfaces  of  the  leaves  in  all  the  plants  enumerated 
have  a  punctate  appearance.  On  closer  inspection,  it  is  evident  that,  corresponding 
to  each  dot,  there  is  a  little  cavity,  the  deepest  part  of  which  is  constructed  of  cells 
with  extremely  delicate  external  walls.  In  quite  young  leaves  only  a  single  thin- 
walled  cell  of  the  kind  is  to  be  seen  at  the  bottom  of  each  shallow  depression.  But 
this  divides,  and,  by  the  time  the  leaf  is  full-grown,  from  two  to  four  cells  are  seen 
to  have  arisen  by  division  of  the  one  cell.  Stomata  are,  in  addition,  intercalated  in 
the  membrane  in  the  neighbourhood  of  these  thin-walled  cells,  and,  in  the  rainy 
season,  when  there  is  no  lack  of  water  in  the  habitats  of  the  plants  in  question,  a 
watery  juice,  containing  a  large  amount  of  salts  in  solution,  exudes  from  these 
stomata.  The  saline  solution  soaks  over  the  whole  surface  of  the  leaf,  and  in  a  dry 
atmosphere  crystals  form  from  it  and  adhere  to  the  leaf  in  the  form  of  little  gland- 
like  patches  or  continuous  crusts. 

If  these  tamarisks,  frankenias,  and  reaumurias  are  observed  during  a  rainless 
season,  the  crystals  of  salt  are  seen  under  the  noon-day  sun  glittering  on  the  leaves 
and  stems,  and  may  be  detached  in  the  form  of  a  fine  crystalline  powder.  But  if 
the  same  place  is  visited  after  a  clear  night,  no  trace  of  crystals  is  to  be  seen;  the 
little  leaflets  have  a  green  appearance,  but  they  are  covered  with  a  liquid  with  a 
bitter  salt  taste,1  and  are  damp  and  greasy  to  the  touch.  The  crystals  have 
attracted  moisture  from  the  air  during  the  night,  and  have  deliquesced,  and  the 
saline  solution  not  only  covers  the  whole  of  the  leaf,  but  also  fills  the  little  cavities 
visible  as  dots  to  the  naked  eye.  The  thin-walled  cells  at  the  bottom  of  the  cavi- 
ties differ  from  the  rest  of  the  epidermal  cells  and  the  guard-cells  of  the  stomata,  in 
that  they  are  susceptible  of  being  wetted,  and  they  may  act  as  absorption-cells,  and 
allow  the  water,  attracted  by  the  salts  from  the  air,  to  pass  through  their  thin 
walls  into  the  interior  of  the  leaves. 

When  the  air  dries  under  the  rising  sun,  crystals  are  again  formed  from  the 
solution  of  salts,  and,  covering  the  leaves  once  more  in  the  form  of  crusts,  fill  up  the 
depressions  and  protect  the  plants  during  the  hot  hours  of  the  day  from  excessive 
evaporation.  Whilst,  therefore,  in  the  dewy  night  these  plants  are  indebted  to 
their  salt  crusts  for  water,  they  are  in  the  day-time  preserved  from  desiccation  by 
the  action  of  the  same  contrivance. 

1  The  salt  incrustations  which  were  removed  from  plants  of  Frankenia  hispida,  collected  on  a  Persian  salt-steppe, 
consisted  principally  of  common  salt  (chloride  of  sodium).  They  contained  in  smaller  quantities,  gypsum,  mag- 
nesium sulphate,  calcium  chloride,  and  magnesium  chloride. 


ABSORPTION-CELLS   ON    LEAVES.  237 

It  is  also  worthy  of  mention  that  papillae  are  developed  near  the  absorption- 
cells,  with  a  view  to  the  retention  of  the  salt  crystals,  similar  to  those  which  hold 
the  calcareous  incrustations  on  the  leaves  of  saxifrages  and  Acantholimon.  The 
leaves  of  plants  covered  with  crystals  of  salt  are  also  for  the  most  part  furnished 
with  little  bristles,  to  which  the  salt  adheres  so  firmly  that  it  is  not  readily  detached, 
even  by  violent  shaking. 

But  however  striking  the  analogy  may  be  between  the  development  and 
significance  of  lime  crusts  and  salt  crusts,  there  is  the  essential  difference  that  the 
former  have  not,  like  the  latter,  the  power  of  attracting  moisture  from  the  air. 
And  on  this  particular  stress  must  be  laid.  In  the  broken  and  hillocky  tracts 
on  the  shores  of  salt-lakes  or  of  the  sea,  where  tamarisks  and  frankenias  are 
especially  wont  to  live,  the  sandy  ground  dries  up  to  such  an  extent  in  the  height 
of  summer  that  it  is  scarcely  conceivable  how  plants  growing  in  it  are  able  to 
preserve  their  vitality.  The  proximity  of  the  sea  has  no  immediate  eflect  on  the 
moisture  of  the  ground  in  such  situations.  The  sea- water  does  not  penetrate  into 
the  ground  far  beyond  the  high- water  line,  and  it  is  out  of  the  question  that  the 
layers  of  soil  serving  as  substratum  to  the  frankenias  and  tamarisks  should  be 
irrigated  by  subterranean  water.  When  in  summer  there  is  an  absence  of  rain  for 
months  together,  these  plants — even  though  in  close  proximity  to  the  sea — would 
necessarily  perish  of  drought.  Only  the  circumstance  that  they  turn  to  account  the 
moisture  of  the  atmosphere  by  means  of  the  excreted  salts  renders  it  possible  for 
them  to  flourish  in  these  most  inhospitable  of  all  inhospitable  sites. 

Many  plants  which  are  periodically  exposed  to  great  dryness  have  the  tips  of  the 
teeth  on  the  leaf -margins  thickened  into  little  cones  or  warts.  They  also  glitter 
somewhat  and  at  times  are  sticky.  The  glitter  and  viscidity  are  due  to  a  resinous 
slimy  substance,  which  often  contains  sugar  and  tastes  sweet.  This  substance 
covers  the  teeth  and  sometimes  spreads  from  the  teeth  inwards  to  a  great  dis- 
tance over  the  face  of  the  leaf  in  the  form  of  a  delicate  film-like  varnish.  The 
greatest  resemblance  exists  between  this  varnish  (sometimes  known  as  "balsam") 
and  the  secretions  of  the  glands  on  the  leaves  of  the  Alpine  Rose  and  of  the 
glandular  hairs  on  those  of  Centaurea  Balsamita.  It  is  excreted  by  special  cells, 
which  are  intercalated  in  the  epidermis  of  the  foliar  teeth,  and  are  at  once  marked 
out  from  the  other  cells  of  the  epidermis  by  the  facts  that  their  protoplasm  is  of  a 
brownish  colour  and  that  their  external  walls  are  easily  permeable  by  water.  The 
excretion  of  the  varnish-like  layer  takes  place  at  a  time  when  the  entire  plant  is  dis- 
tended with  sap,  chiefly,  therefore,  in  the  spring.  When  summer  is  at  its  height 
the  varnish  dries  and  thenceforward  affords  an  excellent  preservative  from  the  risk 
of  too  much  evaporation  from  the  cells  it  covers,  and  especially  from  those  situated 
on  the  teeth  of  the  leaves  by  which  it  was  excreted.  But  if  this  dried  film  of 
varnish  is  wetted  it  saturates  itself  quickly  with  water  and  renders  moisture 
accessible  to  the  cells  beneath  it.  Thus  its  value  is  similar  to  that  of  the  crusts  of 
lime  and  salt  on  the  leaves  of  the  plants  above  described.  When  moist  it  effects 
the  absorption  of  water,  when  dry  it  guards  against  desiccation. 


238  ABSORPTION-CELLS   ON   LEAVES. 

The  reason  for  the  contrivance  just  described  being  exhibited  especially  by  the 
marginal  teeth  of  the  leaf,  lies  in  the  fact  that  dew  is  deposited  particularly  at  those 
spots.  If  one  looks  at  the  leaves  of  the  dwarf  almond  and  plum  trees  in  the 
steppe-districts,  after  clear  summer  nights,  one  finds  a  dewdrop  suspended  to  every 
tooth  on  the  margins;  but  by  noon  all  the  teeth  are  dry  again  and  protected  from 
loss  of  water  by  the  coat  of  varnish.  Moreover,  not  steppe-plants  alone,  but  very 
many  plants  which  grow  in  poor  sandy  soil  on  the  banks  of  streams  and  rivers, 
exhibit  this  contrivance  for  the  direct  absorption  of  water  from  the  atmosphere. 
Instances  are  afforded  by  the  Sweet  Willow,  the  Crack- willow,  Poplars,  the  Guelder- 
rose,  the  Bird-cherry,  and  many  others.  It  is  at  once  evident  that  this  contrivance 
is  observed  chiefly  on  the  leaves  of  trees,  shrubs,  and  tall  herbs,  whilst  incrustations 
of  lime  occur  only  on  shorter  plants  with  rosulate  leaves  spread  out  on  the  ground, 
or  with  rigid  acicular  leaf-structures.  The  grounds  of  this  distinction  may  well 
reside  in  the  fact  that  the  weight  of  a  crust  of  lime  is  many  times  as  great  as  that 
of  the  dry  film  of  varnish.  A  load  capable  of  being  borne  without  hazard  by  the 
leaves  of  a  Statice  plant,  they  being  spread  out  on  the  ground,  or  by  the  rosettes  of 
Saxifraga  Aizoon,  would  be  unfit  for  the  leaves  of  a  Cherry  or  Apricot  tree,  or  for 
those  of  the  Sweet  Willow,  or  the  Crack- willow;  indeed  the  branches  of  these  trees 
would  break  down  under  the  burden  if  their  leaves  were  incrusted  with  lime. 

In  many  cases  only  a  few  of  the  marginal  teeth  of  the  leaf  are  transformed  into 
absorbent  apparatus,  and  special  contrivances  then  always  exist  to  convey  rain  and 
dew  to  those  teeth.  The  Aspen  (Populus  tremula)  serves  as  a  very  good  example 
of  this.  This  tree  has,  as  is  generally  known,  two  kinds  of  leaves.  Those  arising 
from  the  branches  of  the  crown  have  long  petioles  and  laminae  of  roundish  outline  and 
with  somewhat  sinuate  margins;  those  which  are  borne  by  the  radical  shoots  have 
shorter  stalks  and  larger  sub- triangular  laminae  sloping  outwards;  and  the  whole 
leaf  is  so  placed  and  its  margin  so  curved  as  to  oblige  the  rain  which  strikes  the 
upper  surface  in  its  descent  to  flow  down  towards  the  petiole  (see  fig.  55 x).  Now, 
situated  exactly  on  the  boundary  of  lamina  and  petiole  are  two  cup-shaped  structures 
(fig.  55 2)  originating  from  the  lowest  teeth  of  the  leaf,  and  so  arranged  that  every 
drop  of  rain  descending  from  the  lamina  must  encounter  their  shallow  cavities  and 
fill  them  with  water.  These  cups  are  brown  in  colour  and  the  size  of  a  grain  of 
millet;  and  the  cells  of  their  epidermis  are  furnished  with  a  thick  cuticle.  Only 
the  cells  lining  the  shallow  depression  of  each  cup  have  thin  walls,  and  they  excrete 
a  sweet-tasting,  slimy,  resinous  substance  which  in  dry  weather  films  over  the 
cavity  like  a  varnish,  and  protects,  at  all  events,  the  cells  lying  beneath  it  against 
an  injurious  desiccation.  When,  however,  this  coat  is  itself  in  contact  with  water 
it  swells  up,  and  the  moisture  is  then  absorbed  by  the  cells  in  the  pit-like  depression 
and  is  transmitted  to  the  vessels  running  underneath  the  cups  (see  fig.  55  3). 

A  number  of  tall  herbs,  principally  of  the  group  of  Compositse,  have,  like  the 
Aspen,  leaf -teeth  which  are  developed  at  the  part  where  petiole  and  lamina  join  and 
act  as  organs  of  absorption.  In  some,  besides,  the  margin  of  the  green  lamina 
extends  in  the  form  of  a  narrow  ridge  down  the  pale  canaliculate  petiole;  and,  when 


ABSORPTION-CELLS   ON    LEAVES. 


239 


this  is  the  case,  teeth  of  the  kind  are  found  on  this  narrow  green  ridge  which  runs 
along  the  groove.  In  Telekia,  a  handsome  herbaceous  plant  of  wide  distribution  in 
the  south-east  of  Europe,  these  teeth—conical  or  club-shaped— springing  from  the 
margin  of  the  petiole-groove  are  incurved,  and  are  in  general  so  placed  that  their 
blunt  apices  project  into  the  groove.  But  precisely  on  these  obtuse  tips  of  the 
teeth  are  situated  cells  with  very  thin  outer  walls  easily  permeable  to  water,  and 
having  contents  with  a  strong  attraction  for  it.  Thus,  as  soon  as  the  groove  of  the 


Fig.  56.— Water-receptacles. 
1  In  a  Teasel,  Dipsacus  laciniatus.  a  in  the  American  Silphium  perfoliatum. 

petiole  is  filled  with  rain,  collected  from  the  surface  of  the  leaf,  the  tips  of  the 
conical  teeth  are  moistened,  and  they  suck  up  the  water. 

Lastly,  we  have  to  mention  the  curious  receptacles  appertaining  to  foliage- 
leaves  in  which  water  from  the  atmosphere  accumulates  and  continues  to  stand  for 
weeks  without  being  protected  from  evaporation  by  the  excretion  of  special 
substances.  Any  region  or  portion  of  the  leaf  may  participate  in  their  construction. 
In  Saxifraga  peltata  the  lamina  is  shaped  like  a  shield  and  forms  a  shallow  plate 
with  the  concave  surface  turned  to  the  sky.  In  the  Cloud-berry  (Rubus  ChaTnce- 
morus)  the  formation  of  basins  is  brought  about  by  the  margins  of  the  reniform 
lamina  being  superimposed  over  one  another  as  if  to  make  a  spathe.  In  the  various 
species  of  Winter-green,  especially  in  Pyrola  uniftora,  the  pale  cauline  leaves, 


240  ABSORPTION-CELLS   ON    LEAVES. 

inserted  above  to  the  green  leaves,  are  metamorphosed  into  little  saucers.  In  one 
species  of  Teasel,  Dipsacus  laciniatus  (see  fig.  561),  and  in  the  North  American 
Silphium  perfoliatum  (fig.  56 2)  the  two  sheathing  portions  (vagina)  of  every  pair  of 
opposite  leaves  are  connate  and  form  comparatively  large  and  deep  funnel-shaped 
basins,  from  the  middle  of  which  rises  the  next  higher  internode  of  the  stem.  In 
several  Meadow-rues  (Thalictrum  galioides  and  T.  simplex)  the  secondary  leaflets, 
which  are  opposite  one  another  and  shut  close,  almost  like  the  valves  of  a  mussel,  are 
moulded  so  as  to  form  cavities  for  the  retention  of  water,  and  in  many  Umbellif erae, 
such  as  Heradeum  and  Angelica,  the  vagina  of  each  individual  leaf  is  ventricose 
or  inflated,  thus  forming  a  sac  enveloping  the  segment  of  the  stem  which  stands 

above  it. 

These  basins,  saucers,  and  dishes  are  always  so  placed,  relatively  to  their 
surroundings,  that  the  water  derived  from  rain  and  dew  is  directed  into  them  from 
the  surfaces  of  the  leaves,  or  by  the  segment  of  the  stem  which  rises  from  their 
centres,  and  thus  it  is  that  the  depressions  are  filled.  Whether  in  all  cases  much  of 
the  water  accumulated  is  absorbed  is  certainly  open  to  doubt.  In  the  case  of  the 
leaves  of  the  Alchemilla  (fig.  52 2),  which  exhibit  the  phenomenon  so  conspicuously 
that  the  plant  has  received  the  popular  name  of  Dew-cup;  the  absorption  of  water 
is,  at  anyrate,  very  inconsiderable,  and  here  the  retention  of  the  dew  secures 
advantages  of  a  different  kind  to  which  we  shall  presently  have  occasion  to  return. 
On  the  other  hand,  it  is  established  that  in  the  case  of  basins  belonging  to  tall 
herbaceous  plants,  particularly  such  as  grow  on  steppes  and  prairies  where  often 
no  rain  falls  for  a  long  interval,  the  water  collected  is  absorbed  by  the  glandular 
hairs  and  thin-walled  epidermal  cells  developed  within  them.  The  fact  of  this 
absorption  may  be  proved  by  a  very  simple  experiment.  Let  a  stem  of  the 
Silphium,  represented  in  fig.  562,  be  cut  off  beneath  the  pair  of  connate  leaves,  which 
form  a  basin  by  their  union,  and  let  the  cut  surface  le  closed  with  sealing-wax,  so 
that  no  water  can  be  taken  up  by  the  stem  from  below.  If  the  water  accumulated 
in  the  basin  is  now  emptied  out,  the  leaves  shortly  become  flaccid  and  droop;  but  if 
the  basin  is  left  full  of  water,  the  leaves  preserve  their  freshness  a  long  while  and 
do  not  begin  to  wither  until  all  the  water  has  evaporated  and  disappeared  from  the 
basin.  If  oil  is  poured  upon  the  collection  of  water  in  the  basin,  so  that  evapora- 
tion from  the  latter  is  impeded,  a  constant  diminution  of  the  water  in  the  basin  is 
observed  notwithstanding;  this  leads  to  the  conclusion  that  the  water  in  question  is 
really  taken  up  by  the  absorption-cells  at  the  bottom  of  the  basin  and  conveyed  to 
the  tissue  of  the  leaf. 

The  first  thing  that  strikes  one  on  surveying  once  more  all  the  plants  possessing 
on  their  aerial  organs  special  contrivances  for  water-absorption  is  that  a  large 
proportion  of  them  have  taken  up  their  abode  in  swamps  and  on  the  banks  of 
rivers  and  streams,  or  if  not  there,  at  all  events  in  situations  where  no  danger 
exists  of  the  ground  being  thoroughly  dried  up.  No  doubt  this  appears  to  be 
inconsistent.  How  are  we  to  explain  the  fact  that  Gentianeae,  ashes,  willows,  alpine 
roses,  bog-mosses,  &c.,  are  still  in  need  of  water  from  the  atmosphere,  when  they  all 


ABSORPTION-CELLS   ON   LEAVES.  241 

grow  either  in  damp  meadows,  peat-bogs,  on  the  borders  of  never-failing  springs,  or 
in  ever-moist  ravines,  where  their  requirements  in  respect  of  nutrient  water  and 
imbibitious  water  can  be  supplied  all  around  by  means  of  the  roots?  A  glance  at 
the  company  in  which  these  plants  occur  may  perhaps  lead  to  a  solution  of  the 
problem.  In  the  damp  meadows  and  along  the  margins  of  springs  where  gentians, 
the  Sweet-willow,  and  plants  of  that  kind  are  found,  the  Butterwort  (Pinguicula), 
which  has  been  described  in  earlier  pages  amongst  carnivorous  plants,  is  never 
absent;  whilst  wherever  the  pale  cushions  of  the  Bog-moss  spring,  there  also  the 
Sun-dew  is  certain  to  spread  out  its  tentacles  for  the  capture  of  prey. 

With  reference  to  community  of  site  the  assumption  is  warranted  that  all  these 
plants  which  nourish  under  identical  conditions  of  life  endeavour  to  acquire  the 
same  material  by  means  of  their  aerial  parts.  Now,  this  material  cannot  well  be 
other  than  nitrogen,  of  which  they  do  not  find  a  sufficient  store  in  the  substratum. 
What  then  is  more  natural  than  that  those  plants,  which  are  not  adapted  to  the 
capture  of  animals,  should  use  their  aerial  organs,  when  these  are  moistened  with 
rain  or  dew,  to  take  up  direct  nitric  acid  and  ammonia,  which  are  contained — 
though  in  small  traces  only — in  the  atmospheric  deposits,  instead  of  waiting  till 
compounds  of  such  great  importance  to  them  penetrate  into  the  ground  where  they 
may  chance  to  be  detained  at  spots  whence  the  roots  could  only  obtain  them  after 
long  delay  and  by  a  highly  complicated  process  ?  When  one  considers  that  plants, 
growing  amid  the  sand  and  detritus  of  steppes,  on  ledges,  and  in  crevices  of  steep 
rocks,  or  epiphytic  on  the  bark  of  trees,  are  also  able  to  acquire  little  or  no 
nitrogenous  food  from  the  substratum  by  means  of  their  roots,  their  especial  equip- 
ment with  apparatus  for  the  absorption  of  atmospheric  water  becomes  explicable  on 
the  ground  of  the  latter  being  the  medium  of  solution  and  transport  of  nitrogenous 
compounds.  In  the  case  of  epiphytes  and  of  plants  growing  on  steppes  or  rocks, 
there  is  the  additional  consideration  that  a  supply  of  pure  water,  supplemental  to  that 
which  can  be  withdrawn  from  the  substratum,  must  be  very  welcome  to  them  in 
dry  weather,  and  that  at  such  times  it  is  a  great  advantage  for  the  atmospheric 
water  to  be  absorbed  directly  by  the  aerial  organs  instead  of  reaching  them  in  a 
roundabout  manner  through  the  substratum. 

If  this  idea  is  justified,  the  atmospheric  moisture  taken  up  by  the  aerial  organs 
with  the  help  of  the  above-described  contrivances,  would  be  of  value  to  the  plant 
chiefly  in  being  a  carrier  of  nitrogenous  compounds,  and  in  this  acceptation  would 
have  to  be  looked  upon  as  water  of  imbibition.  Whether  it  is  also  used,  at  least  in 
part,  as  food-material  can  neither  be  asserted  nor  controverted.  A  separate  absorp- 
tion of  water  which  serves  only  for  motive  power,  and  of  that  which  is  in  addition 
employed  in  the  construction  of  organic  compounds  does  not  take  place  in  a  plant, 
it  is  not  possible  to  make  any  a  priori  statement  concerning  the  moisture  taken 
up,  as  to  which  part  it  has  to  play  in  the  plant.  Most  probably  the  allotment  of 
functions  is  not  at  all  uniform,  but  varies  considerably  according  to  conditions  of 
time,  place,  and  requirement. 

On   a   former   occasion   it   has   been   mentioned   that   small   animals   are   not 

VOL.  I.  16 


242  ABSORPTION-CELLS   ON   LEAVES. 

infrequently  killed  accidentally  in  the  water  filling  the  larger  kinds  of  basins 
formed  as  parts  of  foliage-leaves,  that  pollen,  spores,  and  particles  of  earth  also  are 
blown  by  the  wind  into  these  basins,  and  that,  after  the  ensuing  solution  and 
decomposition  of  the  organic  and  mineral  bodies  in  question,  the  water  exhibits  a 
brownish  colour  and  contains  organic  compounds  as  well  as  food-salts  in  solution. 
It  is  not  necessary  to  repeat  that  these  compounds  are  able  to  pass  into  the  interior 
of  the  plant  with  the  water  through  the  action  of  the  absorption-cells  which  are 
never  absent  from  the  bottom  of  the  basins;  but  it  seems  proper  to  consider 
specially  in  this  connection  the  most  conspicuous  cases  of  the  phenomenon  which 
have  been  observed.  The  greatest  quantity  of  matter,  dissolved  and  undissolved,  is 
found  in  the  flat,  saucer-shaped  laminae  of  Saxifraga  peltata,  which  grows  on  the 
sites  of  springs  in  the  Sierra  Nevada  of  North  America.  The  water  in  these  saucers 
is  sometimes  coloured  quite  a  dark  brown  by  the  presence  of  decayed  beetles,  wasps, 
centipedes,  fallen  leaves,  and  animal  excreta;  and  when  it  evaporates  a  regular  crust 
is  left  behind  at  the  bottom  of  the  reservoir.  Three  days  after  rain  I  still  found  in 
the  inflated  vagina  of  Heracleum  palmatum,  a  species  of  cow-parsnip,  a  pool  of 
brown  water  2  cm.  deep,  and  at  the  bottom  a  deposit  of  blackish,  oily  mud  in  which 
the  remains  of  decayed  earwigs,  beetles,  and  spiders,  were  still  recognizable.  The 
same  thing  is  observed  in  the  cisterns  of  Bromeliaceae  and  in  the  water-basins  of 
Dipsacus  laciniatus  and  Sttphiwm  perfoliatum  (fig.  56),  and  it  is  interesting  to 
find  there  are  cells  also  at  the  bottom  of  the  basins  of  the  Dipsacus  in  question  from 
which  protoplasmic  threads  radiate  forth,  as  in  the  case  of  the  chambers  of  the 
Tooth  wort,  and  that  numberless  putrefactive  bacteria  always  make  their  appearance 
in  the  water  in  these  basins.  The  quantity  of  organic  residue  is  less  considerable 
in  the  saucer-shaped  leaves  of  pelargoniums,  but,  on  the  other  hand,  earthy  particles 
are  frequently  met  with  in  them  to  such  an  extent  that,  when  the  water  has 
evaporated,  the  concave  surface  of  the  leaf  is  covered  with  an  ashen-gray  layer 
of  earth. 

Observations  of  this  nature  establish  the  conviction  that  no  sharp  line  of 
demarcation  exists  in  respect  of  the  absorption  of  water  either  between  carnivorous 
plants  and  land  plants,  or  between  land  plants  and  saprophytes,  or  between 
saprophytes  and  carnivorous  plants;  and  they  lead  further  to  the  conclusion  that 
water,  mineral  food-salts,  and  organic  compounds  are  susceptible  of  being  taken  up 
not  only  by  subterranean  but  also  by  aerial  absorptive  apparatus. 


LICHENS.  243 


6.    SYMBIOSIS. 

Lichens.— Cases  of  symbiosis  of  Flowering  Plants  having  green  leaves  with  the  mycelia  of  Fungi 
destitute  of  chlorophyll.— Monotropa.— Plants  and  Animals  considered  as  a  vast  symbiotic 
community. 


LICHENS. 


In  describing  the  vegetation  of  a  limited  area  botanical  writers  are  apt  to  desig- 
nate the  various  species  of  plants  as  "denizens"  of  the  country  in  question.  The 
conditions  under  which  the  plants  live  are  likened  to  political  institutions,  and  the 
relations  existing  amongst  the  plants  themselves  are  compared  to  the  life  and  strife 
of  human  society.  By  no  means  the  least  important  factor  in  the  suggestion  of 
these  analogies  is  the  circumstance  that  often  as  a  matter  of  fact  one  has 
opportunities  of  seeing  how  the  species  of  plants  which  live  together  in  a  locality 
are  dependent  in  various  ways  upon  one  another;  how  they  exist  in  continual  con- 
flict for  the  food,  the  ground,  for  light  and  air;  how  some  are  preyed  upon  and 
oppressed  by  others,  whilst  others  are  supported  and  protected  by  their  neighbours ; 
and  how,  not  infrequently,  quite  different  species  join  together  in  order  to  attain 
some  mutual  advantage. 

As  regards  the  preying  of  one  upon  another  the  subject  has  been  treated  in 
detail  in  a  previous  chapter,  and  it  was  also  stated  then  that  the  term  parasite  can 
only  be  applied  to  those  plants  which  withdraw  materials  from  the  living  parts  of 
other  organisms  without  rendering  a  reciprocal  service  in  return.  The  host  attacked 
by  a  parasite  supplies  food  and  drink  without  being  in  any  way  compensated.  One 
might  suppose  that  nothing  would  be  simpler  and  easier  than  to  ascertain  the 
existence  of  this  relationship,  and  yet  many  difficulties  are  encountered  in  the 
determination  of  parasitism  in  individual  cases.  The  main  difficulty  is  due  to  the 
fact  that  one  cannot  always  say  with  certainty  whether  the  host  does  not  perhaps 
get  some  advantage  from  the  parasite  which  drains  its  juices.  Should  this  be  the 
case,  however,  the  latter  would  be  no  longer  a  parasite,  and  the  relationship  between 
the  two  would  rather  be  that  of  simple  commerce  and  mutual  assistance,  an  ami- 
cable association  for  the  benefit  of  both. 

Whilst  discussing  the  second  series  of  parasites,  the  fact  was  mentioned  that  the 
plants  upon  which  the  various  species  of  Eyebright  fasten  their  suckers  suffer  no 
apparent  injury  as  a  consequence  of  this  connection.  The  rootlet  organically 
united  to  the  suckers  does,  it  is  true,  die  away  in  the  autumn;  but  the  Eyebright 
also  withers  at  that  season,  and  it  is  not  inconceivable  that  the  useful  substances 
existing  in  the  green  leaves  of  the  Eyebright  may  be  transferred,  shortly  before  the 
latter  withers,  to  the  host-plant  and  deposited  there  at  a  convenient  time  in  the 
permanent  part  of  the  root  as  reserve-material,  and  that  in  this  way  the  host-plant 
ultimately  derives  benefit  from  the  so-called  parasite.  The  idea  here  suggested  as  a 
possibility  for  the  case  of  Eyebright  and  the  grasses  connected  with  it  is  an  ascer- 
tained fact  in  the  case  of  some  other  plants.  For  plants  are  known  which  unite  to 


244 


LICHENS. 


form  a  single  organism  and  thenceforward  so  co-operate  in  their  functions  that 
ultimately  both  derive  advantage  from  the  arrangement.  The  one  takes  food-stuffs 
from  the  substratum  and  from  the  air  and  transmits  them  to  the  other;  whilst,  in 
the  green  cells  of  the  other,  the  raw  material  is  worked  up,  under  the  influence  of 
sunlight,  into  organic  compounds.  The  organic  compounds  thus  created  are  used 
by  both  for  the  further  production  of  organs,  and  therefore  a  connection  such  as 
this  must  be  looked  upon  as  a  true  case  of  symbiosis,  i.e.  associated  existence  for 

purposes  of  nutrition. 

The  first  place  amongst  social  communities  of  the  kind  must  be  assigned  to 
Lichens,  a  section  of  Cryptogams  possessing  an  extraordinarily  large  number  of 
species  and  differentiated  into  thousands  of  forms,  representatives  of  which  are 


Fig.  57.— Gelatinous  Lichens, 
i  Ephebe  Kerneri;  x450.    2  Collema  pulposum ;  natural  size.    »  Section  through  Collema  pulposum;  x450. 

everywhere  distributed,  from  the  sea-shore  to  the  highest  mountain  peaks  yet 
scaled  by  man,  and  from  the  tropics  to  the  arctic  and  antarctic  zones. 

The  partners  in  the  Lichen  communities  appear  to  be,  on  the  one  hand,  groups 
and  filaments  of  round,  ellipsoidal,  or  discoid  green  cells  belonging  to  plant  species 
included  under  the  general  name  of  Algae;  and,  on  the  other  hand,  pale,  tubular 
cells  or  hyphae,  which  are  destitute  of  chlorophyll,  and  pertain  to  species  of  plants 
comprised  under  the  general  name  of  Fungi  (see  fig.  58). 

The  form  assumed  by  a  large  proportion  of  these  lichens  is  that  of  incrustations 
on  stones,  earth,  bark,  or  old  wood-work;  the  entire  structure  of  the  lichen  is  either 
ensconced  and  imbedded  in  the  depressions  of  weathered  surfaces  of  stone,  or  else 
between  the  cell-walls  of  dead  fragments  of  wood  and  bark,  so  that  it  often  happens 
that  attention  is  only  drawn  to  its  presence  by  the  altered  colour  of  the  substratum, 
or  by  the  fructifications  which  lift  their  heads  above  the  substratum. 

Lichens  of  the  kind  are  termed  Crustaceous  Lichens,  and  the  wide -spread 
Graphic  Lichen  (Lecidea  geographica)  may  serve  as  an  example.  A  second  great 
group  nearly  allied  to  the  first  is  that  of  Foliaceous  Lichens.  The  form  of  the 


LICHENS. 


245 


vegetative  body  in  these  is  best  compared  to  the  foliage-leaves  of  the  Curled  Mint, 
with  their  corrugated  or  sinuate  margins,  or  to  those  of  Malva  rotundifolia.  It 
may  also  be  described  as  a  number  of  lobes  radiating  irregularly  and  bifurcating 
repeatedly,  and  only  lightly  joined  to  the  substratum  by  root-like  fringes,  and  there- 
fore capable  of  being  readily  loosened  and  detached.  The  light-grey  Parmelia 
saxatilis,  which  bear  brown  saucer-shaped  fructifications,  may  be  taken  as  a  repre- 
sentative of  these  Foliaceous  Lichens.  The  Fruticose  Lichens  are  distinguished  as  a 
third  group  in  which  the  thallus  rises  from  the  ground  in  the  shape  of  a  shrub, 
whilst  the  cylindrical,  fistular,  and  ligulate  stemlets,  which  ramify  profusely,  are 
only  adherent  to  the  substratum  by  a  very  small  surface  at  the  base.  With  these 
are  associated  the  Beard  Lichens,  which  hang  down  from  the  bark  of  old  trees  in 
the  form  of  pale,  copiously-branched  filaments.  Lastly,  there  is  a  fifth  group,  the 


Fig.  68.— Fruticose  and  Foliaceous  Lichens. 

i  Stereocaulon  ramulosum  in  conjunction  with  Scytonema;  x650.    *  Cladonia  furcata  with  Protococcus;  x950 
8  Coccocarpia  molybdcea;  section,  x  650  (after  Bornet). 

Gelatinous  Lichens,  which  when  moistened  look  like  dark,  olive-green,  or  almost 
black  lumps  of  wrinkled  and  wavy  jelly  or  as  if  composed  of  variously-divided 
bands  and  strips  packed  together  into  little  cushions. 

In  the  gelatinous  expansions  last  mentioned  the  algal  cells  are  arranged  in 
moniliform  rows  and  are  interwoven  with  the  hyphal  filaments  of  the  fungus 
throughout  the  entire  thickness  of  the  thallus,  as  in  Collema  pulposum  (see 
fig.  57  2  and  57 3),  or  else  they  form  regular  ribbon-shaped  double  rows,  interwoven 
with  few  hyphae,  as  in  Ephebe  Kerneri  (see  fig.  571).  In  crustaceous,  foliaceous, 
and  fruticose  lichens,  the  algal  cells  constitute  a  disorderly  heap  and  are  crowded 
together  in  the  middle  stratum  of  the  thallus,  where  they  are  imbedded  between 
an  upper  and  a  lower  layer  of  densely  felted  hyphal  threads,  as  in  Coccocarpia 
molybdcea  (fig.  58 3). 

Seeing  the  wide  distribution  of  lichens  it  must  be  assumed  that  both  partners 
occurring  in  the  lichen-thallus  are  able  to  range  about  with  extraordinary  ease  and 
latitude.  When  one  observes  how  patches  of  the  most  various  lichens  are  produced 
in  a  few  years  after  a  laudslip  on  the  freshly-broken  surfaces  of  the  stones  which 


246 


LICHENS. 


have  fallen  down  into  the  valley  beneath,  one  can  only  explain  the  phenomenon  by 
supposing  that  the  algal  and  fungal  cells  concerned  have  been  blown  together,  and 
that  the  opportunity  has  been  afforded  them  on  the  blocks  of  stone  of  contracting 
a  union.  Now,  so  far  as  regards  one  of  the  two  partners,  viz.:  the  one  devoid  of 
chlorophyll,  and  known  as  a  fungus— the  idea  that  everywhere  in  the  air  spores  of 
fungi  are  swarming  about  is  so  familiar  to  us  that  the  supposition  of  an  occasional 
stranding  of  individual  spores,  which  are  being  blown  about  by  the  wind,  upon  the 
moist  broken  surfaces  of  stones  can  encounter  no  opposition.  Respecting  those 
spores  in  particular  which  are  ejected  from  the  aerial  fructifications  of  lichens,  the 
discussion  of  their  life-history  and  distribution  must  of  course  be  reserved  for  a 
later  section;  but  it  is  necessary  to  make  here  the  one  statement  that  provision 
exists  for  the  most  profuse  and  distant  dissemination  of  these  spores. 

Thus,  in  the  case  of  one  of  the  partners,  there  is  no  difficulty  in  realizing  its 
ubiquity.  But  when  one  comes  to  the  Algae,  the  name  at  first  calls  up  to  mind  the 
green  filaments  which  occupy  our  pools  and  ponds,  or  the  brown  wracks  and  red 
Floridese  of  the  sea-shore,  and  we  ask  ourselves  how  it  can  be  possible  for  these 
plants  to  occur  on  fractured  surfaces  of  stone,  especially  on  the  debris  of  mountain 
sides.  Indeed,  it  is  certainly  not  Algae  of  these  kinds  that  take  part  in  the 
construction  of  Lichens.  The  name  Algae  is  properly  only  a  general  name  for  all 
Thallophytes  containing  chlorophyll,  and  it  is  applied  to  many  small  organisms 
besides  those  mentioned  above,  namely,  to  numbers  of  Nostocineae,  Scytonemeae, 
Palmellaceae,  Chroolepideae,  and  these  are  the  kinds  which  fall  in  with  the  cells  of 
fungi  and  form  lichens  in  conjunction  with  them.  Owing  to  their  minute  size, 
they  are  apt  to  escape  observation,  and,  in  general,  only  attract  attention  when 
myriads  of  them  clothe  the  bark  of  trees,  cliffs,  stones,  or  earth.  In  these  situations 
they  need  but  little  moisture,  and  it  is  not  necessary  for  any  of  them  to  live  under 
water  like  other  algae;  they  become  desiccated  without  sustaining  the  slightest 
injury  and  make  their  appearance  on  the  substratum  occupied  by  them  at  the  first 
stage  of  their  development,  as  powdery  coats,  and,  in  this  condition  being  extremely 
light,  are  liable  to  be  blown  away  by  a  wind  of  moderate  strength,  and  so 
distributed  over  mountain  and  valley. 

That  this  dissemination  is  not  merely  hypothetical  but  an  actual  fact  has  been 
susceptible  of  easy  proof  by  the  following  experiment,  made  in  a  mountain- valley  in 
the  Tyrol.  A  plane  surface  covered  with  white  filter-paper,  which  was  kept  moist, 
was  exposed  to  a  south  wind;  in  the  course  of  a  few  hours  numerous  particles,  like 
dust,  adhered  to  the  paper,  and  amongst  them  cell-groups  of  Nostocineae  and  others 
of  the  above-mentioned  algae  occurred  regularly,  in  addition  to  organic  fragments 
of  the  most  various  kinds,  such  as  pollen-grains  and  spores  of  all  sorts  of  mosses 
and  fungi.  All  these  bodies  were  deposited  in  the  little  depressions  on  the  sheet 
of  paper,  and  in  the  same  way  they  rest  in  the  grooves,  cavities,  and  cracks  in  the 
surfaces  of  stone,  bark,  and  old  wood-work,  where  they  succeed  in  reaching  a 
further  development  as  soon  as  the  requisite  quantity  of  water  is  provided.  Now, 
if  at  these  places  the  little  algal  cell-groups  meet  with  hyphae  belonging  to  the 


LICHENS.  247 

other  potential  partner,  the  latter  embrace  and  enmesh  them,  as  is  shown  in  the 
above  figures,  and  thus  is  produced  the  confederacy  called  a  Lichen.  The  member 
destitute  of  chlorophyll  takes  up  nutriment  from  the  external  environment;  it 
possesses,  in  particular,  the  property  of  condensing  aqueous  vapour,  and  has,  besides, 
the  power  of  bringing  the  solid  substratum  partially  into  solution  by  means  of 
excreted  substances;  it  effects  adhesion  to  the  substratum,  and,  in  a  majority  of 
cases,  determines  the  form  and  colour  of  the  lichen- thallus  as  a  whole.  The  second 
member,  whose  cells  contain  chlorophyll,  undertakes  the  task  of  producing  organic 
matter,  under  the  influence  of  sunlight,  from  the  materials  conveyed  to  it;  by  this 
means  it  multiplies  the  number  of  its  cells  and  increases  in  volume,  whilst,  at  the 
same  time,  it  yields  to  its  mate  so  much  as  is  necessary  in  order  to  enable  the  latter 
to  keep  pace  with  it  in  growth. 

The  number  of  algae  which  enters  into  a  partnership  of  this  kind  is,  in  any 
case,  much  less  considerable  than  that  of  the  fungi,  and  it  must  be  assumed  that 
one  species  of  alga  may  unite  with  the  hyphae  of  different  lichen-fungi.  The 
extreme  variety,  moreover,  in  the  combinations  of  the  two  sorts  of  confederate 
occurring  on  a  very  small  area  is  obvious  from  the  circumstance  that  it  is  not 
rare  for  half  a  dozen  different  species  of  lichen  to  spring  up  side  by  side  on  a  patch 
of  rock  no  bigger  than  one's  hand.  Whether  they  all  achieve  an  equally  hardy 
development,  or  whether  some  perchance  are  not  crowded  out  and  overgrown 
by  others  depends  on  various  external  conditions — on  the  chemical  composition 
of  the  substratum,  and  particularly  on  the  conditions  of  moisture  and  illumination 
of  the  site  in  question.  Lichens  are  very  sensitive  in  this  respect,  and  the  different 
sides  of  a  single  rock  often  exhibit  quite  different  growths  of  lichens.  A  very 
instructive  example  of  this  is  afforded  by  a  marble  column  near  the  famous  castle 
of  Ambras  in  Tyrol.  This  column  is  octagonal,  and  has  been  standing  in  its  place 
for  more  than  two  hundred  years,  with  all  its  sides  exposed  to  wind  and  weather. 
Lichens  have  settled  on  all  the  eight  faces,  and,  indeed,  are  present  in  such  abund- 
ance that  the  stone  is  quite  covered  by  patches  the  size  of  a  man's  hand.  Many 
of  these  growths  are  but  poorly  developed,  and  not  susceptible  of  being  identified 
with  certainty;  but  altogether  on  this  column  there  must  be  over  a  dozen  different 
species,  the  germs  of  which  can  only  have  been  brought  by  winds.  These  species 
are,  however,  by  no  means  uniformly  disposed;  some  prevail  on  one  side,  some 
on  another,  and  a  few  are  confined  exclusively  to  one  of  the  eight  faces.  Of  three 
species  of  Amphiloma,  the  one  named  A.  elegans  is  restricted  to  the  warmest  side, 
i.e.  the  face  exposed  to  the  south-west;  a  second,  Amphiloma  murorum,  is  to 
be  seen  on  the  upper  part  of  the  southern  face;  whilst  Amphiloma  decipiens 
occurs  on  the  same  face,  but  only  near  the  ground.  On  the  side  with  a  northern 
aspect  Endocarpon  miniatum  predominates,  and  on  the  north-west  face  Calopisma 
citrinum  and  Lecidea  are  the  prevailing  forms. 

What  thousands  of  spores  and  algal  cells  must  have  been  blown  on  to  this 
pillar  to  enable  all  these  combinations  to  arise!  What  complex  processes  must 
have  gone  on  before  the  selection  of  lichens  best  adapted  to  each  different  quarter 


248 


LICHENS. 


of  the  compass  was  effected  on  this  little  marble  column!  It  is  necessary  to  add, 
however,  that  lichens  growing  on  stone,  bark,  or  any  situation  of  the  kind  do 
not  in  all  cases  owe  their  original  appearance  on  the  substratum  to  a  fresh  union 
of  Alga3  and  Fungi,  but  that  there  is  a  second  mode  of  distribution  of  lichens.  This 
method  consists  in  the  transportation  by  air-currents  of  already  completed  social 
colonies  to  places  often  situated  at  a  great  distance  from  the  spots  where  the 
initial  union  between  Alga  and  Fungus  was  contracted.  The  process  is  as  follows: 

in  the  interior  of  an  old,  large,  and  fully  developed  lichen-thallus  certain  groups 

of  cells  separate  from  the  rest,  each  group  consisting  of  one  or  more  green  algal 
cells  enmeshed  in  a  dense  weft  of  hyphae.  When  a  sufficient  number  of  these 
daughter-associations  has  been  formed  the  thallus  of  the  parent  lichen  is  ruptured 
and  the  little  miniature  social-groups,  which  are  termed  "soredia",  come  to  the 
surface.  To  the  naked  eye  a  single  soredium  is  only  visible  as  a  bright  dot,  but 
all  together  they  have  the  appearance  of  a  mass  of  powder  or  meal  lying  loosely 
upon  the  old  lichen-thallus.  In  dry  weather  this  mealy  efflorescence  is  easily 
blown  away  with  other  organic  particles.  If,  then,  a  soredium  thus  removed 
comes  to  rest  in  the  crack  of  a  rock  or  on  any  suitable  substratum,  the  alga  and 
hyphae  composing  it  continue  to  develop,  and  the  organism  grows  into  a  larger 
lichen-thallus,  which  is  able  to  repeat  the  process  just  described.  In  regions  where 
lichens  abound,  soredia  of  the  kind  are  found  regularly  amongst  the  elements 
of  the  organic  dust,  and  occur,  indeed,  mixed  with  fungal  spores  and  algal  cells, 
so  that  it  certainly  happens  not  infrequently  that  two  spots  close  together  in  the 
same  cranny  of  stone  exhibit  both  sorts  of  lichen-growth,  the  one  newly  produced 
by  the  concurrence  and  union  of  algal  and  fungal  cells,  the  other  a  daughter- 
association  which  has  arisen  from  an  old  lichen,  as  a  soredium,  and  is  continuing 
its  development. 

Another  case  of  symbiosis  allied  to  that  of  lichens  is  manifested  by  certain 
Cryptogams  which  live  socially  together  under  water  and  have  received  the 
systematic  names  of  Mastichonema,  Dasyactis,  Enactis,  &c.  In  them  also  a  plant 
containing  chlorophyll,  and  belonging  to  the  group  of  Nostocineae,  appears  as  one 
member  of  the  partnership;  whilst  the  second  is  some  species  of  Leptothrix  or 
Hypheothrix.  The  green  moniliform  rows  of  cells  of  Nostocineae  are  enmeshed 
and  wrapped  round  by  the  delicate,  filamentous  cells  devoid  of  chlorophyll  of  the 
Leptothrix  or  Hypheothrix;  and  later,  by  repeated  processes  of  division,  whole 
colonies  of  green  cell-filaments  ensheathed  in  this  manner  are  produced,  which 
to  the  naked  eye  appear  as  small  soft  tufts,  usually  clinging  to  porous  limestone 
in  the  spray  of  waterfalls.  In  many  cases  the  filaments  destitute  of  chlorophyll 
rest  upon  the  moderately  thickened  cell-membranes  of  the  green  algae,  whilst 
in  other  cases  they  insinuate  themselves  into  the  thick  cell-membranes,  permeate 
them  with  their  webs,  and  form  in  conjunction  with  them  the  sheathing  envelope. 


SYMBIOSIS   OF   PHANEROGAMS   AND   FUNGL  249 


SYMBIOSIS  OF  GREEN-LEAVED  PHANEROGAMS  WITH  FUNGAL  MYCELIA 
DESTITUTE  OF  CHLOROPHYLL. -MONOTROPA. 

Another  instance  of  symbiosis  is  observed  to  exist  between  certain  flowering 
plants  and  mycelia  of  fungi.  The  division  of  labour  consists  in  the  fungus-mycelium 
providing  the  green-leaved  Phanerogam  with  water  and  food-stuffs  from  the  ground, 
whilst  receiving  in  return  from  its  partner  such  organic  compounds  as  have  been 
produced  in  the  green  leaves. 

The  union  of  the  two  partners  always  takes  place  underground,  the  absorbent 
roots  of  the  Phanerogams  being  woven  over  by  the  filaments  of  a  mycelium.  The 
first  root  that  emerges  from  the  germinating  seed  of  the  phanerogamic  plant 
destined  to  take  part  in  the  association  descends  into  the  mould  still  free  from 
hyphae;  but  the  lateral  roots  and,  to  a  still  greater  extent,  the  further  ramifications, 
become  entangled  by  the  mycelial  filaments  already  existing  in  the  mould  or 
proceeding  from  spore-germs  buried  there.  Thenceforward  the  connection 
continues  until  death.  As  the  root  grows  onward,  the  mycelium  grows  with  it, 
accompanying  it  like  a  shadow  whatever  its  course,  whether  the  root  descends 
vertically  or  obliquely,  or  runs  horizontally,  or  re-ascends,  as  is  sometimes  necessary 
when  it  happens  to  be  turned  aside  by  a  stone.  The  ultimate  ramifications  of  roots 
of  trees  a  hundred  years  old,  and  the  suction-roots  of  year-old  seedlings,  are  woven 
over  by  mycelial  filaments  in  precisely  the  same  manner.  These  mycelial 
filaments  are  always  in  sinuous  curves  and  intertwined  in  various  ways,  so  that 
they  form  a  felt-like  tissue,  which  looks,  in  transverse  section,  delusively  like  a 
parenchyma.  As  regards  colour  the  cell-filaments  are  mostly  brown,  sometimes 
they  are  almost  black,  and  it  is  rare  for  them  to  be  colourless.  The  epidermis  of 
many  roots  is  covered  as  if  by  a  spider's  web,  whilst  the  hyphae  form  a  complex 
tangle  of  bundles  and  strands  broken  here  and  there  by  open  meshes  through  which 
the  root  is  visible.  In  other  cases  an  evenly  woven  but  very  thin  layer  is  wrapped 
round  the  root;  and  in  others,  again,  the  fungus-mantle  forms  a  thick  layer  which 
•envelops  uniformly  the  entire  root  (see  fig.  59).  Here  and  there  the  hyphae 
insinuate  themselves  also  inside  the  walls  of  the  epidermal  cells,  and  the  latter  are 
permeated  by  an  extremely  fine  small-meshed  mycelial  net  (see  fig.  59 3). 
Externally  the  mantle  is  either  fairly  smooth  and  clearly  marked  off  from  the 
•environment,  or  else  single  hyphae  and  bundles  of  hyphae  proceed  from  it  and 
thread  their  way  through  the  earth.  When  these  branching  hyphae  are  pretty 
«qual  in  length  they  look  very  much  like  ordinary  root-hairs.  And  they  not  only 
resemble  them,  but  assume  the  function  of  root-hairs.  The  epidermal  cells  of  the 
root,  which  would  in  an  ordinary  way  act  as  absorption-cells,  being  inclosed  in  the 
mycelial  mantle  cannot  exercise  this  function,  and  have  relegated  the  business  of 
sucking  in  liquid  from  the  ground  to  the  mycelium.  The  latter  undoubtedly  acts 
as  an  absorptive  apparatus  for  the  partner  on  whose  roots  it  has  established  itself; 
and  the  water  in  the  soil,  together  with  all  the  mineral  salts  and  other  compounds 


250 


SYMBIOSIS   OF   PHANEROGAMS   AND    FUNGI. 


dissolved  in  that  water,  are  caused  by  the  mycelial  mantle  to  pass  from  the 
surrounding  ground  into  the  epidermal  cells  of  the  root  in  question,  and  thence 
onward,  ascending  into  axis,  branches,  and  foliage. 

Thus  the  fungus-mycelium  not  only  inflicts  no  injury  on  the  green-leaved  plant 
by  entering  into  connection  with  its  roots,  but  confers  a  positive  benefit,  and  it  is 
even  questionable  whether  a  number  of  green-leaved  plants  could  flourish  at  all 
without  the  assistance  of  mycelia.  The  experience  gained  in  the  cultivation  of 
those  trees,  shrubs,  and  herbs,  which  exhibit  mycelial  mantles  on  their  roots,  does 
not,  at  any  rate,  lead  to  that  conclusion.  Every  gardener  knows  that  attempts  to 
rear  the  various  species  of  winter-green,  the  bog-whortleberry,  broom,  heath, 
bilberries,  cranberries,  rhododendrons,  the  spurge-laurel,  and  even  the  silver-fir  and 


Fig.  59. 

1  Roots  of  the  White  Poplar  with  mycelial  mantle.  *  Tip  of  a  root  of  the  Beech  with  closely  adherent  mycelial  mantle;  x  100 
(after  Frank).  »  Section  through  a  piece  of  root  of  the  White  Poplar  with  the  mycelium  entering  into  the  external  cells; 
X480. 

the  beech,  in  ordinary  garden  soil  are  not  attended  with  uniform  success.  Therefore, 
as  is  well  known,  soil  consisting  of  vegetable  mould  from  the  top  layer  of  earth  in 
woods  or  on  heath  is  chosen  for  the  cultivation  of  species  of  the  genera  Erica, 
Daphne,  and  Rhododendron.  But  it  is  not  even  every  kind  of  forest-  or  heath - 
mould  that  can  be  made  use  of.  When  earth  of  that  nature  has  been  quite  dry  for 
a  long  time  it  is  no  longer  fit  for  this  purpose.  On  the  other  hand,  it  is  known  that 
the  above-mentioned  plants  should  be  transplanted  from  their  forest-home  with  the 
soil  still  clinging  to  the  roots,  and  it  is  also  laid  down  as  an  axiom  that  the  roots  of 
these  plants  should  not  be  exposed  and  should  be  cut  as  little  as  possible.  The 
following  reasons  account  for  all  this.  Firstly,  fresh  earth  from  a  heath,  or  mould 
recently  dug  from  the  ground  in  a  wood,  contains  the  mycelia  still  alive,  whereas  in 
dry  humus  they  are  already  dead;  secondly,  the  mycelia  woven  round  the  roots  are 
transferred  together  with  the  balls  of  earthy  matter  suspended  to  them  into  the 
garden;  and,  lastly,  any  considerable  clipping  of  the  roots  would  remove  the 
ultimate  ramifications  which  are  furnished  with  the  absorbent  mycelial  mantle. 

The  failure  of  all  attempts  to  propagate  the  oak,  the  beech,  heath,  rhododendron, 
winter-green,  broom,  or  spurge-laurel,  by  slips  or  cuttings,  if  the  shoot  which  is  cut 


SYMBIOSIS   OF   PHANEROGAMS   AND   FUNGL  251 

off  and  used  for  the  purpose  is  put  into  pure  sand,  is  explicable  in  the  same  way. 
Limes,  roses,  ivy,  and  pinks,  the  roots  of  which  possess  no  mycelial  mantle,  are 
notoriously  propagated  very  easily  by  putting  branches  cut  from  them  into  damp 
sand.  Rootlets  are  at  once  produced  on  those  parts  of  the  branches  which  are 
buried  in  the  sand,  and  their  absorption-cells  carry  on  the  task  of  taking  up 
nutriment  from  the  ground.  But  though  cuttings  of  oak,  rhododendron,  winter- 
green,  bog- whortleberry,  and  broom  strike  root,  no  progress  in  their  development  is 
to  be  observed,  because  the  superficial  cells  of  the  rootlets,  in  these  cases,  have  not 
the  power  of  absorbing  food  when  they  are  not  associated  with  a  mycelium.  It  is 
only  when  the  slips  from  these  plants  are  put  into  sand  with  a  rich  admixture  of 
humus,  the  latter  having  just  been  taken  from  a  wood  or  heath  and  containing  the 
germs  of  mycelia,  that  some  few  are  successfully  brought  to  further  development. 
The  result  is  even  then  often  not  assured,  and  the  cuttings  of  several  of  the  plants 
enumerated  die  even  in  sand  mixed  with  humus  before  they  have  produced 
rootlets. 

Seeing  also  that  the  result  of  attempts  to  rear  seedlings  of  the  beech  and  the  fir 
in  so-called  nutrient  solutions,  where  there  could  be  no  question  of  any  union  with 
a  mycelium,  has  been  that  the  plantlets  dragged  on  a  miserable  vegetative  existence 
for  a  short  time  and  ultimately  died,  we  have  good  grounds  for  assuming  that  the 
envelope  of  mycelial  filaments  is  indispensable  for  the  Phanerogams  in  question,  and 
that  the  prosperity  of  both  is  only  assured  when  they  are  in  social  alliance. 

The  facts  ascertained  in  cases  of  analogous  relationship  lead  one  to  expect  that 
the  fungus-mycelia  also  derive  some  advantage  from  the  flowering-plants,  the  roots 
of  which  they  clothe,  and  to  which  they  render  the  service  of  acting  as  absorption- 
cells.  The  benefit  in  question  is  undoubtedly  the  same  as  that  derived  by  the 
hyphae  of  a  lichen-thallus  from  the  enwoven  green  cells.  The  mycelial  mantles 
withdraw  from  the  roots  of  the  Phanerogams  the  organic  compounds  which  have 
been  elaborated  by  the  green  leaves  in  the  sunshine  above-ground,  and  which  are 
conducted  thence  to  all  growing  parts,  that  is  to  say,  downwards  as  well  as  in  other 
directions,  to  the  tips  of  the  swelling  and  elongating  roots.  According  to  this, 
therefore,  the  division  of  labour  between  the  members  of  the  alliance  for  joint 
nutrition  consists  in  the  mycelium  supplying  the  green-leaved  plant  with  materials 
from  the  ground,  and  the  green-leaved  plant  supplying  the  mycelium  with 
substances  which  have  been  worked  up  above-ground  in  the  sunlight. 

The  range  of  species  which  live  in  a  social  union  such  as  is  here  described  is 
certainly  very  large.  All  Pyrolaceae,  Vaccineae,  and  Arbuteae,  most,  if  not  all, 
Ericaceae,  Rhododendrons,  Daphnoideae,  and  species  of  Empetrum,  Epacris,  and 
Genista,  a  great  number  of  Conifers,  and  apparently  all  the  Cupuliferae  as  well  as 
several  Willows  and  Poplars  are  dependent  for  nutrition  on  the  assistance  of 
mycelia.  We  find,  too,  that  this  condition  recurs  in  every  zone  and  in  every  region. 
The  roots  of  the  Arbutus  on  the  shores  of  the  Mediterranean  are  equipped  with 
a  mycelial  mantle  in  precisely  the  same  manner  as  those  of  the  low -growing 
Whortleberry  of  the  High  Alps. 


252  SYMBIOSIS   OF   PHANEROGAMS   AND   FUNGI. 

Special  importance  is  given  to  the  social  life  by  the  fact  that  the  chief  species  of 
Phanerogams  participating  in  it  are  of  gregarious  growth  and  cover  whole  tracts  of 
country,  forming  boundless  heaths  and  measureless  forests,  as,  for  instance,  the 
various  heaths,  the  oak,  the  beech,  the  fir,  and  the  poplar.  The  conception  of  this 
subterranean  life  affecting  every  moorland  and  vast  timbered  tract  is  one  full  of 
wonder  and  interest. 

We  can  now  see  why  it  is  that  the  ground  in  woods  is  the  abode  of  such  a 
profusion  of  fungi.  No  doubt  some  of  these  fungi  draw  their  nutriment  exclusively 
from  the  store  of  dead  plant-organs  accumulated  there;  but  others,  as  certainly,  are 
in  social  connection  with  the  living  roots  of  green-leaved  plants.  It  is  true  we 
cannot  yet  state  precisely  what  are  the  species  of  fungi  which  contract  this  sort  of 
union,  or  whether  generally  a  definite  elective  affinity  exists  between  certain  fungi 
and  certain  green-leaved  plants.  There  is  much  in  favour  of  this  supposition  in 
a  few  cases:  but,  on  the  other  hand,  it  is  very  unlikely  that  each  of  the  various 
Phanerogams  occupying  a  limited  area  of  ground  in  a  pine-forest,  where  a  few 
square  meters  of  earth  contain  so  many  tangled  roots  belonging  to  pines,  spurge 
laurels,  bilberries,  cranberries,  heath,  and  winter-green,  that  they  can  only  be 
separated  with  difficulty,  should  select  from  the  great  host  of  fungi  growing  in  the 
forest  a  different  partner.  In  instances  of  this  kind  it  seems  just  to  suppose  that 
the  mycelium  of  one  and  the  same  species  of  fungus  enters  simultaneously  into 
connection  with  all  or  several  of  the  plants  growing  close  together;  it  is  similarly 
probable  that  the  mycelia  of  different  species  of  fungi  render  to  one  and  the  same 
flowering-plant  the  service  of  absorption  according  to  the  locality  in  which  it  occurs. 
This  surmise  is  supported  by  the  fact  that  when  certain  species,  brought  from  distant 
parts  and  regularly  exhibiting  mycelial  mantles  on  the  ends  of  their  roots,  are 
reared  in  our  gardens  and  greenhouses  from  seed,  they  unite  in  these  abodes  with 
fungus-mycelia,  which  certainly  do  not  exist  in  the  regions  where  the  Phanerogams 
in  question  grow  wild.  Thus,  for  instance,  the  roots  of  the  Japanese  tree,  Sophora 
Japonica,  and  those  of  the  Epacridese  of  Australia,  are  found  in  European  gardens 
in  social  union  with  fungi,  which  with  us  are  native,  but  which  certainly  do  not 
occur  in  Japan  or  Australia;  and  it  is  therefore  scarcely  open  to  doubt  that  the 
Sophora  Japonica,  to  take  one  example,  associates  itself  with  different  fungi  in 
different  regions. 

Now  that  the  symbiosis  of  fungi  devoid  of  chlorophyll  with  green-leaved 
Phanerogams  has  been  discussed,  we  are  for  the  first  time  in  a  position  to  deal  with 
that  most  remarkable  of  all  cases  of  food-absorption  wherein  the  subterranean  roots 
of  a  flowering-plant  are  completely  wrapped  in  a  mycelial  mantle,  whilst  the  parts 
which  shoot  up  above  ground  bear  no  green  leaves,  and,  in  general,  possess  no  trace 
of  chlorophyll.  Such  is  the  case  of  Monotropa,  the  various  species  of  which  are 
intimately  allied  in  the  structure  of  flowers  and  fruit  with  the  Primrose  and  Winter- 
green,  and  are  met  with  scattered  everywhere  in  shady  woods.  Their  stems,  which 
are  from  10  to  20  centimeters  in  height  and  emerge  from  the  mould  of  the  forest- 
ground  in  summer  time,  are  thick,  fleshy,  succulent,  and  profusely  beset  with 


SYMBIOSIS   OF    PHANEROGAMS    AND    FUNGI.  253 

membranous  and  transparent  scales,  and  the  extremity  of  each  is  bent  back  like 
a  hook.  The  cylindrical  flowers  are  developed  at  the  top  of  the  stem  with  their 
open  ends  turned  to  the  ground,  and  are  half -covered  by  the  scales.  Everything 
about  this  plant  (stem,  leaf-scales,  and  flowers)  is  of  a  pale  waxen-yellow  colour, 
and  the  general  impression  it  produces  is  much  more  that  of  a  Tooth  wort,  or  one  of 
the  colourless  forest  orchids,  than  of  a  species  of  primula  or  winter-green.  Towards 
autumn,  when  ripe  fruits  have  been  produced  from  the  flowers,  the  hitherto 
drooping  extremity  of  the  stem  lifts  itself  into  an  upright  position,  whilst  the 
entire  aerial  portion  of  the  plant  turns  brown  and  dries  up.  Every  disturbance 
caused  by  the  wind,  however  slight,  shakes  out  of  the  spherical  fruits  many 
thousands  of  tiny  seeds  as  fine  as  dust,  which,  like  the  winter-green  seeds,  consist  of 
only  a  few  cells,  and  do  not  admit  of  the  recognition  of  any  differentiated  embryo 
within  them.  Moreover,  underground,  the  rhizomes,  from  which  the  small  group  of 
pale  stems  have  arisen  in  summer,  continue  to  live  through  the  winter,  and  a 
number  of  new  buds  are  developed  on  them.  On  digging  down  to  the  hibernating 
plant  and  removing  the  mould  which  conceals  it,  one  finds  at  a  depth  of  from  10  to 
40  centimeters  bodies  like  coral-stems  consisting  of  dense  masses  of  roots  crowded 
together  and  ramifying  multifariously.  All  the  root-branches  are  short,  thick, 
fleshy,  and  brittle,  and  are  matted  together  to  form  turf -like  masses,  which  are  not 
infrequently  interwoven  with  the  rootlets  of  pines,  firs,  and  beeches,  and  have  all 
their  interstices  filled  with  humus.  Each  rootlet  is  enveloped,  right  up  to  the 
growing  apex,  in  a  thick  mycelial  mantle.  The  hyphal  filaments  of  this  mycelium 
do  not  penetrate  into  the  tissue  of  the  root  of  Monotropa,  nor  do  they  send  any 
haustoria  into  the  superficial  cells  of  these  roots.  The  hyphse  and  the  epidermal 
cells  of  the  root  are,  however,  in  such  close  and  continuous  contact  that  sections 
exhibit  a  complete  continuity  of  the  tissues. 

Monotropa  is  therefore  only  able  to  withdraw  nutriment  from  the  hyphal  weft 
of  the  mycelium  so  far  as  its  subterranean  parts  are  concerned,  and,  seeing  that  it 
is  quite  destitute  of  chlorophyll,  and  its  aerial  stem  and  leaves  display  no  trace  of 
stomata,  the  possibility  of  creating  organic  matter  and  of  adding  in  general  to  its 
substance  by  means  of  its  aerial  parts  is  excluded.  It  therefore  receives  all  the 
materials  of  which  it  is  constructed  from  the  mycelium  of  the  fungus,  whilst  it  is 
not  in  a  position  to  render  anything  in  return  to  this  mycelium  that  it  has  not 
previously  derived  from  the  latter.  If  the  mycelium  subsequently  withdraws  any 
materials  whatever  from  the  still  living  or  decaying  Monotropa,  the  process  is  only 
one  of  restitution  and  not  of  exchange.  Thus,  in  this  case,  there  can  be  no  talk  of 
reciprocity  in  the  processes  of  nutrition  or  division  of  labour  such  as  occurs  when 
there  is  symbiosis.  The  Monotropa  grows  in  height  and  in  circumference  entirely 
at  the  expense  of  the  mycelium  in  which  it  is  imbedded,  so  that  we  have  here  the 
remarkable  phenomenon  of  a  Phanerogam  parasitic  in  the  mycelium  of  a  Fungus. 
We  so  often  come  across  the  converse  process  in  our  experience  that  we  cannot 
easily  familiarize  ourselves  with  the  idea  of  a  flowering-plant  draining  the 
mycelium  of  a  fungus  of  nutriment:  nevertheless  there  is  scarcely  any  other  inter- 


254  ANIMALS   AND   PLANTS   A   SYMBIOTIC   COMMUNITY 

pretation  possible  in  this  case,  for  all  the  other  hypotheses —such  as  that  Monotropa 
enters  into  connection  with  the  roots  of  trees,  or  that  it  is  parasitic  in  the  first 
stages  of  development,  but  subsequently  detaches  itself  from  its  host  and  becomes  a 

saprophyte, rest  on  inaccurate  observations,  and  have  long  been  disproved.     As  a 

parasite  Monotropa  ought  to  have  been  discussed  at  the  same  time  as  others  in 
earlier  pages,  but  it  was  not  without  intention  that  the  description  of  this  plant  was 
reserved  for  this  place,  for  it  would  have  been  difficult  to  state  and  explain  the 
method  of  nutrition  exhibited  by  it  before  some  previous  knowledge  of  the  curious 
phenomena  of  union  of  the  mycelia  of  fungi  with  the  roots  of  green-leaved 
Phanerogams  had  been  acquired. 

ANIMALS  AND  PLANTS  CONSIDERED   AS  A  GREAT   SYMBIOTIC 

COMMUNITY. 

If  we  look  back  at  the  cases  of  symbiosis  already  discussed  and  inquire  what  is 
their  value,  we  find  it  consists  in  an  integration  of  the  functions  of  plants  possessing 
chlorophyll  and  plants  not  possessing  it.  The  reciprocity  here  implied  is,  however, 
at  bottom,  but  a  copy  of  the  complementary  interaction  of  plants  and  animals  which 
takes  place  on  a  grand  scale  in  the  organic  world.  The  associated  plant,  destitute 
of  chlorophyll,  in  which  capacity  fungi  are  always  the  organisms  concerned,  really 
plays  the  same  part  in  the  social  life  as  is  taken  by  animals  in  the  great  economy 
of  nature,  and  this  is  in  harmony  with  the  fact  that  in  other  respects  as  well 
fungi  exhibit  so  many  similarities  to  animals  that  in  many  instances  one  looks  in 
vain  for  a  line  of  division  to  separate  them  from  animal  organisms.  Hence  there  is 
no  need  for  surprise  when  cases  come  under  observation  wherein  a  quite  unmis- 
takably animal  organism  enters,  instead  of  a  fungus,  as  one  of  the  partners  in  a 
symbiotic  community.  Certain  Radiolariae  have  small  yellowish  spots  upon  them, 
which  were  formerly  held  to  be  pigment-cells,  but  have  proved  to  be  little  algae, 
with  cells  furnished  with  true  chlorophyll.  Similar  properties  are  exhibited  by 
the  fresh-water  polyp,  Hydra,  and  by  the  marine  sea-anemones.  Small  algae  occur 
in  social  union  with  these  also  in  the  shape  of  cells  with  membranes  made  of  cellu- 
lose and  containing  chlorophyll  and  starch-grains  in  their  protoplasmic  bodies. 
These  algae  are  in  no  wise  injurious  to  the  animals  with  which  they  are  associated; 
on  the  contrary,  their  presence  is  beneficial,  their  partners  reaping  an  advantage 
from  the  fact  that  the  green  constituents  split  up  carbonic  acid  under  the  influence 
of  the  sun's  rays,  and  in  so  doing  liberate  oxygen  which  may  be  again  taken  in  by 
the  animals  direct,  and  serve  a  useful  purpose  in  their  respiration  and  all  the  pro- 
cesses connected  therewith.  Conversely,  the  alga,  in  association  with  the  animal's 
body,  will  derive  a  further  advantage  from  the  latter,  inasmuch  as  it  receives  at 
first  hand  the  carbonic  acid  exhaled  by  the  animal  in  breathing.  The  small  algae 
living  socially  with  animals  cannot  be  reckoned  as  parasites  in  any  case,  nor 
can  the  animals  be  looked  upon  as  parasites  of  the  algae,  but  we  have  here  the 
phenomenon  of  mutual  assistance  and  of  a  bond  serving  for  the  benefit  of  both 


ANIMALS   AND   PLANTS   A   SYMBIOTIC   COMMUNITY.  255 

parties,  precisely  similar  to  that  noticed  in  the  case  of  lichens  and  in  the  others 
which  have  been  described  above. 

Several  of  the  liverworts  which  live  as  epiphytes  on  the  bark  of  trees  exhibit 
on  the  under  surface  of  their  leaflets  (which  are  inserted  on  the  stem  in  two  rows, 
and  are  pressed  flat  against  the  bark)  little  auricular  structures,  and  in  species  of 
the  genus  Frullania,  these  take  the  form  of  definite  hoods  or  pitchers.  The  rain 
that  trickles  down  the  trunks  of  the  trees,  washing  the  bark  and  wetting  the  liver- 
worts in  its  course,  fills  the  hooded  receptacles  referred  to  with  water,  and  is  retained 
longer  in  these  protected  cavities  than  anywhere  else,  if  a  period  of  drought  ensues 
and  the  liverwort  becomes  dry  again.  Now  these  cowls  are  the  abode  of  tiny 
rotifers  (Callidina  symbiotica  and  C.  Leitgebii),  which  live  on  the  organic  dust 
brought  thither  with  the  water.  In  return  for  the  peaceful  home  thus  afforded 
them  in  the  hooded  chambers  of  the  leaves,  the  rotifers  supply  the  liverworts  in 
question  with  nitrogenous  food.  For  as  such  must  serve  the  matter  excreted  by  the 
rotifers  in  the  interior  of  the  cowls.  Without  the  intervention  of  the  rotifers,  the 
living  organisms  (Infusoria,  Nostocinese,  and  spores)  contained  in  the  water  could 
not  be  converted  into  food  by  the  liverworts,  whereas  the  liquid  manure  arising 
from  the  Infusoria,  Nostocinese,  and  spores,  digested  in  the  bodies  of  the  rotifers, 
contains  highly  nitrogenous  compounds,  which  are  of  great  value  to  the  liverworts 
in  question,  as  indeed  they  are  to  all  epiphytes  living  on  the  bark  of  trees.  It 
stands  to  reason  that  the  symbiotic  liverworts  and  rotifers  derive  also  a  mutual 
advantage  from  the  fact  that  the  oxygen  set  free  by  the  former  comes  into  the 
possession  of  the  rotifers  and  the  carbonic  acid  emitted  by  the  rotifers  into  that  of 
the  liverworts  by  the  most  direct  method. 

Moreover,  these  cases  of  partnerships  further  remind  us  of  other  analogous  rela- 
tions existing  between  plants  and  animals,  which  it  is  necessary  to  refer  to  now, 
although  they  cannot  be  treated  in  detail  till  later  on.  A  great  number  of  flowering- 
plants  excrete  honey  into  their  flowers,  and  so  attract  flying  insects  to  them, 
which  supply  themselves  plentifully,  and  in  their  turn  render  to  the  plants  they 
visit  the  service  of  transferring  the  pollen  from  flower  to  flower,  thus  making 
possible  the  development  of  fruits  and  fertile  seeds.  Certain  small  moths  which 
visit  the  flowers  of  Yucca  bring  the  pollen  to  the  stigmas,  and  force  it  into  the 
stigmatic  orifices  in  order  that  mature  fruits  and  seeds  may  be  produced  from  the 
rudimentary  fruits,  a  result  which  is  indeed  a  matter  of  vital  importance  to  these 
moths.  For  the  moths  lay  their  eggs  in  the  carpels  of  Yucca,  and  from  the  eggs 
larvae  are  developed  which  live  exclusively  on  the  seeds  of  this  plant.  If  the 
Yucca  were  not  fertilized,  and  did  not  develop  any  fruit,  the  larva?  would  die  of 
hunger.  A  similar  phenomenon  occurs  in  many  other  cases  of  the  kind,  where 
both  plant  and  animal  reap  some  benefit.  On  the  other  hand,  in  the  formation  of 
galls,  which  are  produced  by  animals  laying  their  eggs  in  particular  parts  of  plants, 
the  advantage  (with  few  exceptions)  is  all  on  the  side  of  the  animals,  and  these  gall- 
structures  might  most  justly  be  placed  by  the  side  of  parasitic  structures. 

It  is  obvious  from  all  this  that  such  of  the  mutual  relations  of  plants  and  of 


256  ANIMALS   AND   PLANTS   A   SYMBIOTIC   COMMUNITY. 

their  relations  to  animals  as  are  occasioned  by  the  endeavour  to  acquire  nutriment 
are  extremely  various  and  often  linked  together  and  complicated  or  deranged  by 
one  another  in  the  most  curious  manner.  Cases  occur  of  a  particular  plant  being 
socially  connected  with  another,  and  at  the  same  time  also  beset  by  vegetable  and 
animal  parasites.  The  absorption-roots  of  the  Black  Poplar  are  covered  with  a 
dense  mycelial  mantle,  so  that  this  tree  is  associated  for  purposes  of  nutrition  with 
the  fungus  to  which  the  mycelium  belongs.  Such  parts  of  the  roots  of  the  Black 
Poplar  as  are  left  free  from  the  mycelium  are  fastened  upon  by  suckers  sent  forth 
by  Toothwort  plants,  which  withdraw  from  the  roots  the  juices  absorbed  by  the 
latter  from  the  earth  through  the  instrumentality  of  the  mycelial  mantles  clothing 
them.  Meantime,  in  the  cavities  in  the  leaves  of  the  Toothwort  various  small 
animals  are  caught  and  made  use  of  as  nitrogenous  food.  Again,  the  poplar-tree 
bears  Mistletoe  on  its  boughs,  and  its  presence  there  is  due  to  the  missel-thrush. 
The  thrush  takes  the  Mistletoe-berries  for  food,  and,  in  return,  renders  the  plant 
the  service  of  dispersing  the  seeds  and  establishing  them  on  other  trees.  The  para- 
sitic Mistletoe  takes  its  liquid  nutriment  from  the  wood  of  the  poplar- tree;  but,  on 
the  other  hand,  its  own  stems  are  covered  with  lichens,  and  these  lichens  are  them- 
selves a  symbiotic  community  of  algae  and  fungi.  Within  the  wood  of  the  poplar- 
stems  spread  the  mycelia  of  certain  Basidiomycetes  (Panus  conchatus  and  Poly- 
porus  populinus),  whilst  the  foliage-leaves  are  covered  with  a  little  orange-coloured 
fungus,  Melampsora  populina.  In  addition,  no  less  than  three  gall-creating  species 
of  Pemphigus  live  on  the  leaves  and  branches  of  the  Poplar,  and  a  number  of 
beetles  and  butterflies  are  nourished  by  them.  Certain  lichens,  mosses,  and  liver- 
worts regularly  settle  on  the  bark  of  old  trunks,  and  included  amongst  these  may 
be  the  species  of  liverwort  which  is  inhabited  by  rotifers.  If  all  the  plants  and 
animals  which  live  upon  the  poplar-tree,  within  it  or  in  association  with  it,  are 
counted,  the  number  turns  out  to  be  not  much  fewer  than  fifty. 


ACTION    OF   PLANTS   ON   THE   SOIL.  257 

7.  CHANGES   IN   THE   SOIL    INCIDENT   TO   THE   NUTRITION 

OF   PLANTS. 

Solution,  displacement,  and  accumulation  of  particular  mineral  constituents  of  the  soil  owing  to  the 
action  of  living  plants.— Accumulation  and  decomposition  of  dead  plants.— Mechanical  changes 
effected  in  the  soil  by  plants. 

SOLUTION,   DISPLACEMENT,  AND  ACCUMULATION  OF    PARTICULAR  MINERAL 
CONSTITUENTS  OF  THE   SOIL  RESULTING  FROM  THE   ACTION   OF  PLANTS. 

Reference  was  made  in  the  preceding  section  to  a  marble  pillar  on  the  faces  of 
which  a  dozen  different  lichens  have  settled  in  the  course  of  centuries.  I  again 
introduce  to  the  reader's  notice  this  unobtrusive  monument  in  order  to  demonstrate 
in  its  case  the  changes  to  which  stone  is  subjected  by  the  plants  clinging  to  it  or 
nestling  in  its  crevices.  It  may  be  premised,  as  a  matter  of  course,  that  when  the 
marble  column  was  erected  two  hundred  years  ago  the  eight  sides  were  polished, 
and  presented  perfectly  even  surfaces.  But  what  is  its  appearance  to-day?  The 
whole  is  rough  and  uneven;  in  parts  it  is  as  though  corroded,  and  there  are  little 
pits  clustered  together  in  places.  The  idea  might  arise  that  depressions  have  been 
formed  in  course  of  time  by  the  impact  of  drops  of  rain,  but  nearer  inspection 
shows  that  there  can  be  no  question  that  the  inequalities  have  been  produced  in 
this  way;  on  the  contrary,  it  is  by  the  influence  of  the  lichens  adherent  to  the  stone. 
Especially  on  the  two  sides  of  the  pillar  facing  south  and  south-west,  one  sees, 
clearly  how  each  pit  corresponds  exactly  in  size  to  a  species  of  grey  lichen  there 
ensconced,  and  how  this  lichen,  as  it  continues  to  grow  and  extends  radially, 
corrodes  and  etches  the  marble  it  touches  in  ever-widening  circles.  The  expression 
"to  etch"  may  here  be  taken  literally,  for  there  is  no  doubt  that  the  process,  the- 
result  of  which  is  manifested  in  the  formation  of  little  pits,  is  mainly  caused  by  the 
excretion  of  carbonic  acid  from  the  lichen's  hyphse,  whereby  the  calcium  carbonate 
is  converted  into  bicarbonate.  The  latter,  being  soluble  in  water,  is,  in  part,  taken 
up  by  the  lichen  as  nutriment,  whilst  part  is  washed  away  by  the  rain. 

In  addition  to  this  chemical  action,  the  hyphal  filaments  exercise  also  a  purely 
mechanical  influence.  A  growing  hypha  penetrates  wherever  the  merest  particle  of 
carbonate  of  lime  has  been  dissolved  and  accomplishes  regular  mining  operations  at 
the  spot.  Projecting  particles  of  the  carbonate  not  yet  dissolved  are  separated  by 
mechanical  pressure  from  the  main  mass;  and  at  the  places  in  question  where  a 
lichen  is  in  a  state  of  energetic  growth,  tiny  loose  rhombohedral  fragments  of  the 
lime  are  to  be  seen,  which  are  washed  away  by  the  next  shower  or  else  carried  off 
as  dust  by  the  wind.  The  same  process  as  that  which  may  be  so  clearly  traced  on 
the  marble  pillar  at  Ambras  takes  place,  of  course,  also  on  the  limestone  that  has  not 
been  carved  or  polished,  in  every  locality  where  lichens  exist  at  all.  We  notice  it 
in  the  case  of  other  kinds  of  stone  as  well — in  dolomite,  felspar,  and  even  in  pure 
quartz  rock — for  even  quartz  is  not  able  to  withstand  the  long-continued  action  of 

VOL.  I.  17 


258  ACTION   OF   PLANTS   ON   THE   SOIL. 

carbonic  acid  and  the  mechanical  operations  above  referred  to  in  the  performance 
of  which  the  hyphse  act  like  levers.  Some  of  the  powerful  iron  bands  belonging 
to  the  great  suspension  bridge  across  the  Danube  at  Budapest  afford  us  the 
opportunity  of  observing  the  mining  operations  of  lichens  on  a  substratum  of  pure 
iron.  Of  course  in  these  cases  the  decomposition  and  solution  initiated  by  the 
carbonic  acid  varies  according  to  the  nature  of  the  substratum;  the  result  is, 
however,  invariably  the  same;  there  is  always  a  loss  of  substance  on  the  part  of  the 
substratum,  and  a  part  of  the  dissolved  matter  is  always  taken  up  by  the  adherent 
plant,  whilst  another  part  is  carried  away  either  in  solution  or  mechanically  by 
wind  or  rain. 

Mosses  act  in  precisely  the  same  manner  as  lichens.  If  a  tuft  of  Grimmia 
apocarpa  is  lifted  away  from  the  side  of  a  block  of  limestone,  it  becomes  evident 
that  in  the  neighbourhood  of  the  place  where  all  the  stemlets  of  the  little  moss- 
colony  meet,  the  underlying  stone  is  threaded  through  and  through,  and  rendered 
friable.  There  lie  the  rhizoids  imbedded  between  isolated  particles  of  lime,  which 
are  as  fine  as  dust,  and  have  been  disintegrated  by  the  chemical  and  mechanical 
activity  of  the  organs  in  question.  At  spots  where  plants  of  Grimmia  have  died, 
the  limestone  always  exhibits  an  obvious  loss  of  substance  in  the  form  of  unevenly 
corroded  depressions. 

The  fact  that  the  roots  of  Phanerogams  also  alter  the  subjacent  stone  in  a 
similar  manner  may  be  proved  by  the  following  experiment.  A  polished  slab  of 
marble  is  covered  with  a  layer  of  sand,  and  seeds  of  plants  caused  to  germinate  in 
this  sand.  The  roots  of  the  seedlings  as  they  grow  downwards  come  almost 
immediately  upon  the  marble  slab,  and,  turning  round,  creep  onward  in  close 
contact  with  the  stone.  After  a  short  time  the  parts  of  the  slab  against  which  the 
roots  are  pressed  become  rough  as  though  they  had  been  etched;  a  solution  of 
individual  particles  of  the  carbonate  of  lime  takes  place  under  the  influence  of  the 
acid  juice  saturating  the  cell- walls  of  the  root's  cells,  and  this  circumstance  reveals 
itself  to  the  naked  eye  as  a  roughness  which  is  readily  perceptible. 

Whereas  the  loss  of  substance  affecting  the  solid  substratum  of  plants  may  thus 
be  at  once  detected  by  sight,  the  removal  of  constituents  of  the  air  and  of  water 
eludes  direct  observation.  The  ingredients  withdrawn  by  plants  are  instantly 
replaced  in  water  and  still  more  in  the  air  by  influx  from  the  environment,  and 
obviously  no  holes  or  pits  are  the  outcome  as  in  the  case  of  a  surface  of  limestone  rock. 

In  the  discussions  that  follow  it  is  important  to  retain  the  conception  that  in 
the  process  of  vegetable  nutrition  certain  substances  may  undergo  local  displace- 
ment, accumulation,  and  aggregation,  and  temporary  consignment  to  a  state  of 
quiescence.  Ingredients  of  the  earth's  crust  are  borne  upwards  into  atmospheric 
regions,  and  constituent  parts  of  the  air  are  carried  deep  down  into  the  ground. 
Lime,  potash,  silicic  acid,  iron,  &c.,  pass  from  disintegrated  rocks  into  the  realms 
above  ground — into  stems  and  leaves,  and  to  the  tops  of  the  highest  trees,  whilst 
carbon  and  nitrogen  pass  from  the  aerial  shoots  and  from  the  foliage  spread  out  in 
the  sunshine  into  the  deepest  shafts  which  the  roots  have  bored  for  themselves  in 


ACTION   OF    PLANTS   ON   THE   SOIL.  259 

the  ground.  If  one  were  to  mark  out  the  space  of  ground  from  which  the  lime, 
potash,  and  other  nutrient  salts  used  in  the  construction  of  a  birch-tree  were 
derived,  its  bulk  would  certainly  be  found  to  be  much  larger  than  that  of  the  birch; 
and,  if  we  were  to  try  to  estimate  the  volume  of  air  through  which  the  carbon, 
which  has  been  converted  into  organic  compounds  in  the  tree,  was  previously  dis- 
tributed in  the  form  of  carbon  dioxide,  it  would  turn  out  to  exceed  the  volume  of 
the  birch  a  thousandfold.  In  this  sense,  every  plant  may  justly  be  considered  as 
an  accumulator  of  those  substances  which  serve  for  its  nutriment.  Every  plant 
continues,  so  long  as  it  lives,  to  store  them  up  in  ever-increasing  quantities  in  its 
own  body,  and  in  the  case  of  long-lived  plants  there  is  thus  collected  ultimately 
quite  a  considerable  quantity.  When  the  life  of  an  accumulator  of  the  kind  is 
extinguished,  those  materials  which  were  taken  from  the  atmosphere  are  able  to 
return  into  the  atmosphere;  but  such  mineral  food  as  has  been  derived  from  the 
ground  and  lifted  into  the  upper  parts  of  the  plant— particularly  those  above  the 
ground — and  has  there  been  amassed  in  a  confined  space,  does  not  return  to  its 
original  place.  A  dead  tree  breaks  down  on  the  first  provocation,  and  the  trunk 
lies  on  the  ground  and  rots.  Such  part  of  its  substance  as  can  pass  into  the  atmos- 
phere in  gaseous  form  escapes;  but  the  salts  accumulated  within  it,  which  it  raised 
from  deep  under  ground  during  its  lifetime,  are  retained  by  the  surface-layers  of 
the  soil.  Even  though  some  of  them  are  washed  out  of  the  trunk  by  the  lixiviating 
action  of  rain-water,  the  superficial  layers  of  earth  operate  as  a  filter,  and  do  not 
allow  any  part  to  return  to  the  underlying  strata.  So,  too,  the  nutrient  salts  which 
reach  the  foliage  of  plants  are  added  to  the  top  layers  of  the  soil;  for  fallen  leaves 
go  through  much  the  same  process  as  the  trunk  which  is  broken  by  storms  and 
undergoes  decay  as  it  lies  prostrate  upon  the  ground. 

Thus,  wherever  men  do  not  interfere  by  clearing  away  the  accumulative  agents 
in  question  (i.e.  plants),  where  there  is  no  removal  of  the  haulms  of  cereals  from 
fields,  or  of  mown  grass  and  herbs  from  meadows  to  serve  as  hay,  or  of  timber  from 
the  forest — wherever,  in  a  word,  the  vegetable  world  is  left  to  itself  and  the 
natural  progress  of  evolution  is  not  frustrated  by  any  disturbing  element — the 
food-salts  which  have  been  amassed  will  accumulate  in  the  uppermost  layers  of  the 
earth.  Moreover,  seeing  that,  as  has  been  already  pointed  out,  every  plant  has  the 
power  of  possessing  itself  of  substances  of  value  to  it,  even  when  they  are  only 
present  in  the  environment  of  the  roots  in  scarcely  appreciable  quantities,  it  is 
possible  for  the  top  layers  of  soil  to  contain  a  considerable  amount  of  a  substance 
which  only  occurs  in  the  subjacent  rock  in  such  small  measure  as  to  be  detected 
with  difficulty.  The  percentage  of  lime  yielded  by  the  subsoil  on  the  Blockenstein, 
a  granitic  mountain  1383  meters  high,  on  the  borders  of  Bavaria  and  Upper  Austria, 
was  2*7,  whilst  that  of  the  top  layer  was  197;  the  percentage  on  Mount  Lusen, 
situated  to  the  north  of  the  Blockenstein,  was  1/9  for  the  subsoil  and  8'6  for  the 
superficial  layer.  When  one  considers  that  fresh  plants  strike  root  in  the  ground 
near  the  surface  and  these  again  act  as  accumulators,  and  remembers  in  addition 
that  snails  make  their  appearance  in  abundance  wherever  vegetable  food  containing 


2(30  ACTION   OF   PLANTS   ON   THE   SOIL. 

lime  is  to  be  found,  that  these  snails  again  are  to  be  reckoned  as  accumulators,  and 
that  their  shells,  which  consist  almost  entirely  of  lime,  remain  after  the  animals' 
deaths  in  the  top  layer  of  soil,  it  is  not  surprising  to  find  that  the  earth-mould  on  a 
granite  plateau  contains  a  proportion  of  lime  not  much  less  than  that  yielded  by 
mould  resting  on  argillaceous  limestone. 

Still  more  striking  than  the  influence  of  rock  plants  and  land  plants  in  trans- 
posing and  accumulating  lime  is  the  agency  of  hydrophytes  in  causing  the  same 
results.  In  the  trickling  springs  of  mountainous  regions  as  well  as  in  the  standing 
pools  of  level  country  and  no  less  in  the  depths  of  the  sea,  plants  occur  which 
obtain  part  of  the  carbonic  acid  they  require  by  the  decomposition  of  the  bicar- 
bonate of  lime  dissolved  in  the  surrounding  water.  The  monocarbonate  of  lime, 
which  is  insoluble  in  water,  is  then  precipitated  in  the  form  of  incrustations  upon 
the  leaves  and  stems  of  the  plants  in  question.  Many  of  these  hydrophytes  take  up 
carbonate  of  lime  into  the  substance  of  their  cell-membranes;  and  in  other  cases  both 
phenomena  occur,  that  is  to  say,  not  only  are  they  incrusted  externally  with  calcium 
carbonate,  but  the  cell-walls  are  also  thoroughly  impregnated  by  the  same  salt.  In 
the  streams  arising  from  springs  loaded  with  bicarbonate  of  lime  in  solution  derived 
from  the  heart  of  a  mountain,  a  number  of  mosses  regularly  occur — Gymnostomum 
curvirostre,  Trichostomum  tophacewrn,  Hypnum  falcatum,  and  others  besides. 
These  mosses  and  also  several  species  of  Nostocinese  belonging  to  the  genera 
Dasyactis  and  Euactis  become  completely  incrusted  with  lime,  in  the  manner 
referred  to,  but  go  on  growing  at  the  apical  end  as  the  older  and  lower  parts 
imbedded  in  lime  die  off.  In  consequence,  the  bed  of  the  stream  itself  becomes 
calcified  and  elevated,  and,  in  course  of  time,  banks  of  calcareous  tufa  are  formed, 
which  may  attain  to  considerable  dimensions.  Banks  raised  in  this  manner  are 
known  which  are  no  less  than  16  meters  in  height;  to  construct  them  mosses  must 
have  worked  for  more  than  2000  years. 

Numerous  Stoneworts  (species  of  Chara  or  Nitella),  the  Water-milfoil  and  Horn- 
wort  (Myriophyllum  and  Ceratophyllum),  Water-crowfoots  (Ranunculus  divari- 
catus  and  R.  aquatilis),  and  more  especially  many  Pond-weeds  (Potamogeton), 
which  grow  in  continuous  masses  in  still,  inland  waters,  incrust  their  delicate  stems 
and  leaves  with  lime  during  the  summer,  but  in  autumn  shrink  away,  that  is  to  say, 
their  stems  and  leaves  fall  and  decay,  leaving  scarcely  any  trace  of  the  mass  of 
vegetation  till  the  advent  of  the  following  spring.  The  calcareous  deposits,  how- 
ever, are  preserved,  and,  sinking  to  the  bottom  of  the  water  where  the  incrusted 
plants  lived,  form  a  layer  which  year  by  year  increases  in  thickness.  Anyone  who 
undertakes  the  investigation  of  the  sequestered  wastes  of  water  in  the  shallow 
lakes  of  lowland  districts  will  be  convinced  of  the  magnitude  of  the  scale  on  which 
this  kind  of  accumulation  must  take  place.  As  one's  boat  glides  over  places  where 
there  is  a  luxuriant  growth  of  the  lime-incrusted  Chara  rudis  and  G.  ceratophylla, 
there  is  a  crepitating  sound  in  the  water  like  the  snapping  of  dry  sticks  of  birch- 
wood.  Great  numbers  of  stoneworts  are  fractured  by  the  boat  as  it  strikes  against 
them,  and  if  one  takes  hold  of  the  fragments  they  feel  like  a  heap  of  brittle  glass 


ACTION   OF  PLANTS   ON  THE   SOIL.  261 

fibres.  What  a  quantity  of  carbonate  of  lime  must  be  deposited  yearly  at  the 
bottom  of  these  lakes  and  ponds!  Amongst  pond-weeds,  Potamogeton  lucens,  in 
particular,  clothes  its  large  shining  leaves  with  a  very  stout,  uniform  crust, 
which  drops  off  in  scales  as  the  plant  dries,  the  weight  of  which  can  be  exactly 
determined  in  the  case  of  each  separate  leaf.  The  result  of  careful  weighing  showed 
that  a  single  leaf  equal  in  weight  to  0'492  grm.  was  covered  with  a  calcareous  crust 
weighing  1-040  grm.  Now,  supposing  one  shoot  of  this  pond- weed,  having  five 
leaves,  and  covering  an  area  of  1  square  decimeter,  decays  in  the  autumn,  and  lets 
its  lime  sink  to  the  bottom  of  the  pond,  the  approximate  weight  of  lime  deposited 
each  year  on  a  square  decimeter  of  the  ground  at  the  bottom  is  5  grms.,  and,  if 
this  process  is  repeated  every  year,  a  layer  is  deposited  in  ten  years  which  weighs 
50  grms.,  and  consists  of  calcium  carbonate  and  traces  of  iron,  manganese,  and 
silicic  acid.1 

There  is  no  doubt  that  it  is  possible  for  calcareous  strata  of  great  depth  to  be 
produced  in  this  way  in  fresh  water.  That  also  in  times  past  lacustrine  deposits  of 
lime  have  had  a  similar  origin  is  inferred  from  the  fact  that  the  spore-fruits  of 
stoneworts  (Characeae)  and  the  nutlets  of  pond-weeds  have  been  found  over  and 
over  again  inclosed  in  these  formations  of  lime.  Calcareous  deposits  originating 
in  this  manner  are,  at  present  at  least,  less  frequent  in  the  sea.  Still,  the  Aceta- 
bulariaa  undergo  similar  changes  there,  and  may  be  the  cause  of  an  elevation  of 
the  sea  bottom  and  of  an  accumulation  of  lime.  On  the  other  hand,  in  the  sea, 
the  Lithothamnia  and  Corallinas  play  a  predominant  part,  and  form— just  like 
true  corals,  and  often  indeed  in  conjunction  with  these  and  other  marine  animals — 
lime  reefs  of  great  magnitude. 

The  agency  of  plants  may  occasion  accumulations  of  iron  hydroxide,  silicic  acid, 
and  salts  of  potassium  and  sodium  at  particular  places  besides  lime.  The  formation 
of  meadow  iron-ore,  spring  iron-ore,  and  bog  iron-ore,  the  construction  of  tripoli, 
agate,  and  flint,  by  the  conglomeration  of  siliceous-coated  Diatomaceae,  and  the 
accumulation  of  potassium  and  sodium  salts  in  the  superficial  strata  of  salt  steppes 
are  processes  which  take  place  essentially  in  the  same  manner  as  the  piling  up  of 
carbonate  of  lime,  although  upon  a  more  modest  scale. 

The  question  now  arises,  why  it  is  that  the  substances  which  are  stored  in  pre- 
ponderant quantities  in  the  vegetable  frame,  which  are  the  main  constituents  of  the 
living  part  of  plants,  and  represent  the  alpha  and  omega  of  plant  life,  are  not  pre- 
served as  well  as  the  mineral  food-salts  in  question.  Why  do  not  carbon  and 
nitrogen,  materials  so  eagerly  appropriated  by  the  living  plant,  compounded  by  it 
with  the  elements  of  water,  secured  in  some  measure  in  organic  compounds,  and 
constituting  the  fundamental  mass  of  the  vegetable  structure,  remain  behind  in  the 
same  condition  after  the  death  of  the  plant  ?  When  autumn  comes  and  the  lime- 
laden  pond-weed  dies,  only  the  calcareous  crust  falls  to  the  ground,  and,  at  the 
bottom  of  the  pond,  enters  upon  a  period  of  quiescence.  The  tissue  of  the  plant 

AIn  the  case  investigated  96  per  cent  calcium  carbonate,  0'28  per  cent  iron  oxide,  1  '51  manganese  oxide,  and  1'51 
per  cent  silicic  acid ;  the  last,  from  the  Diatomacese,  settled  on  the  calcareous  crust. 


ACTION   OF   PLANTS   ON   THE   SOIL. 

itself— all  its  carbohydrates  and  albuminoid  compounds— cannot  remain  dormant, 
but  are  split  up  without  delay  into  those  simpler  compounds  of  which  they  were 
compounded  in  the  summer;  and,  by  the  following  spring  there  is  nothing  more  to 
be  seen  of  any  of  the  pond-weed's  stems  and  leaves.  Certainly  this  is  only  to  such 
a  conspicuous  extent  true  of  plants  living  under  water;  dead  plants  buried  in  earth 
or  exposed  to  the  atmosphere  are  resolved  less  rapidly,  and  under  certain  circum- 
stances deposits  of  organic  remains  on  limited  areas  are  preserved  even  almost 
unaltered  through  boundless  ages. 

Let  us  try  to  obtain  a  somewhat  closer  knowledge  of  these  various  degrees  of 
preservation.  Thoroughly  dried  wood,  leaves,  and  fruit,  if  protected  from  all  but 
transient  moisture,  are  capable  of  being  preserved  unaltered  for  long  periods  of  time. 
When  wood  is  exposed  in  a  dry  place  to  the  sun,  it  turns  brown,  and  in  the  course 
of  years  becomes  quite  black  outside,  the  most  superficial  layers  being  regularly 
carbonized,  as  may  be  seen  particularly  well  in  the  case  of  woodwork  situated 
under  the  projecting  roofs  of  old  mountain  chalets.  This  wood  exhibits  no  sign  of 
crumbling,  mouldering,  or  rotting.  In  the  dry  chambers  of  old  Egyptian  graves 
fruits,  foliage,  and  flowers  have  been  found  which  were  laid  by  the  side  of  the 
dead  3000  years  ago,  and  they  had  not  undergone  a  greater  change  than  if  they 
had  been  dried  but  a  few  days.  Even  the  colours  of  flowers  of  the  Larkspur,  the 
Safflower,  and  other  plants  of  the  kind,  were  still  apparent,  and  the  separate 
stamens  in  Poppy  flowers  were  in  a  state  of  complete  preservation.  Dry  ness  there- 
fore may  be  looked  upon  par  excellence  as  one  of  the  preventives  of  the  decomposi- 
tion of  organic  matter. 

The  same  result  as  is  secured  by  dry  ness  in  the  cases  cited  is  brought  about  in 
the  ground  of  moors  by  humous  acids.  The  dead  plants  saturated  with  these  acids 
are  not  resolved  into  carbonic  acid,  water  and  ammonia,  but  preserve  their  form  and 
weight  almost  unaltered,  and  are  converted  into  peat.  Above  the  mass  of  peat  new 
generations  of  plants  continue  to  spring  up  and  produce  ever  fresh  organic  matter, 
which,  in  its  turn,  becomes  peat,  and  is  added  to  the  mass  beneath,  so  that  gradually 
a  very  deep  bed  of  organic  matter  may  be  accumulated  in  this  manner.  In  the  low 
country  lying  between  East  Friesland  and  the  Hummling,  from  the  river  Hunte  to 
the  marshes  on  the  Dollart,  there  is  a  stretch  of  nearly  3000  sq.  kilometers  covered 
with  a  layer  of  peat  which  has  an  average  depth  of  10  meters. 

Of  minor  importance  is  the  preservation  of  dead  plants  and  parts  of  plants  in 
snow  and  ice.  The  leaves,  twigs,  and  seeds,  which  are  carried  by  the  wind  on  to 
the  snow-fields  of  the  high  mountains,  remain  there  a  long  time  almost  without 
alteration  in  respect  of  form  or  size;  they  only  turn  brown  under  the  influence  of 
the  intense  sunlight,  and  at  last  become  quite  black  as  though  they  were  carbonized, 
which,  in  fact,  they  are.  So  also  such  insects  as  meet  their  death  on  the  snow-fields 
are  converted  there  into  a  black,  cindery  mass.  Indeed,  even  all  the  minutest 
organic  fragments  lying  on  a  glacier  become  carbonized,  and  this  explains  the  fact 
that  the  so-called  cryokone,  or  snow-dust,  which  we  have  already  had  occasion  to 
allude  to,  has  a  graphitic  appearance. 


ACTION   OF   PLANTS   ON   THE   SOIL.  263 

Dead  leaves,  haulms,  branches,  and  tree-trunks,  when  they  rest  upon  damp 
ground,  as  also  lifeless  roots,  rhizomes,  bulbs,  and  tubers,  buried  in  moist  earth,  pass 
into  a  state  of  putrefaction,  provided  that  their  temperature  does  not  fall  below 
freezing-point,  that  is  to  say,  they  are  resolved  into  carbonic  acid,  water,  and 
ammonia,  the  rapidity  of  the  process  varying  directly  as  the  supply  of  water  and 
the  degree  of  temperature  to  which  the  dead  matter  is  exposed,  and  inversely  as  the 
quantity  of  compounds  of  humous  acid  present.  If  more  dead  fragments  of  plants 
accumulate  within  a  particular  interval  of  time  on  one  spot  than  decay,  a  formation 
of  vegetable  mould  takes  place  there;  on  the  other  hand,  the  ground  remains 
destitute  of  humus  when  the  entire  accretion  of  organic  matter  is  quickly  decom- 
posed as  soon  as  it  is  dead.  The  general  fact  turns  out  to  be  that  the  decomposition 
of  organic  bodies  is  prevented,  or  at  least  limited,  by  a  dry  condition,  and  is 
promoted  by  moisture,  and  that  it  can  only  be  prevented  in  moist  surroundings  by 
the  presence  of  large  quantities  of  humous  acids,  or  by  the  temperature  being  low 
enough  to  turn  water  into  ice. 

This  result  directs  attention  to  those  inconceivably  small  animate  beings,  which, 
as  has  been  proved  by  experience,  are  arrested  in  their  activity  by  scarcity  of  water 
and  are  killed  by  the  antiseptic  substances  referred  to.  That  they  are  the  cause  of 
the  resolution  of  dead  plants  is  corroborated  by  the  facts  that  they  are  always 
present  where  vegetable  putrefaction  is  in  progress,  and  that,  on  the  other  hand, 
decomposition  can  be  prevented  by  rendering  the  access  of  these  minute  organisms 
impossible.  First  in  importance  in  this  respect  of  course  are  bacteria,  the  causal 
connection  of  which  with  processes  of  dissolution,  and  especially  with  those  decom- 
positions, which  are  known  by  the  name  of  putrefaction,  is  established.  Of  these 
bacteria,  Bacterium  Termo,  and  several  micrococci,  bacilli,  vibriones,  and  spirilla, 
are  the  commonest.  Their  multiplication  and  the  withdrawal  for  this  purpose  of 
substances  from  dead  plants  cause  a  splitting  up  of  the  organic  compounds  in  the 
latter.  The  albuminoid  compounds  are  first  of  all  peptonized;  next,  tyrosin,  leucin, 
volatile  fatty  acids,  ammonia,  carbon-dioxide,  sulphuretted  hydrogen,  and  water 
are  formed,  this  stage  of  the  process  being  accompanied  by  the  evolution  of  an 
offensive  odour  of  decomposition,  and  later,  nitrous  and  nitric  acids  are  produced  by 
further  oxidation.  The  carbohydrates,  too,  chiefly  cellulose  and  starch,  are  split  up, 
and  the  products  of  this  analysis,  in  so  far  as  they  are  not  used  up  by  the  bacteria 
for  their  growth  and  reproduction,  pass  in  a  gaseous  condition  into  the  atmosphere, 
or  into  the  water  surrounding  the  dead  plants.  Moreover,  the  bacteria  themselves 
do  not  remain  at  the  spots  where  they  have  been  battening  on  vegetable  matter,  but 
swarm  away  through  the  water,  or  else  come  to  rest  in  a  short  time,  in  which  case 
if  the  seat  of  their  activity  dries  up  they  are  blown  away  by  currents  of  air,  and  so 
conveyed  to  other  dead  plants.  Similar  decompositions  can  be  induced  by  moulds 
(Ewrotium,  Mucor,  Botrytis  cinerea,  Penicillium  glaucum)  as  well  as  by  bacteria, 
and,  in  addition,  the  disintegration  of  wood  occasioned  by  the  mycelium  of  Dry-rot 
(Merulius  lacrymans),  the  green-rot  of  trunks  of  oaks,  and  beeches,  caused  by 
Peziza  ceruginosa,  the  mouldering  of  wood  induced  by  the  mycelium  of  Polyporus 


264  ACTION   OF   PLANTS   ON   THE    SOIL. 

sulfureus  and  various  other  fungi,  the  red-rot,  &c.,  all  depend  on  similar  disruptions 
of  the  organic  compounds  in  dead  plants,  and  result  in  the  ultimate  dispersal  of 
these  in  the  air  in  the  form  of  carbon-dioxide,  ammonia,  nitric  acid,  and  water. 

Thus,  ultimately,  the  exercise  of  this  destructive  activity  only  effects  a  return  of 
the  compounds  just  enumerated— the  most  important  to  plant-life—to  the  regions 
whence  they  had  previously  been  withdrawn  by  the  plants  when  living.  Carbon 
and  nitrogen,  in  particular,  are  set  free  from  their  bonds  and  given  back  to  the 
atmosphere  in  the  form  and  combination  in  which  they  are  capable  of  being 
appropriated  anew  by  living  plants  as  food-material. 

Considered  from  this  point  of  view  the  phenomena  of  putrefaction  and  rotting 
appear  as  important  and  even  necessary  incidents  in  the  history  of  the  substances 
which  are  of  the  greatest  importance  to  plants.  Abhorrence  of  putrefaction  is 
innate  in  us  all,  and  everything  connected  with  it — in  particular,  the  entire  race  of 
bacteria — is  looked  upon  with  aversion.  To  estimate  these  processes  according  to 
their  deserts  requires  a  sort  of  self-abnegation.  But  when  we  overcome  our 
repugnance  and  weigh  the  whole  subject  impartially,  we  come  to  the  conclusion  that 
the  continued  existence  of  vegetable  life  and  of  life  in  general  depends  upon  the 
occurrence  of  putrefaction.  If  the  untold  numbers  of  plants  which  die  in  the  course 
of  a  year  did  not  rot  sooner  or  later,  but  remained  unchanged  as  lifeless  forms,  a 
certain  quantity  of  carbon  and  nitrogen  would  be  idle,  being  withdrawn  from  the 
sphere  of  activity  and  locked  up,  so  to  speak.  Now,  assuming  this  to  be  repeated 
year  by  year,  a  time  must  come  when  all  the  carbon  and  nitrogen  would  be 
imprisoned  in  dead  plants.  Thereupon,  all  life  would  cease,  and  the  whole  earth 
would  be  one  great  bed  of  corpses. 

Not  only  putrefaction,  but  also  the  minute  organisms  which  excite  putrefaction 
appear  in  a  more  favourable  light  when  viewed  from  this  standpoint.  Let  such 
bacteria  as  act  in  the  capacity  of  foes  to  the  human  race,  ravaging  town  and 
village  in  the  form  of  infectious  diseases,  be  exterminated  if  possible;  but  annihila- 
tion of  putrefactive  bacteria  would  mean  a  disastrous  interference  with  the  cycle  of 
life  upon  the  earth.  These  latter  are  not  to  be  reckoned  as  enemies  but  friends  to 
human  beings.  The  effect  of  their  invasion  of  dead  plants  and  animals  is  certainly 
first  made  manifest,  not  in  the  most  .agreeable  manner,  for  some  of  the  substances 
mentioned  as  being  evolved  in  the  early  stages  of  the  onslaught,  viz.:  various 
ammoniacal  compounds,  sulphuretted  hydrogen,  and  the  volatile  fatty  acids,  are 
disgusting  to  us;  but  as  decomposition  advances  these  phenomena,  which  are  so 
unpleasant  to  our  senses,  abate,  and  the  action  of  putrefactive  bacteria  becomes 
ultimately  a  beneficent  process  of  purification  of  the  last  remnants  of  dead 
organisms.  The  final  result  of  the  decomposition  of  organic  bodies  by  bacteria  has 
been  termed  mineralization.  It  is  a  fact  that  nothing  is  ultimately  left  behind,  in 
the  ground  or  water,  of  bodies  decomposed  by  the  indefatigable  exertions  of  bacteria 
excepting  some  nitric  acid  and  the  small  quantity  of  mineral  food-salts  which  has 
been  taken  up  by  the  living  organism  in  its  time  and  are  now  in  the  form  of  dust 
and  ash. 


MECHANICAL   CHANGES   EFFECTED    IN   THE   GROUND   BY   PLANTS.  265 

By  filling  with  water  a  glass  which  contains  vegetable  and  animal  remains  in 
a  state  of  putrefaction  and  swarming  with  bacteria,  one  is  enabled  to  follow  this 
process  of  mineralization  from  day  to  day.  First,  a  decrease  of  the  organic  matter 
clouding  the  liquid,  accompanied  by  simultaneous  increase  of  ammonia  and  nitrous 
and  nitric  acids,  is  observed;  then,  after  about  two  months,  a  complete  clearing  up 
of  the  liquid.  The  water  is  now  colourless  and  odourless,  but  a  precipitate  has 
formed  at  the  bottom,  which  contains,  in  addition  to  insoluble  food-salts,  bacteria  in 
a  state  of  temporary  quiescence  on  the  termination  of  their  task  and  waiting  till 
fresh  prey  becomes  accessible.  No  doubt  these  processes  occur  in  nature  in  just  the 
same  manner  as  in  the  glass  of  water,  and  the  so-called  self-purification  of  rivers, 
for  example,  has  been  rightly  attributed  to  mineralization.  It  was  long  ago  noticed 
that  the  water  of  such  rivers  as  flow  through  great  towns  and  consequently  take  up 
considerable  quantities  of  animal  and  vegetable  refuse  contains  no  discoverable 
trace  of  all  these  impurities  a  few  miles  below  the  mouths  of  the  drainage  pipes  and 
sewers.  The  water  of  the  Elbe,  which  receives  the  refuse  of  the  towns  of  Prague, 
Dresden,  and  Magdeburg,  is  so  pure  at  Hamburg  that  it  is  there  used  for  drinking 
purposes  without  protest1.  The  Seine,  after  taking  up  masses  of  rubbish  in  Paris,  is 
already  by  the  time  it  reaches  Meulan,  a  distance  of  70  kilometers,  clear  and  pure 
again,  and  does  not  even  exhibit  there  any  traces  of  the  organic  matter  received  in 
the  great  city.  Were  it  not  for  the  activity  of  the  putrefactive  bacteria,  this 
purification  would  never  take  place;  and  although  the  statement  that  putrefactive 
bacteria  are  the  best  of  purifiers  sounds  at  first  like  a  paradox,  it  must  be 
acknowledged  to  be  consistent  and  based  on  experience. 

MECHANICAL  CHANGES  EFFECTED  IN  THE  GROUND  BY  PLANTS. 

All  the  alterations  hitherto  spoken  of  as  being  brought  about  in  earth  and 
under  the  influence  of  vegetation  subsisting  therein  are  reducible  to  chemical 
transpositions.  Added  to  these,  there  are  always  certain  purely  mechanical  changes. 
The  penetration  of  the  rhizoids  of  a  rock-moss  or  the  hyphae  of  a  crustaceous  lichen 
into  limestone  is  accompanied,  as  has  been  already  stated,  by  a  solution  of  part  of 
the  substratum  and  a  mechanical  separation  of.  another  part;  the  rhizoids  or  hyphse, 
as  the  case  may  be,  becoming  imbedded  amongst  tiny  detached  fragments  of  the 
underlying  stone.  When  the  hyphse  and  rhizoids  die,  the  corresponding  piece  of 
the  substratum  is  left  porous,  and  admits  air  and  water,  whilst  other  plants  are 
enabled  to  settle  on  it,  although  they  may  not  perhaps  possess  the  power  of  eating 
into  stone  and  pulverizing  it  in  the  same  degree  as  their  predecessors.  This  is  also 
true  of  the  roots  of  Phanerogams.  The  food-seeking  root-tips  and  their  absorption- 
cells  displace  particles  of  earth  as  they  insinuate  themselves,  and  when  they  decay 
later  on,  the  soil  at  those  particular  places  is  intersected  by  passages  of  varying  size. 
No  doubt  these  passages  mostly  collapse  like  the  abandoned  shafts  and  galleries  of  a 
mine,  but  some  trace  of  root-action  always  remains  behind  in  the  shape  of  an 

1  This  was  written  before  the  last  outbreak  of  cholera. 


266  MECHANICAL   CHANGES   EFFECTED   IN   THE   GROUND   BY   PLANTS. 

increased  looseness  of  the  soil  in  the  locality,  a  result  of  the  greatest  importance, 
inasmuch  as  it  enables  air  and  water  to  permeate  to  a  depth  much  more  easily  and 
quickly  by  the  ways  that  the  roots  have  previously  opened  up.  Dead  roots  rotting 
underground  constitute,  moreover,  the  source  of  the  carbonic  and  nitric  acids  which 
help  to  render  available  the  mineral  constituents,  and  so  serve  the  turn  of  subsequent 
generations  of  plant-settlers  on  the  same  spots,  whilst  they  accomplish  fresh  disin- 
tegration of  the  substance  of  the  soil. 

If,  however,  the  subterranean  parts  of  plants  are  continually  engaged  in  mining, 
and  so  change  in  various  ways  the  position  of  the  component  particles  of  soil,  the 
organs  above  ground  exert  an  influence  in  some  measure  opposed  thereto,  in  that 
they  retain  and  bring  to  rest  particles  of  earth  which  are  set  in  motion  by  currents 
of  air  or  water.  In  the  section  that  treats  of  the  absorption  of  nutrient  salts  by 
lithophytes,  attention  was  directed  to  the  fact  that  the  dust  pervading  the  atmos- 
phere, and  blown  from  place  to  place  by  the  wind,  is  arrested  to  a  remarkable  extent 
by  mosses  and  lichens.  One  need  only  detach  a  small  tuft  of  the  common  Barbula 
muralis,  which  everywhere  occurs  on  walls  by  roadsides,  to  convince  oneself  of  the 
extent  to  which  dust  from  the  road  is  lodged  amongst  the  leaves  and  stemlets,  and 
of  the  tenacity  of  its  adhesion.  Moreover,  not  only  such  dust  as  rises  from  roads, 
but  also  that  variety  which,  though  not  easily  observed,  yet  fills  the  air  of  remote 
mountain-valleys,  of  arctic  ice-fields,  and  of  the  most  elevated  parts  of  the  earth's 
crust,  is  arrested  in  those  localities  by  mosses  and  liverworts,  and  by  many  Phanero- 
gams besides,  the  growth  of  which  is  similar  to  that  of  mosses.  There  is  not  much 
less  dust  clinging  amongst  the  stemlets  of  the  dark  Grimmias,  Andreseas,  and  other 
rock-mosses,  which  grow  in  small  cushion-like  tufts  on  weather-beaten  mountain 
crags,  than  is  attached  to  the  Barbula  living  by  the  dusty  roadside.  If  one  of  the 
tufts  in  question  is  detached  from  its  substratum,  a  fine  powder  composed  of  mica- 
scales,  granules  of  quartz,  chips  of  felspar,  and  a  number  of  minute  organic  frag- 
ments pours  out  from  between  the  moss-stems,  whilst  another  portion  of  this  finely 
powdered  earth  is  left  clinging  to  the  leaves  and  stemlets,  and  is  found  to  be  regu- 
larly adnate  to  them. 

It  is  never,  however,  the  still  fresh  and  living  upper  parts  of  these  leafy  moss- 
stems  that  arrest  and  carry  dust,  but  always  the  older  dead  parts  below.  The  lower 
dead  half  of  the  moss,  whether  still  in  a  state  of  preservation  or  already  rotting,  is 
alone  capable  (in  consequence  of  characteristic  alterations  in  the  lifeless  cell-tissue) 
of  holding  fast  the  atmospheric  dust.  The  under  part  of  moderate-sized  cushions  of 
moss  constitutes  a  compact  mass  composed  half  of  imprisoned  dust  and  half  of 
brown  lifeless  moss-stems.  These  little  cushions,  clothing  rocky  crags,  become  a 
favourable  site  for  the  germination  of  a  whole  host  of  seeds,  which  are  conveyed 
thither  by  the  wind  and  detained  in  the  same  manner  as  the  dust.  The  seedlings 
arising  from  these  seeds  send  their  rootlets  into  the  subjacent  portion  of  the  bed 
of  moss,  where  the  interstices  are  full  of  dust  or  finely-divided  earth.  Here  they 
find  all  the  conditions  prevailing  necessary  for  their  nourishment,  and  they  expand, 
and,  little  by  little,  crowd  out  the  mosses  which  received  them  so  hospitably, 


MECHANICAL   CHANGES   EFFECTED   IN   THE   GROUND   BY   PLANTS.  267 

forming  ultimately  a  bed  of  flowering  plants,  including  in  especial  abundance 
representatives  of  the  orders  of  grasses,  pinks,  and  composites. 

Many  water-plants—in  particular,  aquatic  mosses  and  algae— possess,  in  an 
almost  greater  degree  than  lithophytes  or  land  plants,  the  power  of  laying  hold  of 
inorganic  particles,  and  thus  exercise  a  far-reaching  influence  as  mud-collectors  on 
the  conformation  of  the  ground.  It  is  wonderful  how  plants  are  able  to  arrest  large 
quantities  of  the  fine  sand  hurried  along  by  a  flood,  although  they  are  exposed 
to  the  violent  rush  of  the  water.  The  tufts  of  the  dark  green  alga  Lemanea 
fluviatilis  and  of  the  aquatic  moss  Ginclidotus  riparius,  which  cling  to  rocks  in 
the  cascades  of  clear  and  rapid  mountain  torrents,  are  so  conglomerated  by  mud  and 
sand  that  they  cannot  be  freed  therefrom  until  the  tissue  has  become  dry  and 
shrivelled.  Limnobium  molle,  which  grows  in  the  turbid  waters  from  glaciers,  has 
such  an  abundance  of  earthy  particles  adhering  to  it  that  only  the  green  tips  of  the 
leaf-bearing  stems  are  visible  above  the  grey-coloured  cushions  imbedded  in  the 
mud.  The  felted  masses  of  Vaucheria  clavata,  filling  the  channels  of  apparently 
clear,  gently-flowing  streams,  are  so  mixed  with  mud  that  if  a  lump  of  this  alga  is 
fished  out,  the  weight  of  mud  clinging  to  it  exceeds  that  of  the  alga  itself  a 
hundredfold.  In  these  cases  of  submerged  plants,  it  is,  again,  not  the  living  but  the 
dead  parts  which  serve  to  arrest  the  mud.  On  lifting  up  a  lump  one  sees  clearly 
that  only  the  uppermost  and  youngest  prolongations  of  the  filaments — those  situated 
at  the  periphery  of  the  algal  cushion  as  a  whole — are  living  and  filled  with  chloro- 
phyll; the  fundamental  mass  has  become  colourless  and  lifeless.  But  these  dead 
parts,  which  form  a  thick  felt  of  interwoven  filaments,  alone  retain  in  their  meshes 
the  finely-divided  mud  and  sand  in  such  surprising  quantities;  these  particles  slip 
off  the  green  living  parts  without  adhering  to  them.  An  important  consideration 
in  this  connection  is  the  fact  that  the  dead  cell-membranes  swell  up  and  become 
slightly  mucilaginous,  so  that  fine  particles  of  mud  lodge  more  easily  in  the  soft 
swollen  substratum  thus  formed.  Wooden  stakes  stripped  of  bark  and  fixed  in  a 
strong  current  show  this  very  clearly,  as  do  also  the  trunks  of  trees  that  are  thrown 
up  by  floods  and  lie  stranded  on  the  shore  with  their  bared  boughs  projecting  into 
the  stream.  However  strong  the  current  to  which  wood  in  that  condition  is  ex- 
posed, it  covers  itself  in  a  short  time  with  a  grey  coat  consisting  of  earthy  particles 
brought  down  by  the  water.  If  a  piece  is  cut  off  and  exposed  to  the  air,  the  earthy 
deposit  does  not  become  detached  until  the  wood-cells  have  dried  up  and  shrunk. 
As  long  as  they  are  moist  the  particles  of  mud  continue  to  adhere  to  them. 

This  mechanical  retention  and  storage  of  dust  by  rock-plants  and  of  mud  by 
aquatic  plants  is  of  the  greatest  importance  in  determining  the  development  of  the 
earth's  covering  of  vegetation.  The  first  settlers  on  the  bare  ground  are  crustaceous 
lichens,  minute  mosses  and  algae.  On  the  substratum  prepared  by  them,  larger 
lichens,  mosses,  and  algse  are  able  to  gain  a  footing.  The  dead  filaments,  stems, 
and  leaves  pertaining  to  this  second  generation  arrest  dust  in  the  air  and  mud  in 
the  water,  and  thus  prepare  a  soft  bed  for  the  germs  of  a  third  generation,  which 
on  rocks  consists  of  grasses,  composites,  pinks,  and  other  small  herbs,  and  in  water 


268  MECHANICAL   CHANGES   EFFECTED   IN   THE   GROUND   BY   PLANTS. 

of  pond-weeds,  water-crowfoots,  hornwort,  and  various  plants  of  the  kind.  The 
second  generation  is  produced  in  greater  abundance  than  the  first,  and  the  third 
develops  more  luxuriantly  than  the  second.  The  third  may  be  followed  by  a  fourth, 
fifth,  and  sixth.  Each  successive  generation  crushes  out  and  supplants  the  one 
preceding  it. 

As  on  the  rocky  heights  and  in  the  roaring  torrents  of  mountains,  so  also  on  the 
sandy  plain  and  in  the  depths  of  the  sea,  a  perpetual  variation  in  the  nature  of  the 
vegetation  is  taking  place.  At  all  times  and  in  all  places  we  see  younger  genera- 
tions displacing  the  older  and  building  upon  the  foundations  laid  by  their  pre- 
decessors. The  first  settlers  have  a  hard  fight  with  uncompromising  elements  to 
seize  possession  of  the  lifeless  ground.  Years  go  by  before  a  second  generation  is 
enabled  to  develop  in  greater  luxuriance  upon  the  earth  prepared  by  the  first 
occupiers;  but  there  is  no  cessation  in  the  productive  and  regulative  effects  of 
vegetable  life,  and  its  energy  and  aptitude  in  the  work  result  in  the  erection  of  its 
green  edifices  over  wider  and  wider  areas.  New  germs  are  established  upon  the 
mouldered  dust  of  dead  races,  and  others  on  the  plant  forms  adapted  to  the  altered 
substratum,  and  so,  for  hundreds  and  thousands  of  years,  the  changes  go  on,  until 
at  length  the  tops  of  forest-trees  wave  above  a  black  and  deep  soil,  the  battle-field 
of  a  number  of  bygone  generations.  Thus,  the  life  of  plants,  like  that  of  the  human 
race,  has  its  epochs  and  its  history:  as  in  the  one  so  in  the  other  a  continual 
struggle  prevails;  processes  of  ousting  and  of  renovation  are  always  in  progress,  and 
there  are  ever  new  arrivals  upon  and  departures  from  the  scene. 


CONDUCTION  OP  FOOD. 


1.  MECHANICS  OF  THE  MOVEMENT  OF  THE  KAW  FOOD-SAP. 

Capillarity  and  root-pressure.— Transpiration. 

CAPILLARITY   AND   ROOT-PRESSURE. 

Unicellular  plants  make  use  individually  of  the  food  material  which  they 
absorb  from  their  surroundings,  and  work  it  up  into  the  organic  substances  which 
they  require  for  their  structure  and  increase  in  bulk,  and  also  for  the  production  of 
future  generations.  In  all  plants  composed,  on  the  other  hand,  of  aggregates  of 
cells,  there  is  a  division  of  labour.  Of  the  protoplasts  occupying  the  cell-cavities 
of  such  larger  plant-structures,  one  part  provides  for  the  absorption  of  the  water 
and  food-salts,  another  for  the  taking  in  of  the  gases  which  are  used  as  food, 
and  yet  another  part  works  up  this  food  into  organic  substances  for  construc- 
tive purposes.  The  centres  in  which  these  various  industries  are  carried  on  are 
frequently  situated  at  some  distance  from  one  another,  and  it  is  obvious  that 
there  must  not  only  be  some  communication  between  the  various  regions  of  activity, 
but  that  active  forces  must  come  into  play  which  will  effect  the  transport  of  the 
food  from  the  cells  whose  function  it  is  to  receive  it,  to  those  in  which  it  is  to  be 
elaborated  into  building  material.  It  is  evident  that  the  greater  the  distance  is 
between  the  various  centres  of  the  plant  in  question,  the  more  difficult  will  be  the 
performance  of  this  task.  In  aquatic  plants  and  lithophytes,  all  of  whose  superficial 
cells  have  the  power  of  taking  in  nourishment  from  their  environment,  these 
distances  are  proportionately  small,  while  they  attain  their  greatest  dimensions  in 
land-plants  whose  roots  are  embedded  in  the  earth,  and  whose  leaves  are  surrounded 
by  air.  In  trees  the  food  materials  which  are  taken  up  by  the  absorbing  roots 
beneath  the  ground  must  frequently  travel  far  more  than  100  metres  before  reach- 
ing the  topmost  leaves.  The  path  to  the  summit  is  very  steep,  and  the  fluid  in 
rising  must  be  able  to  overcome  the  force  of  gravitation,  which  has  no  inconsider- 
able significance  at  heights  such  as  these. 

Naturally,  desire  for  knowledge  has  at  all  times  directed  attention  to  this 
phenomenon,  and  the  most  diverse  attempts  have  been  made  to  explain  by  what 
means  the  food-sap  taken  in  by  the  roots  of  trees  is  enabled  to  reach  their 
summits.  It  was  first  considered  to  be  in  virtue  of  capillarity,  that  just  as  oil, 
alcohol,  or  water,  is  drawn  up  the  wick  of  a  lamp,  the  liquid  food  can  rise  in  the 
delicate  tubular  cell-formations  called  vessels,  which,  united  together  in  groups  or 


270  CAPILLARITY   AND   ROOT-PRESSURE. 


bundles,  traverse  the  stems  and  leaves  of  plants.  But  the  vessels  are  closed  in 
above  and  below,  and  therefore  it  is  impossible  that  capillarity  should  be  sufficiently 
developed  in  them.  At  best  it  could  only  raise  the  sap  a  trifling  distance,  and 
could  never  convey  fluid  to  a  height  of  many  metres.  It  is  a  striking  fact  that  in 
many  plants  the  ascent  of  the  sap  is  most  vigorous  after  the  evaporation  from  the 
superficial  parts  exposed  to  the  air  has  been  weakest.  The  so-called  "weeping"  of 
vines,  i.e.  the  outflow  of  sap  from  the  flat  surface  of  a  cut  vine-branch,  does  not 
take  place  in  summer  and  autumn,  immediately  after  the  branch  has  been  fully 
adorned  with  foliage,  and  when  its  extensive  leaf-surfaces  have  given  up  large 
quantities  of  moisture  to  the  surrounding  air;  it  occurs  at  the  end  of  the  winter 
sleep  of  the  plants,  when  the  brown  branches  rising  above  the  ground  are  still  in 
a  bare  and  leafless  condition.  The  cause  of  the  ascent,  or  at  least  of  the  ascent 
in  the  lower  leafless  branches,  must  therefore  be  sought  for  in  the  absorbent  roots, 
and  it  may  be  assumed  that  here  the  same  causes  are  at  work  which  induce  the 
fluid  food  materials  of  the  surrounding  earth  to  enter  the  superficial  cells  at  the 
root-tips. 

It  has  already  been  shown  that  the  contents  of  these  cells  suck  up  the  water  of 
the  nutritive  ground  with  great  force  in  consequence  of  the  chemical  affinity  they 
have  for  it,  or  in  other  words,  that  the  fluid  reaches  the  interior  of  plant-cells  by 
endosmosis',  it  has  also  been  mentioned  that  in  consequence  of  the  taking  in  of 
water  the  volume  of  the  cell-contents  increases,  producing  pressure  from  within 
outwards  on  the  cell- wall,  and  the  cell  swells  and  becomes  turgid.  From  this  one  of 
three  cases  might  be  deduced: — first,  suppose  that  the  cell- wall  is  so  composed 
throughout  that  it  allows  the  entrance  of  water  into  the  cell,  but  not  its  exit,  and 
that  consequently  the  cell-contents  absorb  water,  but  that  a  filtration  of  the  same 
towards  the  exterior  cannot  take  place.  Granted  this  hypothesis,  the  cell-wall  by 
virtue  of  its  elasticity  would  yield  to  the  pressure  of  the  cell-contents,  but  only 
within  the  limits  of  that  elasticity;  hence  a  condition  of  tension  would  be  produced, 
in  which  the  reciprocal  pressures  of  the  cell-wall  and  cell-contents  would  be  in 
equilibrium.  In  the  second  case,  suppose  that  the  pressure  of  the  cell-contents  is 
greater  than  the  force  of  cohesion  between  the  molecules  of  the  cell-wall,  this 
consequently  ruptures,  and  the  cell-contents  issue  from  the  rent  which  is  formed. 
This  phenomenon  is  seen  in  certain  pollen  grains  when  placed  in  water.  In  half 
a  second  the  cells  absorb  so  much  water  that  they  double  their  volume;  the  cell- 
contents  still  absorb  the  fluid,  and  the  cell- wall  can  at  length  no  longer  withstand 
the  pressure;  it  bursts,  and  the  contents,  from  which  the  pressure  is  now  removed, 
pour  through  the  opening,  and  are  diffused  in  the  surrounding  water. 

There  is  a  third  case  possible.  Suppose  that  in  a  given  cell  the  opposite  walls 
are  not  of  identical  structure;  that  the  wall  which  is  in  contact  with  the  damp  earth 
is  so  organized  as  to  allow  the  entrance  of  water,  but  not  its  filtration  to  the 
exterior,  while  the  opposite  wall  offers  only  a  slight  resistance  to  such  filtration; 
then  by  the  increasing  pressure  of  the  cell-contents  fluid  will  be  forced  through 
that  wall  which  offers  least  resistance,  and  the  greater  the  affinity  of  the  cell- 


CAPILLARITY   AND   ROOT-PRESSURE.  271 

contents  for  the  fluids  in  the  nutritive  earth,  the  more  abundantly  and  energetically 
will  this  be  carried  on.  The  phenomenon  can  be  well  seen  in  some  moulds, 
especially  Mucor  Mucedo,  which  makes  its  appearance  in  such  quantity  on 
succulent  fruits;  and  in  the  mycelium  of  the  so-called  Dry-rot,  Merulius  lacry- 
mans.  Fluids  are  sucked  up  by  the  lower  portions  of  the  tubular  cells  which  cover 
the  nutritive  substratum,  and  expelled  again  through  the  walls  of  upper  parts 
of  the  same  cells,  which  project  freely  into  the  air.  These  upper  portions  of  the 
mycelium  cells  appear  as  though  ornamented  with  tiny  dewdrops,  which  in  the 
case  of  the  Dry-rot  coalesce  and  attain  to  a  considerable  size.  Damp  woodwork  in 
cellars,  where  this  fungus  has  established  itself,  is  often  thickly  besprinkled  with 
the  drops  which  have  been  excreted  on  the  surface,  and  if  a  lamp  is  brought  into 
the  darkness,  and  the  infected  places  illuminated,  hundreds  of  these  tiny  drops 
sparkle  and  glitter  like  the  "jewels"  in  a  cave  of  stalactites.  Suppose  then  that 
such  a  cell,  one  wall  of  which  allows  fluid  to  enter,  is  attached  by  the  wall  opposite 
to  that  through  which  the  fluid  enters,  to  another  cell;  then  this  second  cell  will 
absorb  the  liquid,  and,  if  tubular,  the  sap  may  rise  higher  and  higher  in  it,  and  by 
the  pressure  of  the  liquid  continually  arising  from  below,  even  be  forced  through 
other  higher  cells  which  are  capable  of  filtration.  Naturally  the  rising  current  of 
sap  thus  generated  follows  the  line  of  the  least  resistance;  if  then  the  cell-tissue 
where  this  action  terminates  is  perforated  by  canals  ending  in  pores  on  the  surface, 
the  fluid  will  emerge  from  these  pores  in  the  form  of  drops.  This  actually  happens 
not  only  in  many  large-leaved  Aroids,  but  also  in  plants  growing  in  the  open 
country  if  the  air  which  passes  over  the  leafy  parts  above  the  ground  be  very 
humid,  and  the  soil  in  which  the  roots  are  buried  proportionately  warm.  In  many 
plants  with  succulent  foliage,  drops  of  water  may  be  seen  issuing  from  the  thin- 
walled  cells  and  pores  of  the  leaves  when  the  almost  saturated  air  becomes  cooled 
after  sunset,  while  the  soil,  round  about  the  absorbent  roots,  having  been  exposed 
all  the  day  to  the  sun's  rays,  retains  its  higher  temperature.  Young  blades  of 
corn  have  rows  of  such  drops,  which  look  exactly  like  dewdrops,  and  have  often 
been  mistaken  for  them.  This  extrusion  of  water  from  the  leaves  can  easily  be 
produced  artificially  by  placing  the  plants  in  a  saturated  atmosphere,  and  at  the 
same  time  slightly  warming  the  earth  round  the  roots.  There  is  no  doubt  that 
the  sap  which  exudes  from  the  leaf -pores  originates  in  the  nutritive  soil,  and  is 
taken  up  by  the  absorbent  cells  of  the  root;  from  these  the  vessels  and  cells  of 
the  main  root  and  stem,  through  which  the  sap  can  filter,  carry  it  up  to  the  leaves. 
If,  therefore,  we  cut  across  a  stem  a  little  distance  above  the  ground,  we  shall  see 
the  sap,  which  has  already  accomplished  half  its  journey,  welling  up  as  drops  on 
the  cut  surface;  i.e.  we  shall  see  the  remarkable  phenomenon  called  "weeping", 
of  which  mention  has  already  been  made.  The  quantity  of  sap  which  flows 
from  such  a  cut  surface  is  in  many  cases  astoundingly  great.  In  Java  certain 
Cissus  plants,  belonging  to  the  family  of  lianes  and  living  in  damp  woods,  are 
actually  made  use  of  as  vegetable  springs.  The  watery  sap  flows  so  abundantly 
from  a  cut  branch  that  in  a  very  short  time  it  will  fill  a  glass,  and  forms  a  cool  and 


272  CAPILLARITY   AND   ROOT-PRESSURE. 

refreshing  beverage.  Many  Araliaceae  also  furnish  a  sap  fit  for  drinking.  Some 
native  Indian  genera  which  are  used  as  vegetable  wells  have  on  this  account 
received  the  name  of  "plant  springs"  (Phytocrene,  e.g.  P.  gigantea  and  bracteata). 
If  the  young  flower-stalk  of  Agave  americana,  an  American  plant  which  is 
cultivated  in  European  gardens  under  the  name  of  the  "hundred  years'  aloe",  be 
cut  across,  in  twenty-four  hours  about  365  grammes,  and  in  a  week  more  than 
2500  grammes  of  sap  will  flow  out.  This  exudation  continues  for  four  to  five 
months,  and  a  vigorous  Agave  will  produce  in  this  time  as  much  as  50  kilogrammes 
of  sap,  which  will  ferment,  since  it  contains  both  sugar  and  albuminous  substances, 
and  is  indeed  used  by  the  Americans  in  the  preparation  of  an  intoxicating  drink 
called  "pulque".  The  quantity  of  sap  which  exudes  from  vines  is  also  very  great. 
A  branch  2J  cm.  thick,  cut  across  1J  m.  above  the  ground,  produced  within  a  week 
over  5  kil.  of  sap.  In  a  week,  from  the  cut  stem  of  a  rose,  more  than  1  kil.  was 
exuded.  From  maples  and  birches  a  proportionately  large  amount  of  sap  can  be 
obtained,  when  the  trunks  are  cut  about  a  metre  above  the  ground.  The  sap  which 
flows  from  species  of  maple  contains  pure  crystallizable  sugar,  and  in  some  North 
American  species  this  is  present  in  such  abundance  that  it  was  found  to  be  worth 
while  to  collect  the  sap,  at  least  in  former  times. 

It  should  be  noticed  that  the  volume  of  the  exuded  sap  is  in  all  these  cases 
greater  than  the  volume  of  the  root  together  with  that  of  the  stump  of  the  stem 
from  which  the  sap  is  forced  out,  and  this  is  a  proof  that  it  does  not  consist  only  of 
the  water  which  was  contained  in  the  root  and  stem  stump  at  the  time  of  cutting, 
but  that  there  is  a  continual  upward  current  of  sap,  and  that  the  absorbent  cells  of 
the  roots,  for  a  long  time  after  the  operation,  continue  to  draw  up  water  from  their 
environment. 

An  ingenious  experiment  was  performed  at  the  beginning  of  last  century  in 
order  to  ascertain  the  amount  of  pressure  by  means  of  which  the  sap  is  forced  from 
the  cut  surface  of  the  vine  and  other  stems.  A  vine  stem  without  branches  and 
about  the  thickness  of  one's  finger  was  cut  across  in  the  spring  at  a  height  of  about- 
80  cms.  above  the  ground,  and  on  the  root-stock  was  fixed  a  glass  tube  with  a 
double  bend,  in  such  a  way  that  one  end  fitted  exactly  over  the  cut  surface  of  the 
stump,  and  the  tube  was  then  filled  with  mercury.  By  the  pressure  of  the  sap 
which  welled  from  the  cut  surface  the  mercury  was  forced  up  the  tube,  and  in  a 
few  days  it  actually  reached  a  height  of  856  mm.  The  weight  of  a  column  of 
mercury  760  mm.  high  is  equal  to  that  of  a  column  of  air  as  high  as  the  atmosphere 
of  the  earth,  or  of  a  column  of  water  about  10*3  m.  high,  and  consequently  the 
pressure  by  which  the  sap  is  forced  out  of  the  vine  is  considerably  greater  than  the 
weight  of  one  atmosphere,  or  of  a  column  of  water  of  the  height  mentioned.  From 
these  data  it  has  been  estimated  that  the  sap  can  be  raised  through  11 '6  m.  by  the 
pressure  originating  in  the  absorbent  cells  of  the  root.  The  pressure  is  naturally 
greatest  in  the  lower  portions  of  a  stem,  and  gradually  diminishes  towards  the 
higher  regions;  the  ascending  current  of  sap  to  which  it  gives  rise  is  also  not 
uniform,  but  shows  daily,  and  even  hourly,  fluctuations.  Moreover,  the  quantity  of 


TRANSPIRATION.  273 

sap  exuded,  neglecting  these  said  fluctuations,  is  greatest  soon  after  the  stem  is  cut, 
and  then  becomes  gradually  less  until  finally  the  outflow  ceases  entirely  with  the 
death  of  the  stump. 

The  magnitude  of  the  pressure,  and  the  quantity  of  the  sap  forced  up  by  the 
absorptive  power  of  the  cells,  vary  with  the  circumstances  of  the  plants  considered. 
The  pressure  appears  to  be  greatest  in  species  of  vine,  and  in  the  vine  stem,  as 
already  remarked,  it  will  support  the  weight  of  a  column  of  mercury  856  mm.  high. 
In  the  stem  of  the  Foxglove  it  equals  the  pressure  of  a  column  of  mercury  461  mm. 
high;  in  the  stem  of  the  nettle  the  column  is  354  mm.;  in  the  poppy  stem  212  mm.; 
in  the  stem  of  a  bean  159  mm.;  and  in  the  trunk  of  the  White  Mulberry  tree  12 
mm.  high.  In  the  majority  of  herbaceous  plants  this  pressure  is  quite  sufficient  to 
drive  the  sap  from  the  root-tips  up  to  the  leaves  and  top  of  the  stem;  but  this  is 
not  the  case  with  leafy  trees  and  pines,  with  palms  and  creeping  and  climbing 
plants.  Although  watery  fluid  can  be  raised  according  to  the  above  calculation  to 
a  height  of  11 '6  m.  by  root-pressure,  there  is  still  a  great  distance  between  this  level 
and  the  leaves  of  such  trees  and  climbing  plants,  which  may  be  as  much  as  160  m. 
high;  and  the  question  which  presents  itself  is  this:  By  what  means  is  the  sap 
carried  to  the  higher  regions  from  this  level  to  which  it  is  raised  by  root-pressure? 

It  may  be  supposed  that  cells  are  present  at  the  various  heights  in  the  stem  to 
which  the  water  is  driven,  which  act  in  a  manner  similar  to  those  of  the  root;  i.e. 
cells  which  actively  absorb,  whose  cell-wall  on  one  side  only  slightly  resists  filtra- 
tion, and  which  therefore  are  able  to  force  up  the  sap  a  little  higher.  The  results  of 
the  following  experiments  seem  to  support  such  a  supposition.  If  a  piece  of  a 
branch  be  cut  from  the  middle  portion  of  a  tree,  and  the  lower  end  be  peeled  and 
placed  in  water,  sap  will  flow  out  from  the  upper  cut  surface  with  considerable 
force.  The  same  thing  occurs  when  a  leafy  branch  is  placed  in  water  so  that  its 
leaves  are  submerged,  while  the  upper  cut  piece  of  the  branch  projects  a  good  way 
out  of  the  water.  In  this  case  the  cells  of  the  leaves  must  function  as  the  absorptive 
cells.  However,  even  if,  as  is  probable,  parenchymatous  cells  are  to  be  found  at  all 
levels  of  the  plant  stem  behaving  exactly  like  the  absorptive  cells  of  the  root, 
this  arrangement  would  scarcely  suffice  in  all  cases  to  carry  the  sap  to  its  destination. 
Atmospheric  pressure  as  well  as  the  rarefaction  of  the  air  observed  in  the  vessels  of 
the  stem  during  the  summer  have  been  made  use  of  in  explaining  the  upward 
current  of  the  sap,  and  this  r61e  may  actually  belong  to  these  factors;  but  all  these 
mechanical  powers  are  quite  overshadowed  by  that  one  which  has  been  termed  by 
botanists  "Transpiration". 

TRANSPIRATION. 

By  transpiration  of  plants  we  mean  the  act  of  giving  off  aqueous  vapour  to  the 
surrounding  air— briefly  and  in  plain  terms,  the  perspiring  of  plants.  Vapour 
escapes  from  the  cells  of  the  plant  which  are  in  contact  with  the  air,  the  formation 
of  these  cells  being  specially  adapted  to  the  process  of  evaporation,  just  as  it  is  given 

VOL.  I. 


274  TRANSPIRATION. 

off  from  moist  inorganic  bodies  and  exposed  liquids.  Of  the  materials  which  are 
held  in  solution  in  the  sap  of  plants,  only  those  which  have  the  property  of  passing 
from  the  fluid  to  the  gaseous  condition,  at  the  same  temperature  which  transforms 
water  into  water-vapour,  can  evaporate  with  this  fluid.  All  the  others  remain 
behind,  and  the  natural  consequence  is  that  the  sap  in  the  transpiring  cells  becomes 
more  concentrated.  If  water,  which  contains  in  solution  extremely  small  quantities 
of  sugar,  organic  acids,  nitric,  sulphuric  and  phosphoric  acids,  and  salts  of  potassium, 
calcium,  and  iron,  be  set  to  evaporate  slowly  in  a  shallow  dish,  it  will  gradually 
come  about  that  only  a  thin  layer  of  fluid  is  left  on  the  bottom  of  the  dish;  but  this 
now  is  seen  to  consist  of  a  very  concentrated  solution  of  the  substances  mentioned ; 
i.e.  of  the  sugar,  organic  acids,  and  the  various  salts.  It  has  also  all  the  properties 
of  such  a  concentrated  solution,  i.e.  it  has  the  power  of  sucking  in  water  in  the 
liquid  condition  from  its  surroundings.  In  the  same  way  the  contents  of  a  cell  in 
contact  with  the  air  become  more  concentrated  by  evaporation,  and  thus  obtain  the 
power  of  abstracting  water  from  the  environment  of  the  cell,  that  is  to  say,  of  suck- 
ing it  up.  If  two  adjacent  cells  contain  sap  of  the  same  density,  whilst  only  one  of 
them  has  the  power  of  exhaling  water,  the  condition  of  equilibrium  between  them 
will  be  destroyed.  However,  the  balance  naturally  tends  to  be  restored,  and  the  cell 
whose  sap  has  become  more  concentrated  by  the  evaporation  of  water,  takes  up 
watery  fluid  from  the  neighbouring  cell.  Now  picture  a  chain  of  cells  containing 
abundance  of  sap  connected  with  one  another  by  cell-walls  through  which  fluid  can 
filter,  and  let  them  be  so  arranged  that  only  the  uppermost  member  of  the  chain  is 
in  contact  with  the  atmospheric  air.  The  sap  of  this  uppermost  cell  having  become 
concentrated  by  evaporation  will  first  of  all  exert  a  suction  on  the  cell  immediately 
below.  As  fluid  is  withdrawn  from  this  second  cell,  its  sap  also  undergoes  concen- 
tration, and  in  consequence  produces  suction  on  the  third  cell,  the  third  in  like 
manner  on  the  fourth,  the  fourth  on  the  fifth,  &c.,  passing  from  above  downwards. 
In  this  way  innumerable  compensating  currents  are  set  up  between  the  adjoining 
cells,  which,  however,  never  lead  to  true  equilibrium  as  long  as  evaporation  con- 
tinues in  the  cell  in  contact  with  the  air,  but  combine  together  to  form  a  single 
ascending  stream. 

Such  a  current  actually  exists  in  all  living  plants  which  evaporate  from  the 
portions  above  the  ground  and  in  contact  with  the  air,  while  their  lower  extremities 
are  embedded  in  a  damp  nutritious  soil.  This  has  been  termed  the  Transpiration 
Current.  Its  source  is  the  fluid  which  has  been  drawn  from  the  earth  by  the 
absorptive  cells  and  brought  within  the  sphere  of  the  living  cells  of  the  plant;  we 
may  retain  for  this  fluid  the  old  and  very  appropriate  name  "crude"  or  "raw  sap". 
Its  direction  and  destination  are  determined  by  the  position  of  the  evaporating  cells, 
and  its  path  is  through  the  wood,  which  in  tree-trunks  is  inserted  as  a  huge  layer 
between  the  bark  and  the  pith;  in  lesser  stems  it  passes  through  the  bundles  and 
strands  of  woody  cells  and  vessels  which  traverse  them,  being  connected,  deep  under 
the  ground,  by  groups  of  parenchymatous  cells,  with  the  absorptive  cells  of  the 
young  rootlets,  or  with  the  hyphse  of  the  mycelial  mantle,  which  replace  the 


TRANSPIRATION. 


Fig.  60.— Olive  Grove  on  the  Shores  of  Lake  Garda. 


276  TRANSPIRATION. 

absorptive  cells  (beech,  &c.).  These  bundles  pass  above  into  the  leaves,  forming  there 
the  "  veins  "  of  the  leaf-blade,  which  spread  out  into  an  extremely  fine  network  of 
tiny  strands,  and  terminate  quite  close  to  the  evaporating  cells  on  the  surface.  That 
the  wood  actually  forms  the  conducting  tissue  of  the  transpiration  current  is  satis- 
factorily demonstrated  by  the  existence  of  old  trees  whose  trunks  have  long  been 
hollow,  whose  pith  is  disintegrated  and  fallen  away,  and  which  have  also  been  de- 
prived of  bark  around  their  base.  In  the  olive  plantations  at  Lake  Garda,  one  of 
which  is  reproduced  in  figure  60,  many  trees  are  to  be  seen  in  which  the  lower  part 
of  the  trunk  is  not  only  hollow  and  without  bark,  but  is  also  often  tunnelled  and 
split,  so  that  the  upper  part  of  the  tree  looks  as  if  it  were  raised  on  stilts.  The  only 
communication  between  the  soil  and  the  upper  part  of  the  tree  is  by  means  of  these 
props,  which  are  continuous  with  the  roots  below  and  are  composed  entirely  of 
woody  cells  and  vessels.  And  yet  these  olive-trees  are  still  vigorous,  putting  out 
new  branches  and  leaves  every  year,  and  blossoming  and  producing  fruit;  and 
they  derive  their  necessary  food  from  the  ground  by  supplies  which  have  no  other 
upward  path  than  the  wood  of  these  props. 

Moreover,  by  repeated  experiments  it  has  been  proved  that  the  bundles  of  woody 
cells  and  vessels  which  are  united  together  into  a  woody  cylinder,  inserted  between 
the  pith  and  the  cortex  in  the  trunks  and  stems  of  trees  and  shrubs,  serve  as  con- 
ductors of  the  transpiration  current.  If  a  ring  of  cortex  is  removed  from  the  stem 
of  a  leafy  plant,  whose  leaves  are  transpiring  in  dry  air,  and  are  supplied  with 
water  from  below  by  the  transpiration  current,  this  flow  of  sap  to  the  leaves  will 
not  be  interrupted,  and  the  leaves  remain  firm  and  tense.  But  as  soon  as  a  piece  of 
the  wood  is  removed  or  the  above-mentioned  strands  are  cut  through,  even  though 
the  cortex  be  left  entire,  the  flow  to  the  leaves  stops  immediately,  and  they  become 
flaccid  and  hang  down  in  a  withered  condition. 

The  cellular  formations  of  the  wood  and  strands,  which  function  as  the  con- 
ductors of  the  crude  nutritive  sap  to  the  leaves,  are — as  already  mentioned — wood- 
cells  and  wood-vessels.  Formerly  the  idea  was  held  that  these  structures  served  for 
the  passage  of  air,  and  it  was  believed  that  they  were  analogous  to  the  respiratory 
organs — the  so-called  tracheae — of  insects;  therefore  these  wood-vessels  were  also 
called  "tracheae",  and  the  wood-cells  "tracheides".  The  wood-cells  are  elongated 
chambers,  on  an  average  1  mm.  long  and  O'05-O'l  mm.  broad,  and  their  walls  are 
unequally  thickened,  either  by  reticulate  or  annular  bands,  or  spiral  threads  project- 
ing slightly  from  the  inner  wall  into  the  lumen,  or  by  so-called  bordered  pits,  which 
are  represented  in  fig.  101  and  fig.  102.  The  wood-vessels  are  tubular,  and  very  long 
in  proportion  to  their  width,  which  is  never  more  than  a  fraction  of  a  millimetre; 
they  extend  uninterruptedly  through  stalks,  branches,  leaves,  perhaps  even  through 
the  entire  plant  from  the  root-tip  to  the  crown.  They  are  composed  of  rows  of  cells 
whose  separation  walls  have  been  broken  down.  The  walls  of  the  wood-vessels 
exhibit  similar  thickenings  to  those  of  the  wood-cells  or  tracheides.  When  the 
chambers  and  tubes  of  the  wood,  with  their  bordered  pits  and  projecting  bands,  are 
fully  developed,  the  living  protoplasm  which  carried  on  the  building  forsakes  the 


TRANSPIRATION.  277 

scenes  of  its  activity,  and  consequently  in  fully  formed  wood-cells  and  vessels  living 
protoplasmic  contents  are  wanting.  They  must  be  regarded  in  a  certain  sense  as 
dead  structures,  for  they  have  no  further  power  of  growth,  and  the  reciprocal 
pressure  of  wall  and  contents  observable  in  absorptive  cells  and  other  cell-cavities 
occupied  by  living  protoplasm,  which  has  been  termed  "turgescence",  is  never  seen 
in  them. 

In  the  walls  of  the  wood-cells  as  well  as  of  the  vessels,  woody  material  (Lignin) 
is  deposited.  It  appears  to  be  in  consequence  of  this  that  they  are  much  less 
capable  of  swelling  than  are  cell-walls  which  consist  chiefly  of  cellulose.  The 
amount  of  sap  which  presses  its  way  in  between  the  groups  of  molecules  of  the 
lignified  walls,  and  with  which  these  walls  are  saturated,  is  also  comparatively  very 
small.  On  the  other  hand,  of  course,  this  imbibed  sap  is  conducted  much  more 
quickly  through  the  lignified  walls  of  the  cell  chambers  and  tubes  than  through  non- 
lignified  walls.  More  fluid  is  carried  up  by  the  intermolecular  stream  through  the 
woody  walls  of  the  cells  and  vessels  than  by  the  ascension  of  the  raw  nutritive  sap 
in  the  interior  of  the  wood-cells  and  tubes.  If  no  evaporation  is  going  on  from  the 
leaves,  or  if  this  is  only  very  slight,  the  vessels  and  cells  become  filled  with  sap.  As 
soon  as  transpiration  becomes  active,  part  of  the  sap  is  taken  up,  and  if  fresh 
supplies  do  not  arrive  quickly  enough  a  limited  amount  of  air  can  get  in  temporarily, 
which  of  course  must  be  in  a  very  rarefied  condition  on  account  of  the  obstacles 
which  oppose  its  entrance.  The  passage  of  the  sap  is  quicker  through  the  non- 
suptate  vessels  than  through  the  much  shorter  woody  cells.  The  sap  on  its  way 
through  the  latter,  to  the  transpiring  leaves,  must  filter  through  innumerable  trans- 
verse walls.  This  filtration  will  of  course  be  materially  helped  by  the  bordered  pits 
with  which  the  wood -cells  are  so  regularly  provided;  for  the  extremely  delicate 
membrane  which  is  stretched  between  the  two  cavities  of  such  an  apparatus  at  any 
rate  allows  the  sap  to  pass  through  very  easily.  The  bordered  pits  are  exactly  like 
clack-valves,  and  they  also  appear  to  regulate  the  sap-stream,  though  the  way  in 
which  they  do  this  is  not  yet  completely  understood.  The  nearer  the  path  of  the 
raw  sap  approaches  to  the  spots  in  which  evaporation  is  being  carried  on,  the  greater 
is  the  number  of  cells  in  the  sap-conducting  strands,  while  the  vessels  in  the  same 
become  fewer  and  fewer.  The  termination  of  the  whole  sap-conducting  apparatus 
consists  entirely  of  cells  whose  walls  are  stiffened  by  spiral  bands  on  the  inside. 
Between  this  termination  and  the  transpiring  cells  some  parenchymatous  cells  with 
living  protoplasmic  cell-contents  are  interposed,  whereas,  it  must  again  be  insisted, 
the  tubes  and  chambers  composing  the  sap-conducting  apparatus  have  no  living 
protoplasm  in  their  interior. 

The  whole  mechanism  for  the  transmission  of  the  raw  nutritive  sap  may  be  con- 
sidered as  a  system  of  tubes  and  chambers  provided  with  clack-valves,  into  which 
the  fluid  taken  up  by  the  absorbent  root-cells  is  forced,  and  through  which  it  is  con- 
ducted to  the  transpiring  cells  of  the  green  leaves  or  of  the  green  cortex,  which  takes 
the  place  of  the  green  leaves  in  leafless  branches.  This  does  not  exclude  the  activity 
of  cells  at  certain  levels,  as  it  were  at  intermediate  stages  of  the  road  traversed  by 


278 


TRANSPIRATION. 


the  current,  which  have  the  power  of  invigorating  the  stream,  of  hastening  it  if 
necessary,  and  also  of  lessening  it  under  certain  circumstances.  Also  it  is  arranged 
that  in  case  of  need  fluid  nourishment  in  the  higher  regions  of  the  stem  may  reach 
the  leaves  by  side  paths. 

The  cells  which  by  means  of  the  exhalation  of  aqueous  vapour  into  the  atmos- 
phere originate  the  transpiration-current  are,  as  already  mentioned,  not  far  from  the 
terminations  of  the  sap-conducting  apparatus.  In  some  mosses  they  are  freely  ex- 
posed to  the  air.  In  the  Polytrichacese  and  several  other  mosses  (Barbula  aloides, 
ambigua,  rigida)  they  form  short  chains  of  cells  like  strings  of  pearls,  or  bands 
projecting  from  the  grooved  concave  upper  surface  of  the  tiny  leaves  (see  fig.  61  2). 
Again,  among  the  liverworts  are  forms,  e.g.  Marchantia  polymorpha,  which  contain 
large  characteristic  air-chambers  in  the  body  of  their  green  leaf -like  thallus  (fig. 
61  *).  On  the  floor  of  this  chamber  are  green  cells  which  are  so  grouped  together 


Fig.  61.— Transpiring  Cells. 

i  Vertical  section  through  an  air-chamber  of  the  Liverwort  Marchantia  polymorpha;  x300.    2  Vertical  section  through 

a  leaf  of  the  Moss  Barbula  aloides;  x  380. 

as  to  remind  one  of  the  shape  of  the  Prickly  Pear  (Opuntia).  These  green  cells  are 
thin-walled,  and  it  is  from  them  that  water  is  evaporated.  They  are  not  quite 
freely  exposed,  like  those  of  the  mosses  mentioned  above,  since  the  roof  of  the 
chamber,  composed  of  transparent  cells,  is  extended  over  them;  a  chimney-shaped 
passage,  however,  is  left  open  through  the  roof  of  each  chamber  by  which  the  water- 
vapour  given  off  from  the  opuntia-like  cells  can  escape.  These  Marchantias  furnish 
a  transitional  form  between  the  freely  exposed  transpiring  cells  on  the  upper  surface 
of  the  leaf  of  the  moss  and  those  of  flowering  plants.  In  flowering  plants  the 
transpiring  cells  are  situated  as  a  rule  in  the  interior  of  the  green  leaves,  and  also 
in  the  green  cortex  of  leafless  branches,  forming  a  part  of  that  green  tissue  which 
has  been  termed  chlorenchyma,  or  when  in  the  leaves,  mesophyll. 

Leaves  may  be  described  as  consisting  of  cells  filled  with  leaf -green,  or  chloro- 
phyll, placed  closely  together  and  joined  into  layers  above  one  another  so  as  to 
form  a  soft  mass  of  tissue  containing  abundance  of  sap;  this  green  tissue  pierced 
by  the  branched  water-conducting  strands  whose  ultimate  divisions  terminate  in  the 
tissue  mass;  the  whole  surrounded  and  shut  in  by  a  firm  cuticle  which  is  perforated 
in  many  places  by  stomata.  Cellular  passages  are  also  regularly  arranged  for  the 
purpose  of  conducting  away  the  organic  materials  manufactured  in  the  green  cells, 
whilst  groups  of  cells  for  the  support  of  the  whole,  serving  as  beams,  strengthening 
props,  and  the  like,  are  placed  at  definite  points. 


TRANSPIRATION. 


279 


'  In  most  thin  membraneous  leaves  the  upper  and  under  sides  are  differently 
constructed,  and  the  difference  is  not  confined  only  to  the  cuticle,  but  is  also  plainly 
recognizable  in  the  green  tissue.  The  green  cells  below  the  epidermis  on  the  upper 
side  of  the  leaf  have  the  form  of  prisms,  cylinders,  or  short  tubes,  and  are  arranged 
very  regularly  in  ranks  and  files.  In  the  leaves  of  plants  belonging  to  the  lily 
tribe,  they  lie  with  their  long  axes  parallel  to  the  surface;  but  in  most  other  plants 
these  cylindrical  cells  have  their  smaller  side  directed  to  the  surface,  and  stand  side 
by  side  like  palisades,  with  only  very  narrow  air-passages  between  them.  Below 
these  palisade-cells,  and  bordering  on  the  epidermis  of  the  under  side  of  the  leaf,  is 
another  stratum  of  cells  of  a  much  looser  texture  (see  fig.  62 1).  The  cells  of  this 
under  layer  are  not  so  crammed  with  chlorophyll,  and  therefore  appear  a  lighter 


Fig.  62.— Spongy  Tissue. 

i  Vertical  section  through  leaf  of  Franciscea  eximia.  2  Spongy  tissue  in  leaf  of  Daphne  Laureola.—the  epidermis  and 
palisade  cells  of  the  upper  side  of  the  leaf  are  removed.  The  epidermis  of  the  under  side  of  the  leaf,  with  its  stomata, 
can  be  seen  through  the  spaces  in  the  spongy  tissue;  X320. 

green  than  the  palisade-cells.  In  shape  they  are  elliptical,  rounded,  angular,  sinuous, 
or  generally  very  irregular;  usually  they  possess  protuberances  which  project  in 
various  directions,  and  they  are  so  arranged  that  the  outgrowths  of  adjoining  cells 
come  into  contact  with  one  another.  It  looks  as  if  the  neighbouring  cells  were 
stretching  out  their  arms  and  extending  their  hands  to  one  another,  and  consequently 
these  cells  have  been  called  "  many-armed  cells  ".  When  several  adjoining  stellate 
cells  are  connected  together  in  the  manner  just  described  so  as  to  form  a  tissue, 
lacunae  and  passages  are  seen  in  the  tissue,  which  are  broken  through  by  the  joined 
arms  of  neighbouring  cells  as  if  by  pillars,  couplings,  and  bridges.  The  whole  tissue- 
has  the  loose  perforated  appearance  of  a  bath  sponge,  and  is  called  accordingly 
spongy  tissue,  or  spongy  parenchyma  (see  fig.  62 2). 

This  spongy  tissue  is  the  proper  place  for  transpiration.  Nowhere  else  in  the 
plant  are  the  conditions  governing  this  process  so  well  fulfilled  as  just  here.  The 
surfaces  of  the  cells  are  rendered  large  in  proportion  to  their  size  by  their  out- 
growths; and  they  impinge  as  far  as  possible  on  the  larger  or  smaller  lacunae, 
gaps,  and  passages  filled  with  air,  which  all  communicate  with  one  another,  thus 
constituting  an  unmistakable  ventilating  system. 

Since  the  spongy  parenchyma  in  the  leaves  described  does  not  lie  freely  exposed, 


280  TRANSPIRATION. 

but  is  shut  off  from  the  atmosphere  by  a  firm  cuticle  through  which  water- vapour 
can  only  penetrate  with  great  difficulty,  the  aqueous  vapour  which  is  exhaled  by 
the  branched  and  other  cells  of  this  parenchyma  would  saturate  the  lacunae,  and 
further  evaporation  would  be  thereby  prevented.  There  must,  therefore,  be  a 
direct  communication  with  the  outer  air  surrounding  the  leaf;  the  epidermis  of  the 
leaf  must  possess  apertures  through  which  the  water- vapour  can  escape.  The  already 
repeatedly  mentioned  stomata  are  to  be  looked  upon  as  such  apertures. 

Stomata  arise  in  this  way;  in  a  particular  epidermal  cell  a  partition  wall  first  of 
all  divides  it  into  two  cells.  This  cell-wall  splits,  and  the  cleft  widens,  forming 
a  short  canal  which  pierces  the  epidermis,  and  constitutes  a  connection  between  the 
outer  air  and  the  air-containing  lacunae  in  the  interior  of  the  leaf.  This  short  canal 
is  called  the  pore  of  the  stoma,  and  the  two  cells  which  border  it  are  termed  guard 
cells.  These  two  cells  regulate  the  outrush  of  aqueous  vapour,  i.e.  of  that  vapour 
which  has  been  excreted  by  the  thin- walled  cells  of  the  spongy  parenchyma,  and 
passed  into  the  adjoining  passages  in  the  interior  of  the  leaf.  That  cavity  which  is 
placed  immediately  beneath  the  narrow,  short  canal  of  the  stoma,  and  is  connected 
by  passages  with  other  spaces  further  within  the  green  tissue  of  the  leaf,  is  termed 
the  respiratory  cavity. 

The  number  of  the  stomata  or  transpiration-pores  which  pierce  the  epidermis  of 
the  leaf  varies  very  considerably.  In  the  leaves  of  cabbages  (Brassica  oleracea)  on 
1  sq.  mm.  of  the  upper  surface  there  are  nearly  400,  and  on  the  under  side  over  700. 
In  the  leaves  of  the  olive-tree,  on  the  same  extent  of  surface  of  the  under  side,  over 
600.  Succulent  plants  have  remarkably  few  stomata.  On  1  sq.  mm.  of  the  leaves 
of  the  House-leek  (Sempervivum  tectorum)  and  of  the  yellow  Stone-crop  (Sedum 
acre)  only  10-20  are  to  be  met  with.  In  the  majority  of  cases,  on  a  similar  extent 
of  surface,  between  200  and  300  stomata  are  to  be  found.  The  under  side  of  an 
oak  leaf,  50  sq.  cms.  in  area,  showed  over  two  million  stomata.  They  are  in  most 
cases  scattered  fairly  uniformly  over  the  surface  of  the  leaf;  on  the  leaves  of  grasses 
•and  pines,  as  well  as  on  the  green  stalks  of  the  horsetails,  they  form  straight 
regular  rows  which  run  longitudinally;  on  the  leaves  of  some  species  of  saxifrage 
(Saxifraga  sarmentosa,  japonica,  &c.)  they  appear  crowded  together  in  small 
isolated  groups;  and  on  the  leaves  of  the  Begonia  they  are  generally  to  be  seen  side 
by  side  in  pairs.  Obviously  they  are  principally  developed  just  where  the  epidermis 
overlies  spongy  parenchyma,  and  as  in  the  majority  of  cases  this  parenchyma  is 
situated  towards  the  under  side  of  the  leaf,  the  greater  number  of  stomata  are  to  be 
found  on  this  side. 

In  most  flat  membraneous  leaves,  which  have  one  side  directed  towards  the  sky 
and  one  towards  the  earth,  stomata  are  entirely  wanting  on  the  upper  surface, 
being  restricted  to  the  under  side.  An  exception  to  this  is  afforded  by  the  orbicular 
flat  leaves  which  float  on  the  surface  of  water,  e.g.  those  of  the  floating  Pond-weed 
(Potamogeton  natans),  of  the  Frogbit  (Hydrocharis  morsus-rance),  and  of  the 
water-lilies  (Nymphcea,  Nuphar,  Victoria).  These  are  covered  with  stomata  on  the 
upper  side,  while  on  the  lower  side,  which  is  in  contact  with  the  water,  stomata  are 


TRANSPIRATION.  281 

entirely  absent.  On  the  upright  leaves  of  flags,  asphodels,  amaryllis,  and  various 
other  bulbous  plants,  and  on  the  vertical  leaf -like  structures  (phyllodes)  of  the 
Australian  acacias,  as  well  as  on  some  of  the  needle -like  leaves  of  conifers,  the 
stomata  occur  on  both  sides  in  almost  equal  number.  In  the  mimosas  and  various 
other  plants,  having,  in  common  with  the  mimosas,  the  characteristic  faculty  of 
altering  the  position  of  their  leaflets  when  stimulated  externally,  numerous 
stomata  are  found  on  both  sides  of  the  leaf. 

Most  stomata  are  elliptical  when  open;  rarely  circular  or  linear.  The  length  of 
stomates  varies  between  0'02  and  0'08  mm.,  the  breadth  between  O'Ol  and  0*08  mm. 
Pines,  orchids,  lilies,  and  grasses  have  the  largest  stomata;  water-lilies,  olives,  and 
some  fig-trees,  the  smallest. 

The  stomata  in  the  epidermis,  the  passages  and  cavities  below  them  into  which 
the  thin-walled  cells  of  the  green  tissue  evaporate  water,  and  the  strands  through 
which  the  sap  is  conducted  from  the  roots  to  the  green  tissue,  all  work  in  connec- 
tion with  one  another  like  the  various  parts  of  a  machine.  Each  portion  of  the 
mechanism  helps  and  depends  upon  the  others,  the  immediate  result  of  the  common 
work  being  always  the  elevation  of  that  nutritive  fluid  which  is  brought  by  the 
absorptive  roots  into  the  plant.  In  the  main,  therefore,  the  result  obtained  by 
transpiration  is  the  same  as  that  which  root-pressure  aims  at,  and  it  might  be 
thought  (taking  for  granted  the  truth  of  the  above  statement)  that  either  root- 
pressure  or  transpiration  is  superfluous.  Or  perhaps  transpiration  and  root-pressure 
work  in  a  complementary  manner  together.  Perhaps  the  conditions  between  the 
two  forces  are  so  arranged  that  the  fluid  taken  in  by  the  absorptive  cells  from  the 
nutritive  soil  is  forced  up  to  a  certain  level  by  root-pressure,  and  from  thence  is 
promoted  to  still  higher  levels  by  means  of  transpiration?  This  would  suggest  a 
comparison  with  the  raising  of  water  from  a  spring  situated  in  a  valley-basin  sur- 
rounded and  shut  in  by  mountains.  In  the  depth  of  the  basin  exists  underground 
water  which  is  fed  by  the  subterranean  supply  coming  from  the  mountains.  Ac- 
cording to  the  pressure  of  this  supply,  the  water  in  the  lower  earth-strata  of  the 
basin  rises  to  a  certain  height.  This  pressure  is  not  strong  enough,  however,  to 
drive  the  water  to  the  surface  of  the  basin,  and  in  order  that  it  may  reach  this,  it 
is  necessary  to  employ  a  pump,  which  will  reach  down  to  that  stratum  of  earth 
which  is  saturated  by  the  underground  water.  But  the  level  of  this  water 
differs  in  summer  and  winter.  It  depends  also  upon  the  amount  of  rainfall  on  the 
neighbouring  mountains,  which  may  undergo  great  fluctuations.  In  some  years  the 
underground  water  in  the  spring  has  almost  risen  to  the  upper  opening;  at  other 
times  only  the  deepest  strata  of  the  valley-basin  contain  water.  The  pump,  by 
which  the  water  has  to  be  raised,  must  be  constructed  with  all  these  possibilities  in 
view,  and  must  be  so  regulated  that  the  absorbent  action  is  felt  as  far  down  as  the 
deepest  position  which  the  underground  water  is  known  to  take. 

Transpiration  behaves  in  like  manner  in  the  portions  of  a  plant  above  ground, 
and  its  action  on  the  fluid  food  taken  in  by  the  roots  may  be  compared  with  that  of 
a  suction-pump.  It  would  be  a  quite  inadequate  arrangement  if  the  sucking  actioo 


282 


TRANSPIRATION. 


produced  by  transpiration  could  only  reach  down  to  the  highest  level  attained  by 
the  water  which  has  been  forced  up  by  root-pressure,  and  precautions  must  be 
taken  that,  in  case  of  the  abatement  of  the  root-pressure,  water  would  be  raised 
from  the  lower  positions  up  to  the  transpiring  cells,  and  that  under  certain  condi- 
tions the  action  of  transpiration  should  reach  even  to  the  absorbent  cells  at  the 
root-tips.  It  has  been  shown  by  experiments  that  plants  with  large  leaves  lose  in 
the  summer  more  water  by  transpiration  than  is  forced  up  into  the  stem  by  root- 
pressure,  and  yet  the  leaves  do  not  become  faded.  The  conclusion  drawn  from  this 
is  that  at  certain  times  the  effect  of  transpiration  makes  itself  felt  down  from  the 
leaves  through  the  stem  as  far  as  the  root-tips.  It  has  also  been  shown  that  in 
many  plants,  just  when  the  most  active  evaporation  is  taking  place  in  the  leaves, 
none,  or  only  very  little  sap  is  forced  into  the  stem  by  root-pressure.  If  the  stem 
of  a  vine  be  cut  across  in  the  height  of  summer,  when  the  green  leaves  have  been 
unfolded  some  time  and  are  transpiring  actively,  no  "  tears "  are  seen  on  the  cut 
surface  of  the  stump,  no  drops  are  pressed  out.  The  vessels  contain  rarefied  air 
but  no  sap,  and  water  can  be  sucked  through  the  stump  by  the  vessels  even  in  the 
direction  of  the  root. 

Let  us  pause  here  in  order  to  get  a  clear  idea  of  the  relations  between  transpira- 
tion and  root-pressure.  Given  the  conditions  for  an  abundant  evaporation  from 
the  aerial  portions  of  a  plant — i.e.  a  fairly  dry  air,  water,  and  an  appropriate 
development  of  transpiring  surface — then  the  action  of  root-pressure  is  diminished, 
while  that  of  transpiration  is  increased,  and  governs  the  whole  of  the  movement  of 
the  sap.  If,  on  the  other  hand,  the  conditions  for  evaporation  from  the  aerial  por- 
tions of  the  plant  are  unfavourable — if  the  air  is  very  damp,  or  if  the  branches  of 
the  plant  are  not  yet  in  leaf — then  root-pressure  comes  into  play,  and,  supported  by 
cells  with  absorbent  contents  which  occur  in  the  higher  regions  of  the  plant,  can 
force  up  the  sap  to  the  tops  of  trees  and  to  the  highest  shoots  of  vine-branches 
which  remain  leafless  all  the  winter.  So  far,  therefore,  root-pressure  can  supersede 
and  replace  transpiration,  a  fact  of  great  importance  in  places  where  the  air  is 
sometimes  very  damp,  and  in  countries  where  the  trees  and  lianes  shed  their  leaves 
in  autumn;  at  the  commencement  of  the  next  period  of  vegetation  they  have  not 
yet  put  out  their  new  foliage,  and  therefore  do  not  possess  a  sufficiently  large  tran- 
spiring surface.  It  is  very  probable  that  in  the  autumn,  when  preparing  for  the 
winter,  certain  cells  in  trees  and  lianes  provide  themselves  with  materials  by  means 
of  which  in  the  coming  spring  they  may  exercise  a  very  strong  sucking  action. 
This  would  also  partly  explain  how  it  comes  about  that  in  the  spring  there  is  such 
a  strong  upward  current  of  sap  in  the  still  leafless  trees  and  vine  branches,  and 
that  the  water  is  conducted  up  even  to  the  topmost  shoots  of  lianes  100  metres 
long,  which  have  shed  all  their  leaves  ir>  the  previous  autumn. 

A  perfect  substitute  for  transpiration  in  the  form  of  the  pressure  produced  by 
the  absorbent  cells  is  seen  in  moulds,  in  the  already-mentioned  dry-rot  fungus,  and 
generally  in  leafless  cryptogams:  possibly  also  in  those  orchids  possessing  neither 
green  leaves  nor  stomata,  and  in  other  humus  plants  Csaprophytes)  such  as  the 


TRANSPIRATION.  283 

Monotropa,  mentioned  earlier  on,  which  stands  in  such  a  peculiar  relation  to  the 
mycelium  of  fungi.  On  the  other  hand,  in  most  green  flowering  plants  which  bear 
leaves,  a  complete  replacement  of  transpiration,  continuing  for  a  long  time,  is  not  an 
advantage.  Experience  has  shown  that  green  leafy  plants,  when  kept  for  a  long 
while  in  an  atmosphere  saturated  with  vapour,  cease  to  grow  and  become  unhealthy; 
they  lose  their  leaves,  and  at  length  succumb  altogether.  This  happens  even  if  the 
amount  of  light,  the  temperature  of  the  atmosphere  and  of  the  earth,  the  composi- 
tion and  humidity  of  the  soil,  in  short,  if  all  the  other  conditions  of  life  are  the 
most  favourable  that  can  be  imagined  for  the  plants  in  question.  It  follows  from 
this  that  it  is  not  immaterial  to  leafy  plants  how  the  sap  reaches  the  leaves, 
whether  it  is  drawn  up  by  transpiration,  or  forced  up  by  root-pressure.  If  the  leaf 
transpires,  water,  in  the  form  of  vapour  only,  is  given  off  to  the  atmosphere ;  all  the 
materials  which  have  been  brought  in  solution  from  below  to  the  leaves  remain 
behind  in  the  cells  of  the  leaf.  If,  on  the  other  hand,  fluid  water  is  pressed  from 
the  pores  of  the  leaves  by  root-pressure,  salts,  sugar,  and  other  compounds  are  always 
to  be  found  in  the  exuded  drops,  having  passed  through  the  cell-wall  in  solution  in 
the  water.  When  it  is  a  question  of  secreting  sugar  as  a  means  of  alluring  insects, 
or  salts  for  a  protective  covering,  such  an  exudation  cannot  advantageously  be  given 
up,  but  is  on  the  contrary  a  fundamental  part  of  the  economy  of  the  whole  plant. 
If  this  is  not  the  case,  and  if  materials  which  have  a  part  to  perform  in  the  leaf  by 
the  formation  of  organic  substances  are  exuded  with  the  drops  of  water,  and  the 
drops  falling  from  the  surface  of  the  plant  trickle  to  the  ground,  there  is  loss  of 
material,  which  does  not  contribute  to  the  advantage,  but  rather  to  the  detriment, 
of  the  organism. 

The  signification  of  transpiration  may  be  explained  in  this  way.  By  transpira- 
tion not  only  is  water  brought  from  below  to  the  more  highly  situated  parts  of  the 
plant,  but  nutritive  salts  in  solution  are  also  conducted  to  the  green  tissue  of  those 
branches  and  leaves  which  are  exposed  to  light  and  air.  The  greater  part  of  the 
ascended  water  is  only  used  as  a  medium  for  the  transmission  of  mineral  salts, 
which  have  been  taken  from  the  soil  into  the  plant.  When  it  has  reached  the 
leaves,  most  of  the  water  evaporates  into  the  atmosphere,  while  the  salts  conducted 
by  it  into  the  green  tissue  remain  behind,  in  order  to  take  part  in  the  chemical 
changes  by  which  organic  compounds  are  manufactured  out  of  the  raw  materials. 
These  salts  are  indispensable  here,  and  transpiration  is  therefore  also  necessary  in  a 
corresponding  degree.  Without  transpiration,  it  would  be  impossible  that  plants, 
whose  green  branches  and  leaves  are  surrounded  by  air,  or  that  trees,  which  rank 
before  all  other  plants  on  account  of  their  superior  size,  could  be  properly  nourished; 
consequently  transpiration  must  be  regarded  as  one  of  the  most  important  life- 
processes  of  terrestrial  plants. 


284  MEANS   OF   ACCELERATING   TRANSPIRATION. 

2.    EEGULATION   OF   TEANSPIRATION. 

Means  of  accelerating  transpiration.— Maintenance  of  a  free  passage  for  aqueous  vapour. 

MEANS    OF   ACCELERATING   TRANSPIRATION. 

Aquatic  plants  do  not  transpire;  therefore  they  do  not  require  either  vascular 
bundles  or  stomata.  Neither  trees  nor  shrubs  grow  under  water,  and  even  the 
largest  Floridese  and  the  most  gigantic  sea-wracks  have  no  wood  nor  stomata. 
These  structures  are  on  the  other  hand  very  important  for  land  plants,  and  in  these 
they  are  developed  in  extraordinary  variety.  When  one  considers  how  much  the 
humidity  and  temperature  of  the  air,  those  very  conditions  which  influence  the 
transpiration  of  plants,  are  continually  changing,  this  diversity  is  not  really 
surprising.  What  endless  series  of  gradations  there  are  between  the  damp  air  of 
a  tropical  estuary,  and  the  arid  wastes  in  the  interior  of  large  continents!  What 
varieties  of  temperature  in  the  different  zones  and  regions  of  the  earth,  and  in  the 
changing  seasons;  what  differences,  even  in  a  narrow  space  in  a  single  small  valley, 
between  the  conditions  of  moisture  of  the  air  and  ground  in  the  depths  of  a  shady 
glen,  and  on  the  sunny,  rocky  slopes!  In  the  one  place  the  air  is  so  saturated  with 
water- vapour  that  even  evaporation  cannot  take  place  from  exposed  pieces  of  water, 
much  less  then  from  plants;  in  the  other  it  is  so  dry  and  the  sun  is  so  strong  that 
plants  can  hardly  suck  up  enough  from  the  ground  to  compensate  for  the  water 
evaporated  from  their  surface.  In  the  former  case  contrivances  must  be  devised 
which  will  promote  transpiration  as  much  as  possible;  in  the  latter,  however,  it  is 
important  that  too  much  evaporation,  which  would  cause  the  drying  up  and  death 
of  the  plant,  should  be  prevented. 

One  of  the  most  important  ways  of  increasing  transpiration  consists  in  the 
development  of  many  cells  whose  surface  is  in  contact  to  the  greatest  possible 
extent  with  the  atmospheric  air,  and  which  are  so  organized  that  water  in  the  form 
of  vapour  can  be  exhaled  from  them.  Further,  it  is  of  importance  that  the  access 
of  air  to  these  cells  is  not  rendered  difficult,  and  that  as  great  a  portion  as  possible 
of  these  cell-groups,  which  help  in  transpiration,  are  reached  by  the  rays  of  the  sun. 
It  is  only  in  the  delicate-leaved  mosses,  which  have  no  stomata,  that  the  whole  of 
the  cells  of  a  leaf,  in  contact  with  the  air,  give  off  unlimited  water,  in  the  form  of 
vapour,  directly  to  the  atmosphere.  In  plants  possessing  leaves  provided  with 
stomata,  the  outer  walls  of  the  epidermal  cells,  which  are  directly  in  contact  with 
the  air,  are  almost  always  rather  thicker  than  the  inner  and  side  walls;  moreover, 
the  outer  wall  is  overlaid  by  the  already  repeatedly  mentioned  covering,  termed 
"  cuticle  ",  through  which  water- vapour  can  pass  only  with  difficulty.  In  tropical 
ferns,  especially  in  the  tree-ferns,  which  grow  in  narrow  wind-sheltered  ravines, 
traversed  by  streams  of  water,  and  which  spread  out  their  fronds  in  the  still,  damp, 
warm  air,  the  outer  walls  are  so  thin  and  delicate,  and  are  covered  by  a  cuticle  of 


MEANS   OF   ACCELERATING   TRANSPIRATION.  285 

such  tenuity,  that  if  the  humidity  of  the  air  sinks  only  a  few  degrees  below  satura- 
tion point,  or  if  a  transient  sunbeam  enters  the  ravine  even  for  a  short  time,  they 
immediately  give  off  water- vapour. 

Apart  from  such  cases,  the  exhalation  of  water- vapour  from  the  superficial  cells 
is  scarcely  worth  noticing;  it  is  almost  entirely  restricted  to  the  cells  of  the  spongy 
parenchyma.  Here  are  to  be  found,  indeed,  very  striking  arrangements,  which  must 
be  regarded  as  contrivances  for  increasing  transpiration.  First  of  all,  where 
transpiration  is  to  be  accelerated,  the  green,  spongy  tissue  is  very  strongly 
developed,  the  air-containing  lacunae  and  passages,  which  penetrate  the  net- work  of 
branched  cells  like  a  maze,  are  enlarged  and  numerous,  and  the  collective  free 
surface  of  all  the  air-bordered  cells  in  the  interior  of  the  leaf  has  a  much  greater 
extent  than  the  mere  outer  surface  of  the  epidermis.  In  the  leaves  of  many  tropical 
plants  which  are  always  surrounded  by  damp  warm  air,  e.g.  in  those  of  the 
Brazilian  Franciscea  eximia,  of  which  a  section  is  represented  in  fig.  62  \  almost 
the  entire  thickness  is  made  up  of  loose  wide-meshed  spongy  parenchyma,  and  it  is 
evident  that  water  will  be  exhaled  from  the  cells  of  this  tissue  as  soon  as  the 
temperature  of  the  leaf  is  raised  even  to  the  extent  of  a  few  degrees  above  that  of 
the  moist  surrounding  air  by  the  sunbeams  falling  upon  it. 

In  many  such  plants  which  urgently  require  a  help  to  transpiration  on  account 
of  their  situation,  the  cavities  of  the  spongy  parenchyma  are  extraordinarily  enlarged 
and  widened  at  certain  points  where  the  greatest  number  of  stomata  are  developed. 
The  difference  in  appearance  between  such  places  and  other  parts  of  the  leaf  having 
dense  spongy  parenchyma  can  indeed  be  recognized  by  the  unaided  vision.  In  such 
a  leaf  looked  at  from  above,  the  large-meshed  portions  of  the  spongy  parenchyma 
appear  as  lighter  spots  in  the  dark-green  grounding;  the  leaf  is  flecked  and  marked 
with  white.  This  is  not  only  the  case  with  many  plants  of  damp,  tropical  forests, 
but  also  in  those  of  temperate  zones,  such  as  species  of  the  genus  Cyclamen, 
Galeobdolon  luteum,  the  Lungwort  (Pulmonaria  officinalis),  and  frequently  also  in 
Hepatica  triloba,  if  they  grow  in  very  shady  places  on  the  damp  ground  of  a  forest. 
It  must,  of  course,  not  be  forgotten  that  all  the  white  spots  and  markings  of  green 
leaves,  which  have  been  named  collectively  "variegations",  are  not  due  to  this  cause. 
In  those  nettle-like  plants,  known  as  Bcehmerias,  the  white  spots  on  the  central 
part  of  the  leaf  lamina  are  caused  by  peculiar  groups  of  crystals  in  the  epidermal 
cells,  the  so-called  cystoliths,  which  reflect  the  light;  in  some  Piperaceaj  they  are 
due  to  groups  of  epidermal  cells  which  are  filled  with  air,  and  below  which  the 
palisade  cells  are  absent;  in  other  plants,  again,  they  may  be  caused  by  the 
formation  of  aqueous  tissue,  a  structure  which  will  be  discussed  later.  In  many 
of  those  plants  with  variegated  leaves,  which  are  so  extensively  cultivated  for 
purposes  of  decoration,  the  variegation  is  not  normal,  but  must  be  considered  as 
pathological,  and  is  in  no  way  connected  with  transpiration. 

Since,  as  we  know  by  experience,  transpiration  of  green  leaves  is  increased  by 
light  and  warmth,  it  is  evidently  an  advantage  for  aU  those  plants  to  which  only  a 
restricted  number  of  sunbeams  can  obtain  access,  if  their  leaf-blades  are  very  large 


286  MEANS   OF   ACCELERATING   TRANSPIRATION. 

and  have  such  a  form  and  position  that  the  small  supply  of  light  can  be  utilized  to 
the  full.  The  resultant  action  is  just  the  same  whether  1000  green  cells  are  only 
moderately  illuminated,  or  if  500  cells  are  illuminated  by  a  light  twice  as  strong. 
If  this  argument  will  not  apply  to  all  plants,  it  certainly  fully  applies  to  some,  and 
it  is  a  fact  that  plants  growing  in  damp,  shady  places  are  characterized  by  their 
comparatively  large,  thin,  delicate  leaves.  These  leaves  are  also  spread  out 
horizontally  in  such  localities;  they  are  smooth  and  not  wrinkled;  neither  rolled 
back  nor  bent  up.  Suppose  we  enter  a  thick  wood  in  the  north  temperate  zone, 
perhaps  in  S.  Germany.  By  the  side  of  delicate-leaved  ferns  grow  species  of 
Gorydalis  (Corydalis  fabacea,  solida,  cava),  together  with  species  of  Dentaria 
(D.  bulbifera,  digitata,  enneaphyllos),  dog's  mercury  (Mercurialis  perennis),  Isopy- 
rum  thalictroides,  bitter  vetch  (Orobus  vernus),  woodruff  (Asperula  odorata), 
Lunaria  rediviva,  herb  Paris  (Paris  quadrifolia),  cuckoo-pint  ( Arum  maculatum), 
spurge-laurel  (Daphne  Mezereum),  and  many  other  species  belonging  to  very 
different  families,  but  all  having  the  common  characteristic  of  possessing  flattened 
leaves  and  no  covering  of  hairs.  If  a  brook  ripples  through  the  shady  wood, 
growing  on  its  banks  will  be  found  the  yellow  balsam  (Impatiens  nolitangere), 
the  broad-leaved  garlic  (Allium  ursinum),  Streptopus  amplexifolius,  and  the 
butter-burr  (Petasites  officinalis),  with  its  huge  foliage,  all  again  characterized  by 
their  large,  smooth,  flat  leaves.  In  such  places  in  S.  Germany  are  generally  to  be 
found  the  largest  leaves.  Those  of  the  butter-burr  attain  to  a  length  of  over  a  metre, 
and  are  almost  a  metre  broad.  The  fronds  of  the  common  bracken-fern  (Pteris 
aquilina)  are  equally  large  in  such  situations;  and  on  the  ground  in  damp,  shady 
alder  woods,  growing  in  comparatively  cold  mountain  glens,  another  fern  (Poly- 
podium  alpestre)  is  to  be  met  with,  whose  frond  is  1J  metres  long.  But  they  only 
possess  these  extended  leaves  when  growing  in  the  situations  described,  in  the  damp 
air  of  cool  and  shady  woods.  One  would  expect  that  under  similar  conditions 
outside  the  wood,  the  leaves  would  exhibit  a  more  luxuriant  growth,  and  would 
attain  to  a  still  larger  size  in  consequence  of  the  influence  of  a  higher  temperature; 
but  this  is  not  the  case.  In  the  drier  air  and  sunshine  on  the  unshaded  banks  of  a 
rivulet,  the  leaves  of  the  butter-burr  are  scarcely  half  as  large  as  those  growing  in 
the  neighbouring  cold  shady  glen,  from  whose  dim  light  the  brooklet  flows  out  into 
the  open  country;  and  on  sunny  ground  neither  of  the  two  above-named  ferns  will 
even  approximately  reach  that  size  to  which  they  grow  when  surrounded  by  the 
cold,  damp  air  in  the  depth  of  the  alder  wood. 

This  difference  in  the  relative  size  of  the  leaves  of  one  and  the  same  species, 
according  as  to  whether  they  grow  in  sunny  places  in  dry  air,  or  in  shady  spots  in 
damp  air,  is  sometimes  carried  so  far  that  the  whole  physiognomy  of  the  plants 
becomes  altered,  and  they  might  easily  be  thought  to  belong  to  distinct  species. 
Thus  plants  of  Convallaria  Polygonatum,  growing  in  shady  meadows  watered 
by  rivulets,  show  leaves  at  least  three  times  as  large  as  those  which  grow  on  the 
rich  damp  earth  on  the  steep  sides  of  rocks  down  which  water  rushes,  where  they 
are  warmed  by  the  sun  all  the  day.  This  comparison  might  be  illustrated  by 


MEANS   OF    ACCELERATING   TRANSPIRATION.  287 

numerous  other  plants  of  the  flora  of  Central  Europe,  which  are  sometimes  to  be 
found  in  damp,  shady  woods,  sometimes  in  sunny  fields;  but  the  above  examples 
will  suffice  to  demonstrate  the  fact  that  in  shady  places  and  damp  air,  in  spite  of 
the  smaller  amount  of  heat,  and  even  when  the  humidity  of  the  soil  is  less,  the 
leaves  will,  notwithstanding,  have  a  greater  size  than  in  sunny  places  where  they 
are  surrounded  by  a  drier  air. 

An  apparent  exception  is  to  be  found  only  when  these  plants  are  situated  above 
the  tree-line  in  Alpine  regions.  On  the  sunny  slopes  of  Monte  Baldo,  in  Venetia,  far 
above  the  wood-line,  Corydalis  fabacea  grows  with  the  same  luxuriance  as  in  the 
shady  forests  of  the  lower  hilly  regions;  and  on  one  place  on  the  Solstein  chain,  in 
the  Tyrol,  at  a  height  of  1800  metres  above  the  sea,  dog's  mercury  and  Galeobdolon 
luteum,  species  of  valerian,  spurge-laurel,  and  ferns  can  be  seen  rising  above  the 
boulders  with  leaves  as  large  as  those  growing  in  the  shade  of  the  woods  below. 
But  this  exception,  as  stated,  is  only  an  apparent  one.  Where  these  plants  flourish 
on  Alpine  heights  flooded  with  light,  the  air  is  just  as  damp  as  in  the  depth  of  the 
woods  1000  metres  lower  in  the  valley.  For  weeks  the  mist  sways  like  drapery 
around  the  heights,  and  the  air,  consequently,  is  certainly  not  drier  than  in  the  woods 
down  in  the  valley.  Indeed,  the  fact  that  plants,  which  one  is  accustomed  to  see 
inhabiting  the  shady  woods  in  the  depth  of  the  valley,  grow  in  Alpine  regions  on 
unshaded  places  with  leaves  of  the  same  size  and  shape  as  before,  is  a  proof  that 
the  large  size  of  their  leaves  in  the  dark  woods  of  these  lower  places  is  not  due  to 
the  absence  of  light,  but  to  the  very  moist  condition  of  the  air  which  prevails  there. 
Plants,  whether  in  the  shade  of  the  forests,  or  on  the  illuminated  heights  of  the 
mountains,  endeavour  to  compensate  for  the  detrimental  influence  of  the  greater 
humidity  of  the  air  by  the  formation  of  an  extensive  transpiring  surface. 

So  far  the  increase  of  leaf  surface  may  be  considered  absolutely  as  a  means  of 
helping  transpiration.  This  method  of  increasing  transpiration  comes  into  action 
in  the  tropics  in  a  much  more  striking  way  than  even  in  the  temperate  zones. 
Especially  in  the  most  characteristic  plant-structures  of  the  tropics  may  it  be 
observed  how  intimately  the  size  of  the  leaves  corresponds  to  the  conditions  of 
moisture  of  the  air,  and  how  it  is  that  palms  develop  the  largest  leaves  just  in  those 
districts  where,  in  consequence  of  the  air  being  saturated  with  aqueous  vapour, 
plants  can  only  transpire  with  difficulty.  In  the  dampest  parts  of  Ceylon  grows  the 
gigantic  Corypha  umbraculifera.  A  copy  of  a  drawing  of  this  tree,  sketched  on 
the  spot  by  Ransonnet,  is  given  in  fig.  63.  It  towers  above  the  tops  of  all  other 
plants,  and  its  leaf-blades  are  from  7  to  8  metres  long,  and  5  to  6  metres  broad. 
In  similar  situations  in  Brazil  the  palm  Raphia  tcedigera  spreads  out  its  fronds  like 
gigantic  feathers.  The  petiole  of  this  leaf  alone  is  4  to  5  metres  long,  and  the  green 
feather-like  blade  is  from  19  to  22  metres  long  and  12  metres  broad— the  greatest 
extent  which  has  hitherto  been  observed  in  any  leaf.  Other  palms  besides  these 
giants,  whose  fronds  wave  all  the  year  round  in  a  damp  atmosphere,  are  but 
little  inferior  to  them.  Under  one  leaf  of  the  Talipot  ten  persons  can  easily 
find  room  and  shelter,  and  if  the  pinnate  leaves  of  the  Sago-palm  be  imagined 


288  MEANS   OF   ACCELERATING   TRANSPIRATION. 

propped  up  against  the  houses  in  the  streets  of  our  towns,  their  tops  would  reach  to 
the  second  story,  and  it  would  be  possible  to  climb  up  to  the  windows  by  them  as 
if  by  the  rungs  of  a  ladder.  Many  of  these  palm  leaves  if  placed  in  an  upright 
position  would  be  equal  in  height  to  our  forest  trees.  In  all  these  leaves  the 
epidermis  is  only  slightly  thickened,  the  spongy  parenchyma  is  well  developed, 
stomata  are  present  in  large  numbers,  and  the  surfaces  of  the  leaves  are  so  directed 
towards  the  incident  sunbeams  that  they  are  abundantly  illumined  and  warmed 
throughout.  The  leaves  become  decidedly  heated  by  the  sun's  rays,  and  thus,  even 
in  the  saturated  air  of  the  tropics,  the  necessary  amount  of  transpiration  becomes 
possible.  Arrangements  similar  to  those  of  the  palms  may  be  observed  in  the 
Aroids  and  Bananas.  These  also  develop  their  most  extended  leaves  in  the 
saturated  or  almost  saturated  atmosphere  on  the  banks  of  still  or  flowing  water, 
and  in  the  moist  heavy  air  of  tropical  primeval  forests. 

It  is  obvious  that  means  of  increasing  transpiration  are  required  in  those  water- 
plants  whose  roots  are  in  the  wet  mud  at  the  bottom  of  lakes  and  ponds,  whose 
stems  and  leaf -stalks  are  directly  surrounded  by  water,  and  whose  leaf -blades  float 
on  the  surface  of  the  water,  as  for  example  the  water-lilies  (Nymphcea,  Victoria), 
the  Frogbit  (Hydrocharis  morsus-rance),  and  the  Nymphaea-like  Villarsia  (V. 
nymphoides).  The  blade  of  the  leaf  is  disc-shaped  in  all  these  plants,  and  the  discs 
lie  side  by  side  flat  on  the  surface  of  the  water.  Frequently  large  areas  of  lakes 
and  ponds  are  covered  with  the  floating  leaves  of  these  plants.  The  whole  of  the 
upper  side  of  such  a  leaf  can  receive  the  rays  of  the  sun,  and  the  leaf  is  thus 
warmed  and  illuminated  throughout.  The  under  side  of  the  leaf  is  coloured  violet 
by  a  pigment  called  anthocyanin,  which  we  will  consider  more  in  detail  later,  and 
of  which  it  need  only  be  mentioned  now  that  it  changes  light  into  heat,  and  thereby 
materially  helps  to  warm  the  leaves. 

The  aqueous  vapour  which  is  in  consequence  developed  cannot  escape  below 
from  the  large  air-spaces  which  permeate  the  leaf,  because  the  under  side,  which 
floats  on  and  is  wetted  by  the  water,  possesses  no  stomata.  The  upper  side  is  so 
richly  furnished  with  stomata  that  on  1  sq.  mm.  460  are  to  be  seen,  and  on  a 
single  water-lily  leaf  about  2J  sq.  dms.  in  area,  about  11 J  millions.  This  upper  side 
alone  provides  a  means  of  exit,  and  it  is  therefore  important  that  the  passage 
should  not  be  obstructed  at  the  time  of  transpiration.  If  the  rain  should  fall  unre- 
strainedly on  the  upper  side  of  the  floating  leaves,  the  collected  rain-water  might 
remain  there  for  a  long  time,  even  while  the  sunbeams  breaking  through  the  clouds 
after  the  shower  are  warming  the  floating  leaves  and  inciting  them  to  transpire. 
In  order  to  avoid  this  an  arrangement  is  made  by  which  it  is  rendered  an 
impossibility  to  wet  the  upper  side  of  the  floating  leaves.  The  falling  rain  is  formed 
into  round  drops  on  reaching  them,  and  does  not  spread  over  the  leaf -surface  so  as 
to  wet  it.  But  in  order  that  the  drops  should  not  remain  long  on  the  leaves  in 
many  of  these  forms,  such  as  in  the  widely  distributed  water-lily  (Nymphcea  alba),ihe 
leaf,  where  it  joins  the  stalk,  is  somewhat  raised,  and  the  edges  are  bent  a  little  up 
and  down  in  an  undulating  manner.  This  gives  rise  to  very  shallow  depressions 


MEANS   OF   ACCELERATING   TRANSPIRATION. 


289 


round  the  edge  of  the  disc,  on  account  of  which  the  drops  of  water  roll  down  from 

the  middle  of  the  leaf  to  the  edge  on  the  slightest  rocking  movement,  and  there 

coalesce  with  the  water  on  which  the  leaves  float. 
This  puckering    x.^;; 

of   the    margin  of 

the  leaf  is  attended 

in  the  water-lilies 

by  a  phenomenon 

which,       although 

not   directly  asso- 
ciated    with     the 

matter  in  hand,  is 

so  full  of  interest 

that  it  cannot   be 

passed        without 

notice.    If  we  take 

a  boat  in  the  bright 

sunshine    at    mid- 
day, and  float  over 

the  calm  inlet  of  a 

lake,  whose  surface 

is  overspread  with 

the  leaves  of  water- 
lilies,  and  if  the 

water   is   clear   to 

the  bottom,  we 
shall  see  the  sha- 
dows of  the  leaves 
which  float  on  the 
surface  sketched 
out  on  the  ground 
below.  But  we  can 
scarcely  believe 
our  eyes — these  do 
not  look  like  the 
shadows  of  the 
leaves  of  water- 
lilies,  but  rather 

of  the  fronds  of  huge  fan-palms.  From  a  dark  central  portion  radiate  out  long  dark 
strips  which  are  separated  from  each  other  by  as  many  light  bands.  The  cause  of 
this  peculiar  form  of  shadow  is  to  be  found  in  the  undulating  margin  of  the  floating 
leaves.  The  water  of  the  lake  adheres  to  the  whole  of  the  under  surface  of  the 
disc  as  far  as  the  edge,  and  is  drawn  up  by  capillarity  to  the  arched  portions 

VOL.  I.  19 


Fig.  63.—Corypha  umbraculifera  of  Ceylon  (after  Ennsonnet). 


290        MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

of  the  undulating  margin.  The  sun's  rays  are  refracted  as  through  a  lens  by  this 
raised  water,  and  so  a  light  stripe  corresponding  to  each  convex  division  of  the 
curved  margin  is  formed  on  the  bed  of  the  lake,  and  a  dark  stripe  corresponding  to 
each  concave  part.  These  are  arranged  in  a  radiating  manner  round  the  dark 
central  portion  of  the  shadow. 

MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

Special  arrangements  are  met  with  in  all  plants  which  possess  stomata,  in  order 
that  the  giving  off  of  aqueous  vapour  may  continue  without  hindrance.  Water 
falling  on  the  upper  side  of  the  leaf,  in  the  form  of  rain  and  dew,  threatens  to 
cause  the  greatest  obstacle  to  this  free  passage  should  it  be  able  to  collect  directly 
in  the  stomata.  The  width  of  an  open  stomate  does  not  render  the  entrance  of 
water  by  capillarity  impossible.  As  long  as  light  and  warmth  exercise  their  power, 
as  long  as  the  temperature  in  the  neighbourhood  of  the  spongy  parenchyma  is 
higher  than  that  of  the  surrounding  air,  and  water- vapour  in  consequence  is  pro- 
duced in  the  spongy  tissue  and  driven  out  with  force  from  the  stomata,  such  an 
entrance  is  indeed  inconceivable.  It  is  impossible  for  aqueous  vapour  to  pass  out 
and  at  the  same  time  for  fluid  water  to  enter  by  the  same  passage  and  through  the 
same  gate.  But  should  the  leaf  become  cooled  by  radiation  after  sunset,  and  dew 
be  deposited  upon  it,  or  should  a  cold  rain  trickle  down  over  the  leaves,  and  the 
stomata  have  been  unable  to  close  quickly  enough,  it  is  quite  possible  that  water 
might  enter,  just  as  it  enters  a  retort  (whose  narrow  mouth  dips  into  water,  and 
whose  contents  have  been  vaporized  by  placing  a  lamp  under  them),  when  the  lamp 
is  removed,  and  the  bulb  of  the  retort  with  its  contents  becomes  cooled.  But  putting 
aside  the  possibility  of  water  thus  pressing  its  way  in,  this  much  is  certain,  that 
the  formation  of  a  layer  of  water  over  the  cells  in  the  immediate  neighbourhood 
of  the  stomata  would  cause  great  injury  to  the  plants;  and  this,  not  only  as  affecting 
transpiration,  but  also  the  free  entrance  and  exit  of  gases.  Therefore,  the  im- 
mediate surroundings  of  the  stomata  must  be  kept  open  as  a  path  for  aqueous 
vapour,  and  no  water  must  be  allowed  to  collect  and  take  up  a  position  there. 

Stomata  are  much  too  small  to  be  seen  with  the  naked  eye.  However,  it  can  be 
ascertained  by  a  very  simple  experiment  whereabouts,  on  a  leaf  or  green  branch, 
stomata  are  to  be  found.  A  twig  or  a  leaf  is  dipped  in  water,  and  then  withdrawn 
after  a  short  time  and  lightly  shaken;  some  spots  will  be  found  wet,  while  other 
places  remain  dry.  Where  water  remains  and  spreads  out  to  form  an  adhering 
film,  no  stomata  will  be  found  in  the  epidermis ;  but  where  the  twig  or  leaf  is  dry, 
one  can  be  sure  of  finding  them.  In  80  per  cent  of  cases  experimented  upon  in 
this  way,  only  the  upper  leaf-surface  became  wetted,  while  the  under  side  kept 
dry;  in  10  per  cent  both  sides  remained  dry;  and  in  the  other  10  per  cent  the 
upper  side  kept  dry,  while  it  was  the  under  side  which  was  wetted.  With  this 
corresponds  the  actual  fact  that  in  far  the  greater  number  of  instances  the  under 
side  possesses  most  stomata,  while  the  upper  side  is  free  from  them.  It  seems  as  if 


MAINTENANCE   OF   A   FREE   PASSAGE   FOR   AQUEOUS   VAPOUR.  291 

this  circumstance  could  be  explained  thus,  that  the  upper  side  is  usually  turned 
towards  the  rain,  and  that  the  stomata  are  on  this  account  collected  together  on 
the  under  side,  which  is  sheltered  from  it.  This  explanation,  however,  which  at 
first  sight  seems  so  plausible,  does  not  quite  correspond  to  the  true  state  of  the  case. 
The  consideration  of  the  reasons  for  believing  that  it  is  an  advantage  for  the  plant 
to  have  the  upper  side  of  the  leaf  free  from  stomata  will  indeed  come  later,  but  one 
thing  must  be  noted  here, — that  the  side  of  the  leaf  turned  towards  the  ground, 
which  in  most  cases  contains  all  the  stomata,  remains  anything  but  dry.  Of 
course  the  rain-water  only  reaches  the  surface  of  the  horizontal  leaf -blade  when 
the  margin  is  so  formed  that  the  adherent  layer  of  water  which  wets  the  surface  is 
drawn  over  gradually  from  the  upper  to  the  under  side,  and  that  is  very  seldom 
the  case;  but  the  wetting  of  this  surface  by  mist  and  dew  is  all  the  more  important 
on  this  account.  On  taking  a  stroll  through  fields  and  meadows  on  a  dewy  morn- 
ing, as  a  rule  only  the  upper  surfaces  of  the  leaves  come  into  view,  and  one  might 
easily  be  led  to  think  that  the  dew  is  deposited  only  on  this  side.  We  constantly 
use  the  expression  that  the  dew  "  falls  ".  Underlying  this  is  the  idea  that  the  dew 
comes  down  like  rain,  and  that  only  the  upper  leaf -surface  becomes  covered  with 
dewdrops.  But  one  has  only  to  turn  the  leaf  over  to  convince  oneself  that  the 
lower  surface  is  likewise  bedewed,  and  on  a  closer  examination  it  will  even  be  seen 
that  dew  is  of  more  importance  in  connection  with  the  lower  than  the  upper  side, 
because  it  remains  there  so  much  longer.  When  the  sun  is  already  high  in  the 
heaven,  and  the  dewdrops  have  long  disappeared  from  the  upper  surface,  and  tran- 
spiration is  in  full  force,  the  under  side  may  still  be  found  studded  with  dewdrops. 
If  in  the  majority  of  cases  the  stomata  lie  on  the  under  side,  and  this  side  is 
exposed  to  the  danger  of  being  covered  with  water  as  much  as  the  upper  one,  it  is 
evident  why  contrivances  for  hindering  the  access  of  water  to  the  stomata  are 
to  be  found  much  more  abundantly  on  the  under  than  on  the  upper  side  of  the 
leaf. 

The  most  important  of  these  arrangements  are  the  following: — 
First  the  coating  of  wax.  This  is  either  in  the  form  of  a  granular  covering;  or  as 
a  fine  crust  which  fits  closely  to  the  epidermis;  or,  most  commonly,  as  a  continuous 
thin  layer  which  is  easily  rubbed  off,  forming  a  delicate  film  popularly  known  as 
<l  bloom  ".  A  group  of  primulas,  belonging  to  mountainous  districts  and  to  the  moors 
of  low  countries,  of  which  Primula  farinosa  may  be  taken  as  the  most  widely 
distributed  and  best  known  representative,  have  a  rosette  of  leaves  spreading  over 
the  damp  ground,  and  on  the  lower  side  of  these  leaves  is  a  white  coat,  which  under 
the  microscope  is  seen  to  consist  of  a  collection  of  short  rods  and  knobs  of  a  waxy 
nature.  If  such  a  leaf  is  plucked  and  placed  in  water  for  a  short  time,  and  then 
withdrawn,  the  upper  side,  which  is  quite  free  from  stomata,  will  be  moistened  by  a 
layer  of  water,  while  the  under  side,  on  which  are  the  stomata  protected  by  the 
granular  coating  of  wax,  remains  quite  dry.  The-  lower  surface  of  the  leaves  is 
covered  with  a  fine  closely  adherent  wax  layer,  in  many  of  the  willows  growing  in 
damp  misty  places  near  rivers  (Salix  amygdalina,  purpurea,  pruinosd),  as  well  as 


292         MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

in  a  great  number  of  rushes,  bulrushes,  and  reed-like  grasses.  If  when  the  dew 
falls  heaviest  one  roams  through  a  thicket  of  willows,  or  across  a  moor,  one  may  see 
plenty  of  drops  hanging  from  the  under  side  of  the  leaves,  but  they  do  not  actually 
wet  this  surface,  and  on  the  slightest  movement  of  the  leaves  they  roll  off  and  fall 
down.  It  is,  indeed,  in  consequence  of  this  that  one  is  more  likely  to  get  thoroughly 
wet  by  walking  through  meadows  and  dwarf  willows  than  by  an  excursion  through 
country  overgrown  with  ordinary  herbs.  The  two  white  stripes,  so  well  known  on 
the  under  side  of  fir  leaves,  are  also  formed  by  a  waxy  coat,  which  prevents  the 
stomata  below  from  being  wetted.  In  species  of  juniper  (e.g.  J.  communis,  nana, 
Sabina)  the  two  white  stripes  occur  on  the  upper  side  of  the  leaf,  and  it  is  interest- 
ing to  see  how  the  distribution  of  the  stomata  again  corresponds;  for  junipers 
belong  to  that  group  of  plants  whose  under  leaf -surf  ace  is  free  from  stomata,  these 
being  present  only  on  the  wax-coated  region .  of  the  upper  side  of  the  leaves. 
Many  grasses,  to  which  we  shall  refer  later  for  other  reasons  (e.g.  Festuca  punctoria), 
only  possess  stomata  on  the  upper  side  of  the  leaf,  and  again  only  where  the  strips 
of  wax  are  situated.  Generally  speaking,  wax  is  a  protection  from  moisture,  and  is 
most  frequently  formed  when  the  stomata  make  their  appearance  on  the  upper  side 
of  the  leaf.  The  leaves  of  peas,  nasturtiums,  woodbine,  poppies,  fumitory,  many 
pinks,  cabbages,  woad,  and  many  other  cruciferous  plants,  which  have  stomata  on 
the  upper  surface,  also  produce  a  covering  of  wax  there.  Water  poured  on  the 
upper  surface  of  a  cabbage-leaf  rolls  off  in  the  form  of  drops,  exactly  as  it  runs  off 
a  duck's  back,  without  wetting  the  surface.  In  the  fronds  of  ferns  (e.g.  Polypodium 
glaucophyllum  and  sporodocarpum),  on  the  upright  leaves  of  Irises  (Iris  ger- 
manica,  pumila,  pallida),  as  well  as  on  the  vertical  leaves  and  leaf -like  branches  of 
many  Australian  acacias  and  myrtles,  and  lastly  in  the  erect  whiplike,  leafless  or 
scantily-leaved  papilionaceous  plants  (Retama,  Spartium),  the  stomata  are  pro- 
tected from  the  wet  by  a  coat  of  wax. 

The  formation  of  hairs  furnishes  another  barrier  to  the  entrance  of  water  into 
the  stomata.  We  shall  come  back  again  to  these  structures,  which  serve  so  many 
different  purposes  in  the  plant  economy,  but  here  only  those  hairy  and  felted 
coverings  whose  task  is  to  hinder  the  wetting  of  the  stomata  will  be  considered. 
Examples  of  these  are  furnished  by  many  Malvaceae  which  grow  in  marshes  and 
ditches  (e.g.  Althcea  officinalis),  and  also  by  some  mulleins  (e.g.  Verbascum  Thapsus, 
phlomoides),  whose  leaves  are  provided  with  stomata  on  both  surfaces,  and  with 
hairy  coverings  which  it  is  impossible  to  moisten.  In  the  damp  meadows  of  the 
valleys  of  the  Lower  Alps  grows  Centaurea  Pseudophrygea,  whose  large  leaves, 
hairy  on  both  sides,  are  very  rough  and  much  wrinkled.  The  stomata  are  only 
situated  in  the  hollows  between  the  ridges.  When  rain  falls,  or  the  leaf  becomes 
bedewed,  the  water  remains  in  the  form  of  drops  on  the  hairs  of  the  elevated  por- 
tions, and  the  cells  in  the  hollows  are  not  wetted.  In  many  alpine  plants,  for 
example  the  Hairy  Hawkweed  (Hieracium  villosum),  after  a  fall  of  rain  or  dew 
the  long  projecting  hairs  of  the  leaves  are  thickly  beset  with  drops  of  water, 
none  of  which  can  reach  the  stomata  on  the  epidermis  beneath. 


MAINTENANCE   OF   A   FREE   PASSAGE   FOR   AQUEOUS   VAPOUR.  293 

It  should  be  particularly  noticed  here  that  plants  with  two-coloured  leaves, 
such  as  those  whose  upper  surfaces  are  green,  smooth,  free  from  stomata  and  easily 
wetted,  while  their  under  surfaces,  covered  with  gray  or  white  hairs,  and  rich  in 
stomata,  which  cannot  be  wetted,  are  generally  to  be  found  on  the  banks  of  rivers 
and  streams. 

In  the  open  woods  which  skirt  the  banks  of  rivers  in  the  valleys  of  moun- 
tainous districts,  i.e.  in  places  where  mist  rises  on  summer  evenings,  and  all  the 
twigs,  leaves,  and  stalks  are  covered  with  drops  of  water,  the  most  characteristic 
plants  are  the  Gray  Alder  (Alnus  incana)  and  the  Gray  Willow  (Salix  incana), 
and  as  undergrowth  everywhere  the  Raspberry — all  plants  adorned  with  the  two- 
coloured  leaves  just  described.  Leaving  the  region  of  woods  growing  on  river 
banks  for  the  neighbouring  meadows,  through  which  ripples  fresh  water  from  a 
spring,  and  where  everything  drips  with  dew  from  evening  until  the  middle  of 
the  following  day,  we  come  to  the  natural  home  of  herbs  and  shrubs  with  leaves 
green  on  the  upper  and  white  on  the  under  sides.  There  Fuller's  Thistles  (Cirsium 
heterophyllum  and  canum)  grow  luxuriantly,  and  the  Meadow-sweet,  with  its 
large  two-coloured  leaves;  whilst  the  whole  course  of  the  brook  is  bordered  by  the 
Colt's-foot  (Tussilago  Farfara)  with  leaves  which  may  be  considered  typical  of  this 
group. 

What  a  contrast  does  this  present  to  the  lofty  vaults  of  a  dense  forest,  perhaps 
only  a  thousand  paces  away,  where  on  the  shady  ground  little  or  no  dew  is 
formed,  and  where  the  leaves  which  canopy  the  brown  soil  are  never  exposed  to  a 
thorough  wetting!  No  parti-coloured  leaf  is  to  be  found  there,  no  leaves  whose 
upper  surface  is  green  and  smooth,  while  the  under  side  is  covered  with  white 
hairs;  and  plants  which  exhibit  a  thick  coating  of  wax  on  their  under  surface,  like 
the  Primula  farinosa  of  the  moors,  are  also  absent.  On  the  other  hand  ferns  are 
here,  as  for  example  the  Hard  Fern  (Blechnum  Spicanf),  whose  leaves  are  furnished 
with  stomata  which  open  quite  without  protection  on  the  tops  of  projecting  undula- 
tions. This  contrast  between  the  leaves  of  plants  in  the  open  marshy  country  and 
in  the  interior  of  forests  is  found,  not  only  in  the  colder  territories  of  the  north, 
but  also  in  tropical  districts.  Moreover,  plants  whose  leaves  are  covered  with  white 
hairs  on  the  under  surface  are  never  to  be  found  under  the  close  leafy  roof  of  huge 
trees  which  prevent  nocturnal  radiation  and  the  formation  of  dew.  Here  occur, 
rather,  plants  having  totally  unprotected  stomata  opening  on  slightly  raised  areas 
of  the  surface,  as  for  example  in  Pomaderis  phylicifolia,  and  on  the  leaves  of  the 
Pepper  family,  e.g.  Peperomia  arifolia  (see  fig.  64 3  and  64  4). 

A  very  remarkable  contrivance  by  which  stomata  are  protected  from  moisture 
consists  in  providing  the  stomata  of  the  upper  surface  with  countless  papillae  and 
cone-shaped  projections ;  between  them,  of  course,  being  innumerable  hollows  and 
depressions.  Falling  water-drops  roll  off  such  surfaces;  the  water  cannot  displace 
the  atmospheric  air  in  the  depressions,  and  therefore  the  leaves  and  stems,  in  so  far 
as  their  epidermis  presents  the  aforesaid  irregularities,  appear  covered  with  a  thin 
layer  of  air.  As  the  stomata  are  situated  in  the  small  hollows,  they  always  remain 


294 


MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 


dry,  and  even  if  that  particular  part  of  the  plant  is  wholly  immersed,  they  do  not 
come  into  contact  with  the  water.  There  are  two  causes  for  the  unevenness  of  the 
leaves:  first,  the  outer  walls  of  a  portion  of  the  superficial  cells  may  become  strongly 
arched  outwards;  or  secondly,  solid  peg-like  projections  may  arise  from  the  cuticle, 
and  to  these  projections  the  air  adheres  so  firmly  that  it  cannot  be  displaced  even 
by  a  considerable  pressure  of  water.  This  protection  of  stomata  against  moisture 
by  papilla-like  outgrowths  is  to  be  found  especially  in  marsh  plants  which  are 
exposed  to  a  changing  water-level.  On  the  banks  of  streams  and  rivers,  and 


Fig.  64.— Stomata. 

i  Surface  view  of  a  portion  of  the  frond  of  the  fern  Nephrodium  Filix-mas.    2  Vertical  section  through  this  portion. 
»  Surface  view  of  a  portion  of  the  leaf  of  Peperomia  arifolia.    *  Vertical  section  through  this  portion ;  X350. 

where  water  welling  up  from  below  forms  pools  and  ponds,  it  may  happen  that 
plants  are  submerged  for  a  week  at  a  time,  and  then  again  remain  dry  for  some 
months. 

Most  of  the  plants  growing  in  such  situations,  particularly  the  sedges  (e.g. 
Carex  stricta  and  paludosa),  the  rushes  (e.g.  Scirpus  lacustris),  most  of  the  tall 
fistular  grasses  (Glyceria  spectabilis,  Phalaris  arundinacea,  Eulalia  japonica),  the 
plants  which  grow  with  the  sedges  (e.g.  Lysimachia  thyrsiflora,  Polygonum 
amphibium),  and  many  other  marsh  plants,  are  all  saved  from  the  danger  of 
having  their  stomata  wetted  during  their  submersion  by  the  papilla-like  out- 
growths of  some  of  the  epidermal  cells,  near  the  stomata,  as  shown  in  the  figures 
on  next  page. 

Bamboos,  and  the  grasses  Arundinaria  glaucescens  and  Phyllostachys  bam- 


MAINTENANCE    OF    A    FREE    PASSAGE    FOR    AQUEOUS   VAPOUR.  295 

busoides,  which  so  much  resemble  the  bamboos,  besides  some  sedges  (e.g.  Carex 
pendula),  exhibit  on  the  other  hand  the  above-named  peg-like  projections  of  the 
cuticle;  these  are  shown  in  the  section  of  a  bamboo  leaf  in  fig.  66 2).  On  plunging 
such  a  bamboo  leaf  in  water,  a  surprising  sight  presents  itself.  The  upper  side, 
covered  by  a  dark  green,  smooth,  flat  epidermis,  with  no  stomata,  becomes  wet  all 
over,  and  retains  its  dark  colour  and  dull  appearance;  but  the  under  surface,  blue- 
green  in  colour,  and  beset  with  stomata  and  thousands  of  cuticular  pegs,  does  not 
allow  the  air  to  be  displaced;  and  this  layer  of  air,  spread  thin  over  the  surface, 
glistens  under  water  like  polished  silver!  The  leaf  may  be  shaken  under  water 
to  any  extent,  and  may  even  be  left  submerged  for  a  week,  but  the  silvery  glisten- 
ing air-stratum  is  not  dislodged.  If  such  a  leaf  is  now  taken  out  of  the  water,  the 
upper  surface  is  quite  wet,  but  the  under  surface  is  dry,  like  a  hand  which  has 
been  dipped  in  mercury  and  then  withdrawn,  and  not  the  smallest  drop  of  water 


Fig.  65.— Protection  of  Stomata  from  Moisture  by  Papilla-like  outgrowths  of  the  Surface. 

i  Vertical  section  through  a  portion  of  the  leaf  of  Olyceria  spectabilis.    a  Vertical  section  through  a  portion  of  the  leaf  of 

Carex  paludosa ;  x200. 

adheres  to  it.  On  placing  a  vessel  of  water,  in  which  some  bamboo  leaves  are 
half  immersed,  under  the  receiver  of  an  air-pump,  and  then  pumping  out  the  air, 
numerous  small  air -bubbles  are  at  once  given  off  from  the  submerged  portions 
of  the  leaves.  At  length  the  silvery  lustre  disappears,  and  the  air  between  the 
cuticular  pegs  is  replaced  by  water.  If  now  the  leaves  be  completely  submerged, 
the  silver  lustre  is  only  shown  on  those  parts  which  were  not  previously  immersed, 
and  where  water  could  not  replace  the  exhausted  air; — the  spaces  round  the  pegs 
in  this  region  having  been  again  supplied  with  air  on  the  opening  of  the  stop-cock 
of  the  pump  in  order  to  submerge  the  leaves.  It  may  be  imagined  from  this 
experiment  how  much  the  stomata  would  be  damaged  by  water  if  the  plants 
mentioned  were  not  protected  from  moisture  by  the  pegs  to  which  the  air  adheres 
so  strongly. 

In  many  plants  which  grow  in  the  sunshine,  and  particularly  in  those  whose 
foliage  is  evergreen  and  only  exposed  to  moisture  at  the  time  of  the  greatest 
activity  of  the  sap  (while  later  it  is  exposed  for  months  to  dry  air),  the  stomata  are 
to  be  found  surrounded  by  an  embankment,  or  sunk  in  special  pits  and  furrows. 
Even  in  the  leaves  of  many  indigenous  plants,  which  are  green  in  the  summer, 
e.g.  those  of  the  Carrot  (Daucus  Carotd),  the  guard-cells  of  the  stomata  are  so 


296         MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

over-arched  by  the  neighbouring  epidermal  cells  that  a  sort  of  vestibule  is  formed 
in  front  of  the  true  pore.  It  can  easily  be  imagined  that  drops  of  water  which 
come  to  such  places  are  not  able  to  press  out  the  air  from  this  vestibule,  and  there- 
fore cannot  penetrate  to  the  guard-cells  of  the  stomata.  In  Hakea  florida  and 
Protea  mellifera,  two  Australian  shrubs  (see  fig.  67),  similar  arrangements  are  met 
with,  but  here  the  stomata  are  still  more  over-arched,  so  that  they  are  only  visible 
to  anyone  looking  at  the  surface  of  the  leaf  through  small  holes  at  the  top  of  the 
dome.  The  stomata  on  the  green  branches  of  various  species  of  Ephedra  are 
surrounded  by  mound-like  projections  from  the  cuticle  of  neighbouring  epidermal 
cells,  and  are  at  the  same  time  somewhat  sunken,  so  that  an  urn-shaped  space  is 


Fig.  66.  —Protection  of  Stomata  from  Moisture  by  Cuticular  Pegs. 

i  Vertical  section  of  a  Bamboo  leaf ;  x  180.      2  Part  of  the  lower  portion  of  the  section ;  x  460.      *  Part  of  the  upper 

portion  of  the  section ;  X460. 

formed  above  each  stoma,  from  which  water  cannot  dislodge  the  air.  On  the 
leaves  of  Dryandra  floribunda,  one  of  the  Proteaceae  which  grows  in  the  thick 
Australian  bush,  several  stomata  occur  at  the  bottom  of  small  pits  on  the 
under  side  of  the  leaf,  and  from  the  side  walls  of  the  depression  spring  hair- 
like  structures  which  interlace  and  form  a  loose  felt- work,  easily  penetrated 
by  gases  but  not  by  fluids  (fig.  68).  The  stomata  on  the  leaves  of  the  Oleander 
(Neriwm,  Oleander)  are  similarly  situated.  These  also  are  at  the  bottom  of  deep 
pits  on  the  lower  side  of  the  leaf,  and  the  entrance  to  them  is  beset  with  extremely 
delicate  hair-like  structures  (see  fig.  73 3).  The  oleander  fringes  the  banks  of 
streams  in  the  sunny  open  country  of  Southern  Europe  and  the  East,  and  in  its 
natural  position  it  is  most  exposed  to  wetting  by  rain,  mist,  and  dew,  just  when 
transpiration  is  an  absolute  necessity  for  it.  But  even  when  the  leaves  are  covered 
on  both  sides  with  a  layer  of  moisture,  none  can  force  its  way  into  the  hair- 
lined  depressions  which  conceal  the  stomata,  and  consequently  transpiration  is  not 
hindered  even  in  the  wettest  season  of  the  year. 


MAINTENANCE   OF   A    FREE   PASSAGE   FOR   AQUEOUS   VAPOUR.  297 

Stomata,  which  are  spread  over  the  green  tissue  of  stems  and  flattened  shoots, 
are  frequently  sunk  in  furrows,  channels,  and  pits,  in  plants  whose  greatest  activity 
occurs  in  the  short  rainy  season,  and  they  are  saved  from  wetting  in  this  position 
by  the  most  varied  contrivances.  On  the  rocky  shores  of  Lake  Garda,  and  up  over 
the  mountain  slopes  to  the  heights  of  Monte  Baldo,  grows  Cytisus  radiatus,  a 
shrub  of  unusual  appearance  (see  fig.  69 l).  Its  branches  only  possess  rudimentary 
green  leaves,  and  are  themselves  furnished  with  green  tissue,  which  plays  the  same 
rdle  as  that  assigned  to  the  mesophyll  of  the  leaf -lamina  in  normal  foliaceous  plants. 


Fig.  67.— Over-arched  Stomata  of  Australian  Proteacese. 

i  Vertical  section  through  a  leaf  of  Hakea  florida.     2  Surface  view  of  the  same  leaf ;  x320. 
of  Protea  mellifera.    *  Surface  view  of  the  same  leaf ;  x360. 


»  Vertical  section  of  a  leaf 


These  green  branches  bear  very  numerous  secondary  branches  inserted  in  decus- 
sating pairs.  On  the  secondary  branches  new  shoots  develop  every  spring  exactly 
similar  in  form,  and  arranged  in  the  same  manner.  At  the  period  when  this 
development  is  taking  place,  the  humidity  in  that  part  of  the  Southern  Alps,  to 
which  Monte  Baldo  belongs,  is  very  great.  In  dull  weather,  rain  and  mist,  or  dew 
in  fine  weather,  deposit  large  quantities  of  water  on  the  soil,  and  on  the  plants 
covering  it,  particularly  in  the  alpine  region  of  the  above-named  mountains,  on  the 
westerly  slopes  leading  down  to  the  lake,  which  are  thickly  clothed  with  the  shrubs 
in  question.  It  is  therefore  important  that  the  rod-like  branches  of  this  Cytisus 
should  be  able  to  breathe  and  transpire  without  hindrance,  and  that  the  short  time 
during  which  the  conditions  for  these  vital  transactions  are  favourable,  should  be 
fully  and  wholly  taken  advantage  of.  Here  again  the  point  above  all  others  to  be 


298 


MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 


aimed  at  is  to  keep  a  free  passage  for  the  water- vapour  which  must  escape  from  the 
stomata.  To  bring  this  about,  the  stomata  are  situated  in  grooves  filled  with  air 
which  are  sunk  in  the  green  tissue,  and  which  give  a  striped  appearance  to  the 
branches.  Water  cannot  force  out  the  air  from  these  narrow  furrows  which  run 
along  the  green  branches  and  twigs,  eight  of  them  to  each  branch.  The  branches  may 
remain  submerged  in  water  for  an  hour  without  a  trace  of  moisture  entering  the 
furrow.  Moreover  hairs  are  present  in  the  furrows  as  a  guard  against  moisture. 
These  cannot  be  wetted,  and  the  air  adheres  to  them  just  as  to  the  cuticular  pegs  of 
the  bamboo  leaf.  A  clear  idea  of  this  arrangement  is  given  in  the  transverse 
section  of  the  stem  shown  in  fig.  69 3  and  69 4.  The  adjacent  section  of  the  green 
branch  of  the  Australian  Casiiarina  quadrivalvis  shows  that  these  curious  plants 
also  have  exactly  the  same  arrangements,  that  the  stomata  lie  at  the  bottom  of 


Fig.  68. — Stomata  in  Pit-like  Depressions. 

i  Surface  view  of  a  leaf  of  Dryandra  floribunda.    A  portion  of  the  hairs  which  fill  the  pit  is  removed,  in  order  to  show 
the  stomata ;  x  350.    2  Vertical  section  through  a  leaf  of  Dryandra  floribunda ;  x  300. 

narrow  furrows  which  run  along  the  green  leafless  branches,  and  that  peculiar  hair- 
structures  are  present  in  the  furrows,  to  which  the  air  adheres,  forming  a  barrier 
against  water,  exactly  as  in  those  of  the  Gytisus.  The  Casuarinse,  which  must 
finish  their  work  for  the  year  during  the  very  short  rainy  period  of  their  native 
country,  require  during  this  time  arrangements  providing  for  unhindered  transpira- 
tion no  less  than  does  the  Gytisus  in  the  Southern  Alps.  Altogether  this  con- 
trivance is  found  to  be  present  in  only  a  limited  number  of  cases;  in  perhaps  only 
twenty  papilionaceous  shrubs,  most  of  which  belong  to  the  Spanish  flora,  of  the 
genera  Eetama,  Genista,  Ulex,  and  Sarothamnus,  in  addition  to  the  Australian 
Casuarinas,  and  in  allied  species  of  Cytisus  (holopetalus,  purgans,  ephedroides, 
equisetiformis,  candicans,  albus,  &c.).  Most  remarkably  also  this  arrangement 
occurs  in  a  small  species  of  Broom  (Genista  pilosa),  which  is  distributed  over  the 
mountains  of  Central  Europe,  over  the  heaths  of  the  Baltic  Lowlands,  Denmark, 
Belgium,  and  England.  And  the  presence  of  this  contrivance  here  is  the  more 
strange,  from  the  fact  that  the  green  branches  with  their  furrows,  in  which  lie 
stomata,  are  not  leafless,  but,  on  the  contrary,  are  provided  with  a  comparatively 
well-developed  foliage. 

Among  the  most  peculiar  plants  whose  stomata  are  concealed  in  hidden  nooks, 


MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 


299 


impenetrable  by  water,  are  two  very  small  orchids,  of  which  one,  Bolbophyllum 
m^nutiss^mum)  grows  in  company  with  mosses  on  blocks  of  sandstone  and  on  the 
bark  of  trees  in  the  rocky  ravines  near  Port  Jackson,  and  on  the  Richmond  River 
on  the  east  coast  of  Australia;  the  other,  Bolbophyllum  Odoardi,  lives  in  similar 


Fig.  69.— Stomata  in  the  Furrows  of  Green  Stems. 

1  Branch  of  Cytisus  radiatus ;  natural  size.  2  Portion  of  a  branch ;  xlO.  «  Cross  section  of  this  branch;  x 30.  «  Part  of  the 
same  section;  x!50.  5  Branch  of  Casuarina  quadrivalvis;  natural  size.  «  Portion  of  a  branch;  x8.  *  Cross  section  of 
this  branch;  xSO.  «  Part  of  the  cross  section ;  X130. 

situations  in  Borneo.  Both  have  a  filamentous  rhizome  from  which  spring  rootlets 
(from  2  to  5  mm.  long  and  0'3  mm.  thick),  arranged  in  pairs,  by  which  they  attach 
themselves  to  the  stone  and  the  bark  of  trees.  Above  the  origin  of  each  pair 
of  rootlets  is  a  little  disc-shaped  tuber,  from  1J  to  3  mm.  in  diameter,  and  J  mm. 
thick,  with  an  aperture  on  the  upper  surface,  scarcely  ^  mm.  broad,  leading  into  a 
hollow  chamber  within  the  disc -shaped  tubers,  about  0*5  mm.  broad  and  0*1  mm. 
high  (see  figure  70).  The  leaves  of  Bolbophyllum  minutissimum  are  reduced 


300        MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

to  tiny  pointed  scales  about  }  mm.  in  length;  two  of  them  are  situated  at  the 
mouth  of  each  cavity,  and  are  inflected  towards  one  another  across  it.  In  Bolbo- 
phyllum  Odoardi,  each  of  the  small  tubers  bears  only  one  small  green  leaf,  which 
is  about  1 J  mm.  long  and  1  mm.  broad,  and  is  placed  close  to  the  opening  of  the 
chamber  (see  fig.  70 4-5'6).  Stomata  are  found  exclusively  in  the  interior  of  the 
hollow  tubers.  Water  cannot  enter  through  the  narrow  mouth  into  the  air-containing 
chamber,  and  even  when,  in  the  rainy  season,  the  whole  of  the  mossy  carpet,  in 
which  these  smallest  of  all  orchids  are  interwoven,  is  saturated  with  water,  their 
transpiration  continues  unhindered,  provided  that  the  other  conditions  on  which  it 
depends  are  fulfilled.  It  is  obvious  that  these  structures  which  prevent  moisture 
reaching  the  stomata  during  the  wet  season  of  the  year  can  take  on  another  function 


Fig.  70.— Orchids  whose  Stomata  lie  in  Hollow  Tubercles. 

»  Bolbophyllum  minutissimum.    2  A  tuber  seen  from  above ;  x8.    8  Vertical  section  through  this  tuber ;  x!5.     *  Bolbophyllum 
Odoardi.    *  A  tuber;  x6.    «  Longitudinal  section  through  this  tuber ;  x6. 

in  a  succeeding  dry  period,  which   may  follow  immediately;    but  this  must  be 
spoken  of  again  later. 

The  occurrence  of  "rolled  leaves",  which  are  observed  in  so  many  plants  of  widely 
different  affinity,  is  also  connected  with  the  keeping  of  water  from  the  stomata. 
The  rolled  leaf  is  always  undivided,  of  small  area,  generally  linear,  but  sometimes 
ovate-linear,  elliptical,  or  even  circular  in  outline;  always  stiff,  and  usually  ever- 
green, and  therefore  living  through  two  or  three  periods  of  vegetation.  Its  edges 
are  bent  down  and  more  or  less  rolled  back,  even  whilst  still  hidden  in  the  bud. 
In  consequence  of  this,  the  lower  side  which  faces  the  soil  is  hollowed  to  a  greater 
or  less  extent,  while  the  upper  side,  turned  skyward,  is  arched.  Frequently  the 
leaf  is  rolled  so  as  to  inclose  an  actual  chamber,  which  only  communicates  with  the 
outer  world  by  a  very  narrow  fissure,  as  is  the  case,  for  example,  in  the  Crowberry 
(Empetrum).  The  rolled-back  margins  of  the  leaves  in  this  plant  almost  touch 
one  another,  and  the  epidermis  of  the  lower  side  of  the  leaf  forms  the  actual 
lining  of  the  cavity  which  resulted  from  the  rolling  of  the  leaf  (see  fig.  71 2). 


MAINTENANCE   OF   A   FREE   PASSAGE   FOR   AQUEOUS   VAPOUR.  301 

If  the  bent-back  margins  do  not  fit  so  closely  together,  a  groove  appears  on 
the  under  side  of  the  leaf,  which  is  more  or  less  sunken  according  to  the  extent  of 
the  rolling,  as  for  example  in  the  Heaths  (Erica  caffra,  vestita,  &c.,  see  fig.  71 l). 
Occasionally  a  groove  is  developed  which  divides  into  two  side  furrows  running 


Fig.  71.— Transverse  Sections  through  Rolled  Leaves. 

i  Erica  caffra;  x280.    2  Empet rum  nigrum ;  X160.    »  Andromeda  tetragona ;  X150.    *  Ty lanthus  ericoides ;  x!30. 

6  Salix  reticulata ;  x  200. 

beneath  the  rolled  edges,  as  for  example  in  the  leaves  of  Andromeda  tetragona  (fig. 
7 13),  and  in  those  of  the  Cape  Rhamnea,  Tylanthus  ericoides  (fig.  71 4).  The  central 
portion  of  the  space  framed  in  by  the  rolled-back  leaves  is  also  frequently  divided 
into  two  longitudinal  grooves,  and  in  such  a  manner  that  the  tissue  below  the 
midrib  of  the  leaf  may  project  as  a  broad  strong  band.  On  the  under  side  of  the 
leaf,  therefore,  are  three  longitudinally  elongated  parallel  projections,  a  central  one 
under  the  midrib,  and  two  lateral,  which  have  been  formed  by  the  rolled-back  margins 


302         MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

of  the  leaf.  On  the  right  and  left  of  the  middle  ridge  lie  two  deep  grooves,  which 
are  apparent  to  the  naked  eye  as  light  stripes  between  the  dark  green  projecting 
portions.  This  is  the  case,  for  example,  in  the  leaves  of  the  Azalea  procumbens, 
also  in  one  of  the  Ericaceae  known  by  the  name  of  Loiseleuria,  which  covers  the 
soil  with  a  close-matted  carpet  wherever  it  makes  its  appearance,  and  is  widely 
distributed  through  Labrador,  Greenland,  Iceland,  Lappland,  and  generally  through 
the  whole  Arctic  region,  as  well  as  over  the  high  mountains  of  Scandinavia,  the 
Pyrenees,  Alps,  and  Carpathians.  The  annexed  figure  72  represents  a  transverse 
section  through  a  single  rolled  leaf  of  Azalea,  a  hundred  and  forty  times  its  natural 

size. 

Occasionally  several  strong  anastomosing  ribs  project  from  the  under  side  of 
the  rolled  leaf,  inclosing  small  pits  and  depressions  in  whose  depth  stomata  are 
situated,  as  may  be  seen  in  the  leaves  of  the  widely  distributed  Willow,  Salix 
reticulata  (see  fig.  71 5). 

Although  all  these  rolled  leaves  have  an  appearance  of  firmness  and  solidity, 
and  frequently  remind  one  of  the  needle-like  leaves  of  the  conifers,  they  are,  unlike 
these,  filled  up  with  a  very  loose  spongy  parenchyma,  which  takes  up  far  more 
room  than  the  palisade  tissue  lying  beneath  the  epidermis  of  the  upper  side.  The 
upper  epidermis  of  all  rolled  leaves  is  easily  wetted,  frequently  uneven  and  finely 
wrinkled,  destitute  of  any  waxy  covering;  the  cells  strongly  thickened  on  their 
outer  walls,  and  pressed  closely  together,  so  as  to  leave  no  spaces  between  them.  On 
the  under  side  it  is  very  different.  Here  stomata  are  present  in  great  number,  and 
the  epidermis  is  either  covered  with  wax,  as  in  the  Marsh  Andromeda,  the  Whortle- 
berry, and  the  Reticulate  Willow  (Andromeda  polifolia,  Oxycoccos  palustris,  and 
Salix  reticulata),  or  it  is  clothed  with  a  fine  felt- work,  as,  for  example,  in  Ledum 
palustre.  Very  often  peculiar  rod-shaped  or  filamentous  projections  of  the  cuticle 
are  present,  which  at  first  sight  might  be  taken  for  hairs,  but  which  differ  from 
hairs  in  being  solid,  not  hollow.  Figs.  72  and  71 lj  2>  3  show  these  structures  (which 
may  be  considered  as  counterparts  of  the  cuticular  pegs  on  the  bamboo  leaf)  on 
the  under  side  of  Azalea  procumbens,  Erica  caffra,  and  Andromeda  tetragona, 
as  well  as  on  the  edges  of  the  fissure  which  leads  into  the  hollow  leaf  of  the  Crow- 
berry  (Empetrum  nigrum).  These  structures  are  to  be  found  almost  without 
exception  in  the  heathers  of  the  northern  moors  as  well  as  in  the  Mediterranean  and 
Cape  flora.  The  importance  of  this  continuous  delicate  coat  lies  chiefly  in  the  fact 
that  air  adheres  to  it  as  to  the  cuticular  pegs  of  the  bamboo  leaf,  and  indeed  so 
firmly  that  even  water,  under  considerable  pressure,  is  not  able  to  displace  it.  On 
placing  a  leaf  of  Azalea  procumbens  under  water,  two  elongated  air-bubbles  are 
seen  along  the  two  longitudinal  furrows,  which  glisten  like  two  strips  of  silver. 
Even  shaking  the  leaf  to  and  fro  will  not  dislodge  these  air- vesicles,  and  even  if  the 
branch  has  been  left  submerged  for  a  week,  this  air  will  still  cling  to  the  depressions 
in  whose  depths  the  stomata  occur.  If  the  branch  be  removed  from  the  water  it 
will  be  seen  that  the  upper  side  of  the  leaves  is  wet,  while  water  has  been  kept 
away  from  the  stomata  of  the  under  side.  And  as  with  Azalea  procumbens,  so  is 


MAINTENANCE   OF   A   FREE   PASSAGE   FOR   AQUEOUS   VAPOUR.  303 

it  with  all  other  rolled  leaves,  whether  they  belong  to  Cape  plants  or  to  heath  plants 
of  the  Baltic  lowlands. 

It  cannot  be  doubted  that  the  mechanism  of  rolled  leaves,  as  just  described, 
furnishes  a  protection  for  the  stomata  against  moisture,  and  keeps  open  a  passage 
for  aqueous  vapour  and  excreted  gases.  The  question  is  now  only  how  it  comes 
about  that  this  arrangement  is  to  be  met  with  in  plants  of  such  widely  distant 
countries  and  under  such  differences  of  climate? 

In  order  to  understand  this  clearly,  let  us  imagine  ourselves  in  some  of  the 
regions  which  are  specially  characterized  by  the  abundance  of  plants  with  rolled 
leaves.  First,  on  one  of  the  high  ridges  of  the  Central  Alps,  where  the  low-lying 
Azalea  spreads  a  thick  covering  over  the  soil,  where  Erica  carnea  in  great  quantity 


Fig.  72.  —Vertical  Section  through  a  Rolled  Leaf  of  the  Trailing  Azalea  (Azalea  procumbent) ;  x  140. 

covers  broad  slopes,  where  Dryas  octopetala,  Salix  reticulata,  Homogyne  discolor, 
Saxifraga  ccesia,  and  many  other  plants  which  possess  evergreen  rolled  leaves 
weave  their  carpet  over  the  stony  earth.  The  ground  in  which  all  these  plants  are 
rooted,  and  from  which  they  draw  their  fluid  nourishment,  has  many  natural  dikes 
and  retains  a  large  quantity  of  water,  not  only  from  the  melting  of  the  heavy 
winter  mantle  of  snow,  but  also  from  the  abundant  rain  of  summer.  For  weeks 
together  the  heights  are  wrapped  in  a  cold  mist  which  saturates  everything  with 
moisture,  and  drops  of  water  hang  from  the  stems  and  leaves,  unable  to 
evaporate  as  long  as  the  air  remains  so  supercharged  with  vapour.  At  length  the 
sky  clears  again,  and  the  water  on  the  plants  begins  to  disappear.  But  even  during 
the  fine  night  following,  all  the  plants  become  covered  with  a  very  heavy  dew  in 
consequence  of  their  rapid  cooling  and  radiation,  and  this  not  unfrequently  remains 
until  the  middle  of  the  next  day.  Transpiration  at  last  occurs  when  the  sun  shines, 
and  particularly  if  dry  winds  sweep  over  the  heights.  But  who  knows  how  long 
this  state  of  things  will  continue  ?  Each  moment  is  precious,  and  every  hindrance 
to  the  evaporation,  so  important  for  the  plants,  would  be  a  distinct  disadvantage. 
The  outlets  for  aqueous  vapour  on  the  under  side  of  the  leaves  especially  should  not 
be  obstructed,  and  the  above  described  contrivance  is  arranged  with  this  end  in 


304         MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

view.  It  can  hardly  be  doubted  that  the  earlier  mentioned  plants  of  high  moun- 
tainous regions  cease  to  transpire  for  weeks  at  a  time  in  the  wet  seasons,  when  a 
thick  unbroken  mist  covers  the  slopes,  and  earth,  stones,  and  vegetation  are  dripping 
with  moisture;  and  of  course  the  conduction  of  food-salts  to  the  green  leaves  is 
interrupted  to  a  corresponding  extent.  If  one  considers  how  short  a  period  is 
afforded  to  plants  of  high  mountain  districts  in  which  to  perform  their  year  s  work, 
it  will  be  understood  how  the  most  active  means  for  promoting  transpiration  must 
be  brought  into  play,  and  how  everything  which  might  interrupt  or  limit  this 
process,  so  important  to  the  welfare  of  the  plant,  must  be  avoided.  A  few  months 
after  the  last  snow  has  melted  on  the  heights,  fresh  snow  again  falls,  and  entirely 
prevents  growth  and  nourishment  during  the  long  winter. 

These  climatic  conditions  account  for  the  fact  that  so  many  Alpine  plants, 
almost  all  those  having  rolled  leaves,  are  evergreen.  It  is  necessary  that  every 
sunbeam  during  the  short  vegetative  period  should  be  utilized,  and  that  the  leaves 
retained  from  the  previous  year  should  be  able  to  transpire  and  to  form  organic 
materials  on  the  first  sunny  day  after  the  winter  snow  has  melted,  although  the  soil 
may  have  become  only  slightly  warmed.  It  may  perhaps  be  urged  against  this 
explanation  that  though,  in  the  steppes  the  period  of  vegetation  is  restricted  to  the 
brief  space  of  three  months,  nevertheless  evergreen  plants  with  rolled  leaves  are 
completely  absent.  But  the  conditions  of  moisture  on  the  steppes  during  this  three 
months'  vegetative  period  are  essentially  different  from  those  of  the  high  mountain 
region.  In  the  steppes,  transpiration  is  never  brought  to  a  temporary  standstill  by 
too  much  moisture ;  evaporation  can  take  place  uninterruptedly  from  the  leaves,  and 
they  have  to  be  protected  not  from  moisture,  but  from  over-transpiration.  With  the 
exception  of  the  halophytes  and  a  few  other  growths  which  are  particularly  well 
protected,  no  plants,  on  account  of  the  extreme  dryness  of  the  air,  can  retain  their 
green  foliage  in  the  height  of  summer  on  the  steppes. 

Some  of  the  plants  which  adorn  the  high  mountains  of  southern  regions  make 
their  appearance  in  the  lower  plains  of  the  extreme  north.  The  same  carpet  of 
Trailing  Azalea,  Dwarf  Willows,  and  Dryas  (Azalea  procumbens,  Salix  reticulata, 
Dryas  octopetala)  is  found  on  the  soil  underfoot.  In  addition  are  other  small  plants 
which  remain  green  during  the  winter  (e.g.  Cassiope  tetragona),  which  are  similarly 
provided  with  rolled  leaves.  Even  if  we  were  not  informed  by  Arctic  explorers 
that  the  number  of  foggy  days  in  the  course  of  the  short  Arctic  summer  is  much 
greater  than  on  the  mountain  heights  of  the  south,  and  that  therefore  a  help  instead 
of  a  hindrance  to  transpiration  is  required,  the  utmost  use  being  made  of  the  short 
time  in  which  it  is  possible  to  draw  food-salts  from  the  soil,  we  might  infer  this  to 
be  the  case  from  the  frequent  appearance  of  these  small  carpet-forming  plants  with 
their  evergreen  rolled  leaves.  Apart  from  other  considerations,  and  disregarding 
the  development  of  the  various  floral  areas  in  point  of  time,  the  above  signification 
of  the  evergreen  rolled  leaves  explains  the  similarity  and  partial  identity  of  the 
arctic  flora  with  that  of  the  heights  mentioned. 

Let  us^turn  now  to  the  low-lying  country  along  the  North  and  Baltic  Seas,  and 


MAINTENANCE   OF   A   FREE   PASSAGE   FOR   AQUEOUS   VAPOUR.  305 

to  the  lowlands,  which  extend  as  far  as  the  northern  slopes  of  the  Alps.  Where  man 
has  not  transformed  the  ground  into  arable  soil,  only  moor  and  heath,  heath  and 
moor,  are  seen  in  wearisome  monotony.  On  the  moors  especially  are  always  the 
same  plants — various  Heaths  (Calluna  vulgaris,  Erica  Tetralix,  Erica  cinerea), 
Black  Crowberry  (Empetrum  nigrum),  Whortleberry  (Oxycoccos  palustris),  Marsh 
Andromeda  (Andromeda  polifolia),  Wild  Rosemary  (Ledum  palustre) — all  plants 
with  evergreen  rolled  leaves,  just  as  on  the  mountain  heights.  Some  of  these  small 
evergreen  bushes,  viz.  the  Crowberry  and  the  common  Ling  (Calluna  vulgaris),  may 
be  traced  in  an  unbroken  range  from  the  plains  up  to  a  height  of  2450  metres  on 
the  slopes  of  the  Alps.  Strange  to  say,  these  plants  do  not  blossom  much  earlier  on 
the  lowlands  than  on  the  high  Alpine  regions,  and  it  has  actually  been  shown  that 
Calluna  blooms  rather  sooner  at  a  height  of  2000  metres  than  in  the  northern  portion 
of  the  Baltic  lowlands.  How  is  this  ?  The  winter  snow  has  long  disappeared  from 
the  lowlands,  while  the  hill-sides  above  are  still  concealed  under  their  cold  white 
covering.  The  winter  snow  has  gone,  to  be  sure,  but  not  the  winter!  WThile  every- 
thing around  is  already  in  blossom,  while  the  ear  is  already  visible  on  the  stalks  of 
rye,  the  neighbouring  moor  is  still  dismal,  waste,  and  lifeless.  A  month  or  so  later 
there  is  a  stir  on  the  dry  soil  of  the  cold  moor,  and  the  absorbent  roots  of  the  plants 
which  have  evergreen  rolled  leaves  commence  their  activity.  When  the  warm  days 
of  midsummer  arrive  and  the  sun  sends  down  its  powerful  rays,  the  temperature  of 
the  soil  quickly  increases,  and  indeed  rises  far  more  than  would  be  thought  possible. 
The  damp  cushion  of  bog-moss  at  mid-day  feels  quite  warm;  and  a  thermometer 
placed  3  cms.  below  the  surface  in  the  uppermost  mossy  layer  of  a  moor  on  a 
cloudless  summer  day  (22nd  June)  showed  a  temperature  of  31°  C.  while  the  tem- 
perature of  the  air  in  the  shade  was  13°!  An  unpleasant  vapour  rises  from  the  damp 
earth,  which  settles  on  the  surface,  and  makes  a  walk  over  the  moor  particularly 
disagreeable.  Scarcely  has  the  sun  set  in  glowing  red  on  the  horizon  when  this 
vapour  condenses  into  patches  of  mist  which  settle  over  the  dark  expanse;  stems, 
branches,  and  leaves  are  covered  with  drops  of  water,  and  next  morning  everything 
is  as  thoroughly  soaked  as  if  it  had  rained  throughout  the  night.  This  process, 
which  is  regularly  repeated  during  the  fine  weather,  is  only  interrupted  when  a 
damp  wind  from  the  sea  blows,  driving  masses  of  cloud  over  the  heath,  or  when 
copious  rain  saturates  the  soil.  It  needs  no  further  showing  that  under  such  condi- 
tions an  abundant  and  continuous  transpiration  from  plants  is  impossible,  and  that 
in  the  short  intervals  which  are  allowed  to  the  leaves  for  transpiration,  the  outlets 
from  the  wide-meshed  spongy  parenchyma  must  not  be  obstructed;  and  it  does 
not  need  further  proof  that  the  evergreen  rolled  leaf  is  the  form  most  suited  and 
adapted  to  these  conditions. 

The  flora  of  the  Cape  of  Good  Hope  may  not  unjustly  be  compared  with  that  of 
the  Baltic  lowlands— countless  low  bushes  which  are  very  like  Heaths,  Wild  Rose- 
mary, and  Crowberry  in  appearance— all  with  small  rigid  evergreen  leaves,  and  entire 
rolled-back  margins;  the  upper  side  of  the  leaf  usually  dark  green,  the  under  side 
having  the  same  arrangements  as  shown  in  the  rolled  leaves  of  plants  growing  on 
VOL.  I. 


306         MAINTENANCE  OF  A  FREE  PASSAGE  FOR  AQUEOUS  VAPOUR. 

moors  which  border  the  Baltic  Sea,  and  in  the  cold  Arctic  tundra.  This  shrubby 
evergreen  vegetation  of  the  Cape  belongs  indeed  in  part  to  the  same  families  as 
these.  Heaths  especially  are  to  be  found  in  abundant  variety;  as  many  as  400 
species  can  be  counted — many  more  than  are  furnished  by  the  whole  of  the  rest  of 
the  world  taken  together.  But  a  great  number  of  species  from  other  families,  viz. 
Rhamnese,  Proteacese,  EpacridesB,  and  Santalaceae,  possess  an  exactly  similar  foliage, 
and  without  blossom  and  fruit  are  often  indistinguishable  from  the  heaths.  This  low 
evergreen  bush  vegetation  is  not  distributed  all  over  the  Cape,  but  is  restricted  to 
the  neighbourhood  of  the  coast,  to  the  country  which  slopes  in  terraces  down  to  the 
south-west,  and  to  the  celebrated  Table  Mountain,  rising  abruptly  above  Cape  Town. 
The  aqueous  vapour  brought  by  the  sea-winds  condenses  directly  over  these  regions, 
and  for  five  months,  from  May  till  the  beginning  of  October,  the  soil  is  not  only 
soaked  by  abundant  rain,  but  what  is  perhaps  of  even  greater  moment,  all  the  ever- 
green bushes  are  kept  in  a  damp  condition  by  the  falling  mist,  and  often  are 
dripping  with  water  just  like  the  heaths  on  the  moors  of  the  Baltic  lowlands. 
When  the  development  of  vegetation  on  the  lower  terraces  of  the  south-west  coast 
is  at  a  standstill  on  account  of  the  increasing  dryness,  the  summit  of  the  Table 
Mountain  is  still  enveloped  in  the  celebrated  mass  of  cloud  known  as  the  "table- 
cloth ",  and  the  plants  growing  on  the  ridges  and  plateaus  are  during  this  time  as 
much  saturated  as  the  Trailing  Azalea,  which  robs  the  south  wind  of  its  moisture  on 
the  mountain  ridges  of  the  Central  Alps.  It  is,  however,  in  this  damp  period  that 
the  growth  of  the  plants  in  question  takes  place.  Most  of  the  plants  on  the  heights 
of  the  Table  Mountain  blossom  and  put  forth  new  shoots  in  February,  March,  and 
April;  on  the  lower  terraces  from  May  to  September.  In  the  northern  regions  and 
on  mountain  heights  the  beginning  and  end  of  the  year's  work  in  plants  is  limited 
by  the  cold,  but  in  the  Cape  the  dryness  of  the  soil  is  the  cause  which  brings  the 
upward  current  of  the  sap  in  vegetation  to  a  standstill  for  so  long  a  time.  At  the 
coast,  however,  this  dryness  is  never  so  severe  that  the  plants  are  exposed  to  the 
danger  of  withering  up  altogether,  as  on  the  steppes. 

As  on  the  south-west  coasts  of  the  Cape,  so  is  it  round  about  the  Mediterranean 
Sea  and  in  the  west  of  Europe,  which  is  swept  by  sea-winds  laden  with  vapour 
from  the  Atlantic;  for  example,  Portugal  and  the  south-west  of  France,  which  are 
in  like  case,  characterized  by  an  abundance  of  low  bushes,  with  evergreen  rolled 
leaves,  and  especially  by  some  gregarious  heaths.  Here  also  the  year's  growth 
takes  place  in  the  wettest  season,  and  arrangements  must  be  made  that  during  this 
period  the  formation  of  organic  materials,  the  withdrawal  of  food-salts  from  the 
soil,  and  consequently  unhindered  transpiration  may  be  carried  on.  Here,  too,  dry- 
ness  interrupts  the  activity  of  the  absorbent  roots,  and  the  evergreen  vegetation  of 
the  coast-line  extends  inland  as  far  as  the  damp  sea- winds  make  themselves  felt; 
while  still  further  inland  a  steppe-like  vegetation  preponderates.  The  analogy  pre- 
sented by  the  Mediterranean  flora  goes  so  far  that,  on  the  southern  point  of  Istria, 
for  example,  which  may  be  compared  as  to  shape  with  the  south  point  of  Africa, 
quantities  of  the  evergreen  Erica  arborea  are  only  to  be  found  on  the  south-west 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS.  307 

coast  district  on  a  comparatively  narrow  strip  of  land;  while  in  the  interior  of  Istria 
the  waste  dry  terraces  of  the  Tschitscherboden  (which  might  be  compared  with  the 
arid  plains  of  the  Cape)  show  no  trace  of  a  heath  vegetation. 

Why  the  plants  with  evergreen  leaves  which  grow  in  the  far  north,  on  the 
heights  of  the  Alps,  on  the  Baltic  lowlands,  on  the  shores  of  the  Atlantic  Ocean,  on 
the  borders  of  the  Mediterranean  basin,  and  at  the  Cape  of  Good  Hope  are  not  all 
of  the  same  species,  is  a  question  which  cannot  be  answered  here;  yet  it  seems 
proper  to  point  out  that  all  plants  furnished  with  evergreen  rolled  leaves,  whose 
year's  work  is  stopped  by  dryness,  would  freeze  in  countries  where  the  earth  in 
winter  is  covered  with  snow,  i.e.  the  molecular  structure  of  their  protoplasm  would 
be  entirely  altered  by  the  frost,  which  would  kill  it;  while  the  protoplasm  of  the 
analogous  northern  forms  would  suffer  no  harm  from  the  cold.  It  is  well  worthy  of 
remark  in  this  connection  that  some  of  the  last-mentioned  plants  have  an  extra- 
ordinarily wide  distribution;  that  they  may  actually  be  found,  quite  similar  in 
appearance,  in  the  bleak  north,  and  in  the  southern  districts,  if  only  those  conditions 
of  moisture  which  we  have  shown  to  account  for  the  form  of  the  leaves  obtain  in 
the  places  mentioned.  Thus  the  Irish  Heath  (Dabeocia  polifolia)  may  be  found 
along  the  Atlantic  coast  as  far  as  Portugal,  and  the  common  Ling  (Galluna  vulgaris) 
grows  just  as  well  at  a  height  of  2450  metres  above  the  sea  beside  the  glaciers  of 
the  (Etzthal  in  the  Central  Alps,  as  further  south  on  the  Abazzia,  surrounded  by 
laurel  groves  on  the  sea-coast  of  Istria. 


3.  PEEVENTION    OF    EXCESSIVE    TRANSPIRATION. 

Protective  arrangements  on  the  Epidermis.— Form  and  Position  of  Transpiring  Leaves  and 

Branches. 

PKOTECTIVE    ARRANGEMENTS    ON    THE    EPIDERMIS. 

The  relation  of  the  form  of  the  evergreen  rolled  leaf  to  transpiration  is  anything 
but  exhausted  in  the  foregoing  account.  The  part  played  by  this  form  of  leaf,  in 
particular  during  the  dry  season  of  the  year,  yet  remains  to  be  discussed.  If  it  is 
necessary  during  the  wet  period  that  transpiration  should  be  increased  as  much  as 
possible,  and  that  everything  which  might  restrict  the  exhalation  of  aqueous  vapour 
from  the  stomata  should  be  kept  away,  it  is  also  of  importance  that  on  the  appear- 
ance of  the  dry  season  the  equilibrium  between  the  water  taken  from  the  soil  and 
the  water  excreted  by  the  leaves  should  not  be  destroyed,  and  that  an  excessive 
evaporation  from  the  portions  of  the  plant  above  ground  should  be  hindered.  New 
seasons  bring  new  problems  to  be  solved.  At  the  time  when  the  water-current 
begins  to  ascend  from  the  soil  saturated  by  the  winter  rains,  we  have  an  aid  to 
transpiration;  later  on,  in  the  dry  period,  we  have  a  protection  against  the  dangers 
which  might  attend  excessive  evaporation.  It  is  certainly  of  great  interest  to  see 


308  PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS. 

how  a  whole  group  of  the  arrangements  discussed  above,  among  which  the  rolled 
leaf  is  not  the  least  noticeable,  serve,  at  different  seasons  of  the  year,  and  often  at 
different  times  of  the  same  day,  two  distinct  purposes,  as  indicated. 

First,  the  stomata  themselves.  While  the  green  tissue  has  need  of  food-salts 
from  the  soil  for  the  manufacture  of  organic  materials,  they  cannot  be  too  widely 
opened;  everything  is  welcome,  then,  which  promotes  transpiration,  and  conse- 
quently assists  in  the  elevation  of  fluid  nourishment  from  the  saturated  soil.  But  if 
the  temperature  and  dryness  of  the  air  increase  after  the  green  parenchyma  has 
finished  its  yearly  task,  or  if  the  soil  from  which  the  absorbent  roots  have  hitherto 
derived  their  supply  of  fluid  become  so  dry  that  the  water  exhaled  from  the  aerial 
positions  can  no  longer  be  replaced,  it  is  of  the  greatest  importance  that  the  stomata 
should  be  closed.  This  is  brought  about  by  the  two  cells  bounding  the  stoma, 
which  have  been  termed  the  "  guard  "  cells. 

In  order  to  clearly  understand  the  mechanism  of  the  opening  and  closing  of 
stomata,  it  is  necessary  to  examine  the  structure  of  these  guard-cells  more  in  detail. 
Both  are  bean-shaped  in  outline,  their  concave  surfaces  being  turned  towards  the 
stoma;  they  are  only  connected  with  one  another  at  their  extremities.  By  their 
convex  sides  they  are  in  contact  with  ordinary  epidermal  cells;  their  outer  walls 
are  in  contact  with  the  atmospheric  air,  and  their  inner  walls  with  the  spongy 
parenchyma.  Both  the  innermost  and  outermost  walls  of  the  guard -cell  are 
strongly  thickened,  but  the  wall  by  which  they  are  connected  with  neighbouring 
epidermal  cells,  as  well  as  that  portion  which  directly  borders  the  stoma,  is  relatively 
thin,  elastic,  and  extensible.  If  the  figure  of  two  such  guard-cells  be  imitated  in 
caoutchouc,  and  they  be  fitted  together  like  an  actual  closed  stomate — water  being 
forced  into  them  under  considerable  pressure — the  curvature  of  the  thin  and  elastic 
portions  of  the  walls  will  be  most  altered.  The  side  wall  in  contact  with  the 
neighbouring  epidermal  cell  bulges  out,  and  at  the  same  time  the  whole  cell  becomes 
elongated  in  a  direction  perpendicular  to  the  surface.  By  this  means  the  two 
guard-cells  are  forced  apart.  When  the  water  is  allowed  to  flow  out  of  the  swollen 
caoutchouc  cells,  they  again  fall  back  into  position,  the  two  portions  of  the  walls 
which  border  the  stoma  coming  into  contact  with  each  other  and  closing  the  opening. 
The  same  thing  occurs  in  the  actual  guard-cells  of  the  living  plant.  As  soon  as 
they  become  distended,  they  separate  from  one  another;  when  they  relax  and 
resume  their  original  position,  they  come  closely  into  contact  again.  This  process 
bears  a  strong  resemblance  to  the  changes  in  the  cells  of  the  pulvini  at  the  base  of 
the  sensitive  leaves  of  Mimosa,  which  will  be  described  later,  and  it  is  highly 
probable  that  it  may  be  traced  back  to  a  similar  stimulation.  That  the  guard-cells 
actually  separate  from  one  another  by  swelling  up,  i.e.  by  absorbing  fluid,  and  then 
close  together  again  in  consequence  of  the  loss  of  water,  can  be  shown  by  first 
supplying  water  and  then  withdrawing  it  by  a  solution  of  sugar.  In  the  former 
case  the  stomata  open,  in  the  latter  they  close,  and  it  may  therefore  be  considered  an 
established  fact  that  a  closing  movement  is  brought  about  by  the  extra  loss  of  water 
in  dry  air.  But  if  these  pores,  through  which  water  vapour  escapes  when  the  plants 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS.  309 

are  full  of  sap,  close  as  soon  as  there  is  a  danger  of  too  much  evaporation,  the 
mechanism  must  be  considered  as  excellently  regulating  transpiration,  and  as'pro- 
viding  a  true  preventative  against  over-evaporation. 

This  closure  of  the  exhalent  chambers  in  the  interior  of  the  leaf,  important  as  it 
is,  would  alone  be  sufficient  in  but  a  very  few  cases  to  ward  off  this  threatened 
danger.  If  the  epidermis  which  stretches  over  the  thin- walled  transpiring  cells  of 
the  spongy  parenchyma  is  itself  thin-walled  and  succulent,  water  will  be  exhaled 
from  it  also  into  the  dry  atmosphere;  this  loss  of  water  from  the  epidermal  cells  is 
compensated  for  by  water  drawn  from  the  neighbouring  parenchymatous  cells  in 
the  interior  of  the  leaf,  and  ultimately  the  leaves  would  wither  up  if  the  supply  of 
water  from  the  roots  were  stopped  or  became  insufficient.  Therefore  the  epidermal 
cells  must  be  adequately  prevented  from  exhaling.  When  this  is  the  case,  and  when 
the  stomata  are  closed,  the  spongy  parenchyma,  and,  generally  speaking,  all  the 
succulent  cells  in  the  interior,  are  securely  protected. 

The  walls  of  the  epidermal  cells  in  the  first  stages  of  their  development  are 
composed  mainly  of  cellulose,  and  are  uniformly  thin  and  delicate  on  all  sides.  The 
outer  wall,  which  is  in  contact  with  the  air,  then  becomes  thickened  and  divided  into 
an  inner  and  an  outer  layer.  The  inner  retains  its  original  character,  but  the  outer 
—the  so-called  "  cuticle  "—undergoes  great  modifications.  The  cellulose  becomes 
changed,  and  is  replaced  by  a  mixture  of  stearin  and  the  glyceride  of  a  fatty  acid 
{suberic  acid),  forming  a  tallow-like  fat  which  is  termed  cutin  or  suberin.  In 
consequence  of  this  metamorphosis  the  cell-wall  becomes  less  and  less  permeable  to 
water,  and  when  it  has  attained  a  considerable  thickness  it  becomes  at  length  almost 
entirely  impervious  to  water  and  aqueous  vapour.  Frequently,  between  the  inner 
cellulose  and  the  outer  corky  layer,  other  so-called  " cuticularized  layers"  are 
formed,  whose  chief  constituent  is  again  suberin,  and  which  often  attain  to  a  con- 
siderable bulk. 

Aquatic  plants,  which  are  not  exposed  to  the  danger  of  excessive  evaporation,  of 
course  do  not  require  this  protection.  Plants  whose  leaves  are  surrounded  by  air, 
on  the  other  hand,  can  never  entirely  dispense  with  it.  The  thickness  of  these 
corky  layers  is  extremely  variable  according  to  the  condition  of  humidity  of  the  air. 
Where  the  air  is  very  damp  throughout  the  year,  the  outer  wall  of  the  epidermal 
leaf -cells  appears  to  be  only  slightly  thicker  than  the  inner,  and  the  cuticle  only 
forms  a  thin  continuous  layer.  On  the  other  hand,  plants  which  are  temporarily 
exposed  to  dry  air  possess  very  highly  developed  cuticular  strata.  Especially  when 
the  leaves  are  evergreen  and  remain  several  years  on  the  branches,  as,  for  example,  in 
the  Holly  (Ilex  Aquifolium,  see  fig.  73 2),  and  in  the  Oleander  (Nerium  Oleander, 
fig.  *733),  the  cuticular  layers  are  so  strongly  developed  that  the  outer  wall  of  the 
epidermal  cells  is  many  times  thicker  than  the  inner  wall.  Evergreen  parasites,  as, 
for  example,  the  Mistletoe  (fig.  73 1),  those  tropical  orchids  and  Bromeliaceae  which 
live  epiphytically  on  the  bark  of  trees  and  are  often  exposed  to  great  dryness  in  the 
hot  season  of  the  year,  cactiform  plants,  and  generally  the  majority  of  succulent 
plants,  possess  epidermal  cells  with  very  strongly  thickened  outer  walls.  This  is  so 


31Q  PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS. 

also  in  the  case  of  the  pines  with  evergreen  needle-shaped  leaves,  where,  as  a  rule, 
the  water  compensating  for  that  exhaled  by  the  leaves  cannot  come  quickly  through 
open  channels,  but  only  slowly  through  the  woody  cells.  Usually  the  cuticle  and 
cuticular  layers  are  of  equal  thickness  over  the  whole  leaf  surface;  this  is  so  espe- 
cially in  smooth,  shiny,  leathery  evergreen  leaves.  Not  infrequently,  however,  an 
irregular  thickening  is  seen,  particularly  in  the  neighbourhood  of  stomata,  where 
circular  ramparts  are  raised,  as  in  Protect,  mellifera  (see  fig.  67 3),  or  peg-shaped  pro- 
jections are  formed,  as  in  the  Bamboo  (see  fig.  66),  or  elongated  hair-like  filaments 
arise,  as  in  the  rolled  leaves  of  Azalea  and  many  Heaths  (see  figs.  71  and  72). 

It  would,  however,  be  erroneous  to  suppose  that  this  formation  of  a  thick  cuticle 
on  the  epidermis  is  a  peculiarity  of  evergreen  leaves.     Plants  which  are  surrounded 


Fig.  73.— Thickened  Stratified  Cuticle. 

i  Vertical  section  of  a  portion  of  the  leaf  of  Mistletoe  (Viscum  album);  x420.    8  Vertical  section  of  a  portion  of  the  leaf 
of  Holly  (Ilex  Aqutfolium);  X500.    »  Vertical  section  of  leaf  of  Oleander  (Nerium  Oleander);  x320. 

all  the  year  by  a  damp  atmosphere,  and  are  never  exposed  in  their  natural  condition 
to  the  danger  of  too  much  evaporation,  very  often  have  evergreen  leaves,  and  yet 
the  outer  wall  of  the  epidermal  cells  is  not  at  all,  or  only  very  slightly,  thicker  than 
the  inner;  and  conversely,  plants  with  apparently  thin  delicate  leaves,  which  are 
green  only  in  the  summer,  have  quite  conspicuous  thickening-layers.  A  knowledge 
of  these  conditions  is  of  the  utmost  importance  in  plant  culture,  and  gardeners  know 
very  well  that  many  plants,  although  they  appear  to  be  capable  of  resistance,  can 
never  be  removed  from  the  damp  air  of  the  greenhouses,  because  the  leaves  then 
become  desiccated  like  those  of  aquatic  plants  which  have  been  taken  out  of  water 
and  exposed  to  the  air.  A  species  of  palm,  Caryota  propinqua,  which  is  repre- 
sented in  its  native  habitat  in  fig.  74  opposite,  was  grown  in  the  botanical  gardens 
at  Vienna,  and  it  developed  in  the  damp  air  a  magnificent  stem  with  fine  large 
leaves.  On  a  summer  day,  when  the  temperature  of  the  open  air  coincided  with 
that  of  the  greenhouse,  this  Caryota,  together  with  the  tub  in  which  it  was  rooted,, 
was  carried  into  the  open  and  placed  in  a  somewhat  shady  place,  but  partly  exposed 


PROTECTIVE   ARRANGEMENTS   ON   THE    EPIDERMIS. 


311 


74— Caryota  propingua. 


312 


PROTECTIVE    ARRANGEMENTS   ON   THE    EPIDERMIS. 


to  the  sun's  heat.  One  day,  after  a  warm  dry  east  wind  had  swept  for  only  a  short 
time  over  the  foliage,  it  became  quite  brown,  and  in  the  evening  all  the  leaves  were 
entirely  dried  up  and  dead.  And  yet  leaf -segments  of  this  palm  appear  to  be  firm, 
leathery,  and  dry,  and  one  would  have  supposed  them  to  be  particularly  well  pro- 
tected against  drying  up.  The  section  of  part  of  a  leaf  which  is  represented  in 
fig.  75,  however,  corrects  this  impression.  This  shows  that  the  epidermal  cells  are 
certainly  very  compact,  by  which  the  firmness  of  the  leaf  is  materially  increased, 
but  that  their  walls  are  not  thickened,  being  only  like  those  of  a  delicate  fern  in 
this  respect.  Under  these  small  thin- walled  epidermal  cells  lie  large  succulent  cells 
which  form  the  so-called  aqueous  tissue,  the  structure  of  whose  walls  likewise 
cannot  limit  evaporation;  below  these  are  the  large  succulent  cells  of  the  green 
tissue.  A  glance  at  this  leaf  section  will  make  it  clear  that  this  palm  is  well 


Fig.  75.— Vertical  section  of  a  portion  of  the  leaf  of  Caryota  propinqua;  x260. 

adapted  to  its  warm  damp  habitat,  where  it  is  never  exposed  to  a  strong  evaporiza- 
tion,  but  not  to  the  dry,  even  if  warm,  air  of  a  Continental  climate. 

To  the  wax-like  excretions  of  the  cell-wall  which  form  a  delicate  bloom,  easily 
rubbed  off,  on  both  sides  of  the  leaf,  frequently  colouring  it  pale  blue,  grey,  or  white 
instead  of  dark  green,  it  has  already  been  stated  that  the  role  is  assigned  of  protect- 
ing the  stomata  from  moisture.  From  what  has  been  said,  one  would  expect  that 
these  waxy  coverings,  which  are  especially  to  be  met  with  in  the  Cruciferse  and 
Rutaceae  of  steppes,  in  many  acacias  and  Myrtaceaa  of  Australia,  and  in  the  pinks 
and  spurges  of  the  Mediterranean  flora,  would  also  be  able  to  limit  transpiration  in 
the  epidermis — that  is,  in  the  structures  over  which  the  bloom-like  covering  extends. 
Experiments  specially  undertaken,  have  also  shown  that  in  the  same  space  of  time, 
and  under  otherwise  similar  conditions,  leaves  whose  bloom  had  been  carefully 
rubbed  off  lost  almost  a  third  more  water  than  others  whose  waxy  covering  had 
been  left  intact. 

That  the  varnish-like  covering  of  the  epidermis,  composed  of  a  mixture  of 
mucilage  and  resin  ("balsam"),  which  is  excreted  from  capitate  hairs  and  other 
glandular  structures,  is  able  to  restrict  transpiration  has  also  been  pointed  out. 
These  coverings  are  especially  developed  in  many  plants  of  the  Mediterranean  flora, 
particularly  in  a  whole  group  of  Cistus  (G.  laurifolius,  populifolius,  Clusii,  ladani- 
ferus,  monspeliensis,  &c.);  further  in  shrubby  plants  which  develop  late,  in  the 
height  of  summer — as,  for  example,  in  Inula  viscosa,  which  is  so  abundant  on  the 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS.  313 

coast.  Plants  of  steppes  and  prairies  (e.g.  Centaurea  Balsamita  of  the  Persian 
steppes  and  Grindelia  squarrosa  in  the  prairies  of  North  America)  are  likewise 
protected  throughout  life  from  over- vaporization  by  varnish-like  coverings  of  this 
kind;  while  the  foliage  of  Cherry,  Apricot,  and  Peach  trees,  as  well  as  of  Birches, 
Sweet  Willows,  Balsam  and  Pyramidal  Poplars,  and  the  Black  Alder,  is  only  covered 
with  such  a  varnish  while  young,  when  it  has  just  burst  from  the  buds,  and  the 
outer  walls  of  the  epidermal  cells  have  not  yet  become  sufficiently  thickened;  later 
on,  however,  when  the  cuticularized  layers  have  become  fully  formed,  this  covering 
which  limits  transpiration  disappears.  Only  on  those  places  of  the  epidermis,  where 
the  outer  walls  of  the  cells  remain  very  thin  and  permeable  by  fluids  and  gases,  is 
this  coat  of  balsam  retained  until  the  leaf  is  to  be  thrown  off;  but  in  this  case  it 
probably  regulates  the  absorption  of  atmospheric  water. 

How  far  the  incrustations  of  lime  and  salt  excretions  take  part  in  the  absorption 
of  atmospheric  water  by  organs  situated  above  the  ground  has  likewise  already  been 
considered  in  the  section  on  water  absorption.  It  is  obvious  that  these  concretions 
and  coverings  of  the  epidermis  must  be  capable  of  restricting  transpiration. 
Incrustations  of  lime  are  principally  found  in  plants  which  grow  in  the  clefts  and 
crevices  of  rocks;  excretions  of  salt  are  only  observed  in  shore-plants  and  those  of 
steppes  and  wastes,  but  then  always  on  low  bushes  and  shrubs  with  small  narrow 
leaves,  and  herbs  whose  foliage  rests  on  the  soil.  The  reason  for  this  is  again  easily 
found.  High  trees  could  not  support  the  weight  of  leaves  loaded  with  incrustations 
of  lime  and  salt,  even  if  their  trunks  and  branches  possessed  the  greatest  strength 
imaginable. 

It  has  been  observed  that  plants  whose  leaves  are  covered  by  incrustations  of 
lime  and  salt,  or  whose  epidermal  cells  are  strongly  thickened  on  their  outer  walls  by 
corky  layers,  are  almost  always  destitute  of  hairs;  while  plants,  on  the  other  hand, 
whose  epidermal  cells  possess  delicate  outer  walls,  if  they  are  not  surrounded 
by  a  damp  atmosphere  throughout  the  year,  nor  submerged  in  water,  are  usually 
furnished  with  structures  known  as  plant-hairs  (trichomes)',  from  which  it  may  be 
inferred  that  the  hairy  covering  of  the  leaf  or  stalk  in  question  is  able  to  protect  it 
from  drying  up  in  just  the  same  way  as  the  corky  layers.  Of  course  only  those 
hairs  are  meant  whose  protoplasmic  contents  have  disappeared,  and  which  have 
become  sapless  and  filled  with  air;  for  those  hair-structures,  which  consist  of  cells 
rich  in  sap  and  osmotic  contents,  would  not  help  in  preventing  evaporation  from  the 
deeper  tissue;  they  are  themselves  in  need  of  protection,  and  special  protective 
arrangements  exist  for  them,  as  already  set  forth  in  the  discussion  on  the  absorption 
of  water  by  aerial  portions  of  the  plant.  Such  structures  would,  if  unprotected, 
give  off  water  to  the  surrounding  air,  and  continually  absorb  fluid  from  adjacent 
cells  below  them.  This  action  does  not  take  place  in  air-containing  cells,  and  if  their 
dry  membranes,  and  the  air  which  they  inclose,  are  interpolated  between  the  dry 
atmosphere  and  the  succulent  tissue  below,  this  latter  will  be  protected  from  evapo- 
ration, like  damp  earth  covered  with  a  layer  of  dry  straw  or  reeds,  or  the  fluid  at 
the  bottom  of  a  bottle  whose  neck  is  closed  with  a  plug  of  cotton-wool. 


314  PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS. 

The  importance  of  air-containing  cells  as  a  covering  for  succulent  tissue  must 
also  be  considered  in  another  relation.  It  is  well  known  that  evaporation  from  th< 
surface  of  fluid  or  a  damp  body  is  much  increased  by  the  warmth  of  the  sun's 
On  the  other  hand,  if  the  heating  is  restricted,  so  also  is  the  evaporation.  If  we  u$ 
a  dry  cloth  to  shade  from  the  sun,  we  lower  not  only  the  temperature,  but  also  th( 
amount  of  evaporation  from  the  shaded  body.  The  covering  of  air-containing  hai] 
on  leaves  may  be  compared  to  such  dry  screens,  and  its  action  may  be  demonstral 
by  the  following  experiment: — Take  two  of  the  bi-coloured  leaves  of  a  Bramble 
bush,  which  are  smooth  on  the  upper  side,  but  covered  with  a  white  felt-work  of 
hairs  on  the  lower,  and  which  are  exactly  similar  in  size  and  position  with  regard  to 
the  sun,  being  situated  very  near  each  other  on  the  stem.  If  these  leaves  are 
wrapped  round  thermometers,  in  such  a  way  that  the  leaf  which  covers  one  thermo- 
meter bulb  has  its  white  felted  side  turned  towrards  the  sun,  that  covering  the  other, 
the  green  hairless  side,  it  will  be  found  that  the  temperature  in  the  leaf  whose 
smooth  green  side  is  directed  towards  the  sun  will  in  less  than  five  minutes  rise 
2°-5°  above  that  of  the  leaf  whose  white  felted  side  is  so  directed.  If  such  leaves 
are  plucked  and  exposed  to  the  sun,  some  with  the  white  felted  side,  others  with  the 
smooth  green  side  uppermost,  the  latter  always  shrivel  and  dry  up  much  sooner  than 
the  former.  There  can  be  no  doubt,  after  this,  that  a  dry  coat  of  hair  over  succulent 
plant  tissue,  which  is  exposed  to  the  sun's  rays,  considerably  restricts  the  heating  ofr 
and  exhalation  from  this  tissue. 

The  significance  of  the  coverings  of  hair  on  portions  of  plants  turned  away  from 
the  sun,  particularly  on  the  under  sides  of  flat  and  rolled  leaves,  has  already  been 
discussed.  These  coverings  are  only  of  slight  importance  as  a  means  of  protection 
against  over-transpiration.  In  rare  cases,  indeed,  it  happens  that  the  hairy  covering 
on  the  side  of  the  leaf  turned  from  the  sun,  the  lining  of  the  leaf,  so  to  speak,  must 
act  as  a  protection,  since  the  flat  leaf -lamina  is  so  twisted  and  turned  that  the  sun's 
rays  strike  not  on  the  upper  but  on  the  under  surface.  There  are  certain  ferns  of 
Southern  Europe  (Ceterach  officinarum,  Cheilanthes  odora,  Notochlcena  Marantce), 
which,  contrary  to  the  habits  of  most  of  this  shade-loving  group,  grow  on  blocks  and 
walls  which  are  exposed  to  the  burning  sun.  In  these  ferns  the  upper  surface  of 
the  leaf  is  smooth,  but  the  under,  on  the  other  hand,  is  thickly  covered  with  dry 
hair-scales.  In  wet  weather  the  leaves  are  spread  out  flat,  with  the  smooth 
surface  uppermost;  in  dry  weather  they  become  rolled  up,  and  the  under  cottony 
side  is  then  exposed  to  the  sun  and  to  dry  winds.  Among  the  low  herbaceous 
growths  of  the  Mediterranean  flora,  a  like  behaviour  is  shown  by  the  widely  distri- 
buted Hawkweed,  Hieracium  Pilosella,  whose  radical  leaves,  forming  a  rosette  on 
the  soil,  appear  green  on  the  upper  and  white  on  the  under  side,  by  reason  of  a  felt- 
work  of  star-shaped  hairs.  In  places  where  the  ground  easily  dries  up,  and  when- 
there  have  been  no  showers  for  a  long  while,  it  is  usually  seen  that  first  the  margins 
of  the  leaves  turn  up,  and  then  by  degrees  the  whole  leaf  becomes  bent  and  rolled, 
so  that  the  lower  side  is  turned  towards  the  sun's  rays,  and  the  white  felt  of  hairs 
functions  as  a  protective  screen  to  the  whole  leaf. 


PROTECTIVE   ARRANGEMENTS   ON   THE    EPIDERMIS.  315 

The  relations  between  the  hairy  covering  on  the  upper  side  of  the  leaf  and  trans- 
piration stand  out,  most  strikingly,  in  those  districts  where  plants  during  their 
vegetation  period  are,  as  a  rule,  exposed  to  dry  air  only  for  a  few  hours  each  day, 
and  where  their  activity  is  not  interrupted  by  a  long  warm  dry  period,  but  by  frost 
and  cold— as  is  the  case,  for  example,  in  the  Alpine  region  of  mountain  heights.  On 
the  Alps,  the  drying  up  of  flowering  plants  by  the  sun  only  occurs  in  a  very  few 


Fig.  76.— Edelweiss  (Onaphalium  Leontopodium). 

cases,  viz.,  where  the  scanty  soil  on  the  narrow  ledges  of  steep  projecting  rocks,  and 
crags,  and  on  rocky  slopes,  &c.,  is  only  watered  by  rain,  mist,  and  dew.  If  no 
showers  fall  for  several  successive  days,  and  the  south  wind  blows  over  the  heights 
with  a  clear  sky  day  and  night,  these  scanty  layers  of  soil  may  dry  up  to  such 
an  extent  that  they  are  unable  to  supply  the  necessary  fluid  food  to  the  plants 
rooted  in  them.  Under  these  circumstances  plants  growing  there  have  most 
pressing  need  of  means  of  lessening  transpiration  in  the  leaves.  In  places  such  as 
these  are  to  be  found,  almost  without  exception,  plants  whose  leaves  and  stems  are 
thickly  covered  on  all  sides  with  hairs,  together  with  succulent  plants  and  saxi- 
frages incrusted  with  lime.  This  is  the  habitat  of  the  felted  Whitlow-grass  (Draba 


316  PROTECTIVE   ARRANGEMENTS   ON   THE    EPIDERMIS. 

tomentosa,  stellata),  of  the  grey-leaved  Ragwort  (Senecio  incanus  and  Carniolicus), 
of  the  magnificent  silky  Cinquefoil  (Potentilla  nitida),  and  of  the  white-leaved 
bitter  Milfoil  (Achillea  Clavennce) ;  especially  is  this  the  habitat  of  the  most 
celebrated  Alpine  plants,  of  the  scented  Edelraut  and  the  beautiful  Edelweiss — the 
former  (Artemisia  Mutellina)  with  a  grey  shimmering  silky  coat,  the  latter 
(Gnaphalium  Leontopodium)  wrapped  in  dull  white  flannel.  On  looking  at  the 
vertical  section  of  the  Edelweiss  leaf  (see  fig.  77  x),  one  sees  that  the  epidermal 
cells  with  their  thin  outer  walls  would  be  unable  to  regulate  exhalation  and  drying 
in  the  sun,  and  that  a  powerful  protection  is  afforded  against  too  rapid  evaporation, 
in  case  of  extraordinary  dryness,  by  the  possession  of  a  layer  of  sapless,  air-filled, 
interwoven  hair-structures.  The  Edelraut,  Ragwort,  and  the  other  plants  named, 
which  grow  on  the  sunny  rocks  of  the  Alps,  show  these  same  characters  of  leaf 
structure,  and  what  has  just  been  said  about  the  Edelweiss  applies  fully  to  them 
also.  It  should  be  mentioned  that  on  the  heights  of  the  Pyrenees,  Abruzzi,  and 
Carpathians,  as  well  as  on  the  Caucasus  and  Himalayas,  the  plants  growing  on 
sunny  ridges  of  rock,  where  they  are  exposed  to  the  wind,  are  covered  with  silk  and 
wool  exactly  after  the  model  of  the  Edelraut  and  Edelweiss,  and  that  there  is  on  the 
Himalayas  an  Edelweiss  which  is  wonderfully  similar  to  that  of  the  European  Alps. 
In  the  far  north,  on  the  other  hand,  where  the  flora  in  other  respects  has  so  much  in 
common  with  that  of  the  Alps,  these  plants  are  absent,  and  generally  a  search  over 
the  rocky  crags  for  herbs  and  shrubs,  whose  leaves  are  furnished  with  silky  or  felt- 
like  coverings  on  the  upper  surface,  is  futile.  The  genera  which  grow  on  these 
places  and  form  a  characteristic  feature  of  the  vegetation  in  consequence  of  their 
great  abundance — as,  for  example,  Diapensia  Lapponica,  Andromeda  hypnoides, 
Mertensia  maritima,  Draba  alpina,  and  others,  possess  remarkably  smooth  green 
leaves.  When  hairy  coverings  are  present,  they  are  restricted  to  the  under  leaf- 
surface,  especially  to  that  of  rolled  leaves.  They  are  never  found  on  the  plants  of 
rocky  slopes,  but  only  on  those  of  damp  marshy  ground,  or  by  the  side  of  water 
which  is  for  a  short  time  free  from  ice.  Here,  however,  they  certainly  do  not  help 
to  lessen  transpiration,  but  function  in  the  way  described  above  in  the  discussion  on 
rolled  leaves.  It  is  indeed  not  too  much  to  connect  these  facts  with  the  conditions 
of  the  climate,  and  especially  to  explain  the  absence  of  plants  whose  foliage  is  silky 
or  felt-like  on  the  upper  surface,  by  saying  that  a  drying  up  of  the  soil  and  a 
limiting  of  the  water  supply  never  occurs  on  the  narrow  terraces  of  steep  rocky 
declivities  in  Arctic  regions,  and  that  therefore  there  is  no  danger  of  over-evapora- 
tion to  plants  growing  in  those  regions. 

It  is  in  keeping  with  this  explanation  that  on  Central  and  South  European 
mountains,  on  whose  heights  an  Alpine  vegetation  is  to  be  found,  the  number  of 
forms  having  silky  and  felted  foliage  increases  as  these  mountains  are  situated 
further  south,  and  the  more  they  are  exposed  to  temporary  dryness.  Plants  of  the 
Edelweiss  type  are  still  wholly  foreign  to  the  Riesen-Gebirge;  in  the  Northern 
Alps  their  number  is  comparatively  small,  in  the  Southern  Alps  they  increase 
in  a  surprising  manner,  and  the  summits  of  the  Magellastock.  the  ridges  of 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS.  317 

the  Sierra   Nevada,   and   the   mountains  of   Greece   are   unusually  rich   in   such 
forms. 

If  plants  growing  in  such  situations  are  protected  against  the  dangers  of  too  rapid 
and  too  abundant  evaporation,  how  much  more  must  this  be  the  case  in  those  regions 
where,  with  the  increasing  warmth  of  summer,  the  number  of  showers  steadily 
diminishes;  and  where  the  soil  becomes  dried  more  and  more  deeply,  so  that  all  the 
plants  whose  roots  are  near  the  surface  are  unable  to  derive  a  drop  more  water  from 
it  ?  All  plants  which  are  to  survive  the  dry  period  in  such  places  must  during  this 
time  entirely  cease  transpiring— they  must,  as  it  were,  turn  into  a  chrysalis  and 
sleep  during  the  summer.  They  actually  do  this  in  all  sorts  of  different  ways,  and 
by  the  most  diverse  means.  One  of  the  commonest  and  most  widely  spread 
methods  is,  without  doubt,  by  having  the  transpiring  organs  clothed  with  a  thick 
covering  of  dry  air-containing  hairs.  Plenty  of  examples  of  this  are  furnished 
by  the  flora  of  the  Cape,  Australia,  Mexico,  the  savannahs  and  prairies  of  the  New 
World,  and  the  steppes  and  deserts  of  the  Old.  In  the  dry  elevated  plains  of 
Brazil,  Quito,  and  Mexico,  there  are  large  tracts  covered  with  gregarious  spurge- 
like  growths  and  grey-haired  species  of  Croton,  and  when  the  wind  blows,  moving 
these  bushes  to  and  fro,  undulations  are  set  up  over  wide  extents  of  country,  the 
whole  appearing  like  a  billowy  sea  of  grey  foliage.  A  similar  picture  is  presented 
by  the  Painciras  belonging  to  the  Composite,  or  by  the  Lychnophora,  on  the  high 
plains  of  Minas  Geraes  in  Brazil.  Nowhere  in  the  whole  world,  however,  does  the 
presence  of  hairs  on  foliage,  as  a  protection  against  exhalation,  appear  in  such  an 
abundant  and  varied  manner  as  in  the  floral  region  surrounding  the  Mediterranean, 
known  as  the  Mediterranean  district.  The  trees  have  foliage  with  grey  hairs;  the 
low  undergrowth  of  sage  and  various  other  bushes  and  semi-shrubs  (for  which  the 
name  "  Phrygian  undergrowth  ",  used  by  Theophrastus,  may  be  retained),  as  well  as 
the  perennial  shrubs  and  herbs  growing  on  sunny  hills  and  mountain  slopes,  are 
grey  or  white,  and  the  preponderance  of  plants  coloured  thus  to  restrict  evapora- 
tion has  a  noticeable  influence  on  the  character  of  the  landscape.  He  who  has  only 
heard  from  books  of  the  evergreen  plants  of  the  Greek,  Spanish,  and  Italian  floras, 
feels  at  the  first  sight  of  this  grey  vegetation  that  he  has  been  in  some  degree 
deceived,  and  is  tempted  to  alter  the  expression  "evergreen"  into  "ever  grey".  Every 
conceivable  sort  of  hair  structure  is  to  be  met  with  in  these  parts — coarse  felt-work, 
thick  velvet,  and  white  wool  mixed  in  endless  variety.  Here  is  a  leaf  looking  as  if 
covered  with  a  cobweb;  there  another  as  if  bestrewn  with  ashes  or  clay;  here  a  leaf 
surface,  covered  with  closely  pressed  hairs  or  scutiform  scales,  glistens  like  a  piece  of 
satin;  and  here  again  is  a  plant  with  such  a  long  flock  of  hair  that  one  might 
imagine  that  sheep  in  passing  had  left  pieces  of  their  fleece  hanging  on  it.  There  is 
hardly  a  family  in  the  flora  of  the  Mediterranean  district  which  does  not  possess 
members  richly  provided  in  this  way.  The  Composites  are  the  most  remarkable  in 
this  respect,  especially  the  genera  Andryala,  Artemisia,  Evax,  Filago,  Inula,  and 
Santolina;  then  come  the  Labiates  of  the  genera  Phlomis,  Salvia,  Teucrium, 
Marrubium,  Stachys,  Sideritis,  and  Lavandula;  rock-roses,  bindweeds,  scabious, 


318  PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS. 

plantains,  papilionaceous  plants,  and  plants  of  the  Spurge-laurel  family— just  those 
plants  which  constitute  the  main  part  of  the  vegetation  on  the  shores  of  the 
Mediterranean  Sea,  and  which  possess  a  thickly-woven  covering  of  hair.  Indeed 
representatives  of  families  such  as  the  Grasses,  whose  members  are  usually  bare, 
here  appear  to  be  quite  shaggy  with  hair.  It  is  also  very  interesting  to  see  that  so 
many  species,  which  have  a  wide  range  of  distribution,  and  which,  from  Scandinavia 
to  the  coasts  of  the  Mediterranean,  have  bare  foliage,  can  in  the  South  protect  them- 
selves from  drying  up,  by  developing  hairs  on  their  epidermis.  For  instance,  from 
Northern  and  Central  Europe  as  far  as  the  Alps,  the  epidermis  of  the  stems  and 
foliage  of  Silene  inflata,  Campanula  Speculum,  Galium  rotundifolium,  and  Mentha 
Pulegium  is  smooth  and  bare;  in  the  South, — particularly  in  Calabria, — the  leaves 
and  stems  of  these  species  are  covered  with  thick  down. 

Next  to  the  Mediterranean  flora,  the  neighbouring  Egyptian  and  Arabian  desert 
regions,  the  elevated  steppes  of  Persia  and  Kurdistan,  as  well  as  the  lowlands  of 
Southern  Russia  and  the  plains  of  Hungary,  show  a  comparatively  large  number  of 
species  whose  leaves  are  thickly  coated  with  hairs  on  both  surfaces.  Their  number 
is  less  than  that  of  the  flora  of  the  Mediterranean  district,  because  in  the  steppes 
and  deserts  the  dryness  of  the  summer  is  greater  than  in  that  region,  and  even 
thick  hairy  coverings  are  not  always  a  sufficient  protection  against  this  dryness,  and 
also  because  in  some  of  these  districts  the  dry  period  passes  directly  into  a  severe 
winter,  and  the  hairs  would  offer  but  a  poor  protection  against  the  cold.  Since  on 
the  coasts  of  the  Mediterranean  Sea  the  winter  temperature  never  falls  below 
freezing  point,  evergreen  and  grey  leaves  remain  there  unmolested,  and  recommence 
their  activity  at  the  beginning  of  the  next  season. 

The  successive  developments  of  certain  plant  forms  are  very  instructive  with 
regard  to  the  relations  existing  between  whole  floral  regions  and  transpiration.  In 
the  steppes,  Mediterranean  district,  and  at  the  Cape,  bulbous  plants  and  annuals 
first  make  their  appearance;  then  follow  the  perennial  grasses  and  woody  plants; 
and  finally  succulent  plants  and  thickly-haired  immortelles.  The  numerous  tulips, 
narcissi,  crocuses,  stars  of  Bethlehem,  asphodels,  amaryllises,  and  all  the  other 
bulbous  growths,  which  begin  to  sprout  immediately  after  the  first  winter  or  spring 
rain,  always  have  bare  foliage.  Their  transpiration  is  very  active  in  consequence  of 
the  rapidly-increasing  temperature  of  the  air,  but  the  saturated  soil  provides  a 
sufficient  substitute  for  the  evaporating  water,  and  also  has  ready  in  a  free  state  the 
food-salts  which  are  required  for  rapid  growth.  The  shrubs  which  sprout  at  the 
same  time,  the  peonies  and  hellebores,  as  well  as  the  host  of  annuals  which  spring 
up,  blossom,  and  fructify  in  an  inconceivably  short  time,  almost  all  possess  bare 
foliage,  especially  in  the  steppes.  Towards  midsummer,  when  the  drought  com- 
mences, all  these  plants  are  already  in  fruit;  their  foliage,  which  until  now  has  been 
actively  at  work,  begins  to  turn  yellow  and  to  dry  up;  their  succulent  tubers  and 
bulbs  are  imbedded  below  the  surface  in  soil  which  is  nqw  as  hard  as  a  stone;  and 
the  seeds  which  have  fallen  from  the  annual  plants  are  easily  able  to  survive  the 
aridity  of  the  summer  and  the  severity  of  the  winter,  since  they  are  inclosed  in 


PROTECTIVE    ARRANGEMENTS   ON   THE   EPIDERMIS.  319 

protective  coverings  of  great  variety.  Any  plants  which  are  still  to  retain  their 
activity  during  the  summer  on  the  steppes  or  in  the  Mediterranean  floral  district 
would  succeed  very  badly  if  only  furnished  with  the  bare  foliage  of  the  spring 
vegetation.  If  such  a  plant  is  to  be  protected  from  drying  up,  its  transpiration 
must  be  lessened.  This  is  effected  by  various  protective  arrangements,  but  best  of 
all  by  a  thick  coating  of  hair.  The  papilionaceous  plants  and  species  of  Orache, 
above  all  the  immortelles  and  wormwoods  (Helichrysum,  Xeranthemum,  Arte- 
misia), which  are  still  in  bloom  in  the  height  of  the  summer  and  can  bear  the 
strongest  heat  of  the  sun,  are,  as  a  rule,  thickly  covered  with  hair,  and  regions, 
which  perhaps  only  a  month  before  were  clothed  in  fresh  green,  are  now  shrouded 
in  dismal  gray.  With  the  transition  from  the  wet  period  of  the  spring  and  winter 
rains  to  the  dryness  of  midsummer,  there  is  a  corresponding  gradual  transition  from 
the  green  of  the  bare,  succulent  hyacinth  leaf  to  the  grey  of  the  rigid  felt-covered 
leaf  of  the  immortelle. 

A  peculiar  appearance  is  shown  in  Mediterranean  floral  districts  by  many 
biennial  and  perennial  plants  which  one  spring  give  rise  to  a  rosette  of  leaves  close 
to  the  soil,  and  in  the  spring  following  to  a  stem  bearing  both  leaves  and  blossom, 
which  arises  from  the  centre  of  the  rosette.  This  rosette  formed  in  the  first  spring 
has  to  live  through  the  dry  hot  summer,  and  is  therefore  covered  with  felted  grey 
hairs;  the  stern  formed  in  the  second  year  which  gives  rise  to  the  blossom,  since  it 
is  formed  during  the  wet  period,  has  no  need  of  the  protective  hairs,  and  is  there- 
fore furnished  with  green  foliage.  The  Salvia  lavandulcefolia  and  Scabiosa 
pulsatilloides  of  Granada,  the  Hieracium  gymnocephalum  of  Dalmatia,  and  in  the 
Mediterranean  flora  the  wide-spread  Helianthemum  Tuberaria  may  be  mentioned 
as  examples  of  such  plants.  Their  appearance  is  so  strange  that  one  involuntarily 
asks  whether  this  green  leafy  stem  really  belongs  to  the  grey  rosette  of  leaves,  or 
whether  some  one  has  not  been  playing  a  joke  by  putting  together  the  stem  and 
rosette  of  two  different  kinds  of  plants. 

These  hair -like  structures,  called  "covering  hairs",  whose  function  is  a  pro- 
tection against  excessive  exhalation,  exhibit  a  very  great  variety  with  regard  to 
form.  Notwithstanding  this  diversity,  however,  a  certain  degree  of  uniformity 
must  not  be  overlooked,  inasmuch  as  in  individual  species  the  same  kind  of  hairs 
are  always  present.  The  coat  of  hair  contributes  not  a  little  to  the  characteristic 
appearance  of  the  species,  and  therefore  has  always  been  considered  of  especial  value 
in  description  and  discrimination.  As  a  help  to  description  the  older  botanists 
introduced  a  series  of  expressions  into  botanical  terminology  by  which  to  denote 
shortly  and  tersely  the  most  pronounced  varieties,  and  this  seems  to  be  the  most 
suitable  place  for  explaining  these  terms — i.e.,  the  forms  of  covering  hairs  which  are 
signified  by  them. 

First,  those  covering  hairs  consisting  of  a  single  epidermal  cell,  which  grows  out 
beyond  the  other  epidermal  cells,  must  be  distinguished  and  set  apart  from  those 
which  have  become  multicellular  by  the  formation  of  separation  walls. 

Unicellular  clothing  hairs  in  many  cases  only  project  slightly  above  the  surface 


320  PROTECTIVE    ARRANGEMENTS   ON    THE    EPIDERMIS. 

of  the  leaf  to  which  they  belong;  they  bend  down  nearly  at  a  right  angle  almost 
immediately  above  their  place  of  insertion,  so  that  the  long  tapering  part  of  the  hair 
cell  lies  on  the  leaf -surf  ace,  as  shown  in  fig.  77s.  When  such  hair-forms,  in  great 
numbers  and  parallel  to  one  another,  entirely  cover  the  surface  of  the  leaf,  light  is 
strongly  reflected  from  them,  and  the  surface  looks  just  like  a  piece  of  silk.  Such 
a  covering  of  hair,  which  is  seen  particularly  well  on  the  shining  foliage  of  the  South 
European  bindweeds  (Convolvulus  Cneorum,  nitidus,  olecefolius,  tenuissimus,  &c.), 
is  termed  "  silky  "  (sericeus).  Two  varieties  of  this  may  be  distinguished,  viz.  the 
more  usual  case  in  which  all  the  hairs  of  the  leaf  lie  parallel  with  the  midrib,  and 
the  rarer  case  where  the  hairs  assume  a  different  position  on  the  right  and  left  of 
the  midrib,  the  whole  of  those  on  either  side  being  respectively  parallel  to  the 
lateral  ribs  of  their  respective  sides.  The  reflected  light  then  only  meets  the  eye  of 
the  observer,  in  any  one  position,  from  one  half  of  the  leaf,  the  other  half  therefore 
appearing  dull.  In  such  a  case  the  whole  leaf  has  that  peculiar  shimmer,  changing 
on  the  slightest  movement,  which  we  admire  on  the  wings  of  certain  butterflies,  and 
which  is  also  shown  by  that  variety  of  silken  material  known  as  satin.  When  the 
unicellular  hairs  do  not  lie  on  the  surface,  but  rise  up  from  it,  the  shimmer  is 
altogether  absent,  or  is  only  present  to  a  small  extent.  If  the  hairs  are  short,  very 
numerous,  and  closely  pressed  together,  they  are  said  to  be  "  velvety  "  (holosericeus); 
if  they  are  of  greater  length  and  situated  further  apart,  the  expression  "  shaggy  " 
(villosus)  is  used.  Hairs  which  consist  of  single  elongated  air-containing  cells, 
much  twisted  and  bent,  with  thin  and  pliant  walls,  are  called  wool-hairs,  and  the 
covering  formed  by  them  is  said  to  be  "  woolly "  or  "  tomentose "  (lanuginosus). 
Woolly  hairs  are  always  twisted  spirally,  sometimes  loosely,  sometimes  tightly— 
frequently  almost  like  a  corkscrew.  As  a  rule  the  spiral  is  in  the  opposite  direction 
to  the  movement  of  the  hand  of  a  watch,  whose  direction  is  said  to  be  to  the  left. 
It  should  also  be  noticed  whether  the  elongated  twisted  cells  of  the  wool-hairs  are 
circular  in  cross  section,  as  in  the  South  European  Centaurea  Ragusina  (see  fig.  77 5), 
or  whether  they  are  compressed  like  a  ribbon,  as  in  Gnaphalium  tomentosum 
(fig.  77  4).  The  latter  case  is  by  far  the  most  common. 

Multicellular  clothing  hairs  originate  by  the  repeated  division  of  certain 
epidermal  cells  caused  by  the  formation  of  separation  walls.  These  dividing  walls 
are  either  all  parallel  to  the  surface  of  the  leaf  or  stem,  or  some  of  them  are  perpen- 
dicular to  the  plane  of  the  leaf.  In  the  first  case  the  cells  are  usually  arranged  like 
the  links  of  a  chain,  and  are  termed  jointed  or  articulated  hairs.  When  such  arti- 
culated hairs  ardfe  short  and  not  interwoven — as,  for  example,  is  the  case  in  the 
beautiful  gloxinias  (see  fig.  77  2),  the  surfaces  clothed  with  them  appear  like  velvet; 
when  they  are  elongated,  curved,  and  twisted  and  entwined,  the  leaf  appears  to  be 
covered  with  wool  (see  fig.  77  1),  and  to  the  naked  eye  this  form  of  covering  is  the 
same  as  that  already  stated  to  be  shown  by  unicellular  covering  hairs.  Silky  coats 
are  also  produced  by  multicellular  hairs,  even  by  such  a  peculiar  form  as  is  repre- 
sented in  fig.  78  3.  These  hairs  are  developed  in  the  following  manner.  A  super- 
ficial cell  by  the  formation  of  a  septum  parallel  to  the  leaf-surface  divides  into  two 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS.  321 

daughter-cells;  the  division  is  repeated  and  gives  rise  to  a  small  chain  of  three,  four, 
or  five  short  cells  which  project  slightly  above  the  surface  of  the  leaf.  The  top  cell 
does  not  divide  further,  but  enlarges  in  a  striking  manner,  not,  oddly  enough, 
lengthening  in  an  upward  direction,  but  transversely,  parallel  to  the  leaf-surface] 
forming  a  lancet-shaped,  rod-like  structure,  which  shades  the  leaf,  and  is  supported 
by  its  sister  cells  as  if  on  a  pedestal.  Thousands  of  such  curious  hair-structures, 


Fig.  77.— Covering  Hairs. 

1  Articulated  woolly  hairs  of  Gnaphalium  Leontopodium.  2  Articulated  velvety  hairs  of  Gloxinia  speciosa.  »  Silky  hairs  of 
Convolvulus  Cneorum.  *  Ribbon-like  flattened  woolly  hairs  of  Gnaphalium  tomentosum.  «  Spiral  woolly  hairs  of  Cen- 
taur'ea  Ragusina.  •  Stellate  hairs  of  Alyssum  Wierzbickii.  f  Umbrella-shaped  hairs  of  Koniga  spinosa;  surface  view. 
»  Vertical  section  of  the  same  hairs.  •  Stellate  hairs  of  Draba  Thomasii.  x  about  60. 

which  may  best  be  compared  to  compass-needles,  clothe  the  surface  of  the  leaf  in 
close  proximity  to  each  other,  and  when  they  are  arranged  in  a  regular  manner, 
they  reflect  the  light  uniformly,  and  produce  a  distinctly  silky  lustre.  If  they  are 
twisted,  this  lustre  is  lessened  to  a  greater  or  less  extent.  This  variety  of  hairs, 
called  T-shaped,  is  distributed  in  a  remarkable  way.  Numerous  species  of  Astra- 
galus, the  scabious  of  the  Mediterranean  flora  (Scabiosa  cretica,  hymellia,  gramini- 
folia),  several  Crucifers  (Syrenia,  Erysimum),  native  on  the  steppes  of  Southern 
Russia,  the  magnificent  Aster  argophyllus  of  Australia,  and  particularly  numerous 

VOL.  L  21 


322 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS. 


species  of  wormwoods ;  the  South  European  Artemisia  arborescens  and  argentea, 
the  Artemisia  sericea  and  laciniata  belonging  to  the  -steppes  and  Siberian  flora,  the 
Common  Wormwood,  Artemisia  Absynthium,  and  the  frequently-mentioned  Edel- 
raut,  Artemisia  Mutellina,  growing  on  the  rocky  crags  of  mountain  heights— all 
owe  their  silky  appearance  to  these  T-shaped  hair-structures. 

It  may  also  happen  that  the  cell  which  is  elongated  transversely  (i.e.  parallel  to 


Fig.  78.— Covering  Hairs. 

1  Floccose  hairs  of  Verbascum  thapsiforme.  2  Tufted  hairs  of  Potentilla  cinerea.  *  T-shaped  hairs  of  Artemisia  mutellina. 
*  Actinia-like  hairs  of  Correct  speciosa.  *  Scutiform  scales  of  Elceagnus  angustifolia.  6  Stellate  hairs  of  Aubretia 
deltoidea.  x  about  50. 

the  leaf -surf  ace),  and  which  is  the  uppermost  of  the  small  group  of  cells  projecting 
above  the  epidermis,  is  prolonged  in  three,  four,  or  even  more  directions,  so  as  to 
have  a  stellate  appearance.  Thus  the  covering  of  the  leaf  is  seen  to  consist  of  three, 
four,  or  many-rayed  stars,  each  supported  on  a  short  stalk  (see  figs.  78 6  and  77 6). 
The  rays  of  the  stellate  cells  are  frequently  forked,  as  in  Draba  Thomasii  (see 
figs.  77  9).  In  rare  cases  they  have  a  comparatively  large  central  portion,  and  are 
only  divided  at  their  circumference  into  short  rays;  they  then  look  exactly  like 
small  sunshades  spread  out  over  the  leaf-surface.  This  elegant  form,  which  is 
represented  in  figs.  77 7  and  778,  has  a  particularly  beautiful  appearance  in 


PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS.  323 

Koniga  spinosa,  a  member  of  the  Mediterranean  flora.  All  these  clothing  hairs, 
with  star-shaped  indented  upper  cells,  are  grouped  together  under  the  name  of 
-stellate  hairs"  (pili  stellati).  In  Crucifera  and  Malvacese  they  occur  in  endless 
variety. 

When  the  uppermost  cell  of  the  group  forming  the  stellate  hair  is  divided  by 
separation  walls,  which  in  part  are  placed  perpendicularly  to  the  leaf-surface, 
branched  hairs  are  the  result.  In  branched  hairs  the  branches,  which  are  almost 


Fig.  79.— Flinty  armour  of  Kochea  falcata. 

i  Section  perpendicular  to  the  leaf-surface.    2  Surface  view ;  on  the  right  hand  the  vesicular  distended  portion  of  a  few 
superficial  cells  is  removed  and  the  stomata  are  brought  into  view ;  x350. 

always  arranged  in  a  stellate  manner  and  are  usually  unicellular,  can  be  dis- 
tinguished from  the  part  which  supports  the  branches.  This  portion  usually  looks 
like  a  pedestal,  and  is  sometimes  multicellular,  sometimes  formed  from  a  single  cell. 
When  the  pedestal  is  very  short,  and  the  cell  supported  by  it  is  divided  by  several 
radiating  divergent  septa,  which  are  either  oblique  or  perpendicular  to  the  leaf- 
surface,  tufted  hairs  (pili  fasciculati)  are  formed.  These  look  like  sea-urchins 
lying  on  the  surface  in  close  proximity  to  each  other;  they  vary  very  much  in  the 
size,  number,  length,  and  direction  of  their  branches,  and  they  are  particularly 
abundant  on  the  cinquefoils  (Potentilla  cinerea  and  arenaria),  cistus  and  rock- 
roses  (Cistus  and  Helianthemum).  A  common  form  is  represented  in  fig.  78 2. 
When  the  foot-stalk  is  very  short,  and  the  radiating  branch -cells  borne  by  it  are 


324  PROTECTIVE   ARRANGEMENTS   ON   THE   EPIDERMIS. 

joined  to  one  another,  a  star-shaped,  ribbed,  multicellular  plate,  indented  at  the 
margin,  is  produced  (see  fig.  78 5).  These  plates  are  generally  flat,  lie  level  on 
the  surface  of  the  leaf  or  stem,  overlap  one  another  with  their  indented  margins, 
and  cover  the  green  surface  of  the  leaf  so  completely  that  it  appears  to  be  white 
instead  of  green,  and  invest  it  with  a  bright,  almost  metallic,  lustre.  Such  leaves 
are  said  to  be  "scaly"  (lepidotus).  The  best  known  examples  of  such  leaves, 
covered  with  shining  silvery  hair-scales,  are  those  of  Elceagnus  and  of  the  Sea 
Buckthorns  (Hippophae).  If  the  plates  are  bent,  irregularly  fringed,  and  lustreless, 
the  leaf  covered  with  them  looks  just  as  if  it  were  strewn  with  bits  of  clay,  and  is 
said  to  be  "  clayey  "  (furfuraceous).  Examples  of  this  are  well  shown  by  the  leaf - 
coverings  of  many  plants  allied  to  the  Pine-apple  (Bromeliaceae).  When  the  top 
cell  of  the  hair  is  supported  on  a  moderately  high  pedestal,  and  is  divided  into 
numerous  radiating  daughter-cells  which  diverge  from  one  another,  a  structure  is 
produced  which  is  somewhat  like  a  knout,  or,  if  the  radiating  cells  are  short,  like  a 
sea-anemone  (Actinia).  This  form  of  hair  is  seen,  for  example,  in  the  Southern 
and  Eastern  European  Phlomis,  in  many  mulleins  (Verbascum  Olympicum),  and, 
with  multicellular  pedicels,  on  the  leaves  of  Gorrea  speciosa,  an  Australian  shrub 
(see  fig.  78 4).  Occasionally  a  branched  hair  produces  several  whorls  of  branches 
above  one  another,  and  then  hair-structures  are  formed  which  resemble  stoneworts 
(Characese)  or  miniature  fir-trees  under  the  microscope.  When  many  such  tiny 
tree-like  hairs  are  placed  close  together  with  interwoven  branches,  they  look  under 
a  magnifying-glass  like  a  small  plantation,  and  the  analogy  is  heightened  if  one- 
storied  tree-shaped  hairs,  like  the  undergrowth  in  a  high  forest,  occur  under  the 
higher  many-storied  ones.  This  is  the  case  in  the  Torchweed,  Verbascum  thapsi- 
forme,  whose  hairs  are  represented  in  fig.  78 1.  Hair-structures  like  these  appear 
to  the  naked  eye  like  flock,  and  are  described  as  "  floccose  "  hairs  (pili  floccosi). 
Many  of  these  have  the  peculiar  habit  of  rolling  themselves  together  into  small 
balls,  which  make  the  leaf -surface  look  as  if  it  were  bestrewn  with  coarse  white 
powder.  This  is  the  case,  for  example,  in  the  mullein  known  as  Verbascum  veru- 
lentum. 

In  the  crowded  condition  of  stellate  and  tufted  hairs,  of  branched  floccose  and 
unbranched  woolly  hairs,  it  is  unavoidable  that  the  neighbouring  hair-cells  should 
cross  one  another,  intertwine,  and  be  more  or  less  interwoven;  and  thus  arises  a 
felted  mass  which  covers  the  surface  of  the  organ  in  question.  Such  hair-masses 
are  termed  "  felt "  (tomentum),  and  the  varieties  are  distinguished  as  "  felted  "  (or 
"  tomentose  ")  stellate  or  woolly  hairs,  &c.  Often  the  felt  only  forms  a  thin  loose 
layer,  through  which  the  green,  of  the  leaf -surfa.ee  can  be  seen:  but  occasionally  it 
is  so  thick  that  the  leaf  appears  snow-white. 

While  in  all  these  cases  the  covering  which  protects  the  leaves  and  stem  of  the 
plant  from  over-transpiration  is  woven  from  air-containing  cells,  cylindrical  and 
elongated — usually,  indeed,  very  much  elongated — in  some  thick-leaved  plants, 
especially  in  species  of  the  genus  Rochea,  a  native  of  the  Cape,  these  cells  become 
vesicular  and  distended;  they  are  arranged  in  rank  and  file  adjoining  one 


FORM    AND   POSITION    OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES.  325 

another,  so  that  taken  together  they  form  a  layer  which  spreads  over  the  other 
epidermal  cells  like  a  coat  of  mail.  The  ordinary  epidermal  cells  are  small  and  only 
slightly  thickened  on  their  outer  walls,  as  shown  in  the  illustration  above.  The 
cells  which  are  placed  together  to  form  the  armour,  however,  are  enlarged  in  quite 
an  unusual  way;  their  stalk-like  base,  looking  as  if  wedged  in  between  the  ordinary 
epidermal  cells,  is  indeed  comparatively  large,  but  the  bladder-like  swollen  portion 
exhibits  dimensions  which  are  about  six  hundred  times  greater  than  those  of  the 
ordinary  epidermal  cells.  The  vesicles  are  closely  packed  together,  and  become 
almost  cubical  by  the  mutual  pressure  they  exert  on  each  other.  Where  a  space 
might  occur,  the  bladders  form  protuberances  and  bulgings  at  the  side  which  fit  in 
together  in  such  a  way  that  a  completely  closed  coat  of  mail  is  the  result.  The 
expression  "  coat  of  mail "  is  the  more  justified  here  since  the  swollen  bladder-like 
cells  of  Rochea  are  as  hard  as  pebbles.  Large  quantities  of  silica  are  present  in  the 
cell-walls,  and  by  burning  them  a  complete  skeleton  in  silica  can  be  obtained,  as  in 
the  case  of  the  silica-coated  Diatomacese.  It  needs  no  further  explanation  that  in 
the  dry  season  such  a  coat  of  armour  affords  to  the  succulent  cells  it  covers  an 
excellent  protection  against  evaporation. 

There  is,  however,  still  another  point  to  be  considered.  The  vesicular  swollen 
cells  on  fully -grown  leaves  are  still  occupied  by  protoplasm,  which  forms  a  thin 
layer  round  the  walls,  while  in  the  centre  is  a  large  cavity  filled  with  cell-sap;  it  is 
only  in  older  leaves  that  the  bladder-like  cells  become  filled  with  air.  As  long  as 
they  contain  watery  cell-sap  they  serve  as  reservoirs  of  water  from  which  the  green 
chlorophyll-bearing  cells  below  can  obtain  supplies  at  the  periods  of  greatest 
drought,  when  all  other  sources  are  exhausted.  This  fact,  that  the  water-reservoirs 
are  situated  on  the  exterior  of  the  plants,  where  there  exist  so  many  aids  to  exhala- 
tion, shows  how  well  the  silicified  walls  of  these  bladders  function.  They  may  be 
compared  to  glass  vessels  whose  mouths  are  directed  towards  the  green  tissue,  and 
whose  walls  allow  absolutely  no  water  to  pass  through. 

FOKM  AND  POSITION  OF  THE  TRANSPIRING  LEAVES  AND  BRANCHES. 

The  enlargement  of  the  green  leaf-surface  has  been  already  explained  as  a  means 
of  increasing  transpiration,  which  is  of  special  importance  when  the  plants  con- 
sidered grow  in  damp  air.  Similarly  a  diminution  of  the  green  surface  signifies  a 
restriction  of  transpiration.  This  relation  is  illustrated  by  the  fact  that  in  all  floral 
areas,  in  which  the  activity  of  the  vegetation  is  restricted  or  entirely  stopped  by 
increasing  dryness,  the  foliage  of  the  plants  is  not  so  widely  outspread,  i.e.  it  under- 
goes a  diminution.  It  is  also  a  well-known  fact  that  one  and  the  same  species, 
if  grown  in  a  dry  sunny  position,  will  exhibit  smaller,  and  in  particular,  narrower 
leaves  than  when  it  has  been  grown  in  a  damp  situation.  This  is  well  seen  in 
passing  from  the  mountainous  districts  bordering  the  Hungarian  lowlands  to  the 
plains  of  the  lower  regions.  A  number  of  shrubs  and  herbs,  Anchusa  officinalis, 
Linum  hirsutum,  Alyssum  montanum,  Thymus  Marschallianus,  &c.,  exhibit  on 


326  FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

the  dry  sands  of  the  plains  much  narrower  leaves  than  in  the  valleys  of  the  moun- 
tainous regions.  In  conjunction  with  the  narrowing  of  the  foliage,  the  wrinkling  of 
the  leaves  has  to  be  considered,  i.e.  the  formation  of  grooved  depressions  on  the  sur- 
face. Strictly  speaking,  there  is  no  lessening  of  the  whole  surface  of  the  leaf,  but 
only  of  that  portion  of  the  surface  which  is  exposed  to  sun  and  wind.  This  is  the 
point  with  which  we  are  concerned.  With  regard  to  the  exhalation  of  water,  only 
the  extent  of  the  surface  directly  influenced  by  the  agents  for  increasing  transpira- 
tion is  to  be  considered;  whilst  the  extent  of  the  grooved  depressions,  which  are  not 
exposed  to  the  sun's  rays,  nor  to  dry  currents  of  air,  may  be  in  a  certain  measure 
neglected.  On  the  whole,  plants  with  wrinkled  and  grooved  foliage  are  not  very 
abundant.  For  the  most  part  the  crumpling  is  to  be  seen  on  quite  young  leaves 
when  first  they  break  through  the  bud-scales,  and  when  their  epidermal  cells  are  not 
yet  sufficiently  thickened  with  cuticular  material.  Later,  when  the  formation  of  the 
cuticle  is  advanced,  the  wrinkles  gradually  become  smooth,  and  the  leaf  becomes- 
flat. 

It  has  already  been  pointed  out  that  those  pit-like  depressions,  on  the  floor  of 
which  stomata  are  concealed  (cf.  figs.  68  and  73),  may  also  serve  to  restrict  trans- 
piration. There  is  no  contradiction  in  the  statement  that  the  same  structure  at  one 
time  hinders  the  entrance  of  water  and  the  wetting  of  the  stomata  at  the  bottom  of 
the  pit,  and  at  another  time  prevents  direct  contact  with  dry  winds  and  consequent 
over-transpiration.  Each  has  its  turn.  When  the  foliage  of  the  Australian  Prote- 
acese,  during  the  summer  sleep,  is  exposed  for  months  to  the  scorching  rays  of  the 
sun  and  to  the  warm  dry  air,  and  when  all  supplies  of  water  from  the  soil  have 
ceased,  evaporation  from  the  leaves  must  be  restricted  as  much  as  possible;  it  is  then 
that  the  pit-like  depressions  perform  their  duty  in  this  respect.  When,  later  on,  the 
plants  are  aroused  from  their  long  sleep,  and  have  to  provide  themselves  with  food, 
to  grow,  blossom,  and  fructify  in  an  extremely  short  space  of  time,  while  violent 
showers  of  rain  are  pouring  down  from  the  clouded  sky,  and  all  the  leaves  are 
dripping  with  wet;  it  is  then  very  important  that,  in  spite  of  these  exceedingly 
unfavourable  conditions  for  evaporation,  an  abundant  transpiration  should  never- 
theless take  place,  and  that  the  function  of  the  stomata  should  be  in  no  way 
impaired  by  the  moisture.  These  pit-like  depressions,  which  in  the  dry  period  pre- 
vented evaporation,  now  have  to  keep  moisture  away  from  the  stomata. 

In  many  plants  evaporation  from  the  superficial  tissue  is  restricted  by  the  close 
contact  of  the  leaves  to  their  supports,  like  the  scales  on  the  back  of  a  fish.  The 
upper  side  of  a  leaf  in  contact  with  the  stem,  and  frequently  adhering  to  it,  is  thus 
deprived  of  the  means  of  exhalation,  and  transpiration  can  only  take  place  on  the 
somewhat  arched  or  keeled  under  side  of  the  green  scale-like  leaf.  This  occurs, 
for  example,  in  the  Tree  of  Life  (Arbor  vitce),  in  several  species  of  Juniper,  in 
Thujopsis,  Libocedrus,  and  various  other  Conifers.  It  is  not  without  interest  to 
notice  that  in  several  of  these  Conifers  the  little  green  scale-like  leaves  only  become 
close  pressed  to  the  stem  when  they  are  exposed  to  the  sun,  whilst  they  project 
from  it  if  the  branches  in  question  are  shaded. 


FORM    AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES.  327 

A  further  reduction  of  the  evaporating  surface  is  brought  about  by  the 
development  of  thickened  or  fleshy  leaves.  In  order  to  render  the  points  under 
consideration  as  clear  as  possible,  it  is  perhaps  well  to  insert  here  the  following 
observations.  By  altering  the  form  of  a  sheet  of  lead  8  cms.  square  and  1  mm. 
thick  into  a  solid  cylinder,  the  diameter  of  this  cylinder  is  seen  to  be  only  1  cm.,  and 
the  whole  surface  of  the  cylinder  is  only  one-fifth  of  that  of  the  previous  flat  sheet. 
The  application  of  these  figures  to  the  tissue  of  a  leaf  demonstrates  how  much 
smaller  is  the  transpiring  surface  of  a  thick  cylindrical  leaf  than  of  a  thin  flattened 
one.  Such  thickened  leaves,  which  approach  more  or  less  to  the  cylindrical  shape, 
are  to  be  found  regularly  where  transpiration  has  to  be  reduced  for  a  considerable 
time — as,  for  example,  in  the  mountainous  districts  of  Central  and  Southern  Europe, 
in  the  genus  Sedum,  growing  on  sandy  soil  which  easily  dries  up,  and  on  stone  walls 
and  battlements  (Sedum  album,  reflexum,  dasyphyllum,  atratum,  Boloniense, 
Hispanicum,  &c.).  They  also  occur  in  a  striking  manner  in  many  tropical  orchids 
which  grow  on  rocks,  or  epiphytically  on  the  bark  of  trees  in  the  East  Indies, 
Mexico,  and  Brazil,  exposed  for  more  than  six  months  to  great  aridity  (Brasavola 
cordata  and  tuberculata,  Dendrobium  junceum,  Leptotes  bicolor,  Oncidium 
Cavendishianum  and  longifolium,  Sarcanthus  rostratus,  Vanda  teres,  and  many 
others);  but  especially  are  they  found  in  aloes  and  stapelias  and  species  of 
Cotyledon,  Crassula,  and  Mesembryanthemum,  whose  habitat  is  in  the  dryest 
districts  of  the  Cape.  Several  TJmbelliferse,  Composite,  and  Portulaceae  (Inula 
crithmoides,  Crithmum  maritimum,  Talinum  fruticosum)  growing  on  stony  places 
of  the  sea-shore  in  the  burning  sun,  and  many  salsolas  of  the  deserts  and  salt 
steppes,  as  well  as  finally  some  Proteacese,  which  for  two-thirds  of  the  year  are 
exposed  to  the  droughts  of  Australia— all  are  characterized  by  their  development  of 
fleshy  leaves. 

Just  as  thick-leaved  plants  have  acquired  their  succulence  by  a  modification 
of  their  foliage,  similarly,  in  the  so-called  cactiform  plants,  it  is  the  stems  which 
become  thick  and  fleshy,  and  take  on  the  functions  of  leaves.  Here  the  green 
tissue  is  situated  in  the  cortex  of  the  stem,  the  epidermis  covering  it  contains 
stomata  just  like  the  epidermis  of  foliage-leaves,  and  the  green  cortex  transpires, 
and  functions  on  the  whole  exactly  as  the  green  leaves  do.  When  the  stems 
of  the  cactiform  plants  are  richly  branched  and  the  branches  are  short,  they 
sometimes  much  resemble  thick -leaved  plants.  Frequently  also  the  separate 
portions  of  the  stem  and  branches  take  the  form  of  fleshy  leaf -like  discs,  as  in  the 
genus  of  the  Prickly-pear  (Opuntia),  and  such  stem-structures  are  usually  mis 
taken  by  the  uninitiated  for  thick  leaves.  Gardeners,  as  a  rule,  group  the  thick 
leaved  and  cactiform  plants  together  under  the  single  term  "succulent  plants". 
To  the  cactiform  plants  belong  the  opuntias  and  cacti,  species  of  Cereus,  Echino- 
cactus  Melocactus,  and  Mammillaria,  which  are  distributed  from  Chili  and  South 
Brazil  over  Peru,  Columbia,  the  Antilles,  and  Guatemala.  These  are,  however, 
especially  developed  on  the  high  plains  of  Mexico  in  astonishing  variety  of 
To  the  cactiform  plants  belong  also  the  leafless  candelabra-like  tree-shaped  spurges 


328  FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

of  Africa  and  the  East  Indies.  These  plants  are  exposed,  far  more  than  the 
thick-leaved  plants,  for  the  greater  portion  of  the  year  to  extraordinary  dryness. 
Their  usual  habitats  are  dry  sandy  and  stony  plains,  waste  rocky  plateaus,  and 
crevices  of  rocks  which  are  almost  completely  wanting  in  soil.  They  always  inhabit 
regions  where  no  rain  falls  for  about  three-fourths  of  the  year,  and  which  usually 
belong  to  the  driest  parts  of  the  earth.  The  whole  organization  of  these  plants 
corresponds  to  these  conditions  of  their  habitat.  Dry  scales  and  hairs  are  produced 
instead  of  foliage-leaves,  or  these  are  often  metamorphosed  into  thorns  which 
project  in  great  numbers  from  the  thick  stem -structures,  and  efficiently  protect 
them  from  the  attacks  of  thirsty  animals.  The  epidermis  of  the  pillar-like,  disc- 
shaped,  or  spherical  stem-portions  is  thickened  on  the  outer  wall,  so  as  to  almost 
resemble  cartilage,  and  frequently  it  forms  a  coat  of  mail  round  the  deeper-lying 
green  tissue  by  the  abundant  deposition  of  oxalate  of  lime  (as  much  as  85  per  cent). 
Most  of  the  succulent  plants,  whose  cell- walls,  which  are  in  contact  with  the  air,  are 
fortified  by  oxalate  of  lime,  silicic  acid,  or  suberin,  have  in  their  tissue  peculiar 
aggregates  of  cells  which  apparently  serve  for  the  storing-up  of  water  for  the  dry 
season,  and  which  have  been  termed  "  aqueous  tissue  " .  The  water  in  these  reser- 
voirs is  always  so  apportioned  that  it  lasts  from  one  rainy  season  to  another;  that 
is  to  say,  the  adjoining  green  tissue  which  exhales  the  stored-up  water  does  not 
suffer  from  drought  during  the  dry  season.  Also,  it  is  contrived  in  these  plants 
that,  immediately  after  the  fall  of  the  first  rain,  the  reservoirs  are  again  filled  with 
water,  and  that  the  emptying  and  filling  of  the  cells  and  the  decrease  and  increase 
of  their  volume  exercise  no  harmful  influence  on  the  adjoining  tissue.  Succulent 
plants  have  been  not  inaptly  compared  to  camels,  the  "  ships  of  the  desert " ,  which 
provide  themselves  with  a  large  quantity  of  water,  and  are  then  able  to  dispense 
with  further  supplies  for  a  long  time  without  injury.  The  cells  of  the  aqueous 
tissue  are  comparatively  large  and  their  walls  thin;  the  active  protoplasm  within 
forms  a  delicate  layer  round  the  walls — that  is  to  say,  a  sac  whose  cavity  is  filled 
with  watery,  often  somewhat  mucilaginous,  fluid.  In  the  cactuses  the  aqueous 
tissue  is  hidden  as  much  as  possible  in  the  interior  of  the  thick  rod-shaped  or 
spherical  stem;  also  in  many  thick-leaved  plants,  such  as  some  of  the  European 
species  of  the  genus  Sedum  (e.g.  Sedum  album,  dasyphyllum,  glaucum);  in  South 
African  species  of  the  genera  Aloe  and  Mesembryanthemum  (e.g.  Mesembry- 
anthemum  blandum,  foliosum,  sublacerum),  the  aqueous  tissue  is  concealed  in  the 
interior  of  the  leaf,  and  is  usually  composed  of  cells  surrounding  vascular  bundles 
there  situated.  In  Sedum  Telephium,  known  by  the  name  of  Orpine,  as  well  as  in 
species  of  House-leek  (Sempervivum),  and  many  salsolas  growing  on  steppes,  the 
ramifications  of  the  vascular  bundles  are  enveloped  in  a  mantle  of  green  tissue, 
and  the  bundles,  which  are,  as  it  were,  overlaid  with  green  cells,  are  so  arranged 
with  regard  to  the  colourless  aqueous  tissue,  that  to  the  naked  eye  they  look 
like  green  strands  in  a  transparent  matrix  which  is.  as  clear  as  water.  In  the 
Mexican  Echeverias  the  aqueous  tissue  is  inserted  as  broad  stripes  in  the  green 
tissue,  and  in  the  thick-leaved  orchids  it  appears  as  if  sprinkled  between  the  green 


FORM    AND   POSITION   OF   THE   TRANSPIRING    LEAVES   AND   BRANCHES.  329 

cells.  The  epidermis  in  numerous  other  thick-leaved  plants  serves  as  a  store-house 
for  water  in  a  marvellous  way.  Individual  epidermal  cells  are  then  greatly 
enlarged  and  project  beyond  the  others  in  the  form  of  sacs,  clubs,  or  bladders,  as 
shown  in  the  picture  of  Rochea  (fig.  79).  These  bladders  either  fit  together  into  a 
one-layered  extended  coat  of  armour,  or  they  are  frequently  placed  irregularly  side 
by  side  or  above  one  another.  In  some  instances  they  form  isolated  groups  or  occur 
singly,  and  appear  then  to  the  naked  eye  like  protuberances  on  the  green  stems  and 
leaves,  where  they  glitter  and  sparkle  in  the  sunshine  like  an  embroidery  of  dew- 
drops.  Many  leaves  and  branches — as,  for  example,  those  of  the  widely-distributed 
Ice-plant  (Mesembryanthemum  cristallinum) — have  the  greatest  resemblance  to 
candied  fruit  covered  with  clear,  colourless,  sparkling  sugar  crystals. 

When  the  walls  of  the  enormously-distended  vesicular  or  bladder-shaped  cells  of 
the  epidermis  are  silicified,  as  are  those  of  the  repeatedly-mentioned  Rochea,  it  is 
easily  understood  that  the  watery  cell-sap  which  they  contain  is  not  exhaled  into 
the  air;  the  fluid  is,  so  to  speak,  inclosed  in  a  glass  bottle  and  can  only  be  given  off 
in  the  direction  of  the  green  tissue.  But  when  the  walls  of  the  bladder-like  giant 
cells  are  not  silicified,  and  not  even  particularly  thickened,  what  is  the  result? 
From  the  aspect  of  the  Ice-plant  one  would  think  that  a  single  warm  dry  day  would 
suffice  to  shrivel  and  dry  up  the  watery  vesicles.  But  this  is  certainly  not  the  case. 
Leafy  twigs  cut  from  the  Ice-plant  may  be  left  all  day  on  the  dry  ground  in  dry  air 
and  sunshine,  and  the  large  bladder-like  cells  on  the  surface  will  not  lose  their 
aqueous  contents.  After  a  week  they  become  collapsed,  having  given  up  their 
water,  not  to  the  atmosphere,  but  to  the  green  tissue  covered  by  this  swollen  coat. 
Without  doubt  this  phenomenon  is  to  be  associated  with  a  peculiar  formation  of  the 
cell- wall;  but  it  is  as  certain  that  the  constituents  of  the  cell-sap,  which  fills  the 
vesicles  are  also  important,  and  it  must  be  assumed  that  substances  are  dissolved  in 
this  aqueous  fluid  which  restrict  the  evaporation  of  the  water. 

These  substances,  which  hold  water  with  great  energy,  and  thereby  enable  the 
plants  in  question  to  survive  through  periods  of  the  greatest  dryness,  are  partly 
viscous,  gummy,  and  resinous  fluids,  partly  salts.  It  is  well  known  that  the  sticky, 
watery  pulp  of  crushed  mistletoe  berries,  used  in  the  manufacture  of  "  bird-lime  ", 
may  be  exposed  to  the  air  for  months  without  quite  drying  up,  and  the  mucilaginous 
juices  of  many  cactuses  and  thick-leaved  plants  behave  in  a  similar  manner,  espe- 
cially those  of  the  Cape  aloes,  which  exhale  no  water,  and  enable  the  plants 
possessing  them  to  withstand  the  drought  for  months.  In  the  thick-leaved  plants 
of  the  salt  steppes  and  deserts,  the  fluids  are  rarely  resinous  or  gummy,  but  they 
frequently  contain  a  surprising  quantity  of  salts  dissolved  in  water,  such  as  common 
salt,  chloride  of  magnesium,  and  the  like;  and  these  also  obstinately  retain  water  in 
proportionately  large  quantities.  It  is  one  of  the  most  surprising  of  phenomena  to 
see  the  thick-leaved  salsolas  rising  above  the  soil  of  salt  steppes,  green  and  succu- 
lent, when  the  ground  is  at  its  driest  in  the  height  of  summer,  when  for  months  no 
clouds  ha  7e  tempered  the  sun's  rays  and  not  a  drop  of  rain  has  fallen,  and  when 
almost  all  other  plants  have  long  ago  turned  yellow  and  faded.  The  large  amount 


330  FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

of  salts  contained  in  the  sap  of  these  plants  renders  them  capable  of  a  resistance 
which  is  almost  greater  than  that  afforded  by  mucilaginous  materials  and  gum-resins. 

It  must,  however,  be  remarked  here  that  not  all  green  leaf-  or  stem-cells  contain- 
ing abundant  water  have  the  function  of  storing  it  up  for  a  dry  season,  and  that  the 
aqueous  cell-groups  and  strands  adjoining  the  green  tissue,  especially  the  so-called 
outer  aqueous  tissue,  in  very  many  cases,  has  another  important  function,  viz.  the 
conducting  of  carbonic  acid  to  places  where  it  can  be  assimilated,  but  this  will  be 
described  in  the  next  chapter. 

An  extreme  reduction  of  the  leaf-surface,  combined  with  a  formation  of  green 
transpiring  tissue  in  the  cortex  of  the  stem,  is  also  shown  in  another  group  of  plants 
known  by  the  name  of  "Switch"  plants.  They  are  characterized  by  thin  rod- 
shaped  stems  and  branches,  while  the  cactiform  plants,  on  the  contrary,  always  have 
their  axes  but  little  branched,  and  massive,  thickened,  fleshy  and  rigid  stem-struc- 
tures which  are  unaffected  by  the  wind.  The  switch-plants  may  be  subdivided  into 
those  which  are  flexible,  hollow,  and  only  slightly  branched — as,  for  example,  the 
horse-tails  (Equisetum),  reeds  (Scirpus),  rushes  (Juncus),  bog-rushes  (Schoenus),  and 
several  cyperuses  (Gyperus)\  and  into  broom-like  shrubs  with  rigid  woody  boughs 
breaking  up  into  innumerable  branches  and  twigs.  The  former  are  distributed  over 
the  whole  world;  the  latter  are  principally  to  be  found  in  Australia  and  in  districts 
bordering  on  the  Mediterranean  Sea.  In  Australia  it  is  chiefly  Casuarinas  and 
some  genera  of  Papilionacese  and  Santalaceae  (Sphcerolobium,  Viminaria,  Lepto- 
meria,  Exocarpus)  which  take  on  this  odd  form,  and  some  of  them  even  attain  to 
the  size  of  trees.  In  the  Mediterranean  flora  isolated  species  and  groups  from  the 
families  of  Asparaginese,  Polygalaceas,  and  Resedaceae  are  seen  with  thin,  stiff,  rod- 
shaped,  leafless  branches,  which  project  stiffly  into  the  air  with  green  cortex;  but 
again,  most  of  these  plants  belong  to  the  Papilionacese  and  Santalacese.  Several 
switch-plants  of  the  papilionaceous  genera  Retama,  Genista,  Cytisus,  and  Spartium, 
growing  together,  often  cover  wide  tracts  of  country  in  densely-crowded  masses, 
and  thus  contribute  not  a  little  to  the  scenic  peculiarity  of  the  district.  Many  small 
rocky  islands  off  the  coast  of  Istria  are  entirely  overgrown  by  Spartium  scoparium, 
which  is  represented  in  the  illustration  opposite.  In  May  large  golden  flowers, 
scented  like  acacias,  appear  on  the  green  rods  of  the  Broom,  and  then  for  a  short 
time  the  dark  green  of  the  switch-plant  is  changed  into  a  brilliant  yellow.  On 
passing  near  the  coast,  just  at  this  time,  the  remarkable  phenomenon  is  seen  of 
golden  yellow  islands  rising  above  the  dark  blue  sea.  This  floral  adornment  is, 
however,  but  transitory,  and  nothing  more  monotonous  and  desolate  than  such  a 
dry  unwatered  islet,  covered  with  these  shrubs,  can  be  imagined 

The  Spartium  belongs  to  those  switch-plants  which  are  not  entirely  leafless,  but 
which  develop  little  green  lancet-shaped  leaves  at  intervals  on  their  long  twigs.  But 
these  are  of  such  secondary  importance  that  their  green  tissue  can  only  form  the 
smallest  portion  of  the  organic  substances  necessary  to  the  further  growth  of  the 
plants,  and  this  duty  chiefly  falls  to  the  share  of  the  cortex  of  the  switch -like 
branches.  The  cortex  is  also  characteristically  formed  in  accordance  with  this  fact. 


FORM    AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 


331 


Under  the  epidermis,  whose  outer  walls  are  much  thickened  and  coated  with  wax, 
is  the  green  transpiring  tissue  or  "  chlorenchyma  " ,  consisting  of  from  five  to  seven 
rows  of  cells.  This  green  tissue  does  not  form  a  continuous  mantle  round  the  stem, 
but  is  divided  into  from  ten  to  fifteen  thick  strands  by  strips  of  hard  bast  (see 
fig.  81).  Below  this  cortex  of  alternating  green  tissue  and  strips  of  bast  are  soft 
bast,  cambium,  wood,  and  a  very  large  pith;  but  these  have  no  further  interest  for 
us  here.  It  is,  however,  worthy  of  remark  that  in  the  green  strands  of  the  cortex 


Fig.  80.— Switch-plants. 
Bushes  of  Spartium  scoparium  near  Rovigno  in  Istria. 

of  the  Spartium,  the  crowded  green  chlorophyll-containing  cells  of  the  chlorenchyma 
closely  adjoin  one  another,  and  that  only  very  narrow  air-passages  ramify  between 
them,  so  that  here  there  is  no  formation  of  a  spongy  parenchyma  penetrated  by 
wide  canals  and  passages.  On  the  other  hand,  large  cavities  occur  where  the  green 
tissue  touches  the  epidermis,  and  these  act  as  substitutes  for  the  wide  ramifying 
canals.  Over  each  of  the  cavities  a  stoma  is  to  be  seen  in  the  epidermis  through 
which  the  water  vapour,  exhaled  chiefly  from  the  green  cells,  can  escape  (see 
fig.  81 2).  The  stomata  are  proportionately  small,  but  their  number  is  very  great. 
Since  the  guard-cells  are  not  so  strongly  thickened  on  their  outer  walls  as  are  the 
other  epidermal  cells,  the  stomata  appear  to  be  somewhat  sunken.  By  this  arrange- 
ment, and  also  by  the  epidermal  coating  of  wax,  they  are  protected  from  moisture. 


332  FORM    AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

In  the  Casuarinese  and  in  Cytisus  radiatus  (see  fig.  69),  the  green  tissue  is  distri- 
buted in  the  cortex  of  the  branches  exactly  as  in  the  case  just  described;  but  the 
strips  of  green  tissue  traversing  the  stem  are  deeply  cut  into  by  longitudinal 
furrows.  In  some  other  leafless  switch-shrubs,  such  as  species  of  the  genus 
Ephedra,  the  chlorenchyma  forms  a  continuous  and  uniform  mantle  round  the 
stem,  uninterrupted  by  strips  of  bast.  But  in  this  case  the  stomata  are  distributed 
uniformly  over  the  whole  surface  of  the  rod-shaped  branches,  while  in  the  brooms, 
Casuarinese,  and  in  Cytisus  radiatus  they  are  absent  from  those  portions  of  the 
epidermis  which  cover  the  strips  of  hard  bast. 

Plants  with  leaf-like  branches  or  cladodes  are  distinguished  from  switch-plants 


Fig.  81.— Switch-shrubs, 
i  Part  of  stem  of  Spartium  scoparium  cut  transversely ;  x30.      2  Part  of  the  transverse  section ;  x'240. 

by  the  fact  that  all  their  shoots  are  not  circular  in  section,  but  some  are  flattened, 
looking  as  though  they  had  been  pressed  out.  When  this  flattening  is  restricted  to 
the  so-called  "  short  branches  ",  i.e.  when  on  a  stem  only  the  ultimate,  comparatively 
short  branches  are  flattened,  the  main  axes  remaining  cylindrical,  like  ordinary 
stalks,  these  structures  have  quite  the  appearance  of  leaves  which  are  sessile  on  the 
rounded  stems.  This  explanation  of  them,  however,  given  by  botanists,  is  not  at 
first  sight  satisfactory  to  the  uninitiated.  Why  should  these  flat  green  structures  be 
branches,  and  not  leaves?  The  illustration  opposite  at  once  makes  the  matter  clear. 
It  represents  two  cladode-bearing  plants,  viz.  two  species  of  Butcher's-broom  (Ruscus 
Hypoglossum  and  aculeatus),  each  at  an  early  stage  of  development  and  also  when 
fully  grown.  On  the  young  shoots,  which  have  just  made  their  way  out  of  the  soil 
(see  figs.  82 l  and  82  3),  the  true  leaves  can  be  seen  in  the  shape  of  small  sessile  pale 
scales  on  the  long,  rounded,  finely-ridged  axis;  and  from  the  angles  which  these  scales 
make  with  the  long  axis  arise  darker,  much  thicker  organs  which  rapidly  increase 
in  size,  while  the  supporting  covering-scales  become  dry,  shrivel  up,  and  finally 


FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 


333 


disappear,  leaving  no  traces.  Since  the  members  which  arise  from  the  axils  of 
leaves  (whether  these  are  small  clothing- scales,  or  large  green  laminae  does  not 
matter)  are  not  considered  to  be  leaves,  but  shoots,  the  flat  leaf -like  structures  of  the 
Butcher 's-broom  are  also  regarded  as  shoots,  and  are  named  "flattened  shoots" 
(cladodes) — or,  considering  their  similarity  to  leaves,  "  leaf-branches  "  (phylloclades). 


Fig.  82.— Plants  with  Leaf-like  Branches  (Cladodes). 

t  Young  shoot,  of  Bv*mu  Hypoglossum.    «  The  same  branch  fully  grown,  with  flowers  on  the  clwlndw 
of  .Rw*wi6  aeuleat«*     *  The  fame  branch  with  flowprs  on  the  cladod** 


»  Young  rtoot 


This  view  is  strengthened  materially  by  the  fact  that  these  leaf-like  structures,  in 
their  further  development,  and   in  the  production  of  shoots,  behave  exactly  ] 
ordinary  cylindrical  axes.     That  is  to  say,  small  scale-like  leaves  spring  from  them, 
and  from  the  axils  of  these  scales  arise  stalked  flowers  (see  figs.  82  2  and  82  *)  which 
ultimately  fructify.     Plants  possessing  such  phylloclades  are  not  very  numerous  on 


334  FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

the  whole.  The  Butchers-brooms,  chosen  above  as  examples,  belong  to  Southern 
Europe,  and  occur  in  large  quantities  on  the  soil  of  dry  woods,  where  everything  is 
wrapped  in  deep  sleep  during  the  height  of  summer.  In  the  Antilles,  and  in  the 
prairies  of  the  East  Indies,  are  about  twenty  shrub -like  species,  belonging  to 
the  genus  Phyllanthus  of  the  Spurge-family.  New  Zealand  also  possesses  one  of 
these  peculiar  phyllocladous  plants,  belonging  to  the  papilionaceous  genus  Car- 
michelia.  In  the  species  of  both  these  genera  (see  fig.  83)  the  flattened  shoots  are 
exceedingly  like  lancet-shaped  foliage-leaves,  and  the  true  leaves  are  transformed 
into  small  pale  scales.  These  tiny  scales  are  situated  on  the  margins  of  the 
phylloclades,  and  from  their  axils  arise  stalks  bearing  the  flowers  and  fruit.  On 
the  Andes  of  South  America  occur  the  remarkable  colletias,  of  which  a  species, 
Colletia  cruciata,  is  represented  in  fig.  83.  The  leaflets  on  these  extraordinary 
shrubs  are  diminutive,  but  not  pale  and  scale-like;  whilst  the  green  phylloclades, 
which  play  the  part  of  the  foliage-leaves,  form  very  strong  flattened  organs,  tapering 
to  a  point,  and  placed  opposite  one  another  in  pairs,  so  that  each  pair  is  always  at 
right  angles  to  the  couple  next  above  or  below.  Yet  another  arrangement  is  seen 
in  Coccoloba  platyclada  (Polygonacese),  a  native  of  the  Salomon  Islands,  and  in 
Cocculus  Balfourii,  growing  in  the  island  of  Socotra.  But  it  is  impossible  here  to 
enter  into  all  these  variations  in  detail;  it  is  enough  to  have  brought  forward 
the  most  striking  forms  of  phyllocladous  plants  which  are  represented  in  figs.  82 
and  83. 

If  in  all  these  peculiar  plants  the  branches  are  flattened  and  spread  out,  it 
cannot  indeed  be  asserted  that  the  surface  of  their  transpiring  tissue  has  undergone 
diminution,  and  thus  far  of  course  this  strange  development  has  nothing  to  do  with 
the  restriction  of  transpiration.  The  arrangement  by  which  this  is  brought  about 
must  be  sought  for  elsewhere.  It  consists  in  this:  the  leaf -like  shoots  are  so 
directed  that  their  surfaces  are  vertical  and  not  horizontal.  Contrary  to  most  flat 
leaves,  which  turn  their  broad  surfaces  fully  to  the  incident  light,  the  flattened 
shoots  are  placed  vertically  so  that  at  mid-day  they  only  cast  a  very  narrow  shadow, 
and  do  not  stop  the  sunbeams  on  their  way  to  the  soil.  It  is  obvious,  however,  that 
such  a  leaf -like  structure  placed  vertically,  as  it  were  on  edge,  will  exhale  much  less 
than  a  foliage-leaf  whose  surface  is  opposed  to  the  mid-day  sunbeams.  The  work 
carried  on  in  the  green  cells,  under  the  influence  of  light,  is  not  hindered  by  this 
position  of  the  leaf -like  organs.  If  the  vertical  green  surfaces  are  not  so  well 
illuminated  by  the  sun's  rays  during  the  warmest  part  of  the  day,  this  is  abundantly 
compensated  for  by  the  fact  that  their  broad  surfaces  are  exposed  to  the  light  both 
of  the  morning  and  evening  sun.  On  the  other  hand,  when  the  sun  rises  and  sets, 
the  heat  is  not  so  powerful,  and  consequently  there  is  no  such  rapid  exhalation  to 
be  feared  as  when  the  sun  is  in  the  zenith.  To  put  the  matter  shortly,  transpiration 
alone — not  illumination — is  restricted  by  the  vertical  position  of  the  green  laminae, 
and  therefore  this  metamorphosis  has  rightly  been  considered  a  protective  measure 
against  excessive  transpiration. 

This  arrangement  is  only  found  in  plants  of  dry  regions,  where  transpiration 


FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES.  335 

requires  no  assistance,  but  where,  on  the  contrary,  the  danger  is  often  imminent  that 
water  cannot  be  drawn  from  the  soil  in  sufficient  quantity  to  replace  that  lost  by 
exhalation. 

The  phylloclades,  moreover,  are  only  a  type  of  a  large  number  of  organs  which, 
in  a  word,  all  agree  in  this;  the  edge  or  narrow  side  of  the  flattened  exhaling 
structure,  not  the  broad  surface,  is  turned  towards  the  zenith.  In  many  of  the 


Fig.  83.— Plants  with  Leaf-like  Branches  (Cladodes). 
i  Colletia  cruciata.     2  Carmichelia  australis.     «  Phyllanthus  speciosu*. 

vetches  of  the  Southern  European  flora  (Lathyrus  Nissolia,  Ochrus),  but  especially 
in  a  large  number  of  Australian  shrubs  and  trees,  principally  acacias  (Acacia  longi- 
folia,  falcata,  myrtifolia,  armata,  cultrata,  Melanoxylon,  decipiens,  &c.),  it  is  the 
leaf -stalks  which  are  extended  like  leaves  placed  vertically,  and  then  the  develop- 
ment of  the  leaf -lamina  is  either  entirely  arrested,  or  has  the  appearance  of  an  appen- 
dage at  the  apex  of  the  flat  green  leaf -stalk,  or  "  phyllode  ",  as  it  is  called.  In  many 
Myrtacese  and  Proteacea3,  especially  in  species  of  the  genera  Eucalyptus,  Leucaden- 


336  FORM   AND   POSITION    OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

dron,  Melaleuca,  Protea,  Banksia,  and  Grevillea,  the  leaf-blades  themselves  are  not 
placed  horizontally  like  those  of  our  maples,  elms,  beeches,  and  oaks,  but  vertically 
on  edge,  like  the  phylloclades  and  phyllodes.  Imagine  now  an  entire  wood  of  such 
eucalypti  and  acacias,  on  which  the  mid-day  sun  is  pouring  down  its  rays.  If  it  is 
not  exactly  literally  true  to  say  that  each  vertical  leaf  only  casts  a  linear  shadow  at 
noon,  it  is  at  least  certain  that  there  is  not  much  shade  on  the  ground  of  such  a 
wood.  The  sunbeams  find  their  way  everywhere  between  the  erect  leaf-blades, 
penetrating  the  depths  below,  and  it  is  impossible  to  speak  of  the  dim  forest-light 
under  such  circumstances.  The  Casuarinese,  which  grow  with  eucalyptus,  acacias, 
and  Proteaceae  do  not  help  to  make  such  woods  shady,  and  thus  one  is  quite 
justified  in  speaking  of  the  shadowless  forests  of  Australia. 

Although  Australia  stands  alone  in  the  variety  and  abundance  of  its  plants 
possessing  vertical  leaf -blades,  other  floral  areas  furnish  numerous  and  remarkable 
examples  of  this  arrangement.  One  has  only  to  think  of  the  curious  shape  of  the 
so-called  "equitant"  leaves  belonging  to  many  plants  of  the  Lily  family 
(Tofieldia,  Narihecium),  numerous  Iridese,  and  the  closely-related  genera,  Gladiolus, 
Ferraria,  Witsenia,  Monibretia,  &c.,  chiefly  natives  of  the  Cape.  The  leaves 
exhibit  the  peculiarity  of  being  folded  together  lengthwise,  and  the  sides  thus 
brought  into  contact  become  fused  with  one  another.  Only  at  the  point  where  they 
join  the  stem  do  the  two  halves  remain  distinct,  forming  a  groove  in  which  is 
inserted  the  base  of  an  upper  leaf.  The  formation  of  such  equitant  leaves  from 
ordinary  leaf -blades  may  perhaps  be  illustrated  by  taking  a  strip  of  paper  smeared 
on  one  side  with  paste  and  folding  it  longitudinally  so  that  the  pasted  sides  are  in 
contact  and  become  joined  together.  Such  equitant  leaves  are  so  directed  that  their 
broad  surfaces  are  much  less  exposed  to  the  perpendicular  rays  of  the  mid-day  than 
to  those  of  the  rising  and  setting  sun. 

In  the  Mediterranean  flora,  and  on  many  steppes,  plants  are  not  seldom  to  be 
met  with  whose  leaves  look  as  if  they  had  not  been  able  to  free  themselves  from 
the  stem.  In  such  plants  the  projecting  portion  of  the  foliage-leaf  is  very  small, 
but  the  margins  are  continued  for  some  way  down  the  stem  as  projecting  strips 
and  wings.  Leaves  of  this  kind  are  termed  "decurrent".  They  are  particularly 
abundant  amongst  Composites,  viz.  in  the  genera  Centaurea,  Inula,  Helichrysum', 
but  they  also  occur  in  many  Papilionaceous  plants  and  Labiates.  The  position  of 
these  vertical  wings,  which  traverse  the  stem,  is  exactly  the  same,  with  regard  to 
the  sun,  as  that  of  the  phyllodes,  phylloclades,  and  equitant  leaves,  and  they  behave 
in  respect  to  transpiration  in  exactly  the  same  way. 

In  many  plants  the  blades  of  the  foliage-leaves  when  young  have  not  a  vertical 
position  but  gradually  assume  it  during  development,  i.e  the  blades  at  first  arc 
turned  so  that  the  flattened  surfaces  are  horizontal  and  face  upwards  and  downwards. 
Later  they  twist  round  at  the  point  where  they  are  inserted  on  the  stem,  so  that 
their  margins  become  directed  upwards  and  downwards.  As  already  stated,  this 
peculiarity  is  observed  in  many  eucalypti  and  various  other  Australian  trees 
and  shrubs.  But  plants  in  sunny  situations  in  other  regions  also  exhibit  this 


FORM    AND    POSITION    OF   THE   TRANSPIRING    LEAVES   AND   BRANCHES. 


337 


peculiarity  In  the  Spanish  flora,  for  example,  is  an  Umbellifer  (Buplturum 
vemcale)  whose  leaves  are  so  twisted  with  regard  to  the  sun  that  they  remind  one 
forcibly _o  the  Australian  acacias.  Many  Composites,  especially  the  widely-distri- 
buted  Wild  Lettuce  (Lactuca  Scariola),  growing  on  dry  soil  in  Central  Europe 


Fig.  84.— Compass  Plants. 

1  Silphium  laciniatum,  seen  from  the  east.      *  The  same  plant  seen  from  the  south.     *  Lactuca  Scariola,  seen  from  the  east. 
*  The  same  plant  from  the  south.      Both  species  are  considerably  reduced. 

exhibit  this  contrivance  in  a  striking  manner.  A  Composite  shrub,  Silphium 
ladniatum,  to  be  found  in  the  prairies  of  North  America,  from  Michigan  and 
Wisconsin  as  far  south  as  Alabama  and  Texas,  has  obtained  a  certain  renown  by 
reason  of  the  remarkable  twisting  of  its  leaf -blades.  It  long  astonished  hunters 
in  the  prairies  that  in  these  plants  (represented  in  fig.  84)  the  leaf -laminae,  especially 
those  springing  from  the  lowest  portions  of  the  stem,  not  only  assumed  a  vertical 

VOL.  I.  22 


338  FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHEb. 

position,  but  that  the  broad  surfaces  of  each  leaf  always  faced  the  rising  and  setting 
sun.  Healthy  living  plants  as  they  grow  in  the  sunny  meadows  look  as  though 
they  had  been  laid  between  two  gigantic  sheets  of  paper,  somewhat  pressed,  and 
dried  for  some  time  in  the  way  plants  are  prepared  for  herbariums,  and  had  then 
been  removed  from  the  press  and  set  up  so  that  the  apex  and  profile  of  the  vertical 
leaf -blades  point  north  and  south,  i.e.  in  the  meridian;  while  their  surfaces  face 
the  east  and  west.  This  inclination  is  so  well  and  regularly  observed  by  the  living 
plants  on  the  prairies,  that  hunters  are  enabled  to  guide  themselves  over  such 
regions,  even  under  a  clouded  sky,  by  means  of  these  plants;  for  this  reason  Sil- 
phium  laciniatum  has  been  called  a  "compass"  plant.  The  life  of  the  compass 
plant  is  assisted  by  this  placing  of  the  vertical  leaves  in  the  meridian,  in  that  the 
broad  surfaces  are  placed  almost  at  right  angles  to  the  incident  sunbeams  which 
illuminate  them  in  the  cool  and  relatively  damp  morning  and  evening,  while  at  the 
same  time  they  are  not  too  strongly  heated  nor  stimulated  to  excessive  transpiration. 
At  mid-day,  on  the  other  hand,  when  the  sun's  rays  only  fall  on  the  profile  of  the 
leaves,  the  heating  and  transpiration  are  proportionately  slight.  It  is  of  interest 
that  the  leaves  of  these  compass  plants,  as  well  as  those  of  the  above-mentioned 
Lettuce  represented  with  the  compass  plant  in  fig.  84,  show  this  inclination  and 
position  when  they  grow  on  level,  moderately  dry,  unshaded  ground,  and  that  in 
damp  shady  places,  where  there  is  no  danger  of  over-transpiration  from  the 
powerful  rays  of  the  noon-tide  sun,  the  twisting  of  the  leaves  does  not  take  place, 
and  they  are  not  brought  into  the  meridian. 

The  placing  of  their  leaf-blades  parallel  to  the  ground  when  in  the  shade,  but 
vertically  when  in  dry  sunny  places,  is,  generally  speaking,  a  phenomenon  which 
may  be  seen  in  very  many  plants,  including  shrubs  and  trees.  A  species  of  lime,  a 
native  of  Southern  Europe,  viz.  the  Silver  Lime  (Tilia  argentea),  is  particularly 
noticeable  in  this  respect.  On  dry  hot  summer  days  the  leaves  assume  an  almost 
vertical  position,  but  only  on  those  boughs  and  twigs  which  are  exposed  to  the  sun. 
If  the  tree  stands  at  the  foot  of  a  wall  of  rock,  or  on  the  edge  of  a  thick  wood,  so 
that  a  portion  of  it  is  shaded,  the  leaves  on  this  shaded  part  remain  extended 
horizontally.  Such  a  tree  then  presents  a  strange  aspect,  as  the  leaves  are  of 
two  colours — dark  green  on  the  upper  side,  and  white  on  the  under  surface  by 
reason  of  a  fine  felt-work  of  white  stellate  hairs — and  it  is  scarcely  credible  at  first 
sight  that  the  shaded  and  sunny  portions  of  the  tree  belong  to  one  another. 

In  the  compass  plants  and  also  in  the  Silver  Lime  the  alterations  in  the 
direction  of  the  leaves  are  brought  about  by  alterations  in  the  turgidity  of  certain 
groups  of  cells  in  the  leaf -stalk.  It  is  exactly  the  same  cause  which  produces  the 
periodic  movements  of  numberless  plants  with  pinnate  or  palmate  leaves,  and  the 
leaf -folding  of  many  grasses;  and  it  is  natural  to  conjecture  that  these  phenomena 
of  movement  are  also  connected  with  transpiration.  This  is  in  part  actually  the 
case.  In  consequence  of  alterations  in  turgidity  of  the  pulvini,  the  pinnate  leaflets 
of  the  Gleditschias  and  some  Mimosas  rise  up  after  sunset,  while  those  of  the  Amor- 
phas  fall  down,  and  assume  a  vertical  position  during  the  night;  but  this  is  con- 


FORM    AND   POSITION    OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES.  339 


nected  with  the  nocturnal  radiation  of  heat  (as  will  be  explained  later)  and  not  with 
exhalation.  It  is,  however,  equally  certain  that  the  placing  together  and  folding  up 
of  leaves  and  leaflets  in  many  other  plants  is  brought  about  in  order  to  prevent 
over-transpiration  and  consequent  withering  up.  Many  shrubby,  thorny  mimosas 
of  Brazil  and  Mexico,  when  in  their  native  habitat  and  position,  extend  their  leaflets 
horizontally  when  evening  approaches,  contrary  to  the  behaviour  of  the  well-known 
Sensitive  Plant  (Mimosa  pudica),  and  they  remain  in  this  position  throughout  the 
night.  Next  morning  they  are  still  widely  outspread.  As  soon  as  the  sun  has 
risen,  and  its  beams  fall  on  the  foliage,  the  leaflets  shut  together;  the  menacing 
thorns,  which  until  now  have  been  hidden  by  the  extended  leaves,  become 
apparent;  and  the  leaflets  remain  in  the  vertical  position  during  the  hottest  and 
driest  hours  of  the  day.  Towards  sunset  they  again  rise  and  are  extended 
horizontally.  There  is  but  one  exception  to  this  cycle  of  changes — if  the  opened 
leaf  is  shaken  by  the  wind,  and  if  the  sky  has  been  gray  and  clouded  all  day.  In 
the  former  case,  under  the  influence  of  the  wind,  a  rapid  closure  occurs;  in  the 
latter  case,  when  the  weather  is  bad,  they  remain  open  all  day.  One  of  the 
Rutacese,  Porliera  hygrometrica,  behaves  like  these  mimosas.  In  Peru,  the  native 
country  of  these  plants,  where  they  abound,  the  opening  and  closing  of  the  leaves 
has  even  been  made  use  of  for  weather  predictions,  for  when  the  vertical  leaves  are 
closed,  dry  hot  weather  can  be  reckoned  upon;  when  they  are  open,  damp  cool 
weather.  In  the  cultivated  Bean  (Phaseolus),  moreover,  alterations  of  position  in 
parts  of  the  leaflets  may  be  observed  to  take  place  during  the  day.  When  the  sun 
is  powerful,  the  leaflets  assume  a  vertical  position,  so  that  at  noon  the  sun's  rays 
only  reach  a  small  portion  of  the  blade. 

In  several  species  of  Wood-sorrel  belonging  to  the  South  African  flora,  and 
also  in  the  widely-distributed  Common  Wood-sorrel  (Oxalis  Acetosella),  it  may  be 
noticed  that  the  leaflets,  as  soon  as  they  are  directly  struck  by  the  sun's  rays,  sink 
down,  so  that  their  under  surfaces — on  which  the  stomata  are  situated — face  one 
another,  the  three  leaflets  together  forming  a  pyramid;  while  these  same  leaflets 
in  damp  shady  places  remain  extended.  The  leaflets  of  the  water  fern,  Marsilea 
quadrifolia,  which  grows  in  marshes  and  is  distributed  through  Central  and 
Southern  Europe,  temperate  Asia,  and  North  America,  are  very  similar  to  those  of 
the  Wood-sorrel,  but  carry  their  stomata  on  the  upper  surface.  As  long  as  they 
remain  floating  on  the  surface  of  water,  these  leaflets  are  extended,  but  as  soon  as 
the  water-level  sinks  and  the  leaflets  become  surrounded  by  air,  they  fold  together 
above  in  the  sunshine,  and  their  position  becomes  vertical,  precisely  as  in  the  • 
compass  plants. 

As  another  phenomenon  of  this  kind  the  periodic  folding  or  closing  of  the  leaves 
of  grasses  must  be  specially  mentioned.  It  has  long  been  noticed  that  certain 
grasses  exhibit  a  very  different  aspect  according  as  they  are  observed  on  a  dewy 
morning  or  in  the  noon-day  sunshine.  In  the  morning  their  long  linear  leaves  are 
fluted  on  the  upper  surface,  or  spread  out  quite  flat.  As  soon  as  the  humidity  of 
the  air  diminishes,  in  consequence  of  the  higher  position  of  the  sun,  they  fold 


340  FORM    AND   POSITION    OF   THE   TRANSPIRING   LEAVES   AND    BRANCHES. 

together  lengthwise;  again  after  sunset  they  widen  and  become  flat  or  fluted.  This 
process  may  be  repeated  twice  on  a  summer's  day  within  twenty-four  hours,  if  a 
storm  intervenes  at  mid-day  and  is  followed  by  a  sunny  afternoon.  How  much 
this  depends  upon  the  conditions  of  humidity  of  the  air,  is  demonstrated  by  the  fact 
that  such  grasses,  when  grown  in  pots,  can  be  easily  made  to  open  and  close  their 
leaves  by  alternately  sprinkling  them  with  water  and  placing  in  damp  air,  and  then 
for  a  short  time  exposing  them  to  dry  air.  The  leaf -folding  in  various  species 
of  Sesleria  is  exceedingly  quick  and  also  very  interesting.  The  species  of  this 
genus  grow  principally  on  the  Alps,  Carpathians,  and  Balkans.  They  always 
grow  together  and  often  cover  wide  stretches  of  hilly  and  elevated  districts  with 
thick  grassy  turf.  One  species  (Sesleria  cosrulea)  is  distributed  over  Northern 
Europe  in  Finland,  Sweden,  and  England.  The  closing  of  the  leaves  of  these  moor- 
grasses  reminds  one  strongly  of  the  Venus  Fly-trap  (Dioncea  muscipula),  which  has 
already  been  fully  described.  It  is  indeed  an  actual  shutting  together  of  the  two 
halves  of  the  leaf.  As  in  the  leaf  of  the  "Fly-trap",  the  midrib  of  the  leaf  of  the 
Sesleria  remains  in  its  original  position  unaltered;  also  the  two  halves  of  the  leaf  do 
not  come  flatly  in  contact,  but  rise  up  obliquely  so  as  to  leave  between  them  a  deep, 
narrow,  groove-like  cavity,  widest  at  its  lowest  part  (see  fig.  85  2).  While  the  open 
leaf  turns  its  upper  surface,  rich  in  stomata,  towards  the  sky,  the  two  raised  halves 
of  the  folded  leaf  are  parallel  with  the  incident  sunbeams,  and  the  folded  leaf  of  the 
moor-grass  may  then  be  compared  to  the  equitant  leaf  of  an  iris.  In  the  cavity 
produced  by  the  closing  up  of  the  leaf  are  the  stomata,  however,  and  thus  the  green 
tissue  next  them  is  excellently  protected  from  the  sun's  rays  as  well  as  from  the 
direct  action  of  the  wind.  The  epidermis  of  the  lower  surface,  which  is  exposed  on 
the  folded  leaf  to  all  the  agencies  which  excite  transpiration,  possesses  no  stomata, 
but  is  provided  with  a  thick  cuticle. 

A  leaf -folding  similar  to  that  of  Sesleria,  along  the  midrib,  has  been  observed  in 
the  leaves  of  Avena  planiculmis,  which  grows  in  sunny  fields  on  the  Sudetics  and 
Carpathians.  It  also  occurs  in  Avena  compressa,  and  many  others  related  to  these 
species.  The  folding  or  closing  of  the  leaves  in  the  large  section  of  fescue-grasses 
(Festuca)  is  carried  on  somewhat  differently.  In  Sesleria,  the  opened  upper  sur- 
face of  the  leaf  forms  only  a  single  shallow  groove,  and  the  folding  only  occurs  at 
the  midrib;  but  on  the  upper  side  of  the  fescue-grass  leaf  several  parallel  grooves 
are  to  be  seen,  and  the  green  tissue  is  divided  up  by  these  grooves  into  several  pro- 
jecting ridges,  exhibiting  a  very  remarkable  structure.  In  each  ridge  can  be  dis- 
tinguished the  base  which  forms  a  part  of  the  under  side  of  the  whole  leaf;  then 
the  apex  opposite  the  base,  belonging  to  the  upper  surface  of  the  entire  leaf;  and 
finally,  the  two  side  portions  forming  the  sloping  sides  of  the  grooves  which  run 
between  the  ridges  (see  figs.  87  and  88). 

The  greater  part  of  each  ridge  consists  of  green  tissue.  The  stomata  on  the 
ridge  only  open  on  the  sloping  sides  facing  the  grooves.  Neither  the  crests  of  the 
ridges  nor  the  lower  surface  of  the  leaf  exhibit  a  single  stomate.  The  apex  is  without 
chlorophyll,  and  almost  always  has '  a  border  of  elongated  cells  with  strong  elastic 


FOKM    AND   POSITION    OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES.  341 

walls  under  the  epidermis;  the  same  thing  occurs  on  the  under  side  of  the  leaf  (ie 
at  the  base  of  the  ridges),  which  is  formed  of  one  or  several  layers  of  cells  without 
chlorophyll,  but  furnished  with  thickened  walls.  The  closing  of  the  leaf  is  not  so 
simple  here  as  in  the  Seslerias.  There  the  leaf-folding  only  produced  a  single  deep 
channel,  widened  at  its  base;  in  the  fescue-grasses  all  the  small  grooves  between 
the  ridges  become  narrowed  by  the  closing,  i.e.  by  the  upward  inclination  of  the 
right  and  left  halves  of  the  leaf,  those  adjoining  the  central  ridge  to  the  greatest 


Fig.  85. — Folding  of  Grass-leaves. 

i  Vertical  section  through  an  open  leaf  of  the  thin-leaved  Moor-grass  (Sesleria  tenuifolia).     *  Vertical  section  through 
a  closed  leaf ;  x40.      *  Portion  from  the  centre  of  an  open  leaf;  xSOO. 

extent,  those  in  the  neighbourhood  of  the  approximated  margins  in  a  lesser  degree 
(see  fig.  88 2).  Since  the  stomata  lie  on  the  sides  of  the  ridges,  it  is  obvious  that 
transpiration  is  checked  to  the  utmost  by  the  closing  and  consequent  approximation 
of  the  opposite  sides  of  each  groove. 

In  individual  cases  among  various  fescue-grasses  are  to  be  found  manifold 
differences  in  the  number  and  shape  of  the  ridges,  also  with  respect  to  the  formation 
of  the  under  surface  of  the  leaf,  and  most  of  all  in  the  form  assumed  by  the  leaf  in 
its  expanded  condition.  There  is  a  large  group  of  festucas  which  are  said  to  be 
poisonous  by  the  shepherds  in  the  mountain  regions  of  Spain,  and  in  the  Alps,  the 
Taurus,  and  the  Elbruz.  These  will  be  spoken  of  again  later.  When  open  in 
damp  weather  they  form  only  a  moderately  narrow  main  furrow,  with  several 


342  FORM   AND   POSITION   OF  THE  TRANSPIRING   LEAVES   AND   BRANCHES. 

narrow  secondary  grooves  leading  from  it,  as  can  be  seen  in  a  vertical  section  of 
an  open  leaf  of  Festuca  alpestris,  a  plant  very  abundant  in  the  Southern  Alps  (see 
fig.  86 6).  In  Festuca  alpestris,  the  blunt  apex  of  each  ridge  has  a  border,  three 
layers  deep,  of  cells  destitute  of  chlorophyll,  and  the  lower  side  of  the  leaf  is  pro- 
vided with  an  actual  armour  of  thick-walled  bast  cells,  covered  by  an  epidermis, 


Fig.  86. -Folding  of  Grass-leaves. 


°    th^C  °Pen  l6af  °f  Stipa  MP*11***;  X240.     *  Vertical  section  through  an  entire  open  leaf. 

Vert  .  a  closed  leaf;  X30.    *  Vertical  section  through  a  portion  of  the  leaf  of  Festuca  alpuMr,  X210. 

tical  section  through  an  entire  open  leaf.     •  Vertical  section  through  a  closed  leaf;  x  30. 

whose  outer  walls  are  much  thickened.  A  vertical  section  through  the  leaf  of 
Festuca  punctoria,  a  native  of  the  Taurus,  is  represented  in  fig.  88.  In  this 
plant,  the  leaves,  when  open,  present  a  fairly  shallow  depression;  the  under  surface 
is  clothed  with  a  protective  mantle  of  five  layers  of  strong  cells  devoid  of  chloro- 
phyll; the  ndges  are  rounded  off  and  possess  only  a  single  layer  of  covering  cells, 
provided  with  an  extremely  strong  wax-like  coat.  The  open  leaves  of  Festuca  Porcii 


FORM   AND    POSITION   OF   THE   TRANSPIRING   LEAVES    AND   BRANCHES.  343 

a  native  of  the  Carpathians,  are  relatively  thin  (see  figs.  87 4  and  87 5).  Below  the 
epidermis  of  the  under  side  is  no  mantle  of  bast  cells  as  in  the  species  already 
described,  but  only  isolated  strands  of  bast;  however,  the  crest  of  each  ridge  is 
furnished  with  a  strand  of  bast  cells;  the  ridges  themselves  project  very  much,  and 
the  whole  leaf  is  traversed  by  six  deep  narrow  grooves. 

In  the  three  fescue-grasses  cited  here  as  examples,  and  in  all  species  of  the 
genus  Festuca,  forming  the  main  part  of  the  turf  of  our  fields,  a  vascular  bundle 


Fig.  87.— Folding  of  Grass-leaves. 

i  Vertical  section  through  a  closed  leaf  of  Lasiagrostis  Calamagrostis.  2  Vertical  section  through  an  open  leaf;  x24. 
»  Vertical  section  through  a  portion  of  the  open  leaf;  X210.  *  Vertical  section  through  a  closed  leaf  of  Festuca  Porciu 
8  Vertical  section  through  an  open  leaf;  x24.  •  Vertical  section  through  a  portion  of  the  open  leaf;  x210. 

surrounded  by  green  tissue  traverses  each  ridge.  In  the  hinged  leaves  of  many 
other  grasses,  the  green  tissue  of  each  ridge  is  divided  into  two  portions.  The 
vascular  bundle  is  bordered  above  and  below  by  strands  of  thick-walled  cells  devoid 
of  chlorophyll,  and  thus  arises  a  strong  septum  in  the  green  parenchyma,  beautifully 
shown  in  the  transverse  section  of  a  leaf  of  Lasiagrostis  Calamagrostis,  illustrated 
in  fig.  87.  In  the  leaves  of  the  Feather-grass  (Stipa  capillata)  are  alternating  higher 
and  lower  ridges;  a  vertical  section  is  shown  in  fig.  861-2'8.  In  the  higher  ridges 
occur  septa  similar  to  those  in  Lasiagrostis,  but  in  the  lower  there  is  only  a  vas- 
cular bundle  surrounded  by  green  tissue  as  in  the  fescue-grasses.  No  less  than 


344  FORM   AND   POSITION   OF   THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

twenty-nine  ridges  can  be  counted  on  the  leaf  of  the  above-mentioned  Lasiagrostis, 
a  plant  widely  distributed  in  the  valleys  of  the  Western  and  Southern  Alps,  where 
it  clothes  the  sunny  slopes  in  thick  masses.  When  the  leaf  folds  up,  the  twenty- 
eight  grooves  between  the  ridges,  on  whose  sides  are  the  stomata,  become  narrowed, 
and  the  entire  leaf  assumes  a  tubular  form,  so  that  transpiration  is  almost  com- 
pletely suspended.  In  Stipa  capillata,  which  is  very  abundant  on  clay  steppes, 
the  same  thing  occurs  (see  fig.  86 3).  In  both  grasses  the  closure  of  the  grooves  on 
whose  sides  are  the  stomata,  is  completed  by  short  stiff  hairs  on  the  summit  of  the 
ridges,  which  interlock  when  the  ridges  approach  one  another,  and  so  block  up 
access  to  the  grooves  (fig.  86s).  It  would  take  us  much  too  far  to  describe  the 
numerous  other  modifications  which  are  to  be  met  with  in  the  structure  of  hinged 
grass -leaves.  The  examples  given  suffice  to  make  it  evident  that  the  danger 
of  over-transpiration  is  avoided  by  the  folding  of  the  leaf,  and  that  amongst 
the  grasses  very  many  arrangements  obtain  in  order,  sometimes,  to  expose  those 
green  parts  of  the  leaf  whose  epidermis  is  supplied  with  stomata  to  the  rays  of  the 
sun,  and  at  other  times  to  withdraw  them,  according  to  the  humidity  of  the  soil 
and  of  the  surrounding  air,  thus  suitably  regulating  transpiration  to  the  existing 
circumstances. 

The  mechanism  by  which  grass-leaves  open  and  close  may  be  explained  in  two 
ways — either  the  process  is  due  to  hygroscopic  changes,  as  in  the  opening  and  clos- 
ing of  the  "  Rose  of  Jericho  ",  or  to  alterations  in  the  turgidity  of  certain  groups  of 
cells,  as  in  the  mimosas.  If  the  former  alone  were  the  case,  a  dry,  dead  grass-leaf 
should  be  still  capable  of  opening  and  closing  in  accordance  with  its  damp  or  dry 
condition;  but  a  leaf  of  any  of  these  when  cut  off  and  dried  no  longer  opens,  even 
after  being  moistened  for  a  considerable  time,  and  therefore  the  first  explanation 
cannot  be  accepted,  at  any  rate  for  most  of  the  grasses.  Apparently,  the  mechanism 
consists  of  alterations  in  the  turgescenee  of  those  groups  of  cells  situated  in  the 
angle  of  the  grooves.  Since  the  floor  of  the  grooves  was  frequently  found  to  con- 
sist of  peculiar  thin- walled  cells  destitute  of  chlorophyll,  and  filled  with  colourless 
watery  sap,  it  was  concluded  that  the  opening  and  closing  of  the  grass-leaves  was 
due  to  the  change  in  turgidity  of  these  cells.  However,  this  was  going  too  far. 
These  cells  in  mosHnstances,  for  example,  in  Festuca  punctoria  (see  fig.  88 2),  would 
be  much  too  delicate  to  effect,  unaided,  the  closure  of  the  leaf  by  their  loss  of 
turgidity,  or  to  open  it  by  their  increasing  turgescenee.  In  many  grasses  these 
cells  are  completely  wanting  (e.g.  in  Festuca  alpestris  and  Stipa  capillata,  fig.  86). 
Moreover,  it  is  observed  that  the  opening  and  closing  of  the  leaf  is  still  carried  on 
when  the  thin-walled  cells  at  the  bottom  of  the  grooves  are  destroyed,  artificially,  by 
puncturing  with  fine  needles.  The  cause  of  the  movement  must  therefore  be  looked 
for  in  the  alteration  of  turgescenee  of  other  cells  below  the  grooves.  When  a 
mantle  of  several  layers  of  thick-walled  cells  is  present  on  the  under  side  of  the 
leaf,  their  walls  are  seen  to  swell  up  simultaneously  with  the  alterations  of  tur- 
gescenee of  the  parenchymatous  cells.  Of  course  the  inner  cell-layers  of  the  mantle 
must  be  capable  of  swelling  up  to  a  greater  extent  than  the  outer,  and  this  has 


FORM    AND   POSITION    OF   THE   TRANSPIRING   LEAVES   AND    BRANCHES.  345 

actually  been  shown  to  be  the  case  in  some  species.  Moreover,  although  the  thin- 
walled  cells  at  the  bottom  of  the  furrows  are  not  conswered  strong  enough  to  bring 
about  the  opening  and  closing  by  changes  in  their  turgidity  alone,  it  is  by  no  means 
asserted  that  they  have  no  other  part  to  play.  When  they  are  constructed  as  in 
the  leaves  of  the  moor-grasses  and  in  the  fescue-grass  of  the  Taurus  (Festuca 
punctoria,  figs.  85  and  88),  they  certainly  are  not  without  a  purpose.  Their  advan- 
tage to  the  plant  lies  in  the  fact  that  they  can  be  much  compressed  without  harm 
by  the  closure  of  the  leaf,  whereby  the  neighbouring  parenchymatous  cells  are  pro- 


Fig.  88.— Folding  of  Grass-leaves. 

1  Vertical  section  through  an  open  leaf  of  Festuca  punctoria,  of  the  Taurus.     »  Vertical  section  through  a  closed  leaf ;  x40. 
«  Vertical  section  through  a  portion  of  the  open  leaf;  x280. 

tected  from  injury;  also  that  by  means  of  these  cells,  which  are  filled  with  watery 
sap,  carbonic  acid  from  the  atmosphere  is  conducted  to  the  underlying  green  tissue; 
and  lastly,  that  in  case  of  necessity,  water  can  be  absorbed  from  the  air.  They  re- 
mind one  strongly  of  the  thin- walled  groups  of  cells  of  foliage-leaves  used  for  the 
•direct  absorption  of  moisture,  and  possibly  they  can  function  in  this  way.  If, 
in  places  where  these  grasses  grow  naturally,  a  slight  shower  of  rain  falls  after  a 
long  period  of  drought,  or  if  dew  falls  during  clear  nights,  little  or  none  of  the 
water  reaches  the  roots,  since  it  is  retained  by  leaves  overspreading  the  soil.  But 
the  water  easily  runs  into  the  furrows  of  the  folded  leaves  of  grass,  and  since  the 
large  thin-walled  cells  at  the  bottom  of  the  grooves  can  be  wetted,  they  offer  to  the 
water  which  can  pass  through  them  the  shortest  path  to  the  green  cells  in  the 
interior  of  the  leaf. 


346  FORM   AND   POSITION   OF  THE   TRANSPIRING   LEAVES   AND   BRANCHES. 

A  process,  very  similar  to  the  opening  and  closing  of  grass-leaves,  is  also  to  be 
observed  in  the  true  mosses,  in  all  species  of  the  genus  Polytrichvm,  and  in  some  of 
the  Barbulas.  The  peculiar  structure  of  the  leaves  of  these  mosses  has  been  already 
treated  of.  In  addition  to  the  description  there  given,  it  may  be  mentioned  that  the 
ridges  of  thin-walled  green  cells,  which  are  present  on  the  upper  surface  of  such  a 
leaf  (see  fig.  89),  only  remain  exposed  to  currents  of  air  as  long  as  this  air  possesses 
the  requisite  degree  of  humidity;  that  is  to  say,  the  blade  of  the  leaf  from  whose 
upper  surface  the  bands  project  only  remains  expanded  while  that  is  the  case 

(fig.  892). 

As  soon  as  the  air  becomes  dry,  the  lateral  portions  of  the  leaf-blade  bend 
upwards,  and  envelop  the  green  ridges  like  a  mantle  (fig.  89 1).  These  are  then 


Fig.  89.— Folding  of  Moss-leaves. 

Transverse  sections  through  the  leaf  of  a  Polytrichum  (Polytrichum  commune). 
2  The  leaf  damp  and  open;  x85. 


i  The  leaf  dry  and  folded. 


inclosed  in  a  hollow  chamber,  and  only  retain  communication  with  the  surrounding 
air  by  a  narrow  slit  above,  which  is  left  open  between  the  inflected  leaf -margins. 
But  here  again  it  should  be  noticed  that  the  highest  cells  in  each  ridge  are  strongly 
thickened,  on  the  part  turned  towards  the  opening,  which  doubtless  helps  to  lessen 
transpiration.  The  opening  and  closing  of  the  Polytrichum  takes  place  very  rapidly. 
By  repeated  hygrometric  changes  in  the  air,  the  process  may  be  performed  naturally 
several  times  in  a  single  day.  In  Polytrichaceae,  which  have  been  plucked  while  their 
leaves  were  open,  the  closure  is  seen  to  be  completed,  in  dry  air,  in  a  few  minutes. 
Dead  and  withered  leaves  are  always  closed,  and  never  reopen,  even  when  kept 
damp  for  a  long  time — from  which  it  may  be  concluded  that  the  mechanism  of  the 
opening  and  closing  cannot  be  due  to  a  simple  hygroscopic  phenomenon.  Probably, 
the  same  mechanical  forces  come  into  action  as  produce  the  folding  of  leaves  of 
grasses;  but  the  process  in  moss-leaves  is  much  more  complicated,  since  it  consists, 
not  merely  in  the  upward  inclination  of  the  leaf-edges,  but  also  in  an  upward  curva- 
ture and  spiral  twisting  of  the  whole  leaf. 


OLD   AND   YOUNG   LEAVES.  347 


4.  TRANSPIRATION   DURING   VARIOUS   SEASONS   OF  THE 
YEAR.     TRANSPIRATION   OF   LIANES. 

Old  and  Young  Leaves.— Fall  of  the  Leaf.— Connection  between   the  structure  of  the  Vascular 

Tissues  and  Transpiration. 

OLD  AND  YOUNG  LEAVES. 

The  various  regulators  of  transpiration,  hitherto  described,  either  persist  in  the 
plant-organs  in  question  throughout  life,  or  only  remain  for  a  comparatively  short 
time.  They  are  present  throughout  life  in  evergreen  leaves,  particularly  in  regions 
where  wet  and  dry  seasons  alternate  during  the  year.  In  this  case  the  plants 
require  powerful  aids  to  transpiration  in  the  rainy  season,  and  in  the  dry  season 
abundance  of  protective  measures  against  excessive  loss  of  water.  Evergreen  leaves 
cannot  afford  to  dispense  with  either  the  promoting  or  inhibiting  arrangements 
after  the  expiration  of  the  first  year,  because  for  several  years  they  still  have  to 
pass  through  both  these  seasons.  It  is  otherwise  with  those  leaves  whose  activity 
only  lasts  for  a  single  summer.  These  burst  from  the  buds  at  the  beginning  of  the 
vegetative  period,  and  then  unfold,  transpire  and  respire  for  a  few  months,  pro- 
ducing organic  materials,  and  conduct  them  towards  the  places  where  they  are 
required.  At  the  commencement  of  the  drought,  however,  or  on  the  appearance 
of  frost,  they  turn  yellow  and  fade,  are  detached  from  the  stems  and  branches 
which  bear  them,  and  die.  In  leaves  of  this  kind,  an  arrangement  which  is  very 
necessary  during  the  first  season  may  become  superfluous  later — it  may  even 
become  disadvantageous  under  changed  external  influences,  and  the  leaf  would  then 
be  benefited  by  freeing  itself  entirely  from  the  contrivance.  It  would  often  be 
useful  to  the  plant  to  substitute  in  the  place  of  a  protective  contrivance,  which  is 
only  beneficial  at  the  commencement  of  the  vegetative  period,  another  arrangement 
fitted  to  the  new  and  altered  conditions.  In  the  so-called  deciduous  leaves,  i.e. 
in  those  which  throughout  the  year  are  only  active  in  the  summer,  often  only 
for  two  months,  it  is  a  fact  that  an  alternation  of  this  kind  may  be  regularly 
observed  in  the  mechanisms  which  govern  transpiration. 

It  will  be  noticed  in  a  young  foliage-leaf  which  has  just  pierced  through  the 
ground,  or  in  one  which  is  still  half -hidden  between  the  cotyledons  of  a  seedling,  or 
surrounded  by  the  loosening  scales  of  a  winter  bud,  that  the  development  of  that 
portion  whose  duty  will  later  on  be  to  transpire  and  assimilate,  is  very  backward. 
The  leaf -veins  are  already  very  prominent,  but  the  green  tissue  is  in  quite  a  rudi- 
mentary condition.  It  is  not  only  that  the  extent  of  the  surface  is  very  small, 
but  that  the  epidermis  which  covers  it  is  not  yet  properly  developed;  the  outer 
walls  of  the  epidermal  cells  are  not  yet  fortified  with  a  cuticle,  and  are  consequently 
neither  water-tight  nor  impermeable  to  aqueous  vapour.  If  exposed  to  sun  or 
wind,  the  green  tissue  would  at  once  dry  up.  When  the  young  foliage-leaf  has 


348  OLD    AND   YOUNG   LEAVES. 

forced  its  way  out  of  the  bud  above  the  soil,  or  from  between  the  cotyledons,  the 
conditions  are  still  the  same,  and  therefore  particularly  efficacious  protective 
arrangements  are  required  that  the  leaves  just  merging  from  the  bud,  and  thus 
exposed  to  the  vicissitudes  of  the  weather,  may  grow  up  properly,  i.e.  that  their 
green  transpiring  tissue  may  be  normally  developed. 

Some  of  these  protective  contrivances  belong  exclusively  to  the  developing 
period  of  the  leaves,  and  are  lost  when  they  become  fully  grown.  Others  may  be 
seen  in  the  adult  leaves.  The  most  striking  instances  are  perhaps  the  diminution 
of  the  surfaces  directly  exposed  to  the  sun  and  wind,  the  vertical  inclination  of 
the  leaf-blades,  and  the  concealment  of  the  green  tissue  under  a  protective  mantle. 

The  diminution  of  the  surface  directly  exposed  to  the  sun  and  wind  is  caused  by 
the  position  which  the  foliage-leaf  takes  up  within  the  bud.  Space  is  very  limited 
here,  and  the  youngest  and  smallest  leaves  appear  to  be  fitted  into  the  space  by 
the  rolling,  or  folding,  or  crumpling  of  their  blades.  This  diminution  is  obviously  of 
great  advantage  when  the  leaves  open  out  into  the  daylight:  it  constitutes  a  special 
protection  against  the  drying  up  of  the  green  tissues,  and  is,  therefore,  retained 
until  other  protective  measures  are  developed,  and  in  some  cases  even  throughout 
life.  In  many  Polygonaceae  (e.g.  Polygonum  viviparum  and  Bistorta),  in  species  of 
Butter-bur  (Petasites),  in  some  Primulacese,  and  especially  in  many  bulbous  plants, 
the  green  portions  of  the  leaf  are  rolled.  The  midrib,  and  frequently  a  fairly  broad 
central  strip  of  the  leaf  in  addition,  remains  flat,  and  the  right  and  left  halves  are 
rolled  up  from  the  margins,  sometimes  towards  the  upper,  sometimes  towards  the 
lower  surface.  The  stomata  are  chiefly,  or  wholly,  to  be  found  on  the  concave  side, 
beneath  which  lies  the  soft  green  tissue  with  its  ramifying  air-passages.  In  the 
Crocus,  the  two  halves  of  the  leaf  are  rolled  outwards;  they  are  connected  togethei 
by  a  broad,  white,  central  stripe  which  is  not  rolled,  and  is  devoid  of  chlorophyll; 
in  the  Star  of  Bethlehem  (Ornithogalum),  whose  leaves  are  traversed  by  a  similar 
white  stripe,  the  leaf  margins  are  rolled  inwards.  In  species  of  Crocus  the  stomata 
are  placed  in  the  two  grooves  on  the  under  surface;  in  the  Star  of  Bethlehem,  in 
the  grooves  on  the  upper  surface.  The  central  stripe  of  the  young  leaves  in  the 
plants  mentioned  always  remains  flat,  but  in  young  fern-leaves,  which  are  also 
rolled,  the  strongly-developed  midrib  is  curled  spirally  inwards  like  a  watch-spring, 
and  thus  the  green  feather-like  pinnae,  springing  from  the  rachis,  are  placed  one 
above  the  other.  Most  ferns  in  their  native  habitat  rarely  require  special  protec- 
tion against  over- transpiration  during  the  first  stages  of  development;  but -when 
this  is  necessary,  it  is  afforded  in  every  case  by  the  form  assumed  by  the  young 
leaf  just  described.  Moreover,  in  such  instances  special  protective  envelopes  are, 
as  a  rule,  to  be  found,  which  will  be  spoken  of  later. 

Leaves  are  not  so  often  crumpled  as  rolled  on  first  emerging  from  the  bud.  In 
crumpled  leaves  the  net- work  of  anastomosing  veins  forms  a  strong  lattice-work,  and 
the  green  leaf-substance,  fitted  into  the  interstices  of  the  lattice,  is  swollen  up  like 
bubbles  or  sunken  into  pits,  giving  the  whole  leaf  the  appearance  of  a  crumpled  sheet 
of  paper  or  cloth.  The  vernation  (or  position  occupied  by  the  leaf  in  the  bud)  is 


OLD   AND   YOUNG   LEAVES. 


849 


therefore  aptly  termed  «  crumpled  ".  Leaves  specially  noticeable  in  this  respect  are 
those  of  the  many  species  of  dock  (Rumex),  rhubarb  (Rheum),  and  also  of  several 
spring  primulas  (Primula  acaulis,  elatior,  denticulata,  &c.).  Frequently  the 
crumpling  and  rolling  occur  together,  leaves  with  crumpled  vernation  having  their 
lateral  margins  also  somewhat  rolled  inwards. 

Young  leaves  which  have  just  burst  from  the  bud,  and  still  retain  the  form  they 
possessed  there,  are  very  often  seen  to  be  "plaited".     The  veins  of  the  leaf  form 


Fig.  90.— Unfolding  of  Leaves. 

i,  2  Wild  Cherry  (Prunus  avium).     »,  *  Walnut  (Juglans  regia).     *,  «  Wayfaring  Tree  (  Viburnum  Lantana). 
1  Lady's-mautle  (AlchemUla  vulgaris).     «  Wood-sorrel  (Oxalis  Acetosetta\ 

as  it  were,  the  fixed  framework,  and  it  is  only  the  green  portions  between  which  are 
laid  in  folds.  From  the  multiplicity  in  form  and  division  of  the  leaf- veins,  the  kind 
and  manner  of  folding  is  also  very  varied.  When  the  leaf-blade  is  traversed,  by 
radiating  veins,  as,  for  example,  in  the  Lady's-mantle  (AlchemUla  vulgaris),  shown  in 
fig.  90 7,  the  leaf  is  folded  in  vernation  just  like  a  fan;  the  veins  which  radiate  out 
in  the  adult  leaf  are  as  yet  parallel  to  one  another,  and  the  green  portions  which  in 
the  fully-formed  leaf  are  stretched  between  the  veins,  form  deep  folds,  which  are 
closely  packed  together.  The  same  arrangement  occurs  when  each  of  the  radiating 


OLD   AND   YOUNG   LEAVES. 

veins  becomes  the  midrib  of  a  leaflet,  as  in  the  cinquefoils,  and  species  of  clover 
and  Wood-sorrel.  Each  leaflet  is  folded  up  along  the  midrib  like  a  sheet  of  paper, 
and  the  folded  leaflets  are  placed  side  by  side  in  the  same  way  as  folded  leaves 

in  a  book. 

When  the  leaves  are  pinnate,  and  the  leaflets  are  arranged  in  pairs  on  a  common 
rachis,  the  latter  are  folded  together  along  their  midribs,  and  placed  side  by  side,  so 
as  to  resemble  the  pages  of  a  book.  This  vernation  occurs  in  roses,  Mountain  Ash 
(Sorbua  aucuparia)  and  Walnut  (Juglans  regia),  see  figs.  90 3  and  90  4.  In  the 
roses  the  rachis  is  so  short  in  the  bud  that  the  leaflets  springing  from  it  appear  to 
originate  from  one  point,  as  in  the  cinquefoils.  In  most  maple-leaves  and  those  of 
Saxifraga  peltata,  the  folding  takes  place  not  along  the  radiating  veins  alone,  but 
along  the  short  lateral  veins  which  spring  from  the  larger  radiating  ribs.  In  this 
way  small  folds  are  inserted  between  the  larger,  and  this  vernation  leads  up  to  that 
which  was  described  before  as  "crumpled".  The  leaf-folding  exhibited  by  the 
foliage  of  the  Beech  (Fagus  silvatica,  see  fig.  92),  the  Hornbeam  and  the  Hop- 
hornbeam  (Carpinus  and  Ostrya),  the  Oak  (Quercus),  and  many  other  plants,  whilst 
in  the  bud,  is  very  characteristic.  Each  foliage-leaf  possesses  a  midrib  and  numer- 
ous strong  lateral  veins,  which  run  right  and  left  from  the  midrib  like  the  bony 
processes  from  the  spinal  column  of  a  fish.  The  green  portions  of  the  leaf  form 
deep  folds  between  these  lateral  veins,  which  are  as  yet  very  close  to  one  another, 
and  the  folds  are  thus  arranged  exactly  as  in  a  fan.  Yet  another  method  of  folding 
occurs  in  the  Cherry  (Prunus  avium).  Each  leaf,  while  in  the  bud,  and  for  some 
time  after  it  has  burst  from  it,  is  folded  along  the  midrib  only  (see  figs.  90 1  and  90  2). 
The  right  and  left  halves  are  so  flatly  folded  together,  and  fit  over  one  another  so 
completely,  that  at  first  sight  they  appear  to  form  only  a  simple  leaf -blade.  More- 
over, the  two  halves  which  are  in  contact  are  actually  joined  by  means  of  a  balsam- 
like  secretion.  At  this  stage  of  development  they  are  always  erect;  and  this 
brings  us  to  another  protective  contrivance  to  be  observed  in  young  undeveloped 
leaves. 

It  may  be  stated  that,  with  the  exception  of  a  few  "  crumpled  "  forms,  all  young 
foliage-leaves  when  they  emerge  from  the  bud-scales,  or  from  between  the  coty- 
ledons, or  as  they  force  their  way  through  the  soil  into  the  light  of  day,  are  so 
directed  that  their  blades  are  not  horizontal.  In  this  first  stage  of  development, 
indeed,  the  green  transpiring,  but  still  delicate,  portions  of  the  leaf  have  always  a 
vertical  position.  Their  blades  usually  exhibit  the  direction  observed  in  phyllo- 
clades  and  phyllodes,  in  the  equitant  leaves  of  irises  and  tofieldias,  in  the  leaves  of 
the  compass  plants  during  their  greatest  activity,  and  in  the  leaves  of  grasses 
when  folded  together  in  dry  air.  Sometimes  the  entire  extended  or  rolled  blade  is 
erect,  as  in  most  bulbous  plants  and  grasses;  or  the  midrib  is  inclined  towards  the 
horizon,  in  which  case  the  halves  of  the  leaf  are  folded  together  and  the  two 
margins  come  into  contact,  forming  a  sharp  edge  which  is  turned  towards  the  sun 
at  noon.  This  is  seen  in  some  grasses  (Glyceria,  Poa),  and  in  the  Cherry  (Prunus 
avium).  If  the  blade  is  not  erect,  the  stalk  of  the  leaf  is  perpendicular  while  the 


OLD    AND   YOUNG   LEAVES.  351 

still  delicate  blade  hangs  from  it  like  a  closed  parasol,  as  in  Podophyllum,  Cortusa, 
Hydrophyllum,  and  several  Ranunculacese.  In  the  Horse-chestnut  (dlsculus 
Hippocastanum)  the  folded  leaflets  are  erect  when  they  emerge  from  the  bud;  they 
then  sink  down  so  that  their  apices  point  to  the  ground;  and  later,  when  the 
epidermis  has  become  more  thickened,  they  again  rise  until  they  are  almost 
horizontal.  Leaves  of  limes  (Tilia  grandifolia  and  parvifolia)  are  vertical  when 
they  first  break  through  the  bud,  the  apex  directed  towards  the  ground;  it  is  only 
later  that  they  become  almost  parallel  with  its  surface.  The  upright  leaf -stalk  is 
often  bent  like  a  hook  at  the  end,  and  the  vertical  folded  leaflets  depend  from  the 
hooked  portion.  This  arrangement  is  shown  in  the  common  Wood -sorrel,  and 
many  other  plants  (see  fig.  90 8). 

A  third  method  of  protecting  these  delicate  undeveloped  green  portions  of 
young  leaves  consists  in  the  formation  of  screens  and  coverings,  which  exhibit  the 
greatest  variety.  The  envelope  is  frequently  furnished  by  the  so-called  stipules. 
In  many  plants  two  lobes  arise  on  the  right  and  left  of  the  leaf -stalk  at  the  point  of 
junction  of  the  leaf  and  stem,  and  these  have  been  termed  "  stipules  "  (stipulce).  In 
figs,  oaks,  beeches,  limes,  magnolias,  and  numerous  other  plants,  the  stipules  are 
membraneous,  pale,  usually  without  chlorophyll,  and  they  appear  like  scales  placed 
as  screens  in  front  of  the  small,  tender  green  leaflets  when  they  burst  through  the 
bud,  and  in  any  case  must  be  considered  to  protect  them  from  the  sun's  rays  (see 
fig.  92).  When  once  the  young  leaf  has  grown  beyond  the  top  of  these  screens  and 
no  longer  needs  them,  they  shrivel  up,  are  detached,  and  fall  to  the  ground. 
Millions  of  such  fallen  scales,  called  in  botanical  terminology  "deciduous  stipules", 
are  to  be  seen  on  the  ground  in  oak  and  beech  forests  shortly  after  the  leaves  have 
attained  their  normal  size.  The  stipules  of  magnolias,  particularly  of  the  Tulip- 
tree  (Liriodendron  tulipifera),  a  native  of  North  America,  but  now  cultivated  all 
over  Europe,  are  very  remarkable  (see  fig.  91).  They  are  comparatively  large  and 
boat-shaped,  and  are  always  so  arranged  in  pairs  as  to  form  a  closed  cup.  Shut 
up  within  this  membraneous,  slightly  transparent  cup  can  be  seen  the  young  leaf, 
its  stalk  being  bent  into  a  hook,  and  the  two  halves  of  the  blade  folded  together 
along  the  midrib  like  those  of  the  Cherry.  In  this  position  the  leaf  grows  gradually 
as  if  in  a  small  greenhouse ;  it  enlarges,  and  as  soon  as  the  epidermal  cells  are  so 
much  thickened  that  there  is  no  further  danger  of  it  drying  up,  the  cup  opens  and 
the  two  boat-shaped  stipules  separate  from  one  another,  shrivel  up,  and  at  length 
fall  off.  Only  two  scars  at  the  base  of  the  leaf  remind  one  that  two  stipules  were 
situated  here  in  the  spring,  whose  function  was  to  protect  the  delicate  young  leaf 
from  desiccation. 

One  of  the  most  noticeable  arrangements  for  the  protection  of  the  tender, 
undeveloped  green  tissue  consists  in  the  peculiar  grouping  of  the  leaf-veins.  This 
may  be  best  observed  in  foliage-leaves  which  are  folded  along  the  lateral  veins  in 
vernation.  Each  individual  leaf  is  erect,  usually  a  little  bent  at  the  apex  and 
margins,  and  slightly  hollowed  so  that  the  upper  surface  is  concave,  and  the  lower 
side,  which  is  turned  towards  the  incident  light,  convex.  Since  the  midrib  of  the 


352 


OLD   AND   YOUNG   LEAVES. 


leaf  is  still  comparatively  short,  while  the  numerous  lateral  veins,  on  the  contrary, 
are  already  strongly  developed,  the  latter  must  lie  so  close  to  one  another  that  they 
actually  come  into  contact.  Consequently  on  the  under  surface  of  the  erect  leaf, 
which  is  turned  towards  the  sun,  nothing  can  be  seen  of  the  delicate  green  tissue ; 


i  <•-.•  ^H  j/a^ 


Fig.  91.— Leaf -unfolding  of  the  Tulip-tree  (Liriodendron  tulipifera) 

only  the  thick  lateral  veins,  devoid  of  chlorophyll,  stand  out  side  by  side  like  the 
upporting  framework  of  a  rush  mat.  The  green  portions  of  the  leaf,  which  extend 
etween  the  veins,  form  projecting  folds  on  the  concave  surface,  i.e.  on  the  surface 

Which  is  turned  from  the  sun.  They  are  thus  hidden  behind  the  close-pressed  layer 
by  a  roof,  and  are  consequently  protected  as  efficiently  as  possible  from 


OLD   AND   YOUNG   LEAVES. 


353 


the  sun  and  wind.  The  ribs  themselves  are  composed  of  cellular  structures  which 
are  not  open  to  the  danger  of  over-transpiration,  and  the  epidermis  which  covers 
them  is  entirely  devoid  of  stomata.  When  the  leaves  at  the  ends  of  the  young 
twigs  are  opposite,  erect,  and  concave,  and  their  margins  are  in  contact,  they  form 
an  actual  capsule  round  the  apex  of  the  shoot.  This  occurs  in  the  Wayfaring  Tree 
(Viburnum  Lantana\  illustrated  in  fig.  90 5.  The  small  folds  of  green  tissue 
project  into  the  interior  of  the  capsule,  and  the  still  closely-pressed  lateral  veins 
form  the  outer  wall,  and  at  the  same  time  furnish  a  protective  covering  for  the 
enlarging  green  portions  of  the  leaf.  As  soon  as  these  are  fully  developed,  and  the 


Fig.  92.— Unfolding  of  Beech-leaves. 

i  The  brown  bud-scales  have  been  loosened,  and  the  membraneous  stipules  surrounding  the  foliage-leaves  are  visible  above. 
2  Further  stage  of  development,  the  folded  foliage-leaves  being  visible  between  the  stipules.  »  The  same  twig  further 
developed.  *  Lower  surface  of  a  young  folded  leaf.  6  Portion  of  the  same  leaf ;  the  depressions  caused  by  the  folding 
are  bridged  over  by  silky  hairs.  «  Surface  view  of  an  unfolded  leaf ;  the  stipules  are  withered  and  about  to  fall. 
i  Vertical  section  of  a  leaf  at  right  angles  to  the  midrib.  •  Vertical  section  parallel  with  the  midrib. 

epidermal  cells  are  correspondingly  thickened,  the  projecting  folds  become  smooth, 
the  veins  separate  from  one  another,  and  the  whole  leaf  becomes  flat,  assumes  a 
horizontal  instead  of  a  vertical  position,  and  turns  the  upper  instead  of  the  lower 
surface  to  the  incident  light  (see  fig.  90  6). 

It  has  already  been  repeatedly  stated  that  coats  of  varnish  as  protective 
coverings  are  especially  to  be  met  with  on  young  leaves,  which  they  guard  from 
over-transpiration  and  desiccation  during  their  development,  and  that  when  the 
leaf-laminas  become  provided  with  a  cuticularized  epidermis,  these  coats  disappear. 
It  has  also  been  pointed  out,  incidentally,  that  coats  of  hairs  are  of  great  use  as 
protections  and  screens  to  the  young  foliage-leaves  when  they  first  emerge  from  the 

VOL.  L  23 


354  OLD   AND   YOUNG   LEAVES. 

buds.  The  leaves  of  a  great  number  of  plants  are  only  hairy  during  the  commence- 
ment of  development.  Long  hair-cells  may  be  seen  inserted  by  their  narrow  bases 
between  the  flattened  epidermal  cells;  these  at  an  early  stage  shrink  up  close  to 
their  origin,  and  then  break  off.  They  may  remain  hanging  to  the  leaf  for  a  little 
while,  but  afterwards  are  thrown  or  pushed  off  by  the  enlargement  and  expansion 
of  the  leaf -blade,  or  are  frequently  blown  away  by  the  wind.  The  leaflets,  which 
were  originally  quite  thickly  clothed  with  hairs,  then  appear  partially  or  entirely 
smooth  and  green  on  both  sides.  A  remarkable  instance  of  this  is  furnished  by 
Amelanchier  vulgaris,  whose  foliage,  early  in  the  spring,  is  folded  along  the  midrib 
and  covered  with  snow-white  wool,  reminding  one  strongly  of  the  Edelweiss,  while 
in  the  summer  no  trace  of  the  covering  remains.  The  White  Poplar  (Populus  alba), 
pear-trees,  and  mountain-ashes  behave  in  like  manner.  Horse-chestnut  leaves, 
when  they  make  their  way  through  the  brown,  loosened  bud-scales,  are  thickly 
covered  with  wool,  but  during  the  spring  they  lose  this  so  completely  that  only 
here  and  there  on  the  fully-formed  leaves  can  remnants  still  be  observed  clinging 
to  the  leaf.  It  is,  however,  not  only  woolly  coverings  that  are  later  either  partially 
or  wholly  thrown  off  as  superfluous.  On  the  foliage-leaves  of  the  already- 
mentioned  Wayfaring  Tree  ( Viburnum  Lantana)  appear  felted  stellate  hairs 
which  fall  off  as  soon  as  the  epidermis  is  sufficiently  thickened.  In  a  species  of 
Rhubarb  (Rheum  Ribes)  brittle,  candelabra-like,  short-branched  trichomes  are 
situated  on  the  edge  of  the  leaf,  which  is  much  crumpled  at  an  early  stage,  and 
later,  when  of  no  further  use,  they  break  away  in  pieces  and  fall  off.  Again,  in 
many  mulleins  (e.g.  Verbascum  pulverulentum  and  granatense),  there  are  branched, 
shrub-like  hair-structures  which  become  detached  from  the  surface  of  the  fully- 
developed  leaves,  and  are  carried  away  in  loose  flakes  by  the  wind. 

The  covering  of  the  young  leaves  of  the  Beech  (Fagus  silvatica)  consists  of 
silky  hairs,  and  the  way  in  which  these  are  arranged  and  utilized  is  so  peculiar  that 
it  is  worth  while  to  inquire  further  into  the  details.  At  first  sight,  the  under 
surface  of  the  young  beech-leaf  appears  to  be  entirely  covered  with  silky  hair; 
on  a  closer  examination,  however,  it  is  seen  that  the  hairs  are  only  inserted  on 
the  margins  and  on  lateral  veins,  and  that  the  green  portions  of  the  leaf  are  in 
reality  perfectly  smooth  and  free  from  hairs.  Since  the  green  portions  of  the 
leaf  are  thrown  into  deep  folds  (see  figs.  92  4  and  92 5),  and  the  veins  are  still 
close  to  one  another,  while  the  tops  of  the  silky  hairs  springing  from  these  veins 
reach  far  beyond  the  vein  next  to  them,  all  the  furrowed  depressions  caused  by 
the  folding  are  completely  covered  over.  Each  groove  is  bridged  over  by  the 
hairs,  which  are  regularly  arranged,  side  by  side,  parallel  to  one  another;  thus 
the  leaf  appears  to  be  clothed  completely  in  a  delicate  silken  coat.  There  can 
be  no  doubt  as  to  the  function  of  these  hairs.  The  green  tissue  overspanned  by 
them  is  protected  from  the  sun  until  its  epidermis  is  sufficiently  thickened,  and 
when  this  is  the  case  the  folds  flatten  out  (fig.  92 6)  and  the  leaf  assumes  a 
horizontal  instead  of  a  vertical  position,  thus  turning  the  lower  surface  away 
from  the  sun,  and  rendering  the  hairs  of  no  further  use.  They  have  become 


FALL   OF   THE   LEAF.  355 

superfluous,  and  usually  fall  off— or,  if  they  still  remain  on  the  lateral  veins,  they 
are  shrivelled,  insignificant,  and  meaningless. 

The  dry  membraneous  scales  seen  on  young  fern-leaves  should  be  mentioned 
here.  Let  us  examine  a  frond  of  the  first  wild  fern  we  meet— say  of  Nephrodium 
Filix-mas.  The  young  frond  is  still  spirally  rolled,  although  it  has  forced  its  way 
through  the  soil,  and  is  now  exposed  to  the  wind.  Moreover,  nothing  is  to  be  seen 
of  the  fresh  green  which  later  adorns  this  fern;  the  lower  part  of  the  midrib  and 
lateral  veins  appear  to  be  strewn  with  chaff,  being  entirely  covered  with  dry 
membraneous  brown  scales  and  shreds.  Later,  as  the  leaf  unrolls  more  and  more, 
its  green  fronds  also  become  expanded,  but  by  this  time  the  cell-walls  are  sufficiently 
strengthened,  and  no  longer  require  the  chaffy  coat.  In  ferns  which  "grow  in 
sunny,  rocky  situations,  and  as  epiphytes  on  the  fissured  bark  of  old  trees  in 
tropical  regions,  this  coat  of  chaffy  scales  is  even  more  noticeable,  and,  as  stated 
earlier,  in  such  plants  it  persists  throughout  life. 


FALL  OF  THE  LEAF. 

Just  as  many  phenomena  of  the  sprouting  and  unfolding  of  foliage  are  dependent 
upon  transpiration  at  the  beginning  of  the  vegetative  period,  so  many  processes, 
but  chiefly  that  of  the  fall  of  the  leaf,  stand  in  causal  connection  with  transpiration 
at  the  close  of  that  period.  Sooner  or  later,  of  course,  the  activity  of  each  leaf 
entirely  ceases;  it  dies,  becomes  detached  from  the  stem  to  which  it  has  rendered 
service,  and  falls  to  the  ground,  where  it  decays.  In  districts  where  the  vegetation 
can  continue  its  activity  uninterruptedly  throughout  the  year,  there  is  nothing  very 
noticeable  about  the  fall  of  the  leaf.  As  a  rule,  as  the  new  leaves  arise  below  the 
growing  apex  of  the  shoot,  the  lower,  older  leaves  wither  up  and  decay;  the  fall 
is  quite  gradual,  and  takes  place,  like  the  development  of  new  leaves,  all  through 
the  year.  In  neighbourhoods,  however,  where  the  changes  of  climate  prevent  the 
uninterrupted  activity  of  plants  throughout  the  year,  it  is  essentially  different. 
Trees  and  shrubs,  and  many  smaller  plants,  shed  the  whole  of  their  foliage  in  a  few 
days  at  certain  annually-recurring  periods,  and  then  remain  with  bare  branches  for 
a  considerable  time,  apparently  quite  lifeless.  This  is  the  case  in  regions  where  a 
long,  hot,  dry  period  follows  the  short  rainy  season,  and  also  in  very  cold  districts 
where  the  long-continued  frost  causes  an  icy  winter,  and  the  plants  are  locked  in  a 
deep  sleep.  In  tropical  and  sub-tropical  regions,  where  no  showers  occur  for  many 
months  at  a  time,  the  branches  become  stripped  of  their  leaves.  Even  at  the  begin- 
ning of  the  dry  hot  season,  they  remain  apparently  dead  for  months,  but  again 
break  out  into  leaf  at  the  commencement  of  the  cooler  rainy  season,  when  invigor- 
ating moisture  is  supplied  to  the  parched  ground.  On  the  other  hand,  in  those 
regions  of  the  temperate  zone  in  which  there  is  no  sharp  distinction  between  the 
rainy  and  dry  seasons,  and  rain  falls  every  month,  the  foliage  is  stripped  from  the 
trees  at  the  beginning  of  the  cold  period,  and  after  the  winter  is  over,  fresh  green 
leaves  once  more  burst  from  the  buds  on  the  branches. 


FALL   OF   THE   LEAF. 

It  certainly  appears  strange  that  the  leaf-fall  should  be  sometimes  connected 
with  the  approach  of  cold,  and  sometimes  with  that  of  hot  weather.  And  yet  this 
is  the  fact.  Heat  and  cold  are  only  the  indirect  causes;  the  primary  cause  of  the 
fall  of  the  leaf  is  the  danger  threatened  to  the  plant  by  the  continuance  of  transpira- 
tion when  either  heat  or  cold  is  excessive.  The  danger  of  transpiration  during  con- 
tinued dryness  of  soil  and  air  scarcely  requires  much  explanation.  The  conditions 
may  be  summed  up  in  a  few  words:  the  throwing  off  of  the  transpiring  surfaces  when 
the  drought  commences,  and  the  temporary  stoppage  of  the  sap-current — i.e.  the  so- 
called  "  summer  sleep  " — furnish  one  of  the  best  protective  measures  in  plants  sur- 
rounded by  air  against  excessive  transpiration  and  withering.  It  is  more  difficult 
to  explain  the  connection  between  the  fall  of  the  leaf  and  the  commencement  of  the 
cold  period.  This  is  best  indicated  by  some  culture  experiments  which  illustrate 
these  relations.  When  the  soil,  in  which  are  cultivated  plants  with  actively  trans- 
piring leaves  (melons,  tobacco,  and  the  like),  is  cooled  down  to  a  few  degrees  above 
zero,  the  leaves  after  a  short  time  become  faded,  even  although  the  temperature  of 
the  air  and  the  humidity  of  both  soil  and  air  are  entirely  favourable.  By  the 
lowering  of  temperature  in  the  soil,  the  absorbing  activity  of  the  roots  buried 
therein  is  so  reduced  that  the  water  which  is  lost  by  transpiration  from  the  foliage- 
leaves  can  no  longer  be  replaced.  The  leaves  wither,  dry  up,  turn  brown  or  black, 
and  appear  to  be  burnt  or  charred.  In  the  ordinary  language  of  gardeners  they  are 
said  to  be  "  frozen " — frozen  at  a  temperature  above  the  freezing  point,  which 
phenomenon  is  said  to  be  due  to  the  peculiar  sensitiveness  of  these  plants.  It  is 
incorrect  to  speak  of  freezing  in  this  case,  however.  The  plants  are  in  reality  dried 
up  by  reason  of  the  low  temperature  of  the  soil  and  consequent  lessening  of  the 
stream  of  fluid  up  to  the  transpiring  foliage-leaves.  In  regions  which  annually  pass 
through  a  long  period  of  cold,  the  leaves  of  the  plants  are  as  liable  to  be  dried  up 
by  the  cooling  of  the  soil  round  their  roots  when  winter  approaches,  as  are  the  trees 
in  the  catingas  of  Brazil  when  the  hot  dry  season  commences.  They  also  denude 
themselves  of  their  leafy  raiment  as  these  do,  since  otherwise  they  would  be 
unable  to  make  good  the  water  exhaled  by  the  leaves.  When  the  temperature  of 
the  air  sinks  below  zero,  frost  ensues,  and  the  water  in  the  plant  stiffens  into  ice, 
this  hastens  the  fall  of  the  leaf,  but  it  was  already  partially  accomplished  before  the 
frost  set  in,  and  where  the  leaves  still  cling  to  the  branches,  preparations  are  already 
made  for  their  detachment,  which  is  brought  about  by  the  limitation  of  transpira 
tion.  It  must  not  be  concluded  from  this  that  plants  foresee  the  approach  of  winter, 
and  that  the  preparations  for  the  fall  of  the  leaves  result  from  such  an  intelligent 
foresight;  the  phenomenon  is  much  more  easily  explained  on  the  assumption  that 
in  a  climate  which  renders  necessary  a  long  cessation  of  transpiration,  those  plants 
flourish  and  multiply  best  whose  natural  characteristic  is  to  follow  a  period  of 
energetic  work  by  a  long  season  of  rest.  The  ultimate  cause  of  this  instinctively 
adaptive  periodicity  is  certainly  not  yet  explained;  it  is  as  mysterious  as  those  life 
processes  and  phenomena  which  regularly  recur  at  certain  periods,  which  are  perhaps 
hastened  or  retarded  by  favourable  or  unfavourable  external  conditions,  but  cannot 


FALL   OF   THE   LEAF.  357 

be  stopped  by  them,  and  which  the  plant  carries  out,  or  endeavours  to  carry  out, 
without  immediate  external  stimulus. 

It  is  highly  interesting,  with  respect  to  the  acceleration  or  retardation  of  the  leaf- 
fall,  to  observe  how  the  same  species  of  plant  will  behave  under  various  favourable 
or  retarding  external  influences;  or  how,  in  each  region  and  locality,  a  selection  has 
been  made  to  a  certain  extent  of  the  plants  best  adapted  to  the  given  conditions. 
First  it  is  to  be  noticed  that,  under  otherwise  similar  circumstances,  the  foliage 
remains  green  for  a  longer  time,  and  is  retained  longer  on  the  branches  in  places 
where  the  soil  and  air  are  more  humid.  In  damp,  shady,  wooded  glens,  not  only 
ferns,  but  the  leaves  of  birches,  beeches,  and  aspens  are  still  green  while  on  the 
sunny  hillocks  close  at  hand  the  brown  leaves  flutter  down  on  to  the  withered 
fronds  of  the  Bracken  Fern. 

The  most  remarkable  fact,  however,  is  that  in  elevated  mountain  regions  a  plant 
loses  its  leaves  much  earlier  than  does  the  same  species  growing  in  the  lowlands. 
From  the  fact  that  in  the  Alps,  the  larches  and  whortleberry  bushes,  on  the  upper 
limits  of  the  woods,  put  forth  their  green  needles  and  leaves  about  a  month  later 
than  in  the  valleys  at  a  height  of  600  metres  above  the  sea,  it  would  naturally  be 
expected  that  this  considerable  delay  would  be  compensated  for  by  a  corresponding 
postponement  of  the  ending  of  the  year's  work,  and  that  the  fall  of  the  foliage  on 
the  upper  limits  of  the  wood  would  also  be  postponed  for  about  a  month.  But  this 
is  far  from  being  the  case.  The  same  species  of  larch  which  becomes  green  a  month 
later,  up  on  the  mountain  slopes,  also  turns  yellow  a  month  earlier  in  the  autumn. 
While  the  whortleberry  bushes  in  the  depths  of  the  valley  are  still  adorned  with 
dark-green  leaves,  the  same  species  growing  in  the  glades  on  the  upper  limit  of  the 
wood,  already,  from  the  valley,  appear  to  be  shrouded  in  deep  crimson.  Their  leaves 
are  becoming  discoloured  above,  and  are  withering  and  dropping  from  the  twigs. 
The  explanation  of  this  phenomenon  follows  naturally  from  what  has  just  been  said. 
In  the  high  mountain  regions  where  tall  trees  find  their  uppermost  limit,  the  ground 
is  frequently  covered  with  frost  at  the  end  of  August;  snow  falls  regularly  in  the 
first  half  of  September,  and  although  this  may  be  melted  in  snnny  places,  the  soil 
is  nevertheless  thoroughly  cooled  by  the  water  so  produced.  The  days  rapidly 
become  shorter,  and  the  sunbeams  can  no  longer  replace  the  heat  lost  by  radiation 
in  the  lengthened  nights.  The  temperature  of  the  soil  in  which  the  plants  are 
rooted  consequently  falls  rapidly,  and  the  immediate  results  are  that  the  absorbent 
roots  stop  working,  the  decolorization  progresses,  and  the  foliage-leaves,  which  are 
no  longer  able  to  repair  the  loss  caused  by  transpiration,  wither  and  fall  away. 
Accordingly,  on  this  upper  tree  limit,  only  those  larches  and  whortleberry-bushes 
can  thrive  which  are  organized  to  commence  their  year's  work  a  month  later, 
and  to  finish  it  a  month  earlier,  than  those  which  have  taken  up  their  po* 

1400  metres  below. 

This  obviously  applies  not  only  to  the  larches  and  whortleberries,  cited  her 
examples,  but  to  all  other  plants  whose  range  of  distribution  extends  from  the 
lowlands  up  to  the  wood  limit  on  the  slopes  of  the  mountains.     It  also  applies 


358  FALL    OF   THE    LEAF. 

further  to  those  plants  which  have  a  wide  horizontal  distribution;  for  example, 
to  those  which  grow  wild  or  are  cultivated  from  the  lowlands  at  the  northern 
foot  of  the  Alps  to  South  Italy,  and  even  further  south,  on  the  further  side  of  the 
Mediterranean.  By  journeying  southwards,  it  will  be  seen  that  the  beeches  and 
elms  which,  on  the  northern  foot  of  the  Alps  near  Vienna,  lose  their  colour  in 
the  beginning  of  October,  are  never  discoloured  before  November  on  the  moun- 
tains of  Madeira,  and  that  whilst  the  planes  already  show  leafless  branches  in  the 
North  Tyrolese  valleys  at  Innsbruck,  they  retain  their  leaves  (although  these  are 
turning  yellow)  on  the  mild  shores  of  Lake  Garda  at  the  southern  foot  of  the  Alps. 
In  Palermo  they  are  still  adorned  with  dark-green  foliage.  Planes,  indeed,  in 
certain  instances  remain  green  all  winter  in  Greece,  and  thus  far  it  was  no  myth 
when  Pliny  spoke  of  evergreen  planes.  The  Elder,  which  in  the  north  is  a  deciduous 
plant,  in  Poti,  on  the  Black  Sea,  retains  its  green  leaves  through  the  whole  winter. 
In  the  oases  of  the  North  African  deserts  the  Peach-tree  keeps  its  foliage  fresh 
and  green  from  one  vegetative  period  to  another,  and  while  the  blossom  of  this  tree 
in  Central  and  South  Europe  unfolds  on  branches  which  have  lost  their  foliage  in 
the  previous  autumn,  in  the  oases  the  flowers  are  situated  amongst  the  still  green 
leaves  of  the  last  period  of  vegetation.  It  may  be  confidently  assumed  that  here 
also  the  cause  is  the  temperature  and  humidity  of  the  ground,  and  that  the  planes 
and  peaches,  whose  roots  at  the  end  of  autumn  and  winter  are  buried  in  a  damp 
and  relatively  warm  soil,  are  the  last  to  throw  off  their  foliage. 

From  all  these  considerations  it  cannot  be  doubted  that  the  stripping  of  the 
foliage  depends  upon  the  stoppage  of  transpiration,  and  primarily  upon  the  dry- 
ing-up  of  those  sources  from  which  the  transpiring  leaves  derive  their  water. 
Plants  which  denude  themselves  of  their  foliage  of  course  lose  with  it  much  organic 
material,  for  whose  production  they  have  toiled  for  months;  but  this  loss  will  stand 
no  comparison  with  the  advantages  gained  by  the  abscission  of  the  leaves.  In 
reality,  it  is  only  a  framework  of  empty  cells — the  dead  envelopes  of  the  living 
portion  of  the  plant — which  is  thrown  away.  The  protoplasm  has  opportunely 
withdrawn,  the  plastids  which  carried  on  their  activity  in  the  cells  of  the  foliage 
have  migrated  thence  and  taken  up  winter  quarters  in  other  sheltered  parts  of  the 
plant — in  the  stem,  roots,  or  tubers,  and  have  there  deposited  everything  which 
will  be  of  use  in  the  following  year,  such  as  starch,  sugar,  &c.  The  empty  cells 
can  thus  be  easily  sacrificed  to  the  common  weal.  The  leaves  fall  to  the  ground, 
where  they  decay  and  help  to  form  natural  mould,  of  which  the  posterity  of  the 
deciduous  plants  reap  the  benefit.  Since,  by  the  formation  of  albuminous  com- 
pounds in  the  leaves,  an  abundance  of  calcium  oxalate  arises  which  is  of  no  further 
use  to  the  plant,  and  is  consequently  stored  up  in  such  quantity  at  the  end  of 
summer  that  it  at  last  becomes  burdensome  to  the  plants,  the  throwing  off  of  the 
foliage  must  really  be  regarded  as  a  method  of  removing  waste  materials,  and  may 
be  compared  to  the  excretion  of  waste  which  occurs  in  animals. 

Finally,  it  should  be  noted  that  only  plants  whose  foliage  lies  flat  on  the  ground, 
or  whose  branches  and  twigs  are  very  elastic  and  bear  needle-shaped  leaves,  are 


FALL   OF   THE   LEAF.  359 

unharmed  by  the  pressure  of  snow.  Trees,  bushes,  and  shrubs  with  broad  out- 
spread leaves,  such  as  planes,  maples,  limes,  beeches,  and  elms,  are  not  capable  of 
supporting  the  weight  of  snow  lying  on  their  large  leaf-surfaces.  When,  as 
occasionally  happens,  mountain  and  valley  are  covered  in  snow  in  the  autumn 
before  the  leaf -fall  has  commenced,  or  when,  late  in  the  spring,  to  the  terror  of  the 
farmer,  snow  falls  on  wood  and  meadow  after  the  young  leaves  have  attained  to  a 
considerable  size,  the  devastation  produced  is  fearful.  The  large-leaved  shrubs  are 
pressed  down  and  their  stems  broken.  Branches  as  thick  as  one's  arm  and  huge 
tree-trunks  are  shattered,  and  in  the  woods  quantities  of  maples  and  beeches  are 
felled,  or  even  uprooted.  Such  devastation  would  recur  every  year  in  regions 
with  snowy  winters  if  the  leafy  trees  did  not  strip  off  their  foliage  in  time,  and 
it  can  easily  be  imagined  what  would  happen  to  the  woods  after  a  series  of  such 
catastrophes. 

There  is,  consequently,  a  widespread  idea  that  the  autumnal  leaf-fall  is  brought 
about  by  frost.  This  idea  is  founded  on  the  observation  that  when  the  temperature 
in  October  and  November  falls  below  zero,  quantities  of  leaves  drop  from  the 
branches  in  the  early  hours  following  the  cold  bright  nights.  Though  it  can 
scarcely  be  denied  that  the  fall  of  the  leaf  is  in  some  measure  connected  with  frost, 
still  that  it  is  not  always  the  immediate  cause,  is  demonstrated  by  the  fact  that 
when  plants  with  leafy  branches  are  exposed  at  the  end  of  August  or  beginning  of 
September  to  a  temperature  below  zero  the  leaves  do  not  fall  immediately;  while, 
on  the  other  hand,  the  foliage  of  limes,  elms,  maples,  cherry-trees,  &c.,  is  at  last 
stripped  off  in  the  autumn  even  though  no  frost  has  occurred.  It  can  only  be  said, 
therefore,  as  already  stated,  that  frost  is  favourable  to  the  fall  of  the  leaf,  and 
that  it  hastens  the  commencement  of  the  process;  but  not  that  the  detachment 
of  the  foliage  is  brought  about  by  its  sole  agency. 

The  detachment  of  the  leaves  from  the  branches  is  brought  about  by  the 
formation  of  a  peculiar  layer  of  cells,  from  the  co-operation  of  a  special  tissue,  which 
has  been  termed  the  layer  of  separation.  As  a  rule,  leaves  cannot  detach  them- 
selves without  the  previous  formation  of  this  tissue,  not  even  if  they  are  exposed 
for  a  long  time  to  a  very  low  temperature,  and  the  sap  in  their  cells  and  vessels  is 
stiffened  into  ice.  That  portion  of  the  leaf  in  which  the  separation  is  to  take  place 
is  made  up  of  a  strong  tough  tissue,  and  the  mechanical  alterations  produced  by  the 
frost  would  not  suffice  to  complete  the  rupture.  The  separation-layer,  on  the  other 
hand,  which  is  formed  within  this  tissue  in  one  or  several  definite  places,  consists  of 
succulent  parenchymatous  cells,  whose  walls  are  so  constructed  that  they  are  easily 
separated  by  mechanical  or  chemical  agencies,  thus  rendering  possible  a  disintegra- 
tion of  the  cell-tissue.  The  incitement  to  the  construction  of  a  layer  of  separation 
is  indeed  usually  the  limitation  of  transpiration  by  the  gradual  cooling  of  the 
ground,  and  the  cessation  of  the  absorbing  power  of  the  roots  in  those  regions 
which  experience  a  cold  winter.  As  soon  as  this  restriction  of  transpiration 
commences-and  it  varies  very  much,  as  shown  in  the  previous  discussion,  with  the 
latitude  and  altitude  of  the  region  in  question-thin-walled  cells  arise  m  the  lower 


FALL  OF  THE   LEAF. 

portion  of  the  leaves  and  leaflets,  which  rapidly  increase  by  division,  and  in  a  short 
time  form  a  zone,  readily  to  be  distinguished  from  the  thick  older  tissue  by  its 
lighter  tint  and  by  the  fact  that  it  is  somewhat  transparent.  Usually  this  zone 
is  formed  in  the  petiole,  and  at  those  places  where  the  vascular  bundles  become 
narrowed  in  passing  from  the  twig  to  the  leaf-blade,  there  to  divide  up  into  the  ribs 
and  veins.  The  growing  tissue  is  inserted  just  at  this  place;  it  actually  presses  and 
tears  the  other  older  cells  apart,  and  even  causes  a  rupture  between  them.  As  soon 
as  the  separation-layer  has  attained  its  proper  thickness,  its  thin-walled  cells 
separate  from  one  another,  but  so  as  not  to  injure  or  burst  their  membranes  in 
any  way.  It  seems  that  the  so-called  middle  lamella  of  the  cell- wall  is  dissolved  by 
organic  acids,  and  that  thus  the  continuity  between  the  cells  of  the  separation-layer 
is  destroyed.  The  most  trifling  cause  will  now  effect  a  splitting  in  the  loose  tissue 
and  a  fracture  between  the  cells  of  the  separation-layer;  and  when  no  other  external 
shock  follows,  the  detachment  ultimately  takes  place  of  itself,  the  weight  of  the 
leaf  helping  to  bring  about  a  complete  severance.  As  a  rule,  however,  the  fall  of 
the  leaf  is  hastened  by  external  influences.  Every  gust  of  wind  brings  down  the 
leaves;  the  alterations  in  volume  dependent  on  the  frost  and  chill  and  the  subse- 
quent thawing  of  the  cell-sap,  aid  the  severance  and  also  hasten  the  tearing  of 
vascular  bundles  which  are  still  entire;  and  thus  it  happens  that  thousands  of 
leaves  fall  to  the  ground  even  in  the  absence  of  wind,  especially  when,  after  a 
frosty  night,  the  rising  sun  illuminates  the  autumn-tinted  leaves,  and  dissolves  the 
frozen  sap. 

The  region  where  the  separation  is  effected  is  usually  sharply  marked  off,  and  it 
looks  as  if  the  leaves  and  leaflets  had  been  cut  through  with  a  knife.  The  severed 
surfaces  present  a  variety  of  contours,  according  to  the  shape  of  the  leaf -stalk. 
Sometimes  it  is  horseshoe-shaped,  sometimes  triangular  or  rounded,  or  it  reminds 
one  of  a  trefoil-leaf,  and  sometimes  it  has  an  annular  shape.  The  stalk  of  the 
plane-tree  leul'  has  at  the  base  a  conical  swelling  which  incloses  a  bud;  when  the 
leaf  falls  a  fissure  is  formed  entirely  going  round  it.  Many  of  the  separation 
surfaces  of  the  leaf-stalks  are  like  the  articular  surfaces  of  the  long  bones  in 
the  human  skeleton  (of  the  radius,  tibia,  and  at  the  elbow).  Vine  leaves  form 
two  layers  of  separation,  one  close  to  the  stem  at  the  base  of  the  leaf-stalk— 
the  other  at  the  upper  end  of  the  leaf -stalk  immediately  below  the  blade.  In  the 
palmate  leaves  of  the  Horse-chestnut  and  Virginian  Creeper  (Ampelopsis),  in  the 
compound  leaves  of  Spircea  Aruncus,  in  the  pinnate  leaves  of  the  Chinese  Tree  of 
Heaven  (Ailanthus  glandulosa),  and  in  the  bipinnate  leaf  of  the  North  American 
Gymnocladus  Canadensis,  a  small  separation  layer  arises  below  each  leaflet,  and  a 
larger  one,  in  addition,  at  the  base  of  the  leaf -stalk.  Such  leaves,  consisting  of 
several  leaflets,  collapse  like  houses  built  of  cards  when  touched,  and  under  the  trees 
late  in  the  autumn  lies  a  confused  heap  of  leaflets  and  leaf-stalks,  the  latter  some- 
times looking  like  long  rods  (as,  for  example,  in  the  Ailanthus  and  Gymnodadus), 
sometimes  almost  like  long  bones  (as  in  the  Horse-chestnut,  fig.  93). 

Frequently  the  layer  of  separation  is  so  situated  on  the  leaf -stalk  that  after  the 


FALL   OF   THE   LEAF. 


361 


detachment  a  small  portion  of  the  stalk  remains  on  the  branch.  This  is  the  case  in 
the  so-called  Syringa,  or  Mock  Orange  (Philadelphus),  where  the  scale-like  part 
which  is  left  has  to  protect  the  bud  situated  just  above  the  leaf -stalk. 

In  some  trees  and  shrubs  defoliation  is  very  rapid,  in  others  only  gradual.  In 
the  Japanese  Maidenhair  Tree  (Ginkgo  biloba),  the  formation  of  the  separation-layer 
and  the  detachment  of  the  leaves  is  completed  in  a  few  days;  in  hornbeams  and 
oaks  the  stripping  of  the  foliage  continues  for  weeks,  and  frequently  only  a  portion 


Fig.  93. -Leaf -fall  of  the  Horse-chestnut  (jEsculus  Hippocastanum). 

of  the  dead  leaves  is  thrown  off  in  the  autumn,  the  remainder  not  until  the  close  of 
the  winter. 

It  is  also  worthy  of  remark  that  in  some  trees  the  leaf-faU  begins  at  the  end  of 
the  branches  and  gradually  proceeds  towards  the  base,  while  in  others  the  contrary 
is  the  case.  In  ashes,  beeches,  hazels,  and  hornbeams,  the  apices  of  the  branches  are 
leafless  when  the  lower  parts  stiU  bear  firmly-fixed  foliage;  in  limes,  willows, 
poplars,  and  pear-trees,  on  the  other  hand,  the  lower  portions  of  the  branches  are 
seen  to  lose  their  leaves  early  in  the  autumn,  the  denudation  gradually  extending 
upwards;  on  the  extreme  ends  of  the  branches  some  leaves,  as  a  rule,  obstinately 
remain  for  a  long  time,  until  they  also  are  at  length  whirled  away  by  the  first 
snowstorm. 


THE   VASCULAK  TISSUES   AND   TRANSPIRATION. 


CONNECTION  BETWEEN  THE  STRUCTUKE  OF  THE  VASCULAR  TISSUES 

AND  TRANSPIRATION 

It  is  naturally  to  be  expected  that  between  the  contrivances  regulating  transpira- 
tion in  the  immediate  neighbourhood  of  the  green  tissue,  and  those  mechanisms  which 
effect  the  transport  of  the  crude  sap  from  the  roots,  through  the  stem  and  branches, 
up  to  the  region  of  this  transpiring  tissue,  a  mutual  co-operation  will  exist. 

Where  much  water  is  exhaled  from  the  surface,  much  water  must  be  supplied, 
and  in  tracts  leading  to  extensive  and  strongly-transpiring  leaf-blades,  the  fluid 
moves  more  quickly  than  in  a  conducting  apparatus  leading  to  green  tissue,  which 
transpires  but  slowly  and  to  a  small  extent.  In  pines,  whose  stiff  acicular  leaves 
transpire  but  little,  the  raw  food-sap  moves  much  more  sluggishly  than  is  the  case 
with  maples,  whose  flat  leaves  give  off  large  quantities  of  water  in  the  form  of 
vapour.  The  quickest  movement,  however,  is  to  be  found  in  twining  and  climbing 
plants,  whose  stems,  a  few  centimetres  in  thickness,  may  attain  to  a  length 
exceeding  100  metres.  This  is  the  case  in  those  peculiar  climbing  palms,  which  at 
first  wind  over  the  ground  in  numerous  snake-like  coils,  and  then  rise  to  the  tops 
of  the  highest  trees,  and  unfold  their  leaves  there  in  the  sunshine.  Climbing  palms 
(Rotang)  are  known  whose  stems  actually  attain  a  length  of  180  metres,  and  which, 
when  they  have  reached  the  summit  of  the  trees  after  numerous  windings,  become 
erect  and  extend  their  larger  pinnate  leaves  just  like  the  straight-stemmed  palms. 
The  illustration  opposite  (fig.  94)  depicts  in  the  background  the  edge  of  a  wood  up 
whose  trees  have  climbed  examples  of  such  a  species  of  Rotang. 

Many  hours  of  the  day  may  pass,  when,  on  account  of  a  clouded  sky  and  the 
great  humidity  of  the  air,  the  transpiration  in  the  wide-spreading  leaves  above  the 
tops  of  the  trees  will  be  extremely  little;  but  when  the  sun  shines  brightly  and  the 
leaves  become  thoroughly  warmed,  a  large  quantity  of  water  vapour  must  be 
exhaled  in  a  very  short  time.  This  quantity  of  water  must  be  replaced,  and  very 
quickly,  but  by  means  of  a  stem  180  metres  long  and  only  some  centimetres  thick. 
In  order  to  render  the  replacement  possible,  everything  which  might  hinder  the 
rapid  movement  of  the  water  and  its  dissolved  food-stuffs  on  its  long  journey, 
especially  the  resistance  of  the  conducting  tubes,  must  be  minimized  as  much  as 
possible.  The  forward  movement  of  fluids  in  a  channel  is,  however,  rendered 
more  difficult  as  the  tube  narrows,  because  in  a  narrower  tube  a  relatively  larger 
amount  of  the  fluid  adheres  to  the  inner  surface,  and  therefore  it  is  necessary,  in 
order  to  obtain  a  rapid  movement,  that  this  adhesion  be  reduced  as  far  as  possible. 
This  is  most  simply  effected  by  widening  the  channel,  since  the  adherent  surface 
is  thus  diminished  in  comparison  with  the  large  amount  of  the  fluid  passing 
through.  As  a  matter  of  fact,  in  the  stems  of  climbing  palms  relatively  very  wide 
tubes  are  to  be  seen,  through  which  a  large  quantity  of  fluid  can  be  brought 
from  the  roots  to  the  transpiring  leaf-surfaces  in  a  very  short  time,  and  this 
actually  occurs.  The  climbing  palm,  Calamus  angustifolius,  has  conducting  tubes 


THE    VASCULAR   TISSUES   AND   TRANSPIRATION.   * 

of  more  than  \  mm.  diameter,  and  in  the  species  of  Rotang  illustrated  in  fig.  94 
they  are  almost  as  wide. 


Fig.  94.— Indian  Climbing  Palms  (Rotang).    From  a  photograph. 

What  has  been  stated  here  with  especial  regard  to  the  Rotang  or  Climbing  Palm 
applies  also  to  all  other  twining  and  climbing  plants  known  by  the  name  of  lianes, 
and  their  sap-conducting  tubes  are  the  wider,  the  longer  their  stems  and  the  larger 


364  THE   VASCULAR   TISSUES   AND   TRANSPIRATION. 

their  transpiring  leaves.  In  very  many  lianes  the  cavities  of  the  conducting  vessels 
can  be  plainly  seen  with  the  naked  eye.  This  is  the  case,  for  example,  in  the  cross- 
section  of  the  liane  represented  in  natural  size  in  fig.  95  5.  A  diameter  of  }  mm.  is 


Fig.  95.—  Lianes. 


»  Portion  of  tne  stem  of  a  tropical  Aristolochia.  *  Cross  section  of  a  liane-like  Aristolochia.  »  Menispermum  Carolinianum. 
*  Cross  section  of  the  twining  stem  of  Menisvermum  (magnified).  5  Portion  of  a  liane  (probably  an  Asclepiad)  gathered  in 
a  tropical  forest  ;  nat.  size. 

not  at  all  rare  in  passion-flowers  and  aristolochias,  and,  generally  speaking,  in  most 
twining  and  climbing  plants;  whilst  in  many  lianes  the  conducting  tubes  have 
even  been  observed  to  be  0'7  mm.  in  diameter. 


THE   VASCULAR  TISSUES  AND  TRANSPIRATION. 


365 


Fig.  96.—  Aroids  (Philodendron  pertusum  aiid  Philodendron  Imbe)  with  co 


3(56  THE   VASCULAR  TISSUES   AND   TRANSPIRATION. 

A  particularly  noticeable  method  of  conducting  water  from  the  soil  to  the  green 
leaf-blades  is  exhibited  by  some  large-leaved  tropical  Aroids  which  climb  up  trees, 
and  are  provided  with  aerial  roots.  These  plants  have  really  two  kinds  of  aerial 
roots,  viz.:  shorter  ones,  which  are  at  right  angles  to  the  stem,  by  means  of  which 
they  climb  up  their  support,  usually  old  tree-trunks;  and  longer  ones,  passing 
down  perpendicularly  to  the  ground  like  ropes  or  strings.  In  the  Mexican 
Tornelia  fragrans  (Philodendron  pertusum)  represented  in  fig.  96,  these  latter 
roots  attain  a  length  of  4-6  metres  and  a  diameter  of  1-2  cm.  They  are  of 
uniform  thickness,  brown,  smooth,  unbranched,  and  quite  straight.  As  soon  as 
they  reach  the  ground,  the  tip  bends  round  almost  at  a  right  angle,  and  sends 
a  number  of  lateral  roots  which  are  covered  with  an  actual  fur  of  root-hairs  into 
the  soil.  The  bent  end  then  enters  the  soil  for  a  short  distance,  and  thus  the 
entire  aerial  root  is  rendered  fairly  tense.  As  a  rule,  two  such  cord-like  aerial 
roots  originate  below  each  new  leaf,  and  it  seems  as  if  this  arrangement  was 
specially  adapted  to  transport  the  necessary  food-sap  from  the  soil  to  the  large 
luxuriant  leaf  above  by  the  shortest  path.  But  it  not  only  seems  so,  for  this  is 
actually  the  case,  and  it  is  especially  remarkable  that  root-pressure  takes  a 
prominent  part  in  the  transport.  On  cutting  through  one  of  these  cord-like  aerial 
roots  about  a  span  above  the  ground,  watery  fluid  is  immediately  seen  to  ooze  from 
the  middle  of  the  cut  surface.  The  woody  portion  of  the  root,  which  here  forms  a 
central  strand,  contains  very  wide  conducting  tubes,  like  those  in  the  stems  of  lianes*, 
and  the  quantity  of  fluid  exuded  in  thirty-six  hours  amounts  to  as  much  as  17  grms. 
It  is  noteworthy  that  the  root-pressure  here,  according  to  all  appearances,  acts  with 
the  same  force  all  through  the  year.  In  the  vine  this  is  not  the  case.  Vines  which 
are  cut  through  in  the  summer,  it  is  well  known,  no  longer  weep;  the  cord-like 
aerial  roots  of  tropical  aroids,  on  the  other  hand,  weep  at  all  seasons  of  the  year 
when  cut  across.  Indeed,  the  vegetative  activity  is  never  entirely  interrupted  in 
these  plants  all  the  year,  and  it  should  be  remembered,  in  connection  with  this  fact, 
that  they  grow  in  places  where  the  air  and  soil  are  always  warm,  and  where  their 
humidity  is  only  subject  to  slight  variations.  It  may  happen  that  in  damp,  warm 
places  transpiration  from  the  leaves  ceases  for  a  time  entirely,  and  then  it  is  very 
necessary  that  the  amount  of  food-sap  should  be  forced  up  to  the  leaves  by  root- 
pressure  in  order  that  they  may  be  supplied  with  the  food-salts  they  require.  The 
water,  which  contained  dissolved  food-salts,  is  of  no  use  when  it  has  given  these  up, 
and  it  is  therefore  forced  out  of  the  stomata,  these  in  consequence  being  trans- 
formed into  water  pores. 

The  aerial  roots,  which  form  the  shortest  and  straightest  channels  for  con- 
ducting the  raw  food-sap  to  the  leaves,  are,  moreover,  of  great  importance  to  these 
tropical  aroids,  since  it  not  infrequently  happens  that  the  lower  portion  of  the 
stem  in  an  old  plant  dies  off,  leaving  the  upper  part,  which  is  fastened  to  the 
trunk  of  a  tree  by  the  earlier-mentioned  short  supporting  roots,  and  therefore  in 
no  direct  connection  with  the  ground.  The  supporting  roots  would  not  be  sufficient 
to  supply  the  fluid  food  required,  and  the  whole  plant  is  therefore  provided 


TRANSMISSION  OF  THE   FOOD-GASES.  367 

with  this  food  only  through  these  cord-like  aerial  roots  which  are  sent  down  into 
the  soil. 

These  few  examples  are  enough  to  show  that  the  construction  of  the  stem  and 
roots  stands  most  intimately  related  to  transpiration,  inasmuch  as  the  transpiring 
green  tissue  is  effected  by  the  structure.  But  since  the  construction  of  these  plant 
members,  i.e.  the  architecture  of  the  stem,  is  also  dependent  upon  various  other  vital 
processes  to  be  described  later,  it  would  not  be  fitting  to  discuss  their  relations  here 
in  detail,  and  their  treatment  must  be  postponed  until  a  later  section. 


5.  CONDUCTION   OF   FOOD-GASES   TO  THE  PLACES  OF 

CONSUMPTION. 

Transmission  of  the  food-gases  in  land  and  water  plants  and   in  lithophytes. — Significance  of 
aqueous  tissue  in  the  conduction  of  food-gases. 

It  has  been  repeatedly  pointed  out  that  a  division  of  labour  occurs  in  all  large 
plants,  so  that  one  portion  of  the  cells  provides  for  the  reception  of  water  and  food- 
salts,  another  for  that  of  food-gases,  and  yet  another  for  the  conduction  and  trans- 
mission of  fluid  and  gaseous  nourishment  to  the  places  where  they  are  consumed 

How  the  aqueous  food-salt  solutions  derived  from  the  soil  are  brought  to  the 
green  tissue,  what  contrivances  are  thereby  brought  into  action,  and  what 
phenomena  of  plant-life  are  related  to  this  conduction  have  been  discussed,  as  far 
as  practicable,  in  the  previous  pages,  and  it  now  only  remains  to  describe  the 
transmission  of  the  gaseous  food-materials.  This  is  far  more  simple  than  the 
conduction  of  the  solutions  of  food-salts.  The  most  important  of  the  food-gases  in 
question  are  carbonic  acid  and  nitric  acid.  Carbonic  acid  is  continually  being 
conducted  by  means  of  water  to  the  green  tissues.  The  shortest  passage  is  to  be 
found  in  aquatic  plants  whose  protoplasm,  provided  with  green  chlorophyll  and  in 
need  of  carbonic  acid,  is  only  separated  from  the  surrounding  water  by  a  thin 
cell-wall,  while  this  water  always  contains  carbonic  acid,  though  perhaps  only  in 
small  quantity.  Under  the  influence  of  sunlight,  the  groups  of  green  cells  in 
hydrophytes  form  a  centre  of  attraction  to  the  carbonic  acid,  which  is  sucked  up 
with  great  energy  from  the  surrounding  water,  passes  easily  through  the  cell-wall, 
and  so  comes  directly  into  the  neighbourhood  of  the  green  protoplasm,  i.e.  that 
place  where  its  decomposition  is  efiected.  The  green  cells  of  water  plants  therefore 
furnish  an  apparatus  for  both  absorbing  and  decomposing  carbonic  acid,  and 
usually  no  further  means  and  no  special  conduction  through  other  cells  are  required. 

In  lithophytes  it  is  otherwise.  Here  we  have  the  remarkable  fact  that  they  are 
only  active  at  times;  only,  that  is  to  say,  when  they  are  sufficiently  moistened  by 
rain,  dew,  and  mist,  and  are  to  some  extent  submerged  for  a  time  by  heavy  down- 
pours. In  dry  air  their  vital  activity  is  suspended;  they  then  adhere  to  the  rocks  like 


368  TRANSMISSION   OF   THE    FOOD-GASES. 

withered  turf  and  dry  scales,  as  if  dead.  But  as  soon  as  they  are  moistened,  or  can 
condense  moisture  from  the  air,  they  are  aroused  to  renewed  vitality,  and  then  suck 
up  with  great  eagerness  atmospheric  water,  which  always  contains  small  quantities 
of  carbonic  acid  gas,  and  also  traces  of  nitric  acid.  In  the  rock-inhabiting  mosses 
the  cells,  which  absorb  water  from  the  atmosphere  containing  carbonic  acid,  are  also 
those  in  which  the  decomposition  of  carbonic  acid  takes  place.  In  this  respect 
these  mosses  behave  exactly  like  aquatic  plants;  nor  is  it  perhaps  superfluous 
here  again  to  point  out  the  interesting  fact  already  mentioned,  that  there  are  mosses 
which  permanently  live  under  water,  and  there  behave  like  true  water  plants, 
though  they  are  able  equally  to  live  on  rocks,  where  they  remain  dried  up  for 
weeks  together,  and  only  resume  their  activity  when  wetted  by  rain.  It  is  to  be 
taken  for  granted  that  such  damp,  water-saturated  mosses  have  the  capacity  of 
absorbing  carbon  dioxide  from  the  surrounding  atmosphere.  The  carbon  dioxide  is 
changed  into  carbonic  acid  by  its  passage  through  the  cell-wall  saturated  with 
water.  Probably  it  is  only  when  carbonic  acid  is  dissolved  in  water  that  it  reaches 
the  active  protoplasm  in  the  cells  in  question.  In  lichens  the  carbonic  acid  which 
reaches  the  protoplasm  provided  with  chlorophyll  is  also  dissolved  in  water; 
however,  in  most  lichens  the  green  cells  do  not  come  in  contact  with  the  atmosphere, 
but  are  separated  from  it  by  a  layer  of  hyphal  threads.  Thus  the  conduction  to 
the  green  cells  takes  place  by  means  of  the  hyphal  layer  destitute  of  chlorophyll. 

In  land  plants  also  the  cells  which  are  filled  with  chlorophyll-bearing  protoplasm 
seldom  come  directly  into  contact  with  the  atmosphere;  usually  the  green  tissue  is 
surrounded  with  an  actual  mantle  of  water.  That  is  to  say,  the  cavity  of  each 
epidermal  cell  contains  very  watery  fluid,  or,  in  other  words,  in  the  fully-formed 
epidermal  cells  the  protoplasm  constitutes  merely  the  parietal  layers  without 
chlorophyll,  their  large  cavities  being  filled  with  water.  These  epidermal  cells  fit 
closely  to  each  other,  and  on  the  upper  side  of  the  leaf  are  only  rarely  interrupted 
by  stomata.  Usually  the  epidermis  on  the  upper  side  of  the  leaf  gives  rise  to  a 
layer  of  cells  with  clear  watery  contents,  directly  bordering  on  the  green  palisade 
tissue;  and  as  the  carbon  dioxide  of  the  atmosphere  has  to  pass  from  the  upper  side 
to  this  green  tissue,  it  must  first  of  all  pass  through  this  watery  cell-layer  of  the 
epidermis.  There  it  becomes  changed  into  carbonic  acid,  and  passes  from  this 
epidermal  sphere  of  activity,  not  in  the  form  of  gas,  but  dissolved  in  water,  to  the 
cells  of  the  palisade  tissue  below.  Since  the  green  palisade  tissue  under  the 
influence  of.  sunlight  uses  up  the  carbonic  acid  in  the  manufacture  of  organic 
material,  it  becomes  a  centre  of  attraction  for  this  acid  as  long  as  the  illumination 
continues.  At  first  the  carbonic-acid-bearing  contents  of  the  contiguous  cells  are 
eagerly  absorbed,  and  indirectly  carbon  dioxide  also  is  drawn  from  the  surrounding 
air  and  made  to  force  its  way  into  the  epidermal  cells.  The  cell-wall  offers  no 
great  resistance  to  this  entrance.  It  has  been  proved  that  carbonic  acid,  or  rather 
carbon  dioxide,  passes  very  easily  through  the  cell-wall.  According  to  all  this,  it  is 
evident  that  the  small  quantity  of  carbon  dioxide  is  drawn  from  the  air  by  the 
green  illumimated  tissue  of  the  leaves  and  stem,  that  carbon  dioxide  streams 


TRANSMISSION    OF   THE   FOOD-GASES.  369 

rapidly  towards  these  parts,  penetrates  into  the  epidermal  cells,  is  changed  into 
carbonic  acid,  and  reaches  the  green  tissue  by  means  of  the  aqueous  contents  of  the 
epidermal  cells. 

According  to  the  previous  statement,  which  has  been  discussed  in  detail,  the 
epidermis  has  also  to  provide  for  the  transmission  of  the  carbonic  acid  to  the  places 
of  consumption,  viz.  to  the  green  tissue. 

In  accordance  with  climatic  and  other  local  conditions,  and  corresponding  to  the 
individuality  of  separate  species,  the  epidermis  presents,  as  is  well  known,  endless 
variations  in  structure.  This  variety  of  formation  is  concerned  chiefly  with  the 
part  which  it  has  to  play  as  a  protective  covering,  as  strengthener,  and  the  like. 
As  a  conducting  apparatus  for  carbonic  acid,  that  is,  in  the  form  of  a  water  mantle 
or  outer  aqueous  tissue,  it  exhibits  comparatively  little  variation.  In  evergreen 
plants  which  grow  in  warm,  damp  situations  where  transpiration  is  limited,  and 
where  the  water  of  the  soil  is  often  conducted  by  root -pressure  to  the  large 
transpiring  leaf -surfaces,  as,  for  examples,  in  tropical  bananas,  palms,  mangroves, 
figs,  and  peppers,  the  aqueous  cells  which  lie  above  the  green  palisade  tissue  are 
always  arranged  in  several  layers.  In  all  those  plants  also  whose  outermost  cells 
in  contact  with  the  air  have  much  thickened  walls,  and  consequently  a  restricted 
lumen,  as,  for  example,  in  the  Oleander,  which  grows  on  the  sides  of  brooks  (see 
fig.  73  3),  and  in  the  proteaceous  Dryandra  floribunda  growing  in  the  Australian 
bush  (see  fig.  68),  the  water  mantle  consists  of  a  double  layer  of  cells.  When  the 
green  tissue  is  penetrated  by  vascular  bundles  and  groups  of  strengthening  cells 
without  chlorophyll,  the  aqueous  epidermal  layer  is  also  interrupted,  and  is  usually 
only  co-extensive  with  the  palisade  cells.  In  the  leaves  of  grasses  the  colourless 
aqueous  cells  form  rows  which  are  placed  above  the  green  assimilating  tissue,  and 
surround  this  tissue  as  an  actual  mantle. 

The  demand  of  the  green  tissue  for  carbonic  acid  regulates  itself  to  the 
consumption  in  the  formation  of  organic  substances.  Bat  the  consumption  is  at  a 
maximum  at  the  time  of  strongest  illumination  and  greatest  warming  of  the  green 
tissue,  and  therefore  coincides  with  the  most  abundant  transpiration.  At  such  a 
time  the  carbonic-acid-bearing  sap  is  drawn  by  the  active  protoplasm  in  the  green 
tissue  with  great  eagerness  from  the  epidermal  cells  lying  above,  often  so 
abundantly  that  a  quick  replacement  is  impossible.  But  in  consequence  of  this 
the  epidermal  cells  lose  their  turgescence;  they  collapse,  and  the  hitherto  tense 
epidermis  presents  a  flaccid  appearance.  In  order  that  this  collapse  may  take  place 
without  injury,  the  following  contrivance  has  been  devised.  The  side-walls  of  those 
cells  which  form  the  epidermis,  i.e.  the  outer  aqueous  tissue,  are  delicate,  thin,  and 
flexible,  and  as  these  cells  give  up  a  portion  of  their  sap,  their  side-walls  are  folded 
together  just  like  a  bellows  from  which  the  air  has  been  expelled.  When  the  cells 
become  again  filled  with  fluid,  the  folds  are  straightened  out  as  in  a  bellows  filled 
with  air,  and  the  cells  regain  their  former  tenseness. 

In  the  course  of  the  foregoing  representation  we  have  only  described  the 
transmission  of  carbonic  acid  through  the  epidermal  cells  rich  in  watery  cell-sap  on 


VOL.  1. 


370  TRANSMISSION   OF   THE   FOOD-GASES. 

the  upper  side  of  the  leaf.  But  it  must  not  be  forgotten  that  the  same  process  also 
takes  place  on  the  under  side  of  the  leaf,  particularly  when  the  green  tissue  is  not 
divided  into  palisade  cells  and  spongy  parenchyma,  and  also  when  the  epidermis  is 
provided  with  stomata  both  on  the  upper  and  under  sides  of  the  leaf.  In  certainly 
70  per  cent  of  all  leafy  plants  the  arrangement  is  such  that  palisade  tissue  occurs 
beneath  the  succulent  epidermis  of  the  upper  side,  under  this  again  spongy 
parenchyma,  and  again  under  this  the  epidermis  of  the  lower  side,  which  is 
abundantly  pierced  by  stomata.  It  can  therefore  be  asserted,  for  the  majority  of 
plants  with  green  foliage,  that  the  epidermis  of  the  upper  side  chiefly  regulates  the 
transmission  of  carbonic  acid  to  the  palisade  cells,  and  that  transpiration  is  chiefly 
regulated  by  the  epidermis  of  the  lower  side. 

It  is  hardly  probable  that  carbonic  acid  finds  entrance  to  the  green  tissue 
through  the  stomata.  At  the  time  when  the  demand  for  carbonic  acid  is  at  a 
maximum  in  the  green  tissue,  a  considerable  quantity  of  food-salts  must  be 
delivered  to  the  green  cells,  and  the  water  which  provides  for  the  transport  of  the 
food-salts  from  the  soil  up  to  the  small  chemical  laboratories,  as  the  palisade  cells 
may  be  called,  is  rapidly  expelled  from  the  stomata  in  the  form  of  vapour.  But 
while  water- vapour  is  streaming  out  of  the  stomata,  the  carbon  dioxide  of  the  air 
can  hardly  stream  in  through  the  same  avenues  at  the  same  time,  and  it  may  be 
concluded  that  when,  generally  speaking,  this  gas  is  absorbed  through  the  stomata, 
the  occurrence  is  exceptional 

Concerning  the  filling  of  the  epidermal  cells  with  water  and  carbonic  acid,  it 
should  be  here  again  pointed  out  that  in  not  a  few  plants  the  absorption  of  rain  and 
dew  takes  place  directly  through  the  foliage-leaves.  Since  rain  and  dew  always 
contain  small  quantities  of  carbonic  acid  and  traces  of  nitric  acid,  this  method  of 
filling  the  epidermal  cells  is  so  much  the  less  to  be  undervalued.  In  very  many 
green  foliage-leaves  the  continuous  epidermis  above  the  palisade  cells  is  capable  of 
being  moistened,  while  the  lower  epidermis,  rich  in  stomata,  on  the  other  hand,  is 
kept  dry  by  the  most  varied  contrivances;  and  it  is  very  probable  that  in  such  cases 
the  water  of  rain  and  dew  is  taken  up  by  the  whole  epidermis  of  the  upper  leaf- 
surface,  especially  when  these  epidermal  cells  have  a  short  time  previously  given  up 
a  portion  of  their  contents  to  the  green  tissue,  and  have  become  consequently 
somewhat  collapsed.  In  many  cases  it  must  be  concluded,  from  their  shape  and 
position,  that  the  filling  of  the  epidermal  cells  is  only  caused  by  the  watery  sap 
brought  from  the  roots,  and  indeed  only  by  means  of  the  green  palisade  tissue, 
i.e.  of  the  same  tissue  which,  on  occasion,  again  receives  watery  fluid  from  the 
epidermal  cells.  This  periodic  alternation  of  absorption  and  expulsion  may  be 
explained  in  the  following  manner.  The  water  arriving  from  the  soil  is  given  off 
by  the  palisade  tissue  to  the  epidermal  cells  at  certain  times,  i.e.  when  no  carbonic 
acid  is  required,  in  order  that  carbon  dioxide  may  there  be  drawn  from  the  air  and 
changed  into  carbonic  acid.  When  this  has  happened,  and  a  demand  for  carbonic 
acid  is  set  up  in  the  palisade  tissue,  this  tissue  takes  back  the  water  it  had 
previously  given  off,  now  of  course  accompanied  by  the  absorbed  carbonic  acid. 


FORMATION  OF  ORGANIC  MATTER  FROM  THE 
ABSOKBED  INORGANIC  FOOD. 


1.  CHLOEOPHYLL  AND  CHLOROPHYLL -GRANULES. 

Chlorophyll-granules  and  the  sun's  rays.— Chlorophyll-granules  and  the  green  tissue  under  the 
influence  of  various  degrees  of  illumination. 

CHLOKOPHYLL-GKANULES  AND  THE  SUN'S  BAYS. 

In  the  former  section  of  this  book  it  has  been  described  how  everything  which 
serves  as  food  for  plants  is  conducted  to  the  green  tissues.  Food-salts,  food-gases, 
and  water  arrive  at  the  same  goal  by  the  most  diverse  contrivances — to  the  green 
cells  as  those  places  where  the  raw  material  is  worked  up  and  organic  substances 
prepared  from  it;  to  the  place  of  need  where  the  materials  for  further  building  and 
development,  for  rejuvenescence,  multiplication,  and  reproduction  of  the  plants  in 
question  have  to  be  provided.  The  question  how  living  plants  manufacture  organic 
substances  in  the  green  cells  from  the  raw  materials  which  stream  to  them,  particu- 
larly from  the  raw  food-sap  and  carbonic  acid,  must  now  be  discussed. 

First,  it  should  be  remembered  that  the  formation  of  organic  materials  always 
commences  with  the  decomposition  of  the  absorbed  carbonic  acid.  This  decom- 
position, however,  is  only  carried  on  by  that  protoplasm  in  which  are  imbedded 
chlorophyll-granules.  The  protoplasm  in  question  can  only  accomplish  the  indi- 
cated task  by  the  help  of  these  structures,  and  the  chlorophyll -granules  are 
therefore  really  the  organs  on  which  everything  depends.  It  is  in  them  that 
those  remarkable  processes  are  carried  on,  upon  which  depends  the  renewal,  and 
ultimately  the  existence,  of  all  life.  The  description  of  these  organs  must,  there- 
fore, precede  all  further  discussion. 

Having  regard  to  the  importance  of  their  function,  the  structure  of  the 
chlorophyll-granules  appears  to  be  simple  enough.  It  is  possible  that  later 
researches,  with  instruments  and  methods  of  observation  more  perfect  than  those 
now  at  our  disposal,  will  furnish  more  accurate  details  about  their  minute  structure, 
and  particularly  as  to  their  dissimilarity  from  the  protoplasm  in  which  they  are 
imbedded.  In  the  meantime,  only  this  much  is  known— that  the  ground- work  of 
the  chlorophyll-granules  differs  but  little  in  its  structure  and  composition  from  the 
surrounding  protoplasm.  Like  all  sharply-defined  protoplasmic  bodies,  chlorophyll- 
granules  exhibit  a  pellicle-like  thickened  outer  layer;  the  inner  portion,  on  the 


371 


372  CHLOROPHYLL -GRANULES   AND   THE   SUN'S   RAYS. 

other  hand,  is  formed  of  a  porous  mass  of  reticular  or  scaffold-like  strands,  which 
may  best  be  compared  to  a  bath  sponge.  The  holes  and  meshes  of  this  spongy 
colourless  ground  substance  contains  a  green  colouring  matter,  which  is  dissolved  in 
an  oily  material,  and  clothes  the  continuous  small  spaces  in  the  form  of  a  parietal 
layer.  This  green  colouring  matter  of  the  chlorophyll -granules,  which  has  been 
called  chlorophyll,  is  easily  soluble  in  alcohol,  ether,  and  chloroform.  If  green 
leaves  are  steeped  in  an  alcoholic  solution,  they  become  blanched  in  a  short  time, 
and  the  colouring  matter  passes  entirely  into  the  fluid.  The  alcohol  assumes  the 
beautiful  green  colour  which  the  leaves  formerly  possessed,  and  the  previously 
green  leaves  are  now  to  be  seen  floating  in  the  green  alcohol.  In  transmitted  light 
the  solution  appears  a  beautiful  green;  but  when  observed  in  reflected  light  it 
appears  blood-red,  and  therefore  the  colouring  matter  displays  a  marked  fluorescence. 
If  a  fatty  oil  is  added  to  the  green-tinted  alcohol,  and  the  two  are  shaken  up 
together,  the  green  colour  passes  into  the  added  medium,  while  in  the  alcohol  a 
yellow  substance  remains,  which  has  been  termed  xanthophyll.  The  chemical 
composition  of  chlorophyll  is  not  yet  so  clearly  understood  as  we  could  wish. 
It  is  asserted  that  it  is  possible  to  obtain  chlorophyll  in  a  crystallized  form.  The 
crystals  obtained  form  green  transparent  rhomboids,  which,  when  exposed  to  the 
light,  slowly  decompose  again.  This  chlorophyll  behaves  like  a  weak  acid;  contrary 
to  earlier  belief,  it  is  free  from  iron,  but  leaves  behind  almost  2  per  cent  of  ash, 
consisting  of  alkalies,  magnesia,  some  calcium,  phosphoric  and  sulphuric  acids.  The 
fact  that  the  production  of  these  crystals  must  be  preceded  by  a  series  of  long- 
continued  operations,  together  with  the  fact  that  chlorophyll  is  extremely  delicate 
and  easily  decomposed,  always  allows  us  to  suppose  that  the  crystals  mentioned  are 
only  a  product  of  decomposition,  and  do  not  belong  to  that  chlorophyll  which 
colours  the  chlorophyll-granules  in  living  cells.  It  was  previously  thought  that 
chlorophyll  was  a  mixture  of  two  colouring  matters,  viz.  a  blue  and  a  yellow,  until 
later  researches  demonstrated  that  this  supposition  was  unfounded,  and  that  a  false 
impression  had  been  received  through  observation  of  the  process  of  decomposition. 
A  characteristic  absorption  spectrum  has  been  obtained  for  chlorophyll,  which  is 
especially  useful  in  all  cases  where  it  is  a  question  of  demonstrating  the  presence 
of  very  small  quantities  of  the  colouring  matter  in  any  parts  of  the  plant.  With 
respect  to  this  it  is  enough  to  say  that  the  whole  of  the  violet  and  blue  and  the 
ultra-violet  rays  are  cut  off  from  the  spectrum,  and  that  it  exhibits  seven  character- 
istically distributed  absorption-bands.  It  may  be  further  remarked  here  that  after 
treating  the  chlorophyll  with  hydrochloric  acid  tiny  crystals  arise,  which  have  been 
called  hypochlorin.  The  results  of  all  these  researches  have  thrown  but  little  light 
upon  the  part  which  chlorophyll  plays  in  those  processes  which  commence  with  the 
decomposition  of  the  absorbed  carbonic  acid  in  the  chlorophyll-granules. 

Compared  with  the  size  of  the  whole  mass,  chlorophyll  forms  only  an  extremely 
small  fraction  of  the  granules  it  colours  green,  and  when  it  is  withdrawn  by  the 
addition  of  alcohol,  only  the  colour  and  not  the  size  of  the  granules  in  question  is 
found  to  be  altered. 


CHLOROPHYLL -GRANULES   AND   THE   SUN'S   RAYS.  373 

Chlorophyll-granules  appear  to  be  imbedded  in  protoplasm  from  their  origin 
until  their  disappearance.  When  the  protoplasm  is  situated  round  the  wall— or,  in 
other  words,  when  the  central  cavity  of  the  protoplasm  is  large  and  filled  with 
watery  cell-sap,  and  the  plasma  which  surrounds  the  sap-cavity  is  sac-like  and  only 
forms  a  thin  covering  to  the  cell  chamber,  then  the  chlorophyll-granules  are  usually 
imbedded  in  the  middle  layer  of  the  parietal  plasma,  so  that  they  are  separated 
from  the  sap-filled  central  cavity,  as  also  from  the  cell-wall,  by  a  layer  of  colour- 
less protoplasm.  The  same  thing  occurs  when  the  chlorophyll-granules  are 
imbedded  in  the  plasma  strands  which  are  stretched  across  the  cell -cavity  (see 
figs.  52  and  53).  Frequently  the  chlorophyll-granules  project  like  warts,  and  thus 
give  a  knotty  appearance  to  the  protoplasmic  strands;  but  even  then  they  are 
always  covered  by  a  thin  colourless  layer  of  protoplasm. 

In  spite  of  this  close  connection,  chlorophyll-granules  always  appear  to  be 
sharply  defined,  and  exhibit  in  their  entire  development  a  certain  separateness  from 
the  protoplasm  in  which  they  may  reasonably  be  supposed  to  take  their  origin. 
They  enlarge,  divide,  and  multiply,  and  occasionally  in  the  course  of  their  life  alter 
their  form.  With  respect  to  their  shape  there  is  little  variety  in  the  green  tissue 
of  the  stem  and  leaves  of  higher  plants.  The  chlorophyll-granules  almost  always 
appear  there  as  rounded  or  occasionally  angular,  sometimes  even  as  lenticular  or 
many-sided  grains.  A  much  greater  diversity  is  observed  in  those  simple  green 
plants  which  live  in  water,  and  have  been  classed  together  under  the  name  of 
Algae.  In  the  cells  of  the  green  filaments  of  Zygnenw,,  which  are  represented  in 
figure  25A,  m,  the  chlorophyll  bodies  are  stellate,  and  are  so  arranged  in  each 
cell  that  there  are  usually  two  stars  side  by  side.  In  species  of  the  genus 
Spirogyra  (fig.  2  5 A,  I)  they  form  spirally  wound,  usually  knotty,  bands,  and  in 
most  species  of  the  genus  only  one  band  in  each  cell;  but  in  some  species  there  are 
two  bands,  whose  spirals  cross  one  another,  whereby  very  ornamental  structures 
come  into  view  under  the  microscope.  In  species  of  the  unicellular  Penium 
(Plate  I.,  fig.  k),  the  chlorophyll  bodies  form  plates  or  bands  parallel  to  the  long 
axis  of  the  cell,  projecting  against  the  cell- wall  in  all  directions.  In  Mesocarpus  a 
single  green  plate  is  observable,  which  divides  the  cavity  of  the  cell  into  two  almost 
similar  halves;  (Edogonium  exhibits  a  latticed  plate;  species  of  the  genus  Ulva 
have  plate-shaped  chlorophyll  bodies  which  lie  close  to  the  wall;  in  the  cells  of 
Podosira  are  seen  disc-shaped  chlorophyll  bodies  which  jut  out  in  all  directions; 
and  in  the  liverwort  Anthoceros  the  chlorophyll  bodies  are  in  the  form  of  hollow 
spheres  surrounding  the  centres  of  the  cells. 

The  number  of  chlorophyll-granules  in  the  protoplasm  of  the  cell  varies  from 
one  to  several  hundreds.  In  the  cells  of  selaginellas  there  are  usually  2-4;  in 
those  of  the  luminous  moss,  Schistostega  osmundacea,  to  be  described  later  more  in 
detail,  4-12  (fig.  25A,  p).  The  green  cells  of  most  leafy  flowering  plants  contain 
20-100,  many  even  200.  In  the  cells  of  Vaucheria  (figure  25A,  a-d),  the  proto- 
plasm is  so  crowded  with  thickly-pressed  small  green  granules  as  to  make  one 
think  that  the  whole  cell-body  contained  but  a  single  chlorophyll  mass.  Foliage- 


374  CHLOROPHYLL -GRANULES   AND  THE   SUN'S   RAYS. 


yma 
,n  in 


leaves,  in  which  a  distinct  separation  between  the  palisade  and  spongy  parench; 
is  completed,  always  show  many  more  chlorophyll-granules  in  the  former  than  i 
the  latter.  Careful  countings  have  shown  that  the  palisade  cells  usually  contain 
three  or  four  times — occasionally  even  six  times — as  many  chlorophyll-granules  as 
the  adjoining  cells  of  the  spongy  parenchyma.  When  the  chlorophyll-granules  in  a 
cell  are  so  many  that  the  whole  inner  wall  of  the  cell  can  be  covered  with  them, 
they  arrange  and  distribute  themselves  very  equally  in  this  manner,  and  such  cells 
appear  uniformly  green.  It  then  seems  as  if  the  whole  cell-chamber  were  entirely 
filled  with  chlorophyll-granules,  but  this  is  not  really  the  case.  The  central  cavity 
of  the  protoplasm  filled  with  cell-sap  never  contains  a  single  chlorophyll  body. 

The  chlorophyll-granules  imbedded  in  the  parietal  protoplasm  can  also  undergo 
the  most  remarkable  displacements,  which  we  will  forthwith  describe. 

With  regard  to  shape,  cells  with  active  protoplasm,  containing  chlorophyll- 
granules,  exhibit  the  widest  variety.  Especially  are  all  imaginable  cell  shapes 
to  be  found  in  the  group  of  Desmids  which  live  in  water :  rod-shaped,  cylindrical 
(fig.  25A,  k),  crescent-shaped  (fig.  25A,  i),  tabular,  stellate,  tetrahedral,  and  many 
others  for  which  it  would  be  hard  to  find  short  and  suitable  names.  The  Algae, 
which  to  the  naked  eye  seem  composed  of  green  threads,  are  built  up  of  cells  which 
are,  for  the  most  part,  tubular  and  cylindrical  (fig.  25A,  a,  b,  and  I,  m).  In  Lichens 
and  Nostocacese  the  cells  which  form  the  tissues  are  spherical;  in  Mosses  and  Liver- 
worts they  are  pentagonal  and  hexagonal. 

As  already  mentioned  in  former  sections,  the  green  tissue  in  the  foliage  of 
Phanerogams  is  formed,  in  the  majority  of  instances,  of  two  kinds  of  cells — of 
branched  cells  forming  the  spongy  parenchyma,  and  of  cylindrical  cells  which  con- 
stitute the  palisade  tissue  (fig.  62,  p.  279).  The  latter  are  often  short,  their  length 
being  not  much  greater  than  their  width,  but  usually  they  are  five  or  six  times,  and 
occasionally  even  ten  or  twelve  times,  longer  than  broad.  In  bulbous  plants  the 
palisade-shaped  cells  are  arranged  parallel  to  the  upper  leaf -surface,  but  in  the 
majority  of  seed-bearing  plants  they  are  at  right  angles  to  the  upper  surface  of  the 
foliage-leaf,  as  shown  in  the  cross-section  of  a  leaf  of  Salix  reticulata,  fig.  7 15, 
p.  301.  The  green  cells  below  the  epidermis  of  pines  and  various  firs  exhibit  a  very 
peculiar  form.  In  contour  they  appear  angular  and  tabular,  and  are  fitted  closely 
to  one  another  without  intercellular  spaces.  From  the  cell-walls  parallel  to  the 
upper  surface  of  the  leaf  trabeculse  project  into  the  interior,  by  means  of  which  each 
cell  is  divided  up  into  niches  usually  of  equal  size.  Such  cells  remind  one  of 
stables  in  which  the  stalls  of  the  different  horses  are  separated  by  boarded  parti- 
tions. The  projecting  trabeculae  are  always  so  arranged  that  the  entire  cell-chamber 
appears  like  a  group  of  palisade  cells  whose  side  walls  separating  one  from  another 
have  been  interrupted.  These  partitions,  which,  as  stated,  are  to  be  found  in  many 
firs,  but  also  in  grasses  and  many  R-anunculacese— especially  in  the  Monkshood 
(Aconitum),  Peony  (Pceonia),  and  Marsh  Marigold  (Caltha)— increase  the  internal 
surface  of  the  chamber,  and  this  appears  to  be  advantageous,  inasmuch  as  by 
this  means  many  more  parietal  chlorophyll-granules  can  find  a  place  than  would 


CHLOROPHYLL -GRANULES  AND  THE  SUN'S  RAYS.  375 

be  possible  in  a  single  cell  of  equal  dimensions,  but  devoid  of  such  projecting 
trabeculse. 

It  is  shown  by  very  accurate  investigations  that  the  quantity  of  organic 
substances  formed  in  a  cell,  by  the  decomposition  of  carbonic  acid,  is  greater  the 
greater  the  number  of  chlorophyll-granules,  provided  that  all  of  them  are  so 
arranged  within  its  protoplasm  that  they  can  discharge  their  functions.  A  heap  of 
chlorophyll-granules  filling  the  cell  irregularly  would  be  little  suited  to  effect  this 
result.  The  small,  green  chlorophyll-granules  must,  on  the  contrary,  be  so  arranged 
that  no  one  deprives  another  of  light,  and  this  is  most  easily  possible,  especially  in 
a  many-storied  plant-structure,  composed  of  numerous  cells,  when  the  chlorophyll- 
granules  are  grouped  together  like  the  stones  in  a  mosaic,  and  are  arranged  along 
the  walls  in  this  order.  When,  moreover,  the  light  falls  unhindered  through  certain 
portions  of  wall,  as  through  a  window  into  the  cell-cavity,  all  the  chlorophyll- 
granules  there  situated  are  almost  equally  illuminated.  The  larger  the  extent  of 
wall  surface,  the  more  chlorophyll-granules  can  be  accommodated  on  it,  and  there- 
fore the  more  abundantly  can  the  decomposition  of  carbonic  acid  be  carried  on  in 
such  cells.  For  such  green  multicellular  tissue,  whose  most  important  function  is 
the  decomposition  of  carbonic  acid  and  the  formation  of  organic  substances,  the 
parietal  grouping  of  the  chlorophyll-granules,  the  above-mentioned  infolding  of  the 
inner  surface  of  the  cells,  generally  the  increase  of  the  inner  surface  of  the  cell- 
walls  clothed  with  chlorophyll,  is  accordingly  the  most  advantageous  arrangement 
for  the  best  possible  utilization  of  the  available  space. 

When  one  speaks  of  the  "  green  "  of  plants  one  thinks  first  of  all  of  the  foliage- 
leaves,  in  which  that  colour  is  especially  noticeable.  The  name  "chlorophyll" 
translated  by  "  leaf -green  "  might  lead  to  the  idea  that  cells  and  tissues  provided 
with  chlorophyll  are  only  to  be  found  in  the  leaves;  but  this  would  not  at  all 
correspond  to  the  true  state  of  the  case.  Those  plants  which  are  not  differentiated 
into  stem  and  leaves,  especially  the  many  kinds  of  green  water-plants  classed  under 
the  name  of  Algse,  generally  consist  entirely  of  chlorophyll-bearing  cells.  In  those 
mutually-nourishing  combinations  named  Lichens,  one  of  the  partners  is  without, 
while  the  other  is  provided  with,  chlorophyll. 

When  stem  and  foliage-leaves  are  clearly  differentiated,  a  portion  of  the  tissue  is 
deprived  of  chlorophyll  while  the  other  portion  is  more  or  less  rich  in  the  same. 
Chlorophyll-containing  tissue  is  found  in  all  the  members  of  these  stem-plants,  in 
roots,  in  stems,  in  foliage,  in  floral  leaves,  in  fruits,  and  seeds.  In  tropical  orchids 
the  aerial  roots  when  dry  appear  white  and  are  seemingly  quite  devoid  of 
chlorophyll;  but  when  moistened  their  green  colour  is  seen,  because  when  the  outer 
porous  covering  is  filled  with  water,  and  its  cells  become  transparent,  the  colour  of 
the  green  tissue-layer  below  shines  through.  There  are  even  orchids,  e.g.  Angrce- 
cum  globulosum,  funale,  and  Sallei,  which,  when  not  flowering,  have  no  other  green 
tissue  than  that  in  the  aerial  roots,  and  in  which  not  only  the  absorption  of 
materials,  but  also  the  working  up  of  the  absorbed  nourishment,  particularly  the 
decomposition  of  carbonic  acid  and  the  formation  of  organic  substances,  is  earned 


376  CHLOROPHYLL -GRANULES   AND   THE   SUN'S   RAYS. 

on  by  the  green  tissue  of  the  aerial  roots.  Green  tissue  is  much  more  frequently  to 
be  met  with  in  stem-structures  than  in  roots.  Hundreds  of  rushes,  bulrushes, 
cyperuses,  and  horse-tails,  as  well  the  Casuarineae  and  species  of  Ephedra,  included 
under  the  switch  plants,  many  papilionaceous  plants  of  the  genera  Retama, 
Genista,  and  Spartium,  a  number  of  Salicornias,  tropical  orchids,  and  cactiform 
plants,  the  Duckweed  (Lemna),  and  all  the  plants  possessing  flattened  shoots  (see 
tig.  82),  contain  green  tissue,  without  exception,  in  the  cortex  of  their  stem  and 
branches.  Also  ovaries  and  fruits  which  are  not  yet  fully  ripe  are  so  universally 
coloured  green  that  in  popular  language  green  fruit  and  unripe  fruit  are  synony- 
mous. Chlorophyll  is  more  rarely  observed  in  seeds.  Those  whose  embryos  are 
differentiated  into  axis  and  leaf  only  seldom — as,  for  example,  in  the  pines — show 
green  tissue  in  the  cotyledons.  The  seeds  of  orchids,  especially  those  living 
epiphytically  on  the  bark  of  trees,  behave  in  a  peculiar  manner.  These  are 
marvellously  small,  consist  of  only  a  single  group  of  parenchymatous  cells,  and  no 
trace  is  to  be  seen  in  them  of  a  radicle  or  cotyledon.  They  only  retain  the  capacity 
of  germinating  a  short  time,  and  it  is  important  to  these  seeds,  which  are  poorly 
supplied  with  reserve  food,  that  immediately  after  leaving  the  fruit-capsule  they 
may  be  able  to  provide  themselves  with  nourishment  from  their  surroundings,  and 
to  manufacture  organic  substances  from  this  food.  This  they  can  naturally  only  do 
by  the  help  of  chlorophyll,  and  it  is  interesting  to  notice  that  they  also  are  actually 
endowed  with  this  substance.  Even  when  they  are  still  inclosed  in  the  capsule  of 
the  parent  plant  these  seeds  become  green,  and  when  they  are  carried  by  the  wind 
into  some  cleft  on  the  bark  of  an  old  tree-trunk  the  chlorophyll  is  able  at  once  to 
function.  After  a  short  time  a  small  green  tubercle  grows  out  of  the  green  seed, 
and  fixes  itself  by  absorbent  cells  to  the  substratum,  then  very  gradually  it  grows 
up  to  form  a  large  plant-stem. 

Large  flowers  whose  petals,  from  the  commencement  to  the  end  of  the  flowering 
period,  exhibit  a  green  colour  are  esteemed  rarities.  On  the  other  hand,  small  floral 
leaves,  rich  in  chlorophyll,  are  of  very  common  occurrence.  The  change  of  the 
floral  colour  also,  during  the  flowering  period,  from  white,  red,  violet,  and  brown,  to 
green,  has  been  frequently  observed  in  small  as  well  as  in  fairly  large  flowers.  A 
very  striking  example  of  this  is  the  Black  Hellebore  (Hellebores  niger).  When  its 
flowers  open,  the  outer  large  leaves  situated  below  the  petals  (which  are  transformed 
into  small  nectaries),  are  snow-white,  and  show  up  conspicuously  from  their  darker 
surroundings.  From  afar  they  attract  the  attention  of  honey-collecting  insects,  by 
whom  they  are  eagerly  sought  out.  When,  by  means  of  these  honey-sucking 
insects,  pollination  is  brought  about,  the  small  nectaries,  as  well  as  the  large 
dazzling- white  outer  floral  leaves  called  sepals,  become  superfluous.  The  nectaries 
forthwith  fall  off,  but  the  large  sepals  remain  and  take  up  another  function. 
Chlorophyll  is  abundantly  developed  in  their  cells;  the  white  colour  disappears; 
fresh  green  appears  in  its  stead,  and  the  same  floral  leaves  which  previously  had 
attracted  insects  by  their  conspicuous  colour  now  function  as  green  leaves  exactly 
like  foliage-leaves.  A  similar  alteration  of  colour,  which  also  has  the  same 


CHLOROPHYLL -GRANULES   AND   THE   SUN'S   RAYS.  377 

significance,  is  observed  in  many  orchids  and  liliaceous  plants,  but  on  the  whole 
such  a  change  of  function  in  floral  leaves  is  not  common.  These  cursory 
observations  must  show  that  chlorophyll  may  appear  in  all  the  members  of  a  plant, 
although  it  is  also  true  that  foliage-leaves  chiefly  contain  the  green  tissue,  so  that 
certainly  in  90  per  cent  of  all  chlorophyll-bearing  plants  the  decomposition  of 
carbonic  acid  is  carried  on  in  the  foliage-leaves. 

If,  now,  after  the  description  of  the  arrangement,  form,  and  distribution  of 
chlorophyll-granules,  we  would  also  learn  something  as  to  how  organic  substances 
are  formed  in  the  cell-chambers  by  means  of  these  structures,  we  find  ourselves 
in  the  position  of  an  inquirer  who  visits  a  chemical  laboratory  without  a  guide, 
and  wishes  to  ascertain  in  what  way  some  material— for  example,  a  pigment— is 
manufactured.  He  notices  apparatus  set  up  there,  sees  the  raw  materials  heaped 
together,  and  also  finds  the  finished  product.  If  the  manufacture  is  actually 
proceeding,  he  can  also  observe  whether  warmth  or  cold  and  greater  or  less  pressure 
are  brought  into  action  as  propelling  forces,  and  he  can,  if  intrusted  with  the 
manipulation  necessary  to  the  production  of  such  pigment,  imagine  the  relation 
of  the  different  parts  to  the  whole.  Of  the  details,  indeed,  much  must  remain 
incomprehensible,  or  quite  unknown.  Especially  with  reference  to  the  quantity  of 
the  transformed  raw  material,  and  with  regard  to  the  propelling  forces,  must  the 
visitor's  knowledge  remain  incomplete. 

It  is  not  otherwise  with  us  when  we  would  inspect  the  processes  carried  on  in 
the  cells  where  chlorophyll-granules  develop  their  activity.  We  see  the  effective 
apparatus,  we  recognize  the  food-gases  and  food-salts  collected  for  working  up,  we 
know  that  the  sun's  rays  act  as  the  motive  force,  and  we  also  identify  the  products 
which  appear  completed  in  the  chlorophyll-granules.  By  careful  comparison  of 
various  cells  containing  chlorophyll,  on  the  ground  of  observations  which  establish 
the  conditions  under  which  the  manufacture  of  organic  substances  succeeds  best 
and  worst,  having  found  by  experience  that  under  certain  external  conditions  the 
whole  apparatus  becomes  disintegrated  and  destroyed;  it  is  indeed  permissible  to 
hazard  a  conclusion  about  the  character  of  the  propelling  forces.  But  what  is 
altogether  puzzling  is  how  the  active  forces  work,  how  the  sun's  rays  are  able  to 
bring  it  about  that  the  atoms  of  the  raw  material  abandon  their  previous  grouping, 
become  displaced,  intermix  one  with  another,  and  shortly  appear  in  stable 
combinations  under  a  wholly  different  arrangement.  It  is  the  more  difficult  to 
gain  a  clear  idea  of  these  processes,  because  it  is  not  a  question  of  that  displacement 
of  the  atoms  called  decomposition,  but  of  that  process  which  is  known  as 
combination  or  synthesis.  Decompositions  and  analyses,  even  of  the  most 
complicated  compounds  into  simple  combinations  are  well  understood,  but  not 
so  the  converse.  It  is  always  considered  a  fortunate  occurrence  when  a  chemist 
succeeds  in  producing  from  its  fundamental  elements,  or  from  the  simplest  com- 
bination of  these,  one  of  those  complicated  bodies,  which  are,  nevertheless,  formed 
with  such  ease  in  plant  cells.  When  sugar  is  "  made  "  in  a  manufactory,  it  is  not 
that  carbon  and  the  elements  of  water  are  used,  although  these  are  so  abundantly 


378  CHLOROPHYLL -GRANULES  AND  THE   SUN'S  RAYS. 

at  disposal,  but  only  that  the  sugar  is  isolated  which  has  been  formed  synthetically 
from  these  substances  in  those  tiny  chemical  laboratories,  the  vegetable  cells. 
Consequently  it  is  really  incorrect  to  say  that  sugar  is  "  made"  in  our  manu- 
factories; we  should  only  say  that  there  the  sugar  manufactured  by  the  plants  is 
separated  from  other  substances  and  prepared  for  further  use. 

Although  it  is  not  possible  to  represent  the  processes  concerned  in  the  synthesis 
of  organic  materials  in  plant  cells  as  a  matter  beyond  all  doubt,  one  is  justified  in 
taking  refuge  in  hypotheses.  And  it  must  be  looked  upon  as  an  hypothesis  when 
we  consider  the  movement  by  which  the  atoms  of  the  food-gases  and  food-salts  are 
displaced  by  the  sun's  rays  in  the  vegetable  cells  as  a  transmission  of  the  vital  force 
of  the  sun.  The  atoms  have  arranged  themselves  by  this  movement  in  a  different 
order,  they  hold  and  support  one  another,  they  are  stable,  and  a  condition  of 
mutual  tension  has  been  set  up.  The  vital  force  of  the  sun  has  become  the  hidden 
spring  of  all  these  changes.  The  now  stable  organic  material  formed  by  synthesis 
is  thus  equipped  with  an  adequate  supply  of  energy,  designated  in  other  words  as 
latent  heat.  If  the  atoms  of  the  organic  material  from  whatever  cause  again  break 
loose,  abandoning  their  combination  and  arrangement,  they  perhaps  so  displace  and 
rearrange  themselves  that  those  groups  which  previously  existed  are  formed  again, 
and  thus  the  potential  energy  is  changed  to  vital  force,  the  latent  heat  to  sensible 
warmth.  When  a  tree-trunk  is  consumed,  the  vital  force  of  the  sun,  which  had 
been  changed  by  the  formation  of  cellulose  and  the  other  organic  materials 
composing  the  wood  of  that  time  into  latent  force,  is  again  transformed  into 
active  energy;  and  when  we  burn  coals,  the  sun's  rays,  which  thousands  of  years 
ago  brought  about  the  formation  of  organic  vegetable  substances  and  were 
imprisoned  in  the  coal,  will  again  be  set  free,  will  warm  our  rooms,  drive  our 
machines,  or  propel  our  steamships  and  locomotives.  Keeping  this  idea  in  view,  it 
is  at  least  possible  to  imagine  the  mechanical  significance  of  the  sun's  rays  in  the 
formation  of  organic  substances  in  plants,  and  it  may  be  reckoned  that  the 
quantity  of  organic  substance  produced  stands  in  a  fixed  proportion  (which  may  be 
expressed  in  figures)  to  the  store  of  energy  in  the  same. 

One  circumstance  on  which  particular  stress  must  here  be  laid  is  that  the 
various  rays  of  which  sunlight  is  composed,  the  rays  with  various  wave-lengths 
and  refrangibility,  which,  some  of  them  at  least,  appear  to  our  eyes  as  the  different 
coloured  bands  in  rainbows,  play  each  a  very  distinct  part  in  the  formation  of 
organic  materials  in  plant  cells.  Under  the  influence  of  the  blue  and  violet  rays, 
i.e.  of  those  which  are  most  highly  refrangible  and  have  the  smallest  wave-lengths, 
the  oxidation  of  the  organic  materials  called  carbohydrates  is  assisted,  that  is  to 
say,  not  the  formation  but  the  decomposition  and  transformation  of  these 
compounds  are  favoured.  The  red,  orange,  and  yellow,  i.e.  those  rays  which  are 
less  refrangible  and  have  a  greater  wave-length,  behave  quite  otherwise.  These 
favour  the  reduction  of  carbonic  acid,  assist  the  formation  of  carbohydrates  from 
raw  materials,  and  are  therefore  chiefly  concerned  in  the  originating  of  such 
organic  substances.  When  a  sunbeam  passes  through  a  colourless  glass  prism  a 


CHLOROPHYLL  AND  LIGHT  INTENSITY.  379 

continuous  spectrum  is  produced— violet,  dark  blue,  light  blue,  green,  yellow, 
orange,  and  red.  If  the  same  sunbeam  passes  through  a  transparent  but  coloured 
body,  which  may  be  either  solid  or  fluid,  whole  groups  of  colour  absent  themselves 
from  the  spectrum.  Dark  bands  appear  in  the  corresponding  places,  and  we  say 
that  the  light  in  question  has  been  absorbed  by  the  coloured  body.  Now,  if 
chlorophyll  has  the  property  of  absorbing  those  colours  of  the  spectrum  which  are 
not  advantageous  in  the  formation  of  organic  substances  from  raw  material,  the 
rdle  of  this  chlorophyll  cannot  be  too  highly  estimated.  It  must  not  be  overlooked, 
moreover,  that  many  bodies  have  the  capacity  of  absorbing  light  rays  of  shorter 
wave-length,  and,  on  the  other  hand,  of  giving  out  rays  of  greater  wave-length. 
It  is  precisely  those  pigments  which  are  distributed  in  plants,  again  above  all, 
chlorophyll,  which  possesses  this  property  called  fluorescence;  and  we  must 
therefore  also  assign  this  significance  to  chlorophyll,  that  it  can  transform  rays  of 
light  which  are  useless  in  the  synthesis  of  organic  materials  into  those  which  show 
the  best  possible  action  in  this  respect.  If  the  fluorescing  pigments  of  plants 
(chlorophyll,  anthocyanin,  phycoerythrin)  can  transform  the  violet  and  blue  rays 
into  yellow  and  red,  it  is  to  be  supposed  that  their  activity  goes  further,  and  that 
they  will  be  able  to  change  rays  of  small  wave-length  and  higher  refrangibility  into 
rays  which  are  found  beyond  the  red,  which  are  imperceptible  to  our  eyes,  and 
which  possess  very  great  heat-giving  powers,  or,  in  other  words,  that  they  will  be 
able  to  transform  light  into  heat.  From  all  this  it  may  be  seen  that  the 
significance  of  chlorophyll  in  the  formation  of  organic  materials  would  be  three- 
fold. First,  a  retention  or  extinction  of  those  rays  which  might  hinder  the 
formation  of  those  compounds  known  by  the  name  of  carbohydrates;  further,  the 
transformation  of  rays  with  short  wave-length  into  those  of  longer  wave-length, 
which,  according  to  experience,  most  favourably  effect  the  production  of  sugar  and 
starch;  and,  finally,  the  conversion  of  light  into  heat,  and  ultimately  into  latent 
heat. 


CHLOKOPHYLL- GRANULES  AND  THE  GREEN  TISSUE  UNDER  THE 
INFLUENCE  OF  VARIOUS  DEGREES  OF  ILLUMINATION. 

If  it  is  beyond  question  that  organic  materials  can  only  be  formed  from  the 
absorbed  carbonic  acid  in  the  presence  of  chlorophyll,  it  is,  on  the  other  hand,  equally 
certain  that  the  sun  creates  and  works  through  these  formative  processes  by  its 
rays,  and  thus,  as  the  propelling  force,  becomes  the  fountain  of  all  organic  life.  The 
sun  rises  and  sets,  day  follows  night,  and  during  the  night  the  process  just  men- 
tioned, upon  which  the  existence  of  the  living  world  depends,  is  interrupted.  But 
even  in  the  daytime  also,  the  strength  of  the  sun  is  very  unequal;  it  is  one  thing  at 
mid-day,  when  the  source  of  light  is  in  the  zenith  and  the  rays  faU  perpendicularly 
on  the  earth,  and  quite  another  in  the  evening,  as  the  illuminating  orb  sinks 
below  the  horizon  and  the  last  rays  spread  almost  horizontally  over  the  surface. 
Clearly  it  is  anything  but  a  matter  of  indifference  to  the  organs  possessing  a  certain 


380  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

amount  of  chlorophyll  in  what  manner  the  sun's  rays  light  upon  them,  or  what 
quantity  of  vital  force  is  transmitted  to  them  in  a  given  time.  Various  species  of 
plants  may  make  very  different  demands  for  sunlight,  but  for  each  individual 
species  the  need  of  propelling  force  fluctuates  only  within  very  narrow  limits,  which 
cannot  be  exceeded  without  injury.  The  greatest  possible  equality  in  the  supply  of 
propelling  force  is  an  indispensable  condition  of  a  successful  career.  In  order  to 
meet  the  inequality  in  the  flow  of  light  on  bright  and  dull  days,  and  also  during 
various  parts  of  the  day,  it  is  arranged  that  the  green  organs  can  turn  towards  the 
sun,  and  that  according  to  the  hour  of  the  day  and  the  strength  of  the  sun's  rays 
at  that  particular  time,  they  can  take  up  a  definite  position,  and  again  alter  this 
position  with  ease.  And,  indeed,  the  green  chlorophyll-granules  in  the  interior  of 
the  cells  also  show  this  capability  of  accommodating  themselves  in  accordance  with 
the  demand  for  light  as  well  as  the  entire  cells,  and,  finally,  even  the  green  leaves, 
together  with  the  stems  and  branches  which  bear  them. 

If  one  would  obtain  a  clear  idea  of  the  withdrawal  of  the  chlorophyll-granules 
from  the  sunlight,  one  must  remember,  first  of  all,  that  these  green  bodies,  what- 
ever may  be  their  form,  are  imbedded  in  the  protoplasm  of  the  cell,  and  that  the 
protoplasm  is  mobile  and  easily  capable  of  displacement — or,  in  other  words,  that 
the  protoplasm  which  contains  the  green  chlorophyll-granules  twists  and  rotates 
within  the  cell  it  inhabits,  and  can  transport  the  granules  hither  and  thither.  Still 
more.  Chlorophyll-granules  can  be  temporarily  heaped  up  and  crowded  together  in 
definite  places;  they  may  again  be  separated  from  one  another,  and  distributed 
equally  throughout  the  whole  cell-body.  In  the  tubular  cells  of  Vaucheria 
clavata,  represented  in  figure  25A,  a,  the  protoplasm  forms  a  lining  layer  on 
the  inner  side  of  the  colourless  transparent  cell-wall,  and  is  so  thickly  studded 
with  round  chlorophyll -granules  that  the  cell  appears  of  a  uniform  dark  green. 
But  this  is  only  the  case  with  light  of  moderate  intensity.  When  strongly  illumi- 
nated the  chlorophyll-granules  move  apart  from  one  another,  arrange  themselves  in 
isolated  balls,  and  in  a  very  short  time,  in  each  tubular  cell,  dark-green  spots  and 
zones  may  be  seen  corresponding  to  the  crowded  granules,  and  light,  irregular 
bands  appearing  in  those  places  from  which  the  chlorophyll  has  been  withdrawn. 
If  the  intensity  of  the  light  diminishes,  the  green  clusters  dissolve,  and  the  former 
equal  distribution  and  colouring  is  resumed.  In  another  filamentous  green  alga, 
which  lives  in  water  and  belongs  to  the  genus  Mesocarpus,  each  of  the  long 
cylindrical  cells  contains  a  plate-like  chlorophyll  body,  which  in  weak  diffuse  light 
turns  itself  at  right  angles  to  the  incident  rays.  In  this  position  the  broad  side,  i.e. 
the  larger  surface  of  the  chlorophyll  body,  is  turned  to  the  sun,  and  the  incident 
light  is  in  this  way  utilized  to  the  utmost  possible  extent.  As  the  plate-like 
chlorophyll  body  usually  extends  right  across  the  cell,  under  the  conditions  indicated, 
the  ceU  appears  of  a  uniform  green  colour.  If  the  full  rays  of  the  sun  fall  on  such 
Mesocarpus  cells,  the  plate-like  chlorophyll  bodies  begin  to  turn  so  that  the  plane 
of  the  plate  is  parallel  to  the  direction  of  the  rays.  Now  the  narrow  side,  i.e.  the 
smaller  surface  of  the  chlorophyll  body,  is  turned  to  the  rays,  and  only  a  dark-green 


CHLOROPHYLL   AND   LIGHT   INTENSITY.  381 

stripe  is  to  be  seen.  This  turning  movement  of  the  chlorophyll  body  is  very 
quickly  performed,  and  can  be  repeatedly  effected  by  darkening  and  illuminating 
the  cells  of  the  Mesocarpus  filament. 

In  cells,  too,  which  are  joined  together  to  form  tissues,  this  displacement  and 
movement  of  the  chlorophyll-granules  often  appears.  It  has  been  noticed  for  a 
long  time  that  in  the  prothallium  of  ferns,  in  the  leaf-like  liverworts,  in  the 
leaflets  of  many  mosses,  and  even  in  the  large,  delicate  foliage-leaves  of  flowering 
plants,  the  green  tissue  appears  to  be  coloured  a  lighter  or  darker  green  according 
to  the  intensity  of  the  incident  light;  that  under  the  influence  of  intense  sunlight 
they  become  blanched  and  yellowish-green,  but  in  weak  light  assume  a  darker  tint. 
If  a  strip  of  black  paper  be  placed  on  a  foliage-leaf,  illuminated  by  the  sun,  so  that 
only  a  portion  of  the  leaf-surface  is  covered  by  it,  and  if  the  paper  be  removed  after 
a  short  time,  the  portion  left  uncovered  and  illuminated  by  the  uninterrupted  rays 
of  the  sun  appears  light  green,  while  that  part  on  which  lay  the  strip  of  paper,  and 
from  which  the  sun's  rays  were  withheld,  is  dark  green.  Careful  investigations 
have  shown  that  this  change  of  colour  is  due  to  displacement  of  the  chlorophyll- 
granules.  In  diffuse  light  the  chlorophyll-granules  group  themselves  on  those  cell- 
walls  on  whose  surface  the  light  falls  perpendicularly,  and  consequently  in  the 
cylindrical  palisade  cells  of  the  foliage-leaf  on  the  small  walls  parallel  to  its  upper 
surface,  and  it  is  clear  that  such  cells  (and  therefore  the  tissue  formed  by  them) 
have  a  dark-green  appearance  when  looked  at  in  the  direction  of  the  incident  light. 
As  soon  as  they  are  illuminated  by  direct  sunlight,  the  chlorophyll-granules  retire 
from  these  walls  and  take  up  their  position  on  the  cell-walls  which  are  parallel  to 
the  direction  of  the  incident  light.  In  palisade  cells  the  chlorophyll-granules  group 
themselves  by  the  side  of  the  long  lateral  walls,  while  the  short  walls,  which  are  at 
right  angles  to  the  rays,  are  colourless  and  free  from  chlorophyll.  In  the  branched 
cells  of  the  spongy  parenchyma  the  chlorophyll-granules,  which  in  diffuse  light 
were  equally  distributed  in  the  cell,  heap  themselves  together  in  groups  in  the 
branches,  while  the  central  portions  of  the  cells  become  clear  and  free  from 
chlorophyll.  The  whole  tissue,  however,  in  which  this  displacement  has  been 
completed  appears  much  paler  than  before,  and  displays  usually  a  decidedly 
yellowish-green  tint.  This  change  of  position  of  the  granules,  according  to  the 
intensity  of  illumination,  may  be  particularly  well  seen  in  the  very  simply 
constructed  leaf-like  duckweed,  especially  in  Lemna  trisulca.  Three  sections  of 
the  green  tissue  of  this  plant,  vertical  to  the  surface,  are  shown  in  fig.  97. 

With  these  phenomena  is  indeed  also  connected  the  alteration  of  shape  which 
is  observed  under  varied  illumination  in  chlorophyll-granules.  In  the  leaflets  of 
Funaria  hygrometrica,  a  moss  very  common  on  piles  of  charcoal,  damp  walls,  and 
rocks,  the  chlorophyll-granules,  which  are  close  to  the  outer  walls  of  the  cells,  are 
flattened  out,  angular,  and  comparable  to  small  polygonal  tablets,  in  diffuse  light. 
They  are  also  so  arranged  that  the  entire  wall  covered  by  them  appears  an  uniform 
green,  and  only  narrow,  colourless  lines  remain  between  them.  As  soon  as  direct 
sunlight  falls  on  them  they  quickly  alter  their  shape,  the  tablets  becoming  hemi- 


3g2  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

spherical  or  spherical  bodies,  which  project  towards  the  centre  of  the  cell-cavity. 
By  this  means  the  area  of  the  chlorophyll-granules  attached  to  the  cell-wall  is 
contracted,  and  consequently  the  green  of  the  leaf -surface  in  question  is  diminished. 
In  the  leaves  of  many  flowering  plants,  also,  the  chlorophyll-granules  which  are 
distributed  in  the  palisade-cells  along  the  elongated  side-walls  appear,  in  diffuse 
light,  hemispherical  or  even  conical,  and  project  towards  the  centre  of  the  cells  so 
that  they  are  illuminated  to  the  greatest  possible  extent  by  the  light  rays  passing 
through.  Under  the  influence  of  direct  sunlight  they  flatten  out,  become  disc- 
shaped,  and  withdraw  to  some  extent  from  the  bright  rays  passing  through  the 
centre  of  the  cells.  The  significance  of  all  these  processes,  the  changes  of  shape 
as  well  as  the  displacements  of  the  chlorophyll-granules,  is  evident  when  it  is 
considered  that  an  over-abundance  as  well  as  a  deficiency  of  light  would  be 
prejudicial,  and  that  for  every  species  the  quantity  of  the  sun's  rays  absorbed 
by  the  chlorophyll-granules  is  definite.  Protoplasm,  provided  with  chlorophyll, 


Fig.  97.— Position  of  the  Chlorophyll-granules  in  the  cells  of  the  Ivy-leaved  Duckweed  (Lemna  trisulca). 
i  In  darkness.     2  in  direct  sunlight,      s  In  diffuse  light. 

tries  under  all  circumstances  to  obtain  this  definite  amount.  When  weakly 
illuminated,  chlorophyll-granules  maintain  a  shape  and  position  in  consequence  of 
which  they  present  the  largest  possible  surface  to  the  light;  when  strongly  illumi- 
nated, they  assume  a  shape  and  position  by  which  the  smallest  possible  surface  is 
so  exposed.  These  processes,  especially  the  displacement  of  the  chlorophyll-granules, 
obtain  a  heightened  interest  from  the  fact  that  they  can  only  be  brought  about  by 
the  streaming  movements  of  the  irritable  protoplasm.  It  must  be  borne  in  mind 
that  it  is  really  living  protoplasm  which  displaces  the  chlorophyll-granules 
imbedded  in  it  in  order  to  bring  them  to  the  places  best  suited  to  the  illumination 
then  existing,  and  to  place  them  in  sunlight  or  shade;  so  that  it  always  happens 
that  the  displaced  green  bodies  are  neither  too  much  nor  too  little  illuminated. 

Many  unicellular  water-plants,  especially  zoospores,  attain  the  same  result  not 
by  displacement  of  the  chlorophyll-granules  in  the  interior,  but  by  movements  of 
the  entire  cells.  These  green  unicellular  organisms  may  be  seen  swimming  towards 
the  light  by  means  of  their  cilia,  and  in  this  way  they  take  up  the  position  always 
best  adapted  to  the  given  conditions.  If  many  swarm-spores  are  collected  together 
in  a  limited  area,  it  may  happen  that  they  all  travel  to  one  particular  place;  there 
they  swarm  about  in  the  water  and  appear  to  the  naked  eye  like  a  little  green 
cloud.  Or  they  may  settle  on  the  bottom  of  the  pool,  there  arranging  them- 
selves side  by  side,  so  that  no  one  deprives  another  of  light,  and  they  then  appear 


CHLOROPHYLL  AND   LIGHT  INTENSITY.  383 

to  the  naked  eye  as  green  stripes  and  patches.  If  swarm-cells  of  SphcBrella 
pluvialis  are  cultivated  in  a  flat  white  china  dish  filled  with  rain-water,  and  one- 
half  of  the  dish  is  darkened  by  means  of  an  opaque  body  while  the  other  half 
remains  illuminated,  the  whole  of  the  swarm-cells  swim  from  the  darkened  to  the 
illuminated  water  in  order  to  take  up  a  position  as  favourable  as  possible  with 
regard  to  the  light.  If  now  the  china  dish  is  turned  round  so  that  the  hitherto 
illumined  portion  becomes  darkened  and  light  falls  on  the  part  previously  obscured, 
the  swarm-cells  forsake  the  position  which  they  had  recently  taken  up,  swim  from 
the  now  darkened  place  to  the  illuminated  side  opposite,  and  arrange  themselves 
there  according  as  the  illuminating  conditions  are  favourable. 

If,  instead  of  the  Sphcerella  pluvialis  discussed  above,  clumps  of  Vaucheria 
clavata  are  cultivated  in  a  china  dish  filled  with  water,  and  the  water  is  again 
partially  darkened,  together  with  the  green  tufts  growing  in  it,  it  will  be  seen  that 
the  cells,  which  are  elongated  and  fixed  at  one  end,  seek  with  the  other  end  those 
places  where  they  can  find  the  best  light.  Vaucheria  clavata,  which  has  been 
repeatedly  cited  as  an  example,  and  which  is  represented  in  the  middle  figure  on 
page  139,  consists  of  long  tubular  cells,  frequently  bulging  and  branched,  whose 
blunt  growing  ends  appear  dark  green,  while  the  lower  dead  portions  are  branched 
and  coloured  yellowish-white.  The  protoplasm  is  so  richly  studded  with  chlorophyll- 
granules  that  the  entire  inner  wall  of  the  tubular  cells  appears  covered  with  a  green 
lining.  At  the  bottom  of  shallow  pools,  which  is  the  natural  habitat  of  these 
plants,  they  form  hemispherical  clumps,  and  all  the  tubular  cells  which  compose 
the  clumps  have  their  green  ends  directed  upwards  towards  the  source  of  light. 
The  same  thing  occurs  when  the  Vaucheria  cultivated  in  the  china  dish  is 
uniformly  illumined  from  above;  but,  if  partially  obscured,  those  filaments  over 
which  the  darkening  shadow  is  thrown  very  quickly  alter  their  position.  They 
bend  towards  the  light  side,  and  then  the  clump  looks  just  as  if  its  filaments  had 
been  combed  in  this  direction.  Moreover,  the  same  thing  is  also  seen  when  the 
china  dish  containing  clumps  of  Vaucheria  (on  which  until  now  diffused  light  has 
fallen  uniformly  from  above)  is  placed  at  the  further  end  of  a  one-windowed  room, 
so  that  the  light  can  only  reach  it  from  one  side.  Here,  again,  all  the  filaments,  or 
rather,  tubular  cells  of  the  clump,  bend  towards  the  source  of  light,  and  if  they 
continue  to  grow,  the  increase  in  length  is  universally  in  a  line  with  the  direction 
of  the  incident  rays.  After  a  few  days  these  Vaucheria  clumps  also  look  as  if  they 
had  been  combed  out. 

The  green  tissues  of  thallophytes,  and  the  green  leaves  and  stems  of  ferns,  and 
phanerogams,  i.e.  those  extensive  combinations  of  green  cells  whose  function  is  to 
work  in  a  harmonious  manner,  and  to  manufacture  organic  substances  for  the 
plant  to  which  they  belong  from  carbonic  acid  with  the  help  of  other  food- 
materials;  these  behave  in  the  same  way  as  the  individual  green  cells  which 
swim  freely  in  water,  and  as  the  tubular  cells  of  Vaucheria,  which  are  attached 
at  one  end.  Arrangements  are  necessary  for  these  likewise,  by  which  they  can 
always  be  placed  in  the  most  favourable  light.  Of  course,  in  these  plants  where 


384  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

division  of  labour  has  been  so  far  developed,  the  conditions  are  not  so  simple 
as  in  those  plants  which  consist  only  of  single  cells,  and  it  is  naturally  to  be 
expected  that,  according  to  the  character  of  the  individual  species  and  the  places 
which  they  inhabit,  the  arrangements  would  be  very  varied.  The  fact  must  also 
be  kept  in  mind  that  each  spot  on  which  a  plant  has  settled  itself  in  the  course  of 
time  may  undergo  alterations  in  consequence  of  which  the  amount  and  strength  of 
the  light  affecting  that  part  varies  considerably.  Long-lived  plants,  which  grow 
vigorously  in  height  and  breadth,  alter  in  their  relation  to  the  sun  in  various 
stages  of  growth,  and  must  also  alter  their  form  in  a  corresponding  manner,  or,  at 
least,  must  alter  the  direction  and  position  of  their  green  tissues.  All  this  requires 
a  multiplicity  of  contrivances  which  are,  as  a  matter  of  fact,  innumerable,  and  the 
exhaustive  treatment  of  which  is  scarcely  possible.  In  order  to  obtain  a  general 
view,  it  will  be  better  to  pick  out  some  of  the  most  remarkable  of  the  long  series 
of  arrangements  whose  significance  lies  in  this,  that  each  species  of  plant  receives 
for  its  green  organs  neither  too  much  nor  too  little  light,  and  to  describe  them  in 
their  relations  to  light  as  types  of  smaller  or  larger  groups. 

We  will  begin  with  those  arrangements  which  are  rendered  necessary  by 
a  peculiar  habitat,  and,  first  of  all,  we  will  investigate  those  plants  which  have 
taken  up  their  quarters  in  caves  or  grottoes,  and  there  pass  through  all  their 
stages  of  development.  In  deep  excavations  shut  off  entirely  from  the  light,  as 
well  as  in  those  which  have  been  formed  without  human  interference,  and  those 
which  have  been  dug  in  order  to  obtain  metal  ore,  coal,  salt,  and  water,  plants 
with  chlorophyll-bearing  cells  and  tissues  are  completely  wanting.  The  plants 
which  we  find  there  consist  only  of  pale  fungi,  which  live  on  the  scanty  organic 
compounds  which  the  infiltrating  rain-water  brings  with  it  into  the  depths  from 
the  surface  of  the  sunny  land  above,  or  which  have  established  themselves  on 
organic  decaying  bodies  brought  there  by  chance  or  intentionally  by  animals  and 
men.  It  is  otherwise  in  caves,  mines,  grottoes,  pits,  and  wells,  where  light  is  able 
to  penetrate  from  above  or  from  the  sides,  even  if  only  through  a  comparatively 
small  aperture.  Truly  the  vegetation  developed  there  is  not  very  luxuriant,  but 
it  is  a  very  remarkable  circumstance  that  there,  as  a  rule,  the  plants  are  still  green. 
What  actually  astonishes  one  at  first  sight  of  this  vegetation,  flourishing  in  caves 
illuminated  only  from  one  side,  is  the  fact  that  they  exhibit  the  most  beautiful 
and  vigorous  green,  a  green  much  fresher,  indeed,  and  more  pronounced  than 
that  displayed  by  the  plants  outside.  Thus  the  Hart's  Tongue  (Scolopendrium 
officinarum),  widely  distributed  in  Southern  Europe,  when  adorning  the  deep  shady 
walls  of  rocky  ravines  is  coloured  a  much  brighter  green  than  when  it  grows  on  stony 
places  in  the  open  country  where  light  can  reach  it  from  all  sides.  Also  the  liver- 
worts which  cover  the  damp  stones  with  their  leaf -like  thallus,  in  grottoes  through 
which  waters  ripple,  show  there  in  the  half-light  a  distinctly  richer  green  than  when 
outside  the  grotto.  But  this  phenomenon  is  most  striking  in  the  prothallia  of  some 
ferns  belonging  to  the  section  of  the  Hymenophyllacese,  and  in  many  mosses. 

A  tiny  moss,  called  popularly  the  Luminous  Moss,  but  which  has  received  from 


CHLOROPHYLL   AND   LIGHT   INTENSITY.  385 

botanists  the  name  Schistostega  osmundacea,  has  even  attained  a  certain  celebrity 
on  this  account.  It  is  found  distributed  throughout  the  Central  European  granite 
and  slate  mountains,  but  is  only  to  be  met  with  in  clefts  of  the  rocks,  caves  and 
similar  spots.  As  a  rule  it  covers  the  yellow,  clayey  earth  and  the  decayed  and 
disintegrated  pieces  of  stone  which  form  the  soil  of  these  caverns  and  small 
grottoes.  On  looking  into  the  interior  of  the  cave,  the  background  appears  quite 
dark,  and  an  ill-defined  twilight  only  appears  to  fall  from  the  centre  on  to  the  si<l<> 
walls;  but  on  the  level  floor  of  the  cave  innumerable  golden-green  points  of  light 
sparkle  and  gleam,  so  that  it  might  be  imagined  that  small  emeralds  had  been 
scattered  over  the  ground.  If  we  reach  curiously  into  the  depth  of  the  grotto  to 
snatch  a  specimen  of  the  shining  objects,  and  examine  the  prize  in  our  hand  under 
a  bright  light,  we  can  scarcely  believe  our  eyes,  for  there  is  nothing  else  but  dull 
lustreless  earth  and  damp,  mouldering  bits  of  stone  of  a  yellowish-grey  colour. 
Only  on  looking  closer  will  it  be  noticed  that  the  soil  and  stones  are  studded  and 
spun  over  in  parts  with  dull  green  dots  and  delicate  threads,  and  that,  moreover, 
there  appears  a  delicate  filigree  of  tiny  moss-plants  rising  star-like,  pale  bluish- 
green  in  colour,  and  resembling  a  small  arched  feather  stuck  in  the  ground.  This 
phenomenon,  that  an  object  should  only  shine  in  dark  rocky  clefts,  and  immediately 
lose  its  brilliance  when  it  is  brought  into  the  bright  daylight,  is  so  surprising  that 
one  can  easily  understand  how  the  legends  have  arisen  of  fantastic  gnomes,  and 
cave-inhabiting  goblins  who  allow  the  covetous  sons  of  earth  to  gaze  on  the  gold 
and  precious  stones,  but  prepare  the  bitter  disappointment  for  the  seeker  of  the 
enchanted  treasure;  that,  when  he  empties  out  the  treasure  which  he  has  hastily 
raked  together  in  the  cave,  he  sees  roll  out  of  the  sacks,  not  glittering  jewels,  but 
only  common  earth. 

It  has  been  mentioned  that  on  the  floor  of  rocky  caves  one  may  discern  by 
careful  examination  two  kinds  of  insignificant-looking  plant-structures,  one  a  web 
of  threads  studded  with  small  crumbling  bodies,  and  the  other  bluish-green  moss- 
plants  resembling  tiny  feathers.  The  threads  form  the  so-called  protonema,  and 
the  green  moss-plants  grow  up  as  a  second  generation  from  this  protonema.  How 
this  comes  about  will  be  described  in  another  place;  here  it  only  interests  us  that 
the  gleams  do  not  issue  from  the  green  moss-plants,  but  only  from  their  protonema. 
If  this  is  viewed  under  the  microscope  a  sight  is  presented  like  that  depicted  in 
fig.  25A,  p.  From  the  much-branched  threads,  composed  of  tubular  cells,  which 
spread  horizontally  over  the  ground,  numerous  twigs  rise  up  vertically,  bearing 
groups  of  spherical  cells  arranged  like  bunches  of  grapes.  All  the  cells  of  a  group 
lie  in  one  plane,  and  each  of  these  planes  is  at  right  angles  to  the  rays  of  light 
entering  through  the  aperture  of  the  rocky  cleft.  The  grape-like  groups  of  cells 
have  longer  or  shorter  stalks,  but  they  always  appear  in  rows  side  by  side  and 
behind  one  another,  placed  cup-like,  that  the  anterior  groups  do  not  deprive  those 
behind  them  of  too  much  of  the  light  which  enters  the  cavity.  Each  of  the 
spherical  cells  contains  chlorophyll-granules,  but  in  small  number;  usually  four, 
six,  eight,  or  ten  and  they  are  always  collected  together  on  those  sides  of  the 

VOL.  I 


386  CHLOROPHYLL  AND  LIGHT   INTENSITY. 

cells  which  are  turned  towards  the  dark  background  of  the  cave.  There  they  are 
grouped  like  a  mosaic,  and  usually  so  that  one  green  granule  forms  the  centre, 
while  the  others  surround  it  very  regularly  in  a  circle.  Such  groups  remind  one  of 
the  arrangement  of  the  floral -leaves  in  Forget-me-not  flowers,  and  give  a  very 
ornamental  appearance  to  the  cells.  Taken  together,  these  chlorophyll-granules 
form  a  layer,  which,  under  a  low  power  of  the  microscope,  appears  as  a  round  green 
spot.  With  the  exception  of  these  chlorophyll-granules  the  contents  of  the  cell 
are  colourless  and  transparent,  and  share  these  characteristics  with  the  unusually 
delicate  cell-wall.  The  light  which  falls  on  such  cells  through  the  opening  of  a 
rocky  cleft  behaves  like  the  light  which  reaches  a  glass  globe  at  the  further  end  of 
a  dark  room.  The  parallel  incident  rays  which  arrive  at  the  globe  are  so  refracted 
that  they  form  a  cone  of  light,  and  since  the  hinder  surface  of  the  globe  is  within 
this  cone,  a  bright  disc  appears  on  it.  If  this  disc,  on  which  the  refracted  rays  of 
light  fall,  is  furnished  with  a  lining,  this  also  will  be  comparatively  strongly- 
illuminated  by  the  light  concentrated  on  it,  and  will  stand  out  from  the  darker 
surroundings  as  a  bright  circular  patch.  This  lining  has  the  power  of  manufac- 
turing organic  substances  in  the  spherical  cells  of  the  protonema  of  the  Luminous 
Moss,  and  in  this  way  the  scanty  incident  light  is  turned  to  the  greatest  possible 
advantage;  it  is  refracted  and  concentrated  on  those  places  where  the  chlorophyll- 
granules  are  situated,  and  consequently  these  receive  in  the  dark  recesses  an 
amount  of  light  which  amply  suffices  for  their  special  functions.  It  is  well 
worthy  of  notice  that  the  patch  of  green  chlorophyll-granules  on  the  hinder  side 
of  the  spherical  cell  extends  exactly  so  far  as  it  is  illumined  by  the  refracted 
rays,  while  beyond  this  region,  where  there  is  no  illumination,  no  chlorophyll- 
granules  are  to  be  seen.  The  refracted  rays  which  fall  on  the  round  green  spot  are, 
moreover,  only  partially  absorbed;  in  part  they  are  reflected  back  as  from  a 
concave  mirror,  and  these  reflected  rays  give  the  cells  of  the  protonema  a  luminous 
appearance.  This  phenomenon,  therefore,  has  the  greatest  resemblance  to  the 
appearance  of  light  which  the  eyes  of  cats  and  other  animals  display  in  half-dark 
places,  only  illumined  from  one  side,  and  so  does  not  depend  upon  a  chemical 
process,  an  oxidation,  as  perhaps  does  the  light  of  the  glow-worm  or  of  the 
mycelium  of  fungi  which  grows  on  decaying  wood.  Since  the  reflected  light-rays 
take  the  same  path  as  the  incident  rays  had  taken,  it  is  clear  that  the  gleams  of 
the  Schistostega  can  only  be  seen  when  the  eye  is  in  the  line  of  the  incident  rays  of 
light.  In  consequence  of  the  small  extent  of  the  aperture  through  which  the  light 
penetrates  into  the  rock  cleft,  it  is  not  always  easy  to  get  a  good  view  of  the 
phenomenon  described.  If  we  hold  the  head  close  to  the  opening,  we  thereby 
prevent  the  entrance  of  the  light,  and  obviously  in  that  case  no  light  can  be 
reflected.  It  is,  therefore,  better  when  looking  into  the  cave  to  place  one's  self  so 
that  some  light  at  anyrate  may  reach  its  depths.  Then  the  spectacle  has  indeed 
an  indescribable  charm.  What  has  just  been  said  about  the  isolated  cells  is 
exemplified  in  groups  of  cells  placed  behind  one  another,  of  which  usually 
many  thousands  are  found  in  a  very  small  area. 


CHLOROPHYLL  AND  LIGHT   INTENSITY. 

Among  the  mosses  which  find  their  home  in  deep  shady  places,  principally  in 
hollow  tree-trunks,  and  are  noticeable  there  for  their  glossy  green,  Ilookeria 
aple-nde-as  is  especially  worthy  of  attention.  To  be  sure,  its  leaves  do  not  shine  as 
brightly  as  the  protonema  of  Schistostega,  but  the  appearance  is,  on  the  whole,  much 
the  same,  and  here  also  a  similar  development  is  the  cause.  The  leaves  of  Hookeria 
are  comparatively  krge,  but  at  the  same  time  very  thin  and  delicate.  They  are 
composed  of  a  single  layer  of  rhombic  cells,  very  convex  above  and  below,  so  that 
the  whole  leaf  may  be  compared  to  some  extent  to  a  window  with  very  small 
so-caUed  "  bull's  eyes "  in  the  glass.  The  chlorophyll-granules  are  here  arranged 
with  far  less  regularity  than  in  the  protonema  of  the  Luminous  Moss,  but  they 
are  heaped  together  just  as  in  that  plant  on  the  side  of  the  leaf  facing  the  ground, 
that  is  to  say,  which  is  turned  from  the  light.  The  side  which  is  turned  hi  the 
direction  of  the  scanty  incident  light  has  no  chlorophyll  layer.  The  hemispherically- 
convex  cells,  opposed  to  this  scanty  light  which  falls  on  one  side  of  the  leaf,  act  like 
glass  lenses;  they  concentrate  the  weak  light  on  the  chlorophyll-granules  heaped 
up  on  the  other  side;  but,  on  the  other  hand,  light  is  also  reflected,  and  this  gives 
rise  to  the  green  lustre  with  which  the  Hookeria  shines  forth  from  its  dim  sur- 
roundings. 

Like  those  plants  which  inhabit  rocks,  grottoes,  and  stony  clefts,  and  the  shady 
obscurity  of  hollow  trunks,  plants  whose  habitat  is  at  the  bottom  of  the  sea,  and 
in  the  depths  of  lakes  and  ponds,  are  only  visited  by  weakened  sunbeams.  The 
illumination  becomes  the  dimmer  the  deeper  the  habitat  in  question  lies  below  the 
surface  of  the  water,  since  the  intensity  of  the  light  penetrating  the  water  dimin- 
ishes with  the  increasing  length  of  the  distance  travelled.  At  a  depth  of  200  metres 
under  the  sea  complete  darkness  reigns;  at  170  metres  the  intensity  of  illumination  is 
like  that  observed  above  the  water  on  a  moonlight  night;  such  an  illumination  is 
insufficient  to  enable  chlorophyll-bearing  plants  to  manufacture  organic  substances 
from  the  absorbed  raw  materials,  even  although  the  plants  were  provided  with  all 
possible  aids  for  the  collection  of  this  exceedingly  weak  light.  It  is  only  at  a  depth 
of  not  more  than  90  metres  that  light  is  sufficient  for  the  chlorophyll  cells  to 
decompose  carbonic  acid,  and  this  depth  is  ascertained  to  be  the  lowest  limit  of 
chlorophyll-bearing  plants.  Moreover,  these  figures  are  only  applicable  in  the  most 
favourable  circumstances  in  broad  daylight,  and  only  when  the  water  is  very  clear 
and  transparent,  which  really  only  seldom  occurs,  we  might  even  say  excep- 
tionally. The  substratum  on  which  the  submerged  plants  are  situated,  whether 
sand,  mud,  or  rock,  is  usually  sloping,  and  is  most  visited  by  the  oblique  rays  of  the 
sun.  Frequently  also  small  solid  particles  are  suspended  in  the  water,  even  in  water 
which  in  the  aggregate  appears  to  be  quite  clear,  and  so  the  light  is  again  con- 
siderably weakened  This  happens  especially  in  the  neighbourhood  of  steep  coasts, 
where  the  seething  of  the  waves  works  uninterruptedly  at  the  destruction  of  the 
solid  shore,  and  consequently  at  a  depth  of  60  metres  on  such  steep  declivities, 
plants  possessing  chlorophyll  are  seldom  met  with. 

Generally  speaking,  the  vegetation  in  the  sea  is  limited  to  a  zone  of  about  30  metres 


388  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

in  depth,  whose  width  varies  with  the  steepness  of  the  shore.  Below  this  narrow 
girdle,  vegetation  is  practically  extinguished,  and  the  depths  of  the  ocean  are  in  all 
parts  of  the  globe  a  plantless  waste.  This  statement  is  not  contradicted  by  the  fact 
that  sea-wracks  have  been  found  showing  a  length  of  100,  it  is  alleged  even  of  200 
and  300  metres,  as,  for  example,  the  celebrated  Macrocystis  pyrifera,  between  New 
Zealand  and  Tierra  del  Fuego.  These  sea- wracks  do  not  rise  perpendicularly  from 
the  bottom  to  the  surface  of  the  sea,  but  proceed  from  steep  declivities,  and  grow 
at  an  angle  to  the  surface,  on  which  account  they  often  take  the  direction  of  the 
current.  One  must  imagine  their  position  in  the  water  to  be  almost  like  that  of  the 
Floating  Pondweed,  or  the  Water  Crowfoot  (Potamogeton  fluitans  and  Ranunculus 
fluitans),  which  occur  in  brooks  only  a  few  decimetres  deep,  and  nevertheless  may 
attain  a  length  of  more  than  a  metre. 

It  is  naturally  to  be  expected  that  plants  which  grow  in  the  dim  light,  deep 
under  the  water  on  a  rocky  reef,  would  behave  exactly  like  the  grotto-inhabiting 
Luminous  Moss;  and  if  the  barrel-shaped  and  spherical  cell-structures  connected 
into  chains,  the  cyst-like  and  berry-shaped  outgrowths  of  the  unicellular  Caulerpas 
and  Halimedas,  as  well  as  the  facetted  cell-walls  of  those  diatoms  living  in  the 
abysses  of  the  sea  in  dim  twilight,  are  accepted  as  contrivances  by  which  light  is 
collected  and  focussed  on  those  places  within  the  cells  where  the  chlorophyll-bodies 
are  heaped  up,  then  no  mistake  will  be  made.  Several  of  the  sea-inhabiting  Floridese 
and  sea-wracks  belonging  to  the  genera  Phylocladia,  Polysiphonia,  Wrangelia, 
and  Cystosira,  even  exhibit  under  the  water  a  peculiar  luminosity  which  may  be 
compared  with  that  of  the  Luminous  Moss,  although  the  optical  apparatus  is  here 
essentially  different.  In  the  superficial  cells  of  the  luminous  Phylocladias  are  to  be 
found  plates  segregated  out  of  the  protoplasm  and  closely  adhering  to  the  outer 
walls,  which  contain  a  large  number  of  small  crowded  lenticular  bodies.  From  these 
minute  lenses  the  blue  and  green  rays  are  chiefly  reflected,  and  thus  the  peculiar 
iridescence  is  produced.  But,  on  the  other  hand,  yellow  and  red  rays  are  refracted 
on  to  the  chlorophyll-granules,  and  consequently  these  plates  must  be  regarded  as 
an  apparatus  for  focussing  the  light,  which,  by  its  passage  through  the  thick  layers 
of  water,  has  undergone  a  considerable  diminution. 

In  the  depths  of  the  sea,  however,  another  optical  phenomenon  must  be  taken 
account  of,  by  which  the  illumination  of  chlorophyll-granules  in  the  plants  growing 
there  becomes  in  the  end  a  favourable  one,  and  that  is  the  fluorescence  of  erythro- 
phyll,  the  fluorescence  of  that  red  pigment  to  which  the  Floridese  owe  their  charac- 
teristic colour.  In  order  to  make  this  phenomenon  clear,  it  seems  necessary  first 
of  all,  to  rectify  a  wide-spread  error  with  regard  to  the  colour  of  water  generally, 
and  particularly  of  sea- water.  In  the  very  attractively- written  work  by  Schleiden, 
The  Plant  and  its  Life,  the  seventh  chapter,  which  treats  of  the  sea  and  its 
inhabitants,  begins  with  the  following  lines:— "O  learn  to  know  them,  the  horrible 
deeps,  which  are  concealed  beneath  the  shining  treacherous  surface.  You  descend, 
the  blue  of  the  sky  vanishes,  the  light  of  day  is  gone,  a  fiery  yellow  surrounds 
you,  then  a  flaming  red,  as  if  you  were  plunged  into  a  watery  sea-hell,  without 


CHLOROPHYLL  AND  LIGHT  INTENSITY.  389 

glow  and  without  warmth.  The  red  becomes  darker,  purple,  finally  black  and 
impenetrable  night  holds  you  enchained".  This  description  is  founded  doubtless 
on  the  account  of  divers  of  the  olden  time,  according  to  which  red  light  should 
predominate  in  the  abysses  of  the  ocean.  These  accounts  must,  however,  be 
retained  only  to  the  following  extent.  The  cliffs  and  the  rocky  bottom  to  which 
the  divers  descended  might  have  been  richly  carpeted  with  red  Floridese,  possibly 
also  just  then  the  strata  of  water  above  were  filled  with  those  unicellular  red 
algse,  which  cause  the  so-called  "flowers  of  the  sea".  In  the  neighbourhood  of 
the  mouth  of  the  Tejo  at  times  a  superficial  area  of  sixty  million  of  square 
metres  is  coloured  scarlet  by  Protococcus  Atlanticus,  a  unicellular  alga,  40,000 
of  which  cover  only  a  square  millimetre;  and  Trichodesmium  Erythrceum, 
another  microscopic  alga  consisting  of  bundles  of  delicate  articulated  threads  in 
innumerable  milliards,  fills  the  watery  strata  in  the  Red  Sea  as  well  as  in  the 
Indian  and  Pacific  Oceans,  so  that  there  immeasurable  stretches  of  water  receive 
a  dingy  red  colouring.  When  these  algae  make  their  appearance  the  sea  is  said 
to  blossom,  and  at  those  times  the  depths  may  appear  to  the  diver  as  shrouded 
in  a  reddish-yellow  twilight.  At  times  the  same  colour  has  even  been  observed 
in  the  Lake  of  Geneva  when  its  waters  had  been  disturbed;  it  is  due  to  the 
fact  that  the  blue  rays  of  the  incident  light  are  weakened  by  the  fine  atoms 
suspended  in  the  water.  With  respect  to  this  occurrence,  we  may  consider  that 
the  above-mentioned  accounts  of  divers  are  not  the  results  of  intentional  decep- 
tion, but  only  refer  to  particular  cases.  They  cannot  be  applied  universally.  As 
a  matter  of  fact,  the  colour  of  sea-water,  in  direct  as  well  as  in  reflected  light, 
is  blue,  and  the  diver  who  carries  on  his  work  at  the  bottom  of  the  untroubled 
and  non- blossoming  sea,  is  not  surrounded  there  by  red,  but  by  blue  light. 
The  greater  the  quantity  of  salt  contained  in  the  water,  the  deeper  the  blue.  This 
blue  nowhere  appears  so  beautiful  and  so  deep  in  tint  as  in  the  Dead  Sea,  and  in 
the  region  of  the  Gulf  Stream  and  of  the  Kurosiur,  where  the  water  is  particularly 
rich  in  dissolved  salts,  and  also  has  in  the  upper  strata  a  comparatively  high  tem- 
perature. The  blue  colour  of  the  water  is  explained  thus:  from  among  the  rays 
which  are  characterized  by  different  wave-lengths  and  different  refrangibility 
(which,  taken  together,  form  colourless  daylight,  and  which  we  admire  separated 
in  the  colours  of  the  rainbow),  the  red,  orange,  and  yellow  are  absorbed  in  their 
passage  through  the  water,  and  only  those  rays  which  are  characterized  by  high 
refrangibility,  viz.  the  blue,  are  allowed  to  pass  through.  The  rays  on  the  further 
side  of  the  red,  not  perceptible  to  our  eyes,  the  so-called  dark  heat-rays,  are  like- 
wise absorbed  in  their  passage  through  the  water,  and  an  object  at  some  depth  under 
water  would  therefore  only  be  reached  by  rays  of  high  refrangibility,  particularly 
blue  rays.  The  conditions  of  illumination  for  plants  growing  in  the  depths  of  the 
ocean  are  consequently  in  reality  quite  unfavourable.  It  is  not  only  that  a  portion 
of  the  light  falling  on  the  surface  of  the  water  is  reflected,  and  the  other  portion  is 
weakened  by  its  passage  through  the  water,  but  besides,  those  rays  which  are 
necessary  to  the  formation  of  organic  matter  by  the  chlorophyll-granules  in  the 


390  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

plant  cells  are  abstracted  from  the  light  which  passes  through;  for  the  chlorophyll- 
granules  need  just  the  red,  yellow,  and  orange  rays  if  they  are  to  perform  their 
functions;  only  under  the  influence  of  these  rays  can  the  decomposition  of  carbonic 
acid,  the  separation  of  oxygen,  and  the  formation  of  carbohydrates,  take  place. 
The  blue  rays  do  not  assist  at  all  in  this  respect;  they  are  even  hurtful  to  these 
processes,  since  they  assist  the  oxidation,  that  is,  the  decomposition  of  organic 
substance.  Consequently,  phycoerythrin,  the  red  pigment  of  the  Florideae,  now 
appears,  and  indeed  so  abundantly,  that  the  chlorophyll-granules  in  the  interior 
are  quite  hidden  by  it.  This  colouring-matter  displays  a  very  marked  fluorescence, 
that  is  to  say,  it  absorbs  a  large  portion  of  the  light  rays  falling  on  it,  and  gives 
out  other  rays  of  greater  wave-length.  The  blue  rays  are  to  some  extent  changed 
by  it  to  yellow,  orange,  and  red,  and  thus  the  chlorophyll-granules  finally  receive 
those  rays  which  act  as  the  propelling  force  in  the  decomposition  of  carbonic  acid. 
But  this  also  affords  an  explanation  of  the  remarkable  phenomenon  that  sea- 
plants  are  only  coloured  green  close  to  the  shore,  and  only  in  the  most  superficial 
layers  of  water,  while  lower  down  they  appear  red.  Only  quite  on  the  surface  the 
emerald-like  Ulvaceae  and  Enteromorphas  sway  hither  and  thither,  forming  thus  a 
light-green  belt;  these  algae  are  to  be  sought  for  in  vain  in  the  depths  beneath;  of 
the  plants  which  flourish  below  this  region  it  can  no  longer  be  said  that  they  grow 
green;  this  mark  of  vegetation  has  entirely  vanished.  Green  has  given  place  to 
red.  All  the  innumerable  Floridese  are  reddened — sometimes  a  delicate  carmine, 
sometimes  a  deep  purple;  then  again  a  light  brownish-red  and  a  dull,  dark  crimson, 
and  as  we  admire  in  the  bush  the  innumerable  gradations  of  green  colour,  so  is  the 
eye  delighted  in  the  manifold  shades  of  red,  in  which  the  different  variegated 
species  of  Floridese,  intermixing  with  one  another,  display  themselves. 

Let  us  now  leave  the  blue  twilight  of  the  sea-depths,  and  set  foot  on  the  strand 
lapped  by  the  blue  waves  sparkling  with  white  foam,  and  climb  up  one  of  the  rocky 
crags  rising  there  above  the  seething  waters.  Around  us  is  the  bright  daylight, 
and  broad  terraces  of  rock  thickly  overgrown  with  plants,  all  brilliantly  illumined 
by  the  unclouded  sun.  But  where  is  that  fresh  green  which  we  expect  to  find  up 
here  according  to  the  foregoing  definitions  in  herbs  and  bushes?  Here  are  not  green, 
but  grey  foliage  and  branches,  white-haired  stems  and  leaves,  and  the  whole 
woven  and  bound  together  into  a  carpet,  which  looks  as  if  it  had  been  strewn  with 
ashes,  or  as  if  the  wind  had  for  a  week  brought  hither  the  dust  from  the  neigh- 
bouring streets  and  deposited  it.  The  plants  here  on  the  sunny  rocks  have  pro- 
vided themselves  with  silky,  woolly,  and  felted  coverings  for  the  purpose  of  softening 
the  too  glaring  light.  In  the  depths  of  the  sea  and  in  the  grottoes  of  the  slate 
rocks,  the  light  was  too  weak;  here,  however,  it  is  too  strong.  The  chlorophyll- 
granules  tolerate  neither  the  one  nor  the  other;  they  require  light  of  a  definite 
intensity.  If  the  limit,  which  in  this  matter  is  exactly  defined  for  each  species,  is 
overstepped,  the  chlorophyll  is  destroyed.  Too  much  light  may  be  no  less  injurious 
to  the  plants  than  if  the  chlorophyll -granules  are  condemned  to  inactivity  on 
account  of  the  want  of  light. 


CHLOROPHYLL   AND  LIGHT   INTENSITY.  391 

How  quickly  a  glaring  light  is  able  to  destroy  the  chlorophyll  can  be  well  seen 
in  the  green  Sea-lettuce  on  the  shore  below.  In  a  high  sea  a  violent  wave  tears 
fragments  of  the  Ulvacese,  known  under  the  name  of  Sea-lettuce,  from  the  coast- 
rocks;  a  second  wave  as  it  rushes  up  washes  the  leaf -like  structures  on  to  the 
shingle  of  the  shore,  and  there  it  remains  with  other  de'bris  lying  amongst  the 
stones.  The  sea  now  becomes  calm,  the  sky  has  cleared,  the  sun's  rays  are  again 
burning  with  undiminished  strength  on  the  shadeless  strand.  As  long  as  the  Sea- 
lettuce  adhered  closely  to  the  rocks  below  the  surface  of  the  water  it  displayed  a 
brilliant  emerald  green;  the  water  in  which  it  was  submerged  to  some  little  depth, 
even  at  a  low  tide,  sufficed  to  somewhat  temper  the  sunlight;  but  the  stranded  Ulva 
is  deprived  of  this  light-regulating  covering  of  water,  and  in  a  few  hours  its 
chlorophyll  is  destroyed.  It  is  turned  yellow,  and  looks  like  a  lettuce-leaf  which 
has  lain  for  a  week  in  a  dark  cellar.  A  similar  appearance  is  also  seen  in  confervas 
and  spirogyras  which  fill  stagnant  pools  of  water  with  their  masses  of  united  fila- 
ments. Two  decimetres  below  the  water  they  display  a  beautiful  dark-green  colour, 
while  close  to  the  surface  they  appear  a  yellowish-green,  and  if  the  pool  dries  up  so 
that  the  masses  of  filament  come  to  lie  on  the  damp  slime,  in  two  days  they  are 
quite  bleached;  the  undimmed  sunlight  has  completely  destroyed  the  chlorophyll  in 
the  cells.  In  the  depth  of  beech-groves  the  Woodruff  (Asperula  odorata)  raises  its 
leaves  arranged  in  whorls  on  the  stem ;  over  it  the  thickly-leaved  branches  of  the 
beeches  bend  together,  forming  a  roof  through  whose  interstices  only  here  and  there 
a  weak  sunbeam  finds  its  way  into  the  depths.  In  the  dim  light  the  leaf -stars  of 
the  Woodruff  appear  of  a  deep,  dark-green  tint.  Now  the  axe  of  the  woodcutter 
resounds  through  the  forest— the  beeches  are  felled,  the  shading  roof  of  foliage  is 
demolished,  and  the  floor  of  the  wood  is  exposed  to  the  glaring  sunbeams.  Within 
two  weeks  the  Woodruff  can  no  longer  be  recognized;  it  has  become  sickly  and  pale; 
the  leaf -stars  have  lost  their  dark  green,  and  the  chlorophyll  has  been  destroyed  by 
the  glaring  light.  The  same  thing  occurs  with  ferns  as  with  the  Woodruff.  In  the 
dimness  of  the  floor  of  the  forest,  between  steep- walled  rocks,  and  on  shady  northern 
declivities  they  are  tinted  dark  green;  in  sunny  situations  they  become  pale, 
and  then  are  noticeably  retarded  in  growth.  All  these  plants  are  not  organized  to 
adapt  themselves,  in  the  case  of  an  alteration  of  the  illumination  of  their  habitat,  to 
the  new  conditions  and  to  protect  themselves  from  the  undimmed  rays  falling  on 
them.  They  are  only  fitted  for  the  shady  floor  of  the  wood,  and  an  over-abunda 

of  light  is  their  death. 

But  how  is  the  vegetation  protected  in  a  hahitat  where  during  the  who! 
the  vegetative  period  full  light  predominates,  where  the  sun  makes  itself 
rise  to  setting  with  uninterrupted  power?    It  has  already  been  pointed  out  that , 
plants  on  the  broad  ridges  and  terraces  of  the  rocky  shores  of  the  Mediterranean  are 
shrouded  in  dull  grey,  clothed  in  silk  or  wool,  or  else  oversown  with  chaff- 
scales,  and  consequently  have  lost  their  fresh  green  colour.     In  reality  , 
quite  correct  to  say  that  they  have  "lost"  the  green,  for  their  parenchymatous  eel 
especially  those  of  the  palisade  and  spongy  tissues  in  the  foliage-leaves,  are  no 


392  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

rich  in  chlorophyll-granules  than  those  of  shaded  plants,  only  they  have  developed 
from  their  epidermal  cells  those  structures  which  have  been  previously  described 
as  covering  hairs.  These  cellular  structures,  devoid  of  chlorophyll,  cover  over  the 
green  tissue,  and  thus  give  to  the  leaf  in  question  a  grey  or  white  colour.  They 
play  the  part  of  awnings  and  light-extinguishers,  and  when  they  are  removed  the 
leaf  appears  just  as  green  as  one  that  has  been  plucked  from  the  shade  of  the  wood. 

Silky,  velvety,  and  woolly  coats  may  thus  doubtless  take  on  the  function  of 
extinguishers.  We  meet,  therefore,  the  same  contrivances  apparently  which  already 
on  a  previous  occasion  have  been  treated  of,  viz.  when  describing  the  protective 
measures  against  excessive  transpiration.  Thus  through  these  structures  two  birds 
are  killed  with  one  stone.  All  contrivances  which  keep  off  too  glaring  sunbeams, 
and  thereby  hinder  the  destruction  of  chlorophyll,  at  the  same  time  diminish  trans- 
piration; and  inasmuch  as  these  contrivances  perform  two  such  important  functions 
for  the  life  of  plants,  their  wide  distribution  and  great  diversity  is  accounted  for. 
Suited  to  the  conditions,  adapted  to  the  habitat  and  season  of  the  year,  and  in 
harmony  with  other  developments,  they  change  in  a  thousand  ways,  and  thus 
display  a  diversity  which  can  scarcely  be  treated  exhaustively.  Besides  the 
covering  hairs  which  are  placed  above  the  green  tissue,  as  a  protection  and  shade 
against  too  intense  light,  and  at  the  same  time  against  excessive  transpiration, 
obviously  all  the  other  contrivances  previously  described  are  to  be  taken  into 
account.  The  development  of  one  or  several  layers  of  cells,  filled  with  watery 
cell-sap,  above  the  tissue  exposed  to  the  sun's  rays,  the  thickening  of  the  cuticular 
layers,  the  waxy  and  varnish-like  coatings,  the  lime  incrustations  and  salt 
excretions,  the  diminution  of  the  illuminated  portion  of  the  leaf-surface,  the 
formation  of  wrinkles,  folds,  pits,  and  grooves  on  the  illumined  surface  of  the 
foliage — all  these  are  able  to  interrupt  and  diminish  the  rays  and  to  reduce  their 
intensity  to  the  right  degree. 

The  number  of  the  special  contrivances  which  simply  secure  chlorophyll  from 
destruction  by  too  glaring  light,  without  at  the  same  time  protecting  the  green 
tissue  from  excessive  transpiration,  must  indeed  be  very  small.  First  of  all,  we 
may  mention  the  dry  thin-skinned  scales  which  in  many  plants  are  inserted  between 
the  green  leaves.  These  are  seen,  for  example,  in  species  of  the  genus  Paronychia, 
which  in  masses  have  their  habitat  in  sunny  places,  and  produce  silver-glittering 
transparent  scales,  devoid  of  chlorophyll,  close  to  that  portion  of  the  stem  from 
which  the  small  green  leaves  originate.  These  scales,  which  are  designated  stipules, 
and  which,  here,  are  usually  as  large,  occasionally  even  larger,  than  the  green  leaves, 
take  up  naturally  such  a  position  in  the  plants  growing  on  shadeless  hillocks  that 
the  sun's  rays  first  of  all  fall  on  them,  and  only  reach  the  green  leaflets  in  a 
weakened  state. 

Another  arrangement,  which  indeed  is  able  to  restrict  the  destruction  of  the 
chlorophyll  by  the  sun's  rays,  without  affecting  transpiration,  consists  in  the 
development  of  a  blue  or  violet  colouring-matter  in  those  cells  which  compose  the 
superficial  covering  of  the  leaves  and  stem  which  is  directly  illuminated  by  the  sun's 


CHLOROPHYLL   AND   LIGHT   INTENSITY.  393 

rays.  Such  an  arrangement  is  found,  for  example,  in  the  leaves  of  the  aromatic 
Satureja  hortensis,  originally  growing  wild  in  the  Mediterranean  floral  district,  and 
cultivated  in  gardens  under  the  name  of  Summer  Savory,  of  which  leaves  a  small 
portion  is  represented  in  cross  section  on  page  139,  figure  25  A,  q.  Before  the 
sunbeam  reaches  the  chlorophyll-granules  of  the  green  cells  in  the  middle  of  the 
leaf,  it  must  pass  through  these  epidermal  cells  filled  with  violet  sap,  and  here  it 
becomes  so  weakened  and  also  so  changed  that  an  injurious  influence  on  the 
chlorophyll-granules  is  out  of  the  question.  We  must  not  omit  to  notice  here  that 
the  violet  light-reducing  colouring-matter  in  the  epidermal  cells  is  more  abundantly 
developed  the  intenser  the  light  to  which  the  plants  in  question  are  exposed.  If 
plants  of  the  Summer  Savory  grow  in  shady  places,  their  leaves  remain  green  on 
the  upper  sides,  and  scarcely  any  traces  of  the  violet  colouring-matter  are  to  be 
discovered  in  the  epidermal  cells.  If,  on  the  other  hand,  they  have  germinated  in 
shadeless  districts,  both  stem  and  leaves  are  coloured  dark  violet,  and  the  cell-sap  in 
the  epidermal  cells  is  then  of  a  deep  tint  (see  fig.  25  A,  q  on  page  139).  Some  years 
ago  I  cultivated  seeds  of  the  Summer  Savory  in  my  experimental  garden  at  a  height 
of  2195  metres  above  the  sea-level  in  the  Tyrol.  As  is  known,  the  sun's  rays  are 
much  more  powerful  in  the  Alpine  heights  than  in  the  valley,  and  it  was  therefore, 
indeed,  to  be  expected  that  the  leaves  of  the  germinating  plants  would  be  of  a  much 
darker  tint  than  in  the  shadeless  gardens  of  the  valley  below.  In  fact,  the  colouring- 
matter  developed  in  extraordinary  abundance;  even  the  stems  and  leaves  actually 
became  a  dark  brownish  violet.  It  is,  therefore,  beyond  question  that  the  quantity 
of  colouring-matter  in  the  epidermal  cells  directly  exposed  to  the  sun  increases 
with  the  increase  of  the  intensity  of  the  light.  Obviously  this  protection  of  the 
chlorophyll  can  only  occur  when  the  plants  possess  the  materials  for  forming  the 
pink  colouring-matter  in  their  green  organs.  When  this  is  not  possible,  when  the 
characteristic  constitution  of  the  protoplasm  does  not  permit  the  development  of 
the  colouring-matter  named  in  the  foliage-leaves,  the  chlorophyll  must  be  pro- 
tected against  the  glaring  light  in  another  way,  and  if  the  plant  species  is  not 
able  to  ward  off  the  over-abundance  of  sunlight  in  the  new  position,  it  perishes 
entirely.  Flax  (Linum  usitatissimum)  was  sown  next  to  the  Summer  Savory  in 
the  Alpine  experimental  garden — a  plant  which  bears  the  direct  sunlight  quite 
well,  and  flourishes  in  the  valley  as  well  as  in  the  plains  in  sunny  situations. 
However,  the  light  of  the  Alpine  region  was  too  brilliant  for  the  germinating  flax- 
plants;  the  leaves  turned  yellow,  their  chlorophyll  was  destroyed,  and  the  seedlings 
became  pale  and  perished.  Flax  has  not  the  capacity  of  manufacturing  the 
colouring-matter  in  its  superficial  cells,  and  it  is  also  not  organized  to  produce 
covering  hairs  on  the  leaves  and  stem,  or  to  thicken  its  cuticular  strata  suitably- 
in  a  word,  to  adapt  itself  to  the  position  and  to  provide  itself,  under  the  increased 
light  intensity,  with  corresponding  sun-shades  and  light-extinguishers.  While  close 
at  hand,  the  Summer  Savory,  which  requires  just  as  much  warmth,  and  an  equally 
long  vegetative  period  as  flax,  reached  the  flowering  stage,  and  even  produced  ripe 
fruits  capable  of  germinating,  the  flax  died  before  the  development  of  its  flowers. 


394  CHLOROPHYLL   AND   LIGHT   INTENSITY. 

From  these  culture  experiments  two  things  may  be  learned :  first,  that  a  very 
brilliant  light  is  able  to  influence  the  distribution  of  plants  and  to  set  up  an 
impassable  barrier  for  many  of  them ;  and  secondly,  that  many  plants  have  the 
capacity  of  adapting  themselves  to  various  degrees  of  light  intensity;  but  in  conse- 
quence of  this  they  occasionally  develop  such  a  varying  character  that  they  might 
be  mistaken  for  wholly  different  species.  But  I  shall  return  again  later  when 
speaking  of  the  origin  of  new  species  to  this  result  of  cultivation.  Here  we  shall 
only  discuss,  in  order  to  prove  and  make  clear  the  connection  between  certain  plant 
characteristics  and  the  conditions  of  illumination,  how  it  happens  that  the  surface 
of  foliage  exposed  to  the  direct  rays  of  the  sun  is  so  frequently  coloured  violet  or 
red,  or  is  completely  covered  over  with  hairs,  while  the  leaves  of  the  same  species  if 
they  have  been  developed  on  shady  soil  in  dispersed  light  are  coloured  green,  and 
remain  almost  bare;  how  it  happens  that  plants  of  one  and  the  same  species  in  the 
deep  valleys  possess  but  few  hairs,  or  are  provided  with  but  thin  cuticular  layers, 
but  on  the  sunny  slopes  of  high  mountains  are  shrouded  in  thick  grey  or  white  fur, 
or  appear  thick  and  almost  leathery  in  consequence  of  strongly-developed  cuticular 
layers.  In  order  to  prevent  misconception,  it  must  indeed  be  pointed  out  here  that 
all  this  only  refers  to  the  epidermis  over  the  green  tissue  which  is  exposed  to  direct 
or  diffuse  sunlight,  chiefly,  therefore,  to  the  upper  side  of  the  foliage-leaf,  and  that 
when  the  blue  colouring-matter  and  also  the  covering  hairs  are  developed  on  the 
under  side  of  the  leaf,  or  in  floral  leaves  devoid  of  chlorophyll,  they  have  then  an 
essentially  different  significance,  which  will  be  described  in  the  next  section. 

When  describing  the  protective  measures  of  the  green  tissues  against  the 
dangers  of  over-transpiration,  the  vertical  direction  of  branches,  flattened  shoots, 
phyllodes,  and  especially  of  the  green  leaf -surfaces,  was  pointed  out.  The  leaves  of 
irises,  and  of  the  so-called  compass-plants,  the  flattened  outspread  petioles,  with 
their  edge  directed  towards  the  zenith,  in  so  many  Australian  trees  and  shrubs, 
were  there  more  especially  described,  and  finally  it  was  pointed  out  that  the 
leaflets  of  many  papilionaceous  plants,  and  the  leaves  of  numerous  grasses, 
temporarily  take  up  a  position  by  sinking,  rising,  and  folding  together,  in  which 
not  the  broad  side,  but  the  narrow  edge,  is  exposed  to  the  vertical  rays  of  the 
mid-day  sun. 

A  leaf-surface  which  assumes  one  of  these  positions  with  regard  to  the  sun 
will  transpire  much  less  than  a  foliage-leaf  on  whose  broad  surface  the  mid-day 
sun  falls  vertically,  or  almost  vertically;  but  by  such  a  position  the  leaf  is  also 
afforded  a  protection  against  the  too  vivid  light  of  noon.  The  rays  which  reach  a 
vertical  leaf -surface  at  morning  and  evening  are  not  so  intense  as  to  be  able  to 
destroy  chlorophyll;  they  have  rather  just  that  intensity  which  the  chlorophyll- 
granules  require  for  their  activity.  Therefore,  by  this  arrangement  the  function  of 
the  chlorophyll-granules  is  not  restricted,  but  is  actually  assisted,  and  in  this  sense 
the  vertical  direction  of  the  green  surfaces  is  to  be  looked  upon  also  as  an 
arrangement  for  regulating  the  activity  of  the  chlorophyll-granules. 

It  is  evident  after  this  explanation  that  herbs  with  vertically-directed  leaf- 


CHLOROPHYLL  AND  LIGHT  INTENSITY.  395 

surfaces  are  never  to  be  met  with  in  shady  places.  On  the  floor  of  a  thick  wood 
grow  no  irises  and  no  compass-plants;  these  are  at  home  on  the  ridges  of  rocky 
mountains,  and  on  treeless  prairies,  and  if  it  happens  that  a  seed  of  such  a  plant 
falls  into  the  shade  of  a  wood  and  germinates  there,  developing  foliage-leaves,  then 
the  leaf-surfaces  do  not  assume  a  vertical  position,  and  twist  and  bend  themselves 
until  their  broad  surface  is  turned  towards  the  scantily-penetrating  diffuse  light. 
If  the  light  falls  from  above  through  the  interstices  of  the  leafy  covering,  the 
leaf -surf  ace  becomes  horizontal  and  parallel  to  the  ground;  if  the  crests  of  the  trees 
close  together  to  form  a  thick,  opaque  canopy,  and  the  diffuse  light  penetrates  from 
the  side  between  the  trunks  of  the  trees,  the  leaf-laminae  bend  and  turn  to  the 
openings  of  the  wood,  giving  the  impression  that  they  are  looking  out  longingly 
to  the  sunny  country  which  borders  the  dense,  deep-shaded  forest. 

The  same  thing  is  seen  under  every  small  shady  bush,  and,  generally  speaking,  in 
all  places  where  dissimilar  tall  plants  overlap  one  another,  and  where  the  leaves  of 
the  lower  are  arched  over  by  those  of  the  higher  plants.  If  they  belong  to  different 
species,  they  cannot  be  said  to  have  any  consideration  for  one  another.  Each  looks 
out  only  for  itself,  and  the  lofty  species  do  not  trouble  themselves  about  the 
inferior  stuff  which  arises  from  the  soil  under  their  leaves.  If  in  the  depths  below 
there  are  plants  which  find  all  they  require  in  the  diffuse  light  and  the  green  rays 
passing  through  the  leafy  roof,  very  well;  if  not,  these  lower  plants  must  perish  in 
the  shade.  But  it  is  otherwise  if  the  leaves  overlapping  each  other  belong  to  one 
and  the  same  branch,  to  one  and  the  same  plant;  where  they  must  co-operate  for 
the  weal  of  the  whole  plant,  and  the  whole  can  only  maintain  itself  in  the  struggle 
for  existence  by  harmonious  division  of  labour.  Therefore  care  must  be  taken  that 
no  leaf  shall  take  too  much  light  away  from  another;  that  one  shall  protect  and 
support  the  other;  that  neighbours  shall  not  jostle  if  one  or  the  other  has  to  bend, 
turn,  and  extend  itself  in  order  to  best  adapt  itself  to  the  incident  light. 

And  this  foresight  actually  occurs.  It  is  exhibited,  first  of  all,  in  the  position 
of  the  leaves  on  the  stem,  or  in  other  words,  in  the  regulation  of  the  intervals 
between  the  places  of  origin  of  neighbouring  leaves;  secondly,  by  the  fact  that  the 
stalks  of  the  green  leaf -blades  have  the  capacity  of  rising  and  sinking,  twisting  and 
bending,  and  also  of  elongating  if  required;  and  thirdly,  through  the  form  which  the 
leaf -surfaces  possess. 


396  DISTRIBUTION   OF   THE   GREEN   LEAVES   ON    THE   STEM. 


2.   THE   GEEEN    LEAVES. 

Distribution  of  the  green  leaves  on  the  stem.— Eelation  between  position  and  form  of  green  leaves. 
—Arrangements  for  retaining  the  position  taken  up.— Protective  arrangements  of  green  leaves 
against  the  attacks  of  animals. 

DISTRIBUTION  OF  THE  GKEEN   LEAVES   ON  THE  STEM. 

Landscape  painters  tell  us  how  difficult  it  is  to  treat  foliage  correctly,  and  at  the 
same  time  artistically;  how  hard,  for  instance,  so  to  reproduce  the  leafy  crown  of 
maples,  beeches,  elms,  limes,  and  oaks  that  they  shall  immediately  be  recognized 
for  that  which  they  are  intended  to  represent,  and  at  the  same  time  that  that  effect 
and  tone  shall  be  produced  which  is  aimed  at  in  the  picture.  The  variety  of  the 
foliage  is  caused  not  least  by  the  distribution  of  the  green  leaves  on  the  branches, 
and  by  the  branching  dependent  upon  this;  things  as  definite  as  possible  for  each 
species  of  tree,  and,  generally  speaking,  for  every  plant. 

On  cutting  various  leafy  branches  and  observing  the  distribution  of  the  leaves 
on  them,  the  following  differences  first  strike  the  eye.  In  numerous  plants  it  is 
seen  that  two  or  more  leaves  originate  at  the  same  height  on  a  branch,  while  in 
many  other  plants,  at  a  particular  level  of  the  stem  or  branch,  only  a  single  leaf 
is  produced.  In  order  to  be  able  to  understand  these  circumstances,  it  is  advisable 
to  imagine  the  leaf-bearing  shoot  or  stem  as  a  cone.  The  apex  of  the  cone 
corresponds  to  the  upper  end,  and  the  base  of  the  cone  to  the  lower  portion,  i.e.  to 
the  oldest  part  of  the  shoot.  The  whole  shoot  is  not  at  any  time  in  a  completed 
state;  it  continues  to  grow  at  the  apex,  and  at  the  upper  part  is  not  only  younger, 
but  is  also  less  bulky  than  the  older  portions  lying  nearer  to  the  base.  It  can, 
therefore,  indeed  be  quite  well  compared  to  a  cone,  although  this  form  is  only 
seldom  so  noticeably  to  be  met  with  as  in  the  following  diagrammatic  figures. 

That  which  applies  to  the  age  of  the  various  portions  of  the  shoot  naturally 
applies  also  to  the  leaves  projecting  from  the  shoot.  That  is  to  say,  the  lower 
leaves  of  the  shoot  are  the  older,  the  upper  leaves  are  the  younger.  On  looking  at 
the  top  of  the  cone  (see  fig.  98),  the  places  of  insertion  of  the  older  leaves  appear  to 
arise,  first  of  all,  from  the  circular  disc  which  forms  the  base  of  the  cone,  while  the 
younger  leaves  originate  close  to  the  apex,  therefore  close  to  the  centre.  The  stem 
is  to  a  certain  extent  divided  up  by  the  leaves  into  sections  one  above  another. 
Usually  it  is  somewhat  thickened  or  knotted  at  those  places  where  the  leaves 
project  from  it,  and  therefore  the  places  of  origin  of  the  leaves  are  designated  as 
nodes.  Each  portion  of  the  stem  lying  between  two  successive  nodes  is  called  an 
internode.  When  two  leaves  project  at  the  same  height  from  the  stem,  they  are 
inserted  opposite  one  another,  not  unlike  the  two  extended  arms  of  a  human  body, 
and  they  appear  on  the  cone-shaped  stem  (whose  cross  section  at  all  heights 
forms  a  circle)  at  a  distance  from  one  another  of  exactly  half  the  circumference 
of  the  circle  (180°),  (fig.  981).  If  three  leaves  spring  together  from  the  stem, 


DISTRIBUTION   OF  THE  GEEEN   LEAVES  ON   THE  STEM.  397 

as,  for  example,  in  the  Oleander,  these  are  separated  from  one  another  in  a 
horizontal  direction  by  one-third  of  the  circumference  of  the  circle  (120°)  Several 
leaves  springing  from  the  same  height  form  together  a  whorl,  and  the  distance  of 
the  individual  members  of  a  whorl  from  one  another  is  called  the  horizontal 
distance,  or  the  divergence.  The  divergence  amounts  to  i  in  fig  98 '  and  i  in 
fig.  98  \  of  the  circumference  of  the  circle,  and  can  be  thus  shortly  expressed  bv 
means  of  these  fractions. 

It  is  very  remarkable  that  the  whorls  which  follow  after  and  above  one 
another  according  to  their  age  on  one  and  the  same  shoot  do  not  originate  at 
corresponding  places  of  the  circumference,  but  are  displaced  regularly  with  regard 
to  one  another.  Thus  the  point  of  origin  of  the  second  two-membered  whorl  in 


Fig.  98. -Plan  of  Whorled  Phyllotaxis. 
i  Two-membered  WhorL     a  Three-mem  bered  WhorL 

fig.  98 *  is  shifted  through  a  quarter  of  the  circumference  (i.e.  through  90°,  a  rig}  it 
angle)  from  the  point  of  origin  of  the  first,  oldest,  and  lowest  two-membered  whorl. 
The  third  whorl  is  again  shifted  through  a  right  angle  with  regard  to  the  second, 
and  so  it  continues  up  the  stem  as  far,  generally  speaking,  as  foliage-leaves  are  to 
be  found  on  it.  If  the  stem  is  elongated  in  the  case  described,  four  rectilineal  lines 
(orthostichies)  appear  to  be  developed  on  it  (fig.  98 J).  If  a  whorl  is  composed  of 
three  leaves,  and  if  the  successive  whorls  be  displaced  through  one-sixth  of  the 
circumference,  as,  for  example,  in  the  Oleander  (see  fig.  98 2),  six  rectilineal  series  of 
leaves  or  orthostichies  originate,  running  parallel  to  one  another  down  the  stem. 

The  leafy  stem  can  also  be  imagined  as  divided  into  stories,  each  of  which 
displays  the  same  number,  position,  and  distribution  of  the  leaves,  and  agrees 
completely  in  the  plan  of  its  construction  with  the  adjoining  story.  In  one  such 
case  (fig.  981),  each  story  possesses  four  leaves  in  the  form  of  a  cross;  in  another 
case  (fig.  98  *),  it  possesses  two  sets  of  three  leaves  separated  from  one  another  by 
a  distance  of  60°.  If  the  stories  standing  above  one  another  are  separated,  they 
would  be  so  alike  in  arrangement  as  to  be  easily  mistaken  for  one  another.  Each 


398  DISTRIBUTION   OF   THE   GREEN   LEAVES   ON   THE   STEM. 

originates  below  and  ends  above  exactly  like  the  one  over  and  the  one  under  it,  and 
the  only  difference  rests  in  the  fact  that  the  sections  closer  to  the  summit  of  the 
branch  have  smaller  diameters,  and  often  also  a  somewhat  different  outline  of  their 
members.  The  plan  of  construction  is,  however,  as  stated,  exactly  the  same  in  the 
successive  stories. 

In  those  instances  where  each  story  consists  of  two  whorls  of  leaves,  which  are 
displaced  with  regard  to  one  another  through  a  certain  angle,  especially  in  the  very 
common  case  where  the  whorl  is  two-membered,  i.e.  where  the  leaves  are  opposite 
one  another  in  pairs,  and  where  the  successive  pairs  of  leaves  are  alternately 
displaced  through  a  right  angle  from  one  another,  appearing  thus  like  a  cross,  the 
leaves  are  said  to  be  decussate.  This  arrangement  is  seen  especially  in  maples  and 
ashes,  in  lilac  and  olive-trees,  in  elder  and  honeysuckle,  in  labiates,  gentians, 
Apocynacese,  and  numerous  other  families  of  plants. 

Still  more  common  than  this  arrangement  of  the  leaves  is  that  which  has  been 
called  the  spiral.  Here  at  one  and  the  same  height  only  a  single  leaf  originates 
from  the  stem,  and  therefore  all  the  leaves  of  a  stem  are  not  only  shifted  with 
respect  to  one  another  in  a  horizontal,  but  also  in  a  vertical  direction.  If  one 
imagines  the  nodes  of  a  stem  with  decussate  leaves  to  be  so  arranged  longitudinally 
that  the  leaves  are  inserted  no  longer  at  the  same  heights,  but  at  definite  intervals 
above  one  another,  then  from  the  decussating,  i.e.  whorled,  arrangement  a  spiral  is 
produced.  In  many  willows  (e.g.  Salix  purpurea),  and  very  regularly  also  in  some 
buckthorns  (e.g.  Rhamnus  cathartica),  in  the  speedwells  (e.g.  Veronica  spicata  and 
longifolia),  and  also  in  many  composites  leaves  arranged  partly  in  whorls  and 
partly  in  spirals  occur  on  the  same  axis,  and  doubtless  the  one  merges  into  the 
other,  but  for  the  sake  of  clearness  it  is  better  to  keep  them  distinct,  and  to  draw 
a  line  between  them,  even  though  it  be  an  imaginary  one. 

It  may  be  observed  that  stems  with  spirally-arranged  leaves  are  constructed 
exactly  like  those  which  bear  leaf -whorls,  and  that  they  consist  of  many  stories 
each  displaying  a  similar  plan  of  construction,  so  that  the  number,  position,  and 
distribution  of  the  leaves  is  repeated  in  each  story,  and  as  a  matter  of  fact  the 
following  plans  of  construction  are  actually  to  be  found  very  frequently. 

First  case.  In  each  story  only  two  leaves  arise  from  the  circumference  of  the 
stem.  These  two  leaves  are  displaced  with  regard  to  one  another  in  a  horizontal 
as  well  as  vertical  direction,  and  their  horizontal  divergence  amounts  to  half  the 
circumference  of  the  circle  (180°)  as  shown  in  the  plan  in  fig.  99 x.  If  a  continuous 
line  be  drawn  from  the  point  of  insertion  of  each  lower  older  leaf  to  the  younger 
one  next  above  it  on  the  surface  of  the  stem,  this  will  display  the  form  of  a  spiral. 
It  has  been  called  the  genetic  spiral  In  the  first  case  here  discussed  it  forms  in 
each  story  only  a  single  spiral  band.  This  arrangement  is  repeated  in  the  second, 
third,  and  perhaps  in  many  other  stories  which  follow  successively  on  the  same 
axis.  In  this  way  the  lower  leaf  of  the  second,  third,  or  fourth  story  always  lies 
exactly  above  the  lower  leaf  of  the  first  story.  The  same  applies  to  the  upper 
leaves  of  all  the  stories.  Thus  two  rectilineal  lines  or  orthostichies  are  formed  on 


DISTRIBUTION   OF  THE  GREEN   LEAVES  ON   THE  STEM  399 

the  circumference  of  the  stem  by  the  leaves  situated  vertically  above  one  another. 
The  two  lines  are  opposite,  or,  what  comes  to  the  same  thing,  they  are  separated 
from  one  another  by  half  the  circumference  of  the  stem.  This  arrangement  of  the 
leaves,  which  may  be  observed,  for  example,  on  the  branches  of  elms  (Ulmus)  and 
limes  (Tilia),  is  called  the  one-half  phyllotaxis. 

Second  case.  Three  leaves  are  developed  in  one  story,  each  at  a  definite  height, 
an  under,  a  middle,  and  an  upper  leaf.  In  a  horizontal  direction  two  of  the  leaves 
following  one  another  in  age  are  always  shifted  from  one  another  through  a  third 
part  of  the  circumference  (see  fig.  99  2).  If  the  point  of  insertion  of  the  lower  leaf 
is  connected  with  that  of  the  middle  leaf,  and  this  again  with  that  of  the  upper 
leaf  by  a  line,  and  this  line  is  continued  to  the  beginning  of  the  next  story,  a  single 
spiral  is  thus  formed  surrounding  the  stem.  Now  above  the  story  just  described, 
which  we  will  call  the  lowest,  a  second  follows,  which  is  again  provided  with  three 
leaves  arranged  in  exactly  the  same  way.  The  lower  leaf  of  the  second  story  is 
situated  vertically  above  the  lower  leaf  of  the  first  story,  the  middle  above  the 
middle,  and  the  upper  above  the  upper  leaf,  and  the  same  arrangement  is  continued 
through  all  the  stories.  In  this  manner  three  rectilineal  lines,  or  orthostichies, 
arise  on  the  circumference  of  the  stem  from  the  leaves  situated  above  one  another, 
and  each  of  the  lines  is  separated  from  the  other  two  by  £  of  the  circumference. 
This  arrangement,  which  is  to  be  found  on  the  upright  branches  of  alders,  hazels, 
and  beeches,  is  called  the  one-third  phyllotaxis. 

Third  case.  Five  leaves  originate  in  each  story,  which  are  designated  according 
to  age  as  the  first,  second,  third,  fourth,  and  fifth,  the  lowest  being  the  oldest,  the 
highest  the  youngest.  These  five  leaves  give  place  to  one  another  in  a  horizontal 
direction,  and  the  shifting,  i.e.  the  horizontal  distance  between  two  leaves  next  in 
age,  amounts  to  -f  of  the  circumference  of  the  circle  (see  the  plan,  fig.  99 8).  If 
the  five  leaves  are  joined  together  in  succession  according  to  their  age,  a  spiral 
line  is  obtained  consisting  of  two  revolutions,  and  the  "  genetic  spiral "  consequently 
forms  two  circuits  round  the  stem.  If  a  stem,  whose  leaves  are  arranged  in  this 
manner,  is  built  up  of  two  or  several  stories,  then  the  similarly  numbered  leaves 
are  situated  in  straight  lines  above  one  another,  the  first  (lowest)  leaves  of  all  the 
stories  form  together  one  straight  line  (orthostichy);  in  the  same  way  the  second,  the 
third,  &c.  Thus  five  lines  are  developed  on  the  circumference  of  the  stem  by  the 
leaves  situated  one  above  the  other,  and  each  line  is  separated  from  another  by 
i  of  the  circumference.  This  arrangement,  which  is  found  in  oaks,  round-leaved 
willows,  and  in  many  buckthorns,  is  designated  the  two-fifths  phyllotaxis. 

Fourth  case.  Eight  leaves  are  to  be  found  in  each  story,  which  may  again  be 
numbered  from  one  to  eight  according  to  their  age.  Any  two  successive  leaves  are 
separated  from  one  another  horizontally  by  f- of  the  circumference  (see  fig.  99 4). 
If  a  line  be  drawn  starting  from  the  first  and  lowest  leaf,  joining  all  the  eight  leaves 
of  the  story  in  the  order  of  their  ages,  this  forms  a  spiral  line,  or  "genetic  spiral", 
which  traverses  the  stem  three  times.  In  a  stem  consisting  of  several  such  stories, 
the  leaves  named  by  the  same  numbers  are  placed  in  straight  lines  above  one 


400 


DISTRIBUTION   OF  THE  GREEN   LEAVES  ON   THE   STEM. 


another,  and  accordingly  eight  rectilineal  lines  (orthostichies)  run  down  the  stem. 
Each  line  is  separated  from  its  neighbour  by  i  of  the  circumference.  This  arrange- 
ment, which  occurs  in  roses,  raspberries,  pears,  and  poplars,  in  laburnuns,  and  in  the 
barberry,  is  called  the  three-eighths  phyllotaxis. 

Yet  a  fifth  case  is  very  often  to  be  found  in  trees  and  bushes  with  narrow 
leaves,  viz.  in  the  Almond-tree,  in  the  Goafs-thorn,  in  the  Sweet  Willow,  in  the 
Sea  Buckthorn,  and  many  Spiraea  bushes.  Each  story  contains  thirteen  leaves. 


Fig.  99.— Plan  for  Spiral  Phyllotaxis. 

i  One-half  Phyllotaxis.     a  One-third  Phyllotaxis.     »  Two-fifths  Phyllotaxis.     <  Three-eighths  Phyllotaxis.     The  conical  stem 
horizontally  projected ;  the  points  of  insertion  of  the  leaves  on  the  circumference  of  the  stem  marked  by  a  dot. 

which  may  be  connected  by  a  spiral  line,  i.e.  a  "  genetic  spiral ",  with  five  revolu- 
tions. The  number  of  the  straight  lines  here  amounts  to  thirteen,  and  the  distance 
between  two  leaves  following  one  another  in  age  is  -£$  of  the  circumference,  i.e. 
138°  (see  fig.  100). 

Not  so  common,  or  rather  not  demonstrable  with  the  same  precision,  are 
instances  in  which  one  story  shows  twenty-one  leaves  which  are  connected  by  a 
genetic  spiral  with  eight  revolutions;  and  where  a  story  includes  thirty- four 
leaves  which  are  connected  by  a  genetic  spiral  with  thirteen  revolutions.  In  the 
one  case  any  two  leaves  next  one  another  in  age  in  a  story  are  separated  from  one 


DISTRIBUTION    OF   THE   GREEN   LEAVES   ON   THE   STEM. 


401 


another  -fa  of  the  circumference;  in  the  other  case  by  £J;  and  from  this  it  follows 
that  in  the  one  instance  there  are  twenty-one,  and  in  the  other  thirty-four  orthostichies. 

If  we  place  these  actually-observed  instances  together,  we  have  the  series 
4,  4,  *»  t»  A,  A,  4* 

But  the  variety  of  the  conditions  on  which  the  leaves  are  arranged  is  not 
exhausted  by  a  long  way.  Although  but  seldom,  still  cases  have  been  observed 

which  can  be  placed  together  in  the  series  i,  1,  £,  A»  A ,  and  also  in  the  series 

i>  f»  A»  A ^n  a^  these  series  this  very  remarkable  peculiarity  occurs,  that 


Fig.  100.— Plan  of  Five-thirteenths  Phyllotaxia. 

in  each  individual  fraction  the  denominator  is  equal  to  the  sum  of  the  denom- 
inators, and  the  numerator  is  equal  to  the  sum  of  the  numerators  of  the  two 

preceding  fractions. 

Moreover  it  must  be  here  particularly  mentioned  that  the  divergence,  by  which 
the  leaves  following  one  another  in  age  are  separated  in  a  horizontal  d.recl 
the  more  difficult  to  establish  the  smaller  it  becomes.     The  one-third,  two-fi 
three-eighths  arrangements   are  the  most  easily  demonstrable  on  the  full-gro 
shoots,  although  occasionally  doubt  arises  as  to  whether  the  three,  five,  and  e.gh 
orthostichies  represent  completely  straight  lines.     But  the  demonstrate  of  A  a 
the  tf  arrangements,  especially  in  green  herbaceous  stems,  is  very  < 

uncertain.  26 

VOL.  I. 


402  DISTRIBUTION   OF   THE   GREEN   LEAVES   ON   THE    STEM. 

There  are  only  few  plants  on  whose  branches  or  axes  several  stories  occur  with 
twenty-one  or  thirty-four  successive  leaves  in  each.  On  the  other  hand,  it  happens 
that  on  many  shoots,  not  even  one  story  is  completely  formed,  or  in  other  words, 
that  in  more  than  a  hundred  leaves  which  project  from  the  axis,  no  two  are  to  be 
found  situated  quite  vertically  above  one  another,  and  consequently,  in  these  cases, 
rectilineal  orthostichies  are  out  of  the  question.  In  many  fir-cones,  for  example, 
rectilineal  lines  are  sought  for  in  vain,  and  it  is  impossible,  even  approximately,  to 
estimate  how  many  leaves  are  included  in  one  story.  It  has  been  also  conjectured 
that  in  such  cases  the  leaves  of  a  story  are  innumerable,  and  if  so,  the  fraction  by 
which  such  a  system  of  leaf -insertion  would  be  represented  would  be  an  absurd 
figure. 

In  such  shoots  it  is  anything  but  easy  to  establish  the  successive  ages  of  the 
leaves,  that  is,  to  number  them  in  their  proper  order  of  development,  especially 
when  the  leaves  are  thickly  crowded  together.  This  becomes  the  more  difficult 
when  the  leaves  on  such  very  crowded  axes  arrange  themselves  in  spiral  series,  or 
lines  which  are  much  more  apparent  to  the  eye  than  the  lines  of  development  or 
genetic  spirals.  These  spiral  series,  which  are  seen  on  shoots  of  many  succulent 
plants  (Sedum,  Sempervivum),  on  species  of  Pandanus  and  Yucca,  on  the  branches 
of  lycopodiums  and  conifers,  and  especially  also  in  the  inflorescence  of  crucifers 
and  the  cones  of  many  firs,  of  which  a  pine-cone,  represented  in  fig.  101,  may  be 
taken  as  an  example — these  series  are  called  parastichies.  They  may  be  utilized  in 
order  to  ascertain  which  leaves  succeed  one  another  in  age,  thus — by  first  of 
all  ascertaining  how  many  such  parallel  spiral  lines  ascend  to  the  right,  and  how 
many  to  the  left  on  the  axis  examined.  In  a  pine-cone,  for  example  (see  illustration 
below),  eight  such  lines  or  parastichies  are  seen  to  ascend  in  a  somewhat  sharply 
oblique  direction  to  the  left,  and  five  to  the  right  in  a  rather  less  sharply  oblique 
direction.  In  order  to  find  out  which  leaves  succeed  one  another  in  age,  the  lowest 
leaf  is  called  1,  and  the  numbers  8  and  5  are  used  in  the  following  manner.  The 
leaves  of  those  steep  parastichies,  on  the  left  adjoining  1,  are  numbered  by  additions 
of  8  respectively,  9,  17,  25,  33,  41,  &c.  The  leaves  of  the  less  steep  parastichies  on 
the  right,  which  adjoin  1,  are  numbered,  on  the  other  hand,  by  additions  of  5 
respectively,  6,  11,  16,  21,  26,  &c.  The  numbering  of  the  other  parastichies  is  then 
easily  completed  by  subtractions  and  additions  of  the  numbers  8  and  5,  and  the 
numbers  so  obtained  represent  the  successive  ages  of  the  leaves  on  the  cone.  This 
somewhat  complicated  arrangement  may  be  best  demonstrated  by  imagining  the 
surface  of  a  leafy,  almost  cylindrical  axis,  e.g.  of  a  pine-cone,  to  be  slit  up  longi- 
tudinally, rolled  out  flat,  and  extended  so  that  all  the  leaf -scales  lie  in  one  plane,  as 
represented  in  the  plan  illustrated  in  the  right-hand  figure  opposite. 

Naturally  the  most  lively  interest  has  been  aroused  at  all  times  by  the  geo- 
metrical ratios  of  phyllotaxis  here  generally  reviewed,  and  it  could  not  fail  to  follow 
that  the  most  diverse  speculations  should  have  been  connected  with  them.  This  is 
not  the  place  to  consider  these  in  detail,  but  in  so  far  as  the  remarkable  and 
actually  existing  conditions  of  the  geometric  arrangement  of  the  leaves  have  a 


DISTRIBUTION    OF   THE   GREEN    LEAVES   ON   THE   STEM. 


403 


significance  in  the  life  of  the  plant,  the  attempts  to  explain  them  must  not  be 
passed  over.  First  of  all,  it  must  be  pointed  out  that  the  number  of  orthostichies, 
i.e.  of  the  leaf -members  of  a  story,  as  well  as  the  number  representing  the  circuits 
made  by  the  genetic  spiral  in  each  story,  is  connected  with  the  extent  of  the 
horizontal  divergence  between  consecutive  leaves.  In  order  to  make  this  clear,  let 
us  draw  a  spiral  line  on  the  surface  of  a  cone,  as  shown  in  fig.  99,  and  let  us  place 
dots  on  this  line  at  regularly  recurring  intervals.  The  length  of  the  interval 
between  the  dots  is  quite  immaterial,  it  is  only  of  importance  that  the  successive 
dots  shall  remain  separated  from  each  other  by  the  distance  originally  fixed  upon. 


Fig.  101.—  Parastichies  of  a  Pine-cone 

The  eight  parastichies  turning  steeply  to  the  left,  start  from  the  points  1,  6,  8,  8,  6, 
2,  7, 12 ;  the  five  turned  less  steeply  to  the  right,  from  the  points  4,  1,  3,  6,  2. 


Suppose  that  the  dots  are  placed  on  the  spiral  line  at  intervals  of  -jV  of  the  circum- 
ference of  the  circle  (36°),  then  in  each  revolution  of  the  spiral  there  will  be  10 
dots,  separated  by  equal  distances  from  one  another.  With  the  tenth  -fa,  however, 
the  spiral  line  has  completed  the  circuit  of  the  cone,  i.e.  of  the  axis.  The  eleventh 
dot  lies  vertically  above  the  first  dot,  and  with  it  begins  a  new  revolution  and  a 
new  story.  On  such  a  stem  ten  orthostichies  would  necessarily  be  produced,  and  if 
we  substitute  actual  leaves  for  the  dots,  the  phyllotaxis  will  be  represented  by  ^ 
As  another  example,  let  us  place  the  dots  on  the  spiral  line  at  horizontal  distances 
of  |  of  the  circumference.  How  will  the  dots  then  be  arranged?  Dot  2  is  -f  of  the 
circumference  of  the  circle  from  dot  1;  dot  3,  -f +  f  =  -f;  dot  4,  -f+-f+£=-f;  dot  5, 
.2  +  2.4.2.+  2  =  8.  Of  the  circumference  from  dot  1,  measured  along  the  genetic  spiral. 
Dot  4  is  not  quite  vertically  above  dot  1,  and  dot  5  lies  beyond  it,  neither  of  the 
two,  therefore,  coming  exactly  above  1.  More  dots  are  now  placed  at  the  same 
intervals  on  the  second  revolution  of  the  spiral  line;  first  dot  6,  which  is  \&,  then 


404  DISTRIBUTION   OF   THE   GREEN   LEAVES   ON   THE   STEM. 

dot  7,  which  is  ty,  and,  finally,  dot  8,  which  is  Y-  of  tne  circumference  from  dot  I 
along  the  genetic  spiral.  Dot  8  is  found  to  lie  exactly  above  dot  1,  and  here  the 
second  revolution  of  the  spiral  line  is  completed.  This  is  the  termination  of  the 
first  story,  and  with  dot  8  a  new  one  commences.  On  a  stem  whose  leaves  are 
distributed  in  the  same  way  as  the  dots  in  the  example  just  described  —  any  two  of 
which  are  always  separated  from  one  another  by  f-  of  the  circumference  in  a 
horizontal  direction  —  seven  orthostichies  will  be  produced,  and  the  genetic  spiral, 
i.e.  the  line  which  connects  the  leaves  consecutively  following  one  another  according 
to  their  age,  will  make  two  revolutions  round  the  stem.  Such  an  arrangement 
would  be  designated  as  a  two-sevenths  phyllotaxis.  From  these  examples  it  follows 
that  a  definite  phyllotaxis  corresponds  to  each  horizontal  divergence  between  leaves 
following  one  another  in  age,  whatever  this  may  be,  as  long  as  it  only  remains 
constant.  The  divergence  measured  along  the  circumference  of  the  stem  may  be 
large  or  small.  Finally,  there  will  be  an  equal  distribution  of  leaves  around  the 
stem,  and  they  will  project  at  equal  horizontal  distances  in  as  many  directions  as 
are  given  by  the  denominator  of  the  fraction  representing  the  divergence.  But  the 
spiral  line  which  connects  all  the  leaves  represented  by  the  denominator  with  one 
another  will  make  as  many  circuits  round  the  stem  as  the  number  constituting  the 
numerator  of  the  fraction.  In  other  words,  the  extent  of  the  horizontal  divergence 
always  gives  us  the  phyllotaxis.  The  denominator  of  the  fraction  is  equal  to  the 
number  of  orthostichies,  and  the  numerator  is  equal  to  the  number  of  revolutions 
made  by  the  genetic  spiral  in  each  story. 

The  observation  already  alluded  to  above,  according  to  which  those  fractions 
by  which  the  phyllotaxes  actually  found  in  plants  may  be  expressed  as  members 
of  a  definite  series,  must  now  be  considered  further.  It  has  been  found  that  the 
horizontal  divergences  between  consecutive  leaves  respectively  form  part  of  a 
continued  fraction  of  the  form 


in  which  z  is  a  whole  number.     If  for  z  we  substitute  the  number  1,  the  successive 
parts  of  the  fraction  will  give  us  the  series  £,  f,  f,  -J,  A>  £f,  ........     If  0  =  2,  the 

series  i,  i,  f  ,  f,  A.  A  ......  is  obtained.     If  z  =  3,  the  series  £,  J,  f  T3T,  A.  A  ......  > 

and  if  z  =  4,  the  series  becomes  £,  fc  $,  A.  A,  A  ........     It  is  remarkable  here  that 

among  all  the  phyllotaxes,  those  represented  by  the  numbers  £,  •§,  |,  £,  A  ...... 

occur  most  frequently,  while  phyllotaxes  belonging  to  the  other  above-quoted 
series  are  only  occasionally  met  with.  Thus,  as  a  matter  of  fact,  the  series  occurs 
oftenest  in  which  2  is  substituted  for  z.  The  advantage  offered  by  the  series 
produced  from  this  number  has  been  explained  in  this  way:  by  it,  on  the  one  hand, 
phyllotaxes  are  produced  by  which  an  equal  distribution  of  the  leaves  is  obtained 
by  the  smallest  possible  number  in  each  story;  and,  on  the  other  hand,  phyllotaxes 
again  in  which  leaves  may  project  from  the  stem  in  very  many  different  directions. 
The  reason  why  each  species  of  plant  arranges  its  leaves,  even  while  m  the 


DISTRIBUTION   OF   THE   GREEN   LEAVES   ON   THE   STEM.  405 

bud,  in  the  most  advantageous  manner,  quite  independently  of  external  influences 
without  the  knowledge,  so  to  speak,  of  the  conditions  to  which  its  foliage-leaves 
will  be  exposed  in  the  future,  can  only  be  explained  by  the  specific  constitution 
of  its  protoplasm.  Just  as  crystals  are  formed  from  the  aqueous  solution  of  a 
salt  which,  according  to  the  nature  of  the  salt,  are  sometimes  six-sided,  sometimes 
three-sided,  whose  surfaces  are  always  the  same  in  outline,  and  whose  edges  always 
form  exactly  the  same  angles,  so  bands,  bars,  and  partition-walls  arise  in  the 
growing  cells,  by  which  these  cells  become  articulated  and  divided;  and  the  shape 
and  position  of  these  intercalated  walls  and  their  geometrical  ratios  are  no  less 
definite  in  the  most  diverse  plant  species  than  are  the  surfaces  of  the  crystals 
arising  from  the  salt  solution.  But  that  which  applies  to  the  plan  of  construction 
of  the  individual  cells  must  also  apply  to  the  plan  according  to  which  a  group 
of  cells— a  tissue,  a  growing  shoot,  a  stem  with  its  leaves,  even  the  entire  plant- 
is  constructed.  The  position  on  the  circumference  of  the  stem  at  which  a  leaf 
originates  is  certainly  not  determined  by  chance,  but  is  based  upon  the  molecular 
constitution  and  composition  of  the  protoplasm  of  the  species  of  plant  in  question; 
and  if  the  leaves  on  an  oak-branch  always  arrange  themselves  in  f-  phyllotaxis, 
the  constancy  of  the  arrangement  is  neither  more  nor  less  remarkable  than  the 
constancy  of  the  size  of  the  angles  in  an  alum  octahedron. 

It  should  be  noted  here,  in  this  connection,  that  the  geometrical  arrangement 
of  the  cells  in  simple  elongated  tissues,  easily  accessible  to  observation,  is  exactly 
similar  to  the  arrangement  of  the  leaves  on  stems.  For  example,  the  cells  on  the 
hair-like  stigmas  of  grasses  follow  the  one-third  arrangement  very  beautifully.  A 
connection  between  the  geometrical  arrangement  of  the  cells  at  the  apex  of  a 
growing  stem,  and  the  geometrical  arrangement  of  the  leaves  on  the  same,  may 
now  also  be  considered.  A  group  of  cells  is  formed  out  of  each  cell  at  the  growing 
point  of  the  stem  by  the  repeated  intercalation  of  division-walls.  If  the  position 
of  these  dividing  cells  is  geometrically  defined,  and  if  the  partition-walls  resulting 
from  their  division  assume  definite  directions  in  each  species  of  plant,  then  the 
arrangement  of  the  cell-groups  produced  from  these  cells  which  build  up  the  stem 
must  also  be  geometrically  defined.  Supposing  now  that  from  each  of  these  groups 
of  cells  which  build  up  the  stem  a  leaf  arises,  then  the  distribution  of  the  leaves 
on  the  circumference  of  the  stem  will  be  only  a  repetition  of  the  distribution  of 
the  cells  at  the  growing  point  of  the  stem.  In  the  simplest  of  all  leafy  stems, 
in  that  of  a  moss-plant,  this  relation  is  noticeable  enough;  but  in  plants  of  more 
complicated  construction  it  is  not  so  easily  demonstrated.  In  these  the  constancy 
of  the  geometric  ratios  of  the  cells  at  the  growing  point  is  beset  with  many 
difficulties,  and  the  groups  of  cells  produced  from  them  are  also  much  displaced 
and  distorted.  Nevertheless  in  each  form  of  plant  a  uniform  plan  of  construction 
very  probably  exists;  and  it  may  be  taken  for  granted  that  in  each  species  the 
,  arrangement  of  the  atoms  in  the  protoplasm,  the  arrangement  of  the  cells,  and  tho 
arrangement  of  the  leaves,  are  based  upon  the  same  symmetrical  construction. 

Indeed,  even  the  displacements  and  torsions  of  the  cells  which  occur  in  leafy 


406  DISTRIBUTION    OF   THE   GREEN   LEAVES   ON   THE   STEM. 

stems  without  doubt  take  place  according  to  rule,  although  they  may  be  in  part  due 
to  external  causes.  Numerous  comparative  observations  have  shown  that  the 
building,  and  especially  the  lengthening  of  the  growing  stem,  does  not  always 
follow  the  direction  of  a  straight  line;  that,  rather,  a  spiral  torsion  of  the  cells  and 
tissues  not  infrequently  occurs,  so  that  the  idea  that  such  a  stem  by  its  growth 
bores  its  way  through  the  air  is  quite  justified.  This  does  not,  indeed,  refer  to  the 
twining  of  the  stem,  which  will  be  discussed  later,  but  to  the  torsion  of  the  tissue 
mass  of  a  straight  stem  which  remains  straight  after  the  torsion  has  been  effected, 
and  which  may  best  be  compared  to  the  twisting  of  a  bundle  of  rectilineal  strands 
to  form  a  string.  In  every  bud  from  which  a  leafy  branch  arises,  the  points 
of  origin  of  the  leaves  may  be  seen  on  the  periphery  of  the  still  very  short 
conical  axis;  frequently,  also,  the  shape  and  outline  of  the  leaves  are  perceptible, 
and  the  position  and  divergence  of  the  leaf  -  insertions  can  be  geometrically 
established.  If  the  axis  has  elongated,  and  an  extended  branch  been  produced 
from  the  bud,  the  arrangement  displayed  by  the  fully-formed,  displaced  leaves  does 
not  always  coincide  with  that  in  the  bud.  The  phyllotaxis  has  become  altered 
by  reason  of  the  pressure  which  the  individual  groups  of  cells  exercise  on  one 
another  in  their  increase  in  length  and  breadth,  and  in  consequence  of  displace- 
ments connected  with  these  pressures,  i.e.  torsions  arise.  If  the  torsion  is  restricted 
to  one  portion  of  the  stem  only,  an  actual  transition  of  one  phyllotaxis  into  another 
is  seen,  and  occasionally  it  is  very  pronounced. 

In  order  to  make  clear  the  alterations  arising  in  this  way,  it  is  only  necessary 
to  remove  the  leaves  from  a  herbaceous  leafy  stem,  to  hold  it  by  the  two  ends, 
and  to  twist  it  as  a  bundle  of  threads  might  be  twisted  into  a  string.  The  points 
of  insertion  of  the  leaves  are  thus  mutually  displaced,  parastichies  are  formed 
from  the  orthostichies,  and  new,  often  very  complicated,  leaf-arrangements  come 
into  view.  The  alterations  produced  by  the  torsion  of  the  stem  may  also  be 
rendered  evident  by  a  consideration  of  fig.  102.  Let  us  suppose  that  the  black 
dots  on  the  three  thick  lines  of  the  young  conical  stem,  horizontally  projected 
in  this  illustration,  indicate  leaf-positions  which  are  separated  from  one  another 
by  a  distance  of  £  of  the  circumference  of  the  circle  (120°).  Suppose  now  that 
the  stem  has  undergone  a  torsion  as  it  lengthened,  which  is  quite  definite  and 
equally  distributed  over  all  portions  of  the  stem.  Each  portion  of  the  stem 
between  two  consecutive  leaves,  following  one  another  in  age,  is  twisted  through, 
say  ^  of  the  circumference  (24°),  and  in  consequence  of  this  the  divergence  of  the 
leaves  is  no  longer  £  of  the  circumference,  i.e.  120°,  but  120°+  24°  =144°,  or,  as 
much  as  £  of  the  circumference.  By  reason  of  this  the  points  of  origin  of  the 
leaves  come  to  lie  in  the  positions  indicated  by  the  thinner  lines,  and  a  two-fifths 
is  produced  from  a  one-third  phyllotaxis.  In  the  same  way  the  three-eighths 
arises  from  the  one-third  phyllotaxis  if  the  consecutive  dots  are  displaced  -fr  of  the 
circumference  (15°)  by  the  torsion,  and  the  horizontal  divergence  no  longer  amounts 
to  £  of  the  circumference,  but  to  f .  The  one-third  becomes  changed  into  the 
one-half  phyllotaxis  if  the  second  leaf  of  a  story,  which  in  the  bud  was  separated 


DZSTRIBCTION   OF  THE  GREEN  LEAVES  ON  THE  STEM.  407 

from  the  first  by  £  the  circumference,  in  consequence  of  the  torsion  of  the 
stem,  „  d1Splaced  about  t  the  circumference  (60°);  that  is  to  say,  exactly  so 
tha    lt  „  now  separated  from  the  first  by  half  the  circumference  (180'       S 
pabular  alteratum  can  be  very  well  seen  in  the  developing  branch  s  of  beeches 
hornbeams  hasels,  and  many  other  trees  and  shrubs.     In  the  buds  the  leaves  ha"e" 
a  one-third Arrangement,  in  the  fully  formed,  now  woody  branches  the  phyllotaxil 
appears  to  be  one-half.    Since,  as  a  rule,  amongst  buds,  the  simplest  cases f espec Tally 


Fig.  102.— Displacement  of  the  leaf-positions  in  consequence  of  torsion  of  the  stem. 
Transformation  of  the  one-third  into  the  two-fifths  phyllotaxis.    Dot  2  is  displaced  by  torsion  to  2';  dot  S  to  V,  Ac. 

the  one-third  arrangement,  are  most  frequently  observed,  it  appears  probable  that 
the  number  of  original  phyllotaxes  is  really  only  very  small,  and  that  complicated 
leaf  arrangements,  which  are  represented  by  fractions  whose  numerator  consists  of 
two  figures,  frequently  are  produced  by  torsion  of  the  individual  parts  of  the  stem 
during  their  growth.  It  still  remains  to  point  out  here  that  the  phyllotaxis  becomes 
the  more  complicated,  the  less  the  amount  of  torsion  undergone  by  an  internode,  which 
is,  indeed,  evident  from  the  preceding  representation.  It  is  also  worthy  of  note,  that 
in  plants  whose  foliage-leaves  originate  2,  3,  or  more  together  at  the  same  height  on 
the  stem  (which  therefore  possess  whorled  leaves),  such  torsions  of  the  inter-nodes, 
and  alterations  of  the  phyllotaxis  dependent  upon  them,  very  frequently  occur. 


408        RELATION  BETWEEN  POSITION  AND  FORM  OF  GREEN  LEAVES. 


EELATION  BETWEEN  POSITION  AND  FORM   OF  GREEN   LEAVES. 

Now  that  the  distribution  of  the  green  leaves  on  the  surface  of  the  stem  has 
been  generally  described,  it  is  possible  to  discuss  the  relation  of  the  phyllotaxis  to 
the  length  and  breadth,  as  well  as  to  the  shape  and  direction,  of  the  leaf -blades. 

If  a  small  leafy  moss-plant,  or  a  huge  densely-leaved  tree  be  examined,  it  will 
always  be  found  that  the  number  of  orthostichies  on  the  straight  stems  becomes 
smaller  as  the  leaf -blades  become  broader.  If  the  leaf -blades  are  circular,  like 
those  of  the  Judas  Tree  (Cercis  Siliquastrum),  or  if  they  are  broadly  ovate  or 
cordate,  being  broadest  at  the  base,  like  those  of  limes  and  elms,  or  if  they  are  not 
perhaps  borne  on  very  long  petioles,  i.e.  like  those  of  the  Aspen  (Populus  tremula), 
then  they  pass  down  the  stem  in  two  lines,  thus  displaying  a  one-half  phyllotaxis. 
If  the  leaf -blades  are  broadly  elliptical,  and  therefore  broadest  about  the  middle, 
and  also  have  but  short  stalks,  like  those  of  beeches,  alders,  and  hazels,  then  they 
are  arranged  regularly  in  three  rows  on  the  erect  branches  and  display  a  one-third 
phyllotaxis.  If  the  leaves  are  obovate,  i.e.  broader  at  the  top  than  at  the  base, 
and  at  the  same  time  have  only  short  stalks,  as,  for  example,  those  of  oaks,  then 
they  are  arranged  in  five  lines,  according  to  the  two-fifths  phyllotaxis.  If  they 
are  lanceolate  or  oval,  like  those  of  the  Almond-tree,  they  usually  have  the  three- 
eighths  phyllotaxis;  and  finally,  the  narrow  linear  leaves  on  the  twigs  of  the 
Genista  tinctoria,  as  well  as  the  long  narrow  leaves  on  the  stems  of  the  Golden- 
rod  (Solidago),  are  regularly  arranged  in  a  five -thirteenths  phyllotaxis.  In  the 
mosses  the  same  relations  hold  good;  the  broad  leaves  of  the  Mnium  species 
display  the  one-third;  the  elliptical  and  oval  leaves  of  many  earth-mosses  (Barbula) 
the  two-fifths;  and  the  narrow  linear  leaflets  of  polytrichums  the  three-eighths, 
five-thirteenths,  and  more  complex  phyllotaxis.  This  connection  between  the 
breadth  of  the  leaf -blade  and  the  number  of  rectilineal  leaf -rows  on  the  erect  stem 
is  very  noticeable  even  in  members  of  the  same  genus,  and  in  this  respect  perhaps 
no  genus  is  so  instructive  as  the  willow.  There  are  willows  with  circular,  elliptical, 
oval,  and  narrow  linear  leaves,  and  in  these  it  can  be  plainly  seen  that  the  number 
of  orthostichies  increases  in  proportion  as  the  leaves  become  narrower.  Salix 
herbacea  with  circular  leaves  has  a  one-third,  Salix  Caprea  with  elliptical  foliage 
a  two-fifths,  Salix  pentandra  with  lanceolate  foliage  a  three-eighths,  and  Salix 
incana  with  linear  leaves  a  five-thirteenths  phyllotaxis. 

If  we  take  erect  branches  from  each  of  these  willows,  placing  them  all  together, 
and  look  down  upon  them  from  above,  we  see  how  the  three,  five,  eight,  and 
thirteen  rows  of  leaves  radiate  out  from  their  respective  axes.  But  it  is  also 
plainly  evident  that  in  each  case  the  neighbouring  rows  so  adjoin  one  another  as 
to  leave  no  gaps  between  them,  so  that  the  space  round  the  stem  may  be  utilized 
to  the  greatest  possible  extent.  In  one  case,  therefore,  we  have  three  rows  of  very 
broad  leaves,  in  other  cases  five  or  eight  rows  of  moderately  broad  leaves,  and  again 
in  another  instance  thirteen  rows  of  very  narrow  leaves. 


RELATION   BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES.  409 

All  the  rows  of  leaves,  whether  there  be  three,  five,  eight,  or  thirteen  of  them, 
are  equally  illumined  by  the  sunbeams  which  strike  them  from  above  in  the 
direction  of  the  axis  of  the  branch;  no  row  throws  another  into  the  shade,  and  only 
the  upper  individual  members  of  a  row  standing  above  one  another  can  deprive  the 
lower  members  of  light.  But  even  this  shading  is  avoided,  chiefly  by  the  adapta- 
tion of  the  length  and  direction  of  the  foliage-leaves  to  the  height  of  a  story. 

If  the  stories  are  low,  so  that  the  consecutive  leaves  of  a  rectilineal  row  are 
separated  only  by  short  distances,  then  the  leaves  are  short;  if  the  stories  are  high, 
then  the  leaves  are  long;  the  length  is  always  so  arranged  that  the  sunbeams  can 
penetrate  into  the  space  between  every  two  leaves  of  a  row,  and  can,  so  to  speak, 
illuminate  the  interior  of  the  story. 

It  should  be  remembered  here  that  the  sun  does  not  shine  down  vertically  upon 
branches  having  an  upward  direction,  that  its  rays,  even  at  the  equator,  fall 
obliquely  in  the  morning  and  evening,  and  at  these  times  illuminate  the  space, 
bounded  above  and  below  by  two  consecutive  leaves  of  a  row,  just  like  the  rays  of 
the  rising  and  setting  sun  which  enter  a  room  through  the  window.  But  this  does 
not  say  that  no  leaf  is  thrown  into  the  shade  throughout  the  entire  day.  This 
would  be  impossible,  from  the  fact  that  the  sun's  rays  at  each  hour  of  the  day  fall 
at  a  different  angle  on  the  plants  which  remain  firmly  fixed  and  immovable  in  the 
soil.  The  leaves  of  one  side  are  partially  shaded  in  the  morning,  and  those  of  the 
other  side,  in  the  afternoon;  or  they  are  only  illuminated  by  diffuse  light;  and  the 
upright  stem,  which  is  set  round  about  with  projecting  leaves,  must  necessarily 
shade  a  portion  of  them  for  a  short  time  during  the  day.  But  these  shadows,  like 
the  dark  lines  thrown  by  the  gnomon  of  a  sun-dial,  must  continually  move  forward 
with  the  sun,  and  only  remain  in  one  place  for  a  little  while. 

The  entrance  of  the  sun's  rays  between  the  leaves  situated  above  one  another  is, 
moreover,  materially  influenced  by  the  direction  of  the  leaf -blades.  A  leaf  pro- 
jecting obliquely  upwards  from  the  stem,  with  its  midrib  in  the  plane  of  the 
incident  rays,  will  not  at  any  hour  of  the  day  deprive  its  lower-placed  neighbour  of 
too  much  light,  or  at  any  rate  to  a  much  less  extent  than  will  a  leaf  whose  blade  is 
•extended  horizontally  or  sloped  a  little  in  an  outward  direction,  and  which  presents 
its  broad  side  to  the  incident  sunbeams.  This  explains  a  phenomenon  which  is 
.seen  very  often  in  annual  and  biennial  composites  and  crucif ers  with  straight,  erect 
rstems.  The  lowest  leaves  of  these  plants  form  a  right  angle  with  the  axis  of  the 
stem,  and  lie  with  their  broad  surfaces  on  the  soil,  completely  covering  a  larger  or 
smaller  area.  These  can  obviously  not  take  away  the  light  from  any  other  leaves 
•of  the  same  plant.  The  leaves  inserted  higher  up  the  stem  are,  on  the  other  hand, 
no  longer  extended  horizontally,  but  rather  in  an  upward  direction,  and  form  an 
single  with  the  stem  which  is  less  than  a  right  angle;  and  the  highest  leaves  even 
approach  the  upright,  their  midribs  lying  in  the  plane  of  the  incident  noonday 
rays. 

In  accordance  with  this  adjustment,  an  alteration  of  the  dimensions,  particularly 
of  the  length  of  its  leaves,  may  be  observed  at  different  heights  of  an  erect  thickly- 


410  RELATION    BETWEEN   POSITION   AND   FORM   OF   GREEN    LEAVES. 

leaved  stem.  The  lowest  leaves  originating  next  the  soil  are  the  longest;  the  leaves 
next  above  these  are,  on  the  other  hand,  visibly  shorter,  and  often  in  the  region  of 
the  flowers  are  changed  into  insignificant  scales  closely  applied  to  the  stem.  It  can 
easily  be  seen  in  every  plant  of  the  Shepherd's  Purse  (Capsella  Bursa  pastoris),  on 
every  mullein  (Verbascum),  and  every  hawkweed  (Hieracium),  that  such  small 
upwardly-directed  leaves  cannot  injure  by  overshadowing  the  leaves  growing  below 
them  either  in  the  same  or  in  adjacent  rows. 


Fig.  103.— Leaf-mosaic. 

i  Leaf -rosettes  of  a  Crane's-bill  (Geranium  Pyrenaicum)  seen  from  above.  2  Leaf -rosettes  of  a  Saxifrage  (Saxifraga  Aizoon). 
s  Leaf-rosette  of  a  Bell-flower  (Campanula  pusilla)  seen  from  above.  *  Adpressed  scale-like  leaves  on  the  twig  of  an 
Arbor  Vita  (Thuja). 

Many  plants  produce  within  a  year,  at  the  ends  of  their  upright  shoots,  a  large 
number  of  leaves  which  radiate  out  from  the  stem  with  very  small  horizontal 
divergencies,  standing  close  above  one  another,  and  forming  a  so-called  rosette.  In 
order  that  all  the  leaves  of  such  a  rosette  may  receive  an  equal  proportion  of  light, 
it  is  absolutely  necessary  that  the  upper  leaves  should  be  considerably  shorter  than 
the  lower.  And  in  all  rosettes  this  is  actually  the  case.  However,  some  very 
interesting  modifications  are  to  be  seen.  In  rosette-forming  succulent  plants  (e.g. 
Echeveria  and  Sempervivum),  and  in  many  saxifrages  (Saxifraga),  of  which  a 
species  (Saxifraga  Aizoon)  is  represented  in  fig.  103  2,  the  leaves  are  tongue-shaped 
or  spatulate,  and  about  twice  as  broad  near  the  further  end  than  at  the  point  of 


RELATION   BETWEEN  POSITION  AND   FOBM  OP  GREEN  LEAVES. 

insertion    upon    the    abbreviated    axis.      It    is   unavoidable   that   ^ 
proximal  halves  of  most  of  the  leaves  should  be  covered  by  the  leaves  aZeTd 

hi        I'  7™  T    T1*  %ht     ^  theS6  C°Vered  P°rtions  «  1«y  deZLof 
chlorophyll  and  so  have  no  need  of  direct  sunlight.    The  distal  halves,  on  the  ol 

«  which  display  green  tissue,  can  by  this  arrangement  be  all  well  illu±d 
s,mu  taneousy  by  the  sun.  In  many  other  instances  the  increase  in  length  is  ± 
found  m  the  leaf-stalks  of  the  lower  leaves  of  the  rosette.  These  increase  in  length 
that  u,  to  say,  until  the  blades  borne  by  them  are  moved  out  of  the  shadow  of  "the 


Fig.  104.— Formation  of  a  Leaf -Mosaic  by  the  lengthening  (relative  shortening)  of  the  Leaf-stalks, 
i  Small-leaved  Balsam  (Impatiens  parviflora).    a  Green  Amaranth  (Amarantu*  Blitum).  »  Thorn-apple  (Datura  Stramonium). 

leaves  above.  This  is  the  case,  for  example,  in  the  leaf-rosettes  of  Geranium 
Pyrenaicum,  represented  in  fig.  103  \  and  in  the  leaf -rosettes  of  the  dainty  little 
bell-flower  (Campanula  pusilla,  fig.  1033)  growing  on  the  debris-covered  slopes  of 
the  sub-alpine  regions.  In  these  bell-flowers  the  great  difference  in  shape  between 
the  rosette-leaves  and  those  clothing  the  flower-stalk  is  worthy  of  remark.  The 
latter,  which  spring  at  an  acute  angle  from  the  stem,  are  narrowly  lanceolate,  and 
have  very  short  stalks,  while  the  lower  rosette-leaves,  extended  flatly  over  the  soil, 
have  long  stalks,  and  possess  a  broad,  ovate  blade.  It  is  no  disadvantage  to  the 
leaf -stalks,  which  have  no  chlorophyll,  if  they  are  placed  in  the  shade.  But  by  this 
arrangement  all  the  broad,  green  leaf-blades  are  well  illumined,  and  this  applies 
also  to  the  more  loosely-arranged,  upwardly-directed,  narrow  leaves  of  the  stem. 


412  RELATION   BETWEEN   POSITION   AND   FORM   OF   GREEN    LEAVES. 

The  leaves  of  many  plants  with  elongated,  erect  stems,  though  at  a  moderate 
distance  from  one  another,  are  often  arranged  in  a  kind  of  rosette,  and  this  is 
effected  by  the  stalks  of  the  lower  leaves  becoming  considerably  longer  than  those 
of  the  leaves  situated  near  the  apex.  This  condition  is  especially  seen  in  the 
marsh-plants,  whose  flat  leaves  lie  on  the  surface  of  the  water,  viz.  in  Villarsia, 
Hydrocharis,  Polygonum  amphibium,  many  species  of  the  genus  Callitriche,  and 
many  water-inhabiting  Ranunculaceae.  Among  terrestial  plants  this  grouping  of 
the  leaves  is  displayed  particularly  by  many  Amarantacese.  In  the  erect  shoots  of 
Amarantus  Blitum,  illustrated  in  fig.  104  2,  the  stalks  of  the  lower  leaves  of  a  row 
are  six,  seven,  or  eight  times  as  long  as  those  of  the  upper  leaves.  In  this  way  the 
whole  of  the  green  foliage  of  the  plant  can  be  spread  out  almost  at  the  same  level 
without  any  one  overshadowing  another. 

In  plants  with  elongated  stems,  the  mutual  encroachment  of  the  numerous 
leaves  situated  one  above  another  is  also  prevented  by  a  further  arrangement.  We 
mean  the  development  of  the  leaves  in  the  form  of  green  scales  adpressed  to  the 
stem,  as  observed  in  so  many  conifers,  e.g.  in  the  twigs  of  a  Thuja,  as  represented 
in  fig.  103 4.  It  is  true  that  only  the  under  surface  of  the  small  leaflets  can  meet 
the  sun's  rays,  but  the  effect  is  the  same  as  if  only  the  upper  side  had  been 
illumined,  as,  for  example,  in  those  leaves  projecting  from  the  erect  stems  at  a  right 
angle,  or  inclined  with  their  apex  towards  the  soil.  Since  the  small  green  leaflets 
clothing  the  stem  are  arranged  side  by  side,  like  the  tiles  on  a  roof,  and  the  greater 
portion  of  the  under  surfaces  remains  uncovered  by  the  adjoining  leaves,  no 
mutual  withdrawal  of  light  can  be  said  to  occur,  in  spite  of  the  crowded 
arrangement. 

The  arrangements  of  green  leaves  as  just  described  relate  exclusively  to  instances 
in  which  the  blade  of  the  leaf  is  neither  lobed  nor  compound,  but  entire.  A  leaf 
can  deprive  another,  originating  a  little  below  it  from  the  erect  stem,  having  the 
same  shape  and  size,  and  the  same  inclination,  either  entirely  or  almost  entirely  of 
the  sun's  rays,  only  when  entire.  A  leaf  whose  green  lamina  is  sinuous,  lobed, 
divided,  or  incised,  will  always  allow  abundant  sunlight  to  pass  between  the  lobes 
and  segments  on  to  the  leaves  below;  and  the  deeper,  wider,  and  more  numerous  the 
incisions  producing  the  separation  into  lobes  and  segments,  the  more  will  be  the 
light  passing  through.  Of  course  strips  of  shadow  will  be  formed,  but  they  move 
their  position  during  the  day,  remaining  in  one  spot  only  for  a  short  time;  and  it 
would  appear  that  such  a  rapidly  passing  shadow  has  anything  but  an  injurious 
effect  on  the  green  tissue.  From  this  it  follows  that  in  plants  with  divided  foliage, 
the  adjustment  described  previously  for  the  case  of  entire  leaves  is  superfluous. 
As  a  matter  of  fact,  in  plants  whose  foliage-leaves  have  a  much-divided  blade,  the 
fully-grown  upper  and  lower  leaves  are  of  equal  length;  they  all  project  from  the 
erect  stem  at  the  same  angle,  and  the  stem  is,  generally  speaking,  never  clothed 
with  lobed  or  pinnate  leaves  closely  covering  it  like  scales.  In  the  Fennel  and  Dill, 
in  Chamomile,  Larkspur,  and  species  of  the  genus  Adonis,  the  lower  and  upper 
leaves  of  the  stem  are  so  alike  that  it  is  hardly  possible  to  say  whether  an  isolated 


RELATION   BETWEEN    POSITION   AND   FORM   OF   GREEN   LEAVES.  413 

single  leaf  had  been  plucked  from  the  lower  or  upper  part  of  the  stem  Only  the 
lowest  leaves  of  all,  whose  shadow  falls  on  the  ground  and  not  on  neighbouring 
leaves,  are  divided  into  broader  sections;  the  others  are  equally  divided  and  project 
at  equal  intervals  round  the  stem.  While  the  Mullein,  with  its  entire  foliage- 
leaves,  rapidly  diminishing  in  size  towards  the  summit,  presents  a  pyramidal 
appearance  from  a  distance;  the  Fennel  and  Larkspur,  whose  finely-divided  leaves 
are  similar  all  along  the  stem,  rise  up  like  cylindrical  columns.  In  other  words,  if 
the  extreme  outer  point  of  all  the  leaves  of  the  last-named  plants  are  connected 
together  in  a  surface,  this  will  take  the  form  of  a  cylinder.  Only  when  projecting, 
divided  leaves  are  crowded  above  one  another  on  a  very  short  stem,  as,  for 
example,  in  ferns,  and  where  the  plants  are  growing  in  shady  places  where  the 
light  is  very  scanty,  it  happens  that  the  lower  leaves  are  raised  above  the  upper 
in  order  not  to  miss  too  much  of  the  enjoyment  of  the  light. 

The  perforation  of  the  leaf-blades,  which  is  observed,  though  but  seldom,  in 
many  aroids,  has  now  to  be  considered.  The  best  known  in  this  respect  are  the 
Brazilian .  Monstera  egregia,  and  the  Tomelia  fragrans,  illustrated  in  fig.  96, 
which  has  also  been  called  by  gardeners,  in  consequence  of  the  gaps  in  the  leaves, 
Philodendron  pertusum.  The  circular  or  elliptical  holes  do  not  originate  late 
on  in  the  leaf-blade,  but  can  actually  be  seen  when  the  small  and  undeveloped 
leaves  are  yet  folded.  They  are  always  formed  on  the  upper  leaves  of  older 
plants;  the  leaves  of  younger,  shorter  specimens  do  not  possess  them.  This 
circumstance  suggests  that  the  holes  have  the  same  significance  as  that  previously 
assigned  to  the  deep  incisions  and  clefts  between  the  leaf-lobes.  They  are  chinks 
in  the  broad  upper  leaves  whose  shadow  extends  over  a  large  area,  through 
which  a  portion  of  the  obliquely  falling  rays  of  light  can  reach  the  more  deeply 
situated  leaves.  The  peculiar  notches  in  the  blades  of  certain  leaves  of  the  Black 
Mulberry -tree  (Morus  nigra),  as  well  as  of  the  Japanese  Paper  Mulberry 
(Broussonetia  papyri/era),  may  be  explained  in  like  manner.  They  are  only 
found  on  the  upper  leaves  of  a  branch,  and  are  best  seen  on  the  erect  slender 
shoots  which  spring  from  the  base  of  old  trunks.  Sometimes,  in  these  highest 
leaves,  only  one  half  has  an  incision  extending  almost  as  far  as  the  midrib;  some- 
times again  both  halves  are  provided  with  deep  clefts;  in  the  highest  shoot- 
leaves  of  the  Black  Mulberry-tree  the  blade  is  often  divided  up  into  fairly  narrow 
segments  by  several  incisions  on  both  sides.  If  such  developing  shoots,  crowded 
closely  together,  are  observed  at  mid-day,  when  they  are  directly  illumined  by 
the  sun,  the  shadow  of  the  upper  leaves  can  be  seen  sketched  out  on  the  leaves 
below,  but  .to  each  incision  and  indentation  of  a  leaf  at  the  apex,  a  patch  of  light 
corresponds  on  the  leaf-surfaces  in  the  stories  next  below.  Suppose  now  that 
the  holes  above  had  been  closed;  immediately  it  would  become  darkened  under- 
neath, the  spots  of  light  which  continually  move  according  to  the  position  of 
the  sun  from  place  to  place  and  from  leaf  to  leaf  would  be  wanting,  and  the 
activity  of  the  green  tissue  in  the  leaves  of  the  lower  region  would  be  to  some 
extent  impaired. 


414  RELATION    BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES. 

It  was  not  without  reason  that  in  each  separate  instance  hitherto  described, 
emphasis  has  been  laid  on  the  fact  that  the  foliage-leaves  in  question  were  situated 
on  erect  stems,  and  this  must  again  be  particularly  pointed  out  here.  The  con- 
ditions on  horizontal  branches  are  entirely  different,  and  what  is  suitable  for 
one  is  not  always  fitted  to  the  other.  It  is  easy  to  make  this  evident.  It  is 
only  needful  to  bend  down  an  erect  leafy  maple-branch  until  it  becomes  horizontal, 
and  it  will  immediately  be  seen  that  the  surfaces  of  the  leaves  on  the  branch 
assume  a  position  and  direction  very  different  from  their  previous  attitude.  The 
narrow  side,  instead  of  the  broad  side  as  previously,  is  directed  towards  the 
incident  light,  and  the  leaves  now  stand  above  one  another  which  formerly  stood 
opposite  at  the  same  height  from  the  ground.  If  the  arrangement  of  the  foliage- 
leaves  on  the  erect  branch  was  previously  suitable  and  beneficial,  the  contrary 
is  now  the  case.  Such  alterations  in  the  position  of  the  foliage-leaves  or  shoots 
and  branches  of  plants,  however,  occur  not  only  by  way  of  exception,  but  very 
frequently.  It  signifies  the  less  that  strong  winds  bend  and  incline  the  leaf- 
stalks and  twigs,  since  this  alteration  of  position  is,  as  a  rule,  only  of  short  dura- 
tion, and  when  the  storm  is  past,  the  former  position  is  again  taken  up.  The 
pressure  which  snow  exerts  on  plants  in  regions  where  in  winter  the  fall  is  heavy, 
is,  indeed,  of  more  importance,  and  can  produce  alterations  in  the  position  of  the 
twigs  and  branches  which  are  of  longer  duration.  But  most  important  of  all  is 
the  fact  that  perennial  plants  add  a  new  portion  to  the  end  of  their  shoots  every 
year,  that  they  always  develop  each  year  new  sprouts  above  those  already  existing, 
and  not  only  at  the  apex,  but  also  from  buds  which  arise  laterally  on  the  branches. 
Let  us  observe  a  young  maple  whose  topmost  branch  terminates  in  three  buds. 
Twigs  issue  from  the  three  buds  with  the  renewal  of  activity  in  the  spring;  the 
central  bud  grows  directly  upwards,  the  two  lateral  rise  obliquely;  all  three  are 
thickly  leaved,  and  the  foliage  of  the  three  twigs  covers  over  and  shades  three, 
four,  perhaps  ten  times  as  large  a  space  as  the  pair  of  leaves  from  whose  base 
the  buds  had  developed  in  the  previous  summer. 

Now,  above  the  centre  of  the  maple  as  it  was  in  the  previous  year,  what 
may  be  termed  a  new  richly-leaved  and  thickly  overshadowing  little  maple-tree 
grows  up.  That  mutual  consideration,  which  is  otherwise  observed  by  members 
of  the  same  plant,  and  which  was  described  earlier,  here  ceases.  The  leaves  of 
the  topmost  shoot  are,  of  course,  so  arranged  that  no  mutual  injury  is  done; 
but  very  little  attention  appears  to  be  paid  to  the  leaves  below,  as  little  perhaps 
as  to  the  lower  grasses  and  herbs  which  grow  under  the  maple-tree  on  the 
ground. 

But  what  are  the  branches  to  do  which  spring  from  the  buds  in  the  centre 
of  the  maple- tree  under  consideration?  If  they  take  the  same  direction  as  the 
branches  at  the  extreme  summit,  they  will  come  into  the  area  of  the  dark  shadows 
thrown  by  the  numerous  broad  leaves  of  the  top  branches.  They  are,  therefore, 
compelled  to  take  up  another  direction  if  their  leaves  are  not  to  perish  from 
want  of  light.  And,  as  a  matter  of  fact,  this  is  what  they  do.  They  arrange 


RELATION    BETWEEN    POSITION    AND    FORM   OF   GREEN    LEAVES.  415 


Fig.  105.— Spruce-Fir  Trees. 


416  RELATION   BETWEEN    POSITION   AND   FORM   OF   GREEN    LEAVES. 

themselves,  that  is  to  say,  more  or  less  horizontally,  and  increase  in  length  in 
this  direction  until  their  leaves  project  outside  the  shadow  of  the  topmost  leafy 
branches  so  that  they  may  be  able  there  to  catch  the  sunlight.  All  this  is 
observed  not  only  in  maples,  which  have  been  selected  as  examples,  but  in  n 
richly-leaved  trees  and  shrubs;  the  topmost  branches  are  directed  vertically 
upwards,  the  next  lower  rise  obliquely,  those  still  lower  extend  horizontally, 
and  the'  lowest  of  all  frequently  even  incline  earthwards.  The  twigs  of  the 

older,  lower  branches 
which  have  grown 
out  beyond  the 
shaded  area  often 
again  try  to  rise, 
and  assume  a  direc- 
tion which  is  almost 
similar  to  that  of 
the  highest  branches 
at  the  summit.  Such 
branches  and  twigs 
then  display  a  cur- 
vature which  is  like 
a  Roman  f\J  lying 
sideways.  Oaks  and 
horse-chestnut  trees 
furnish  striking 
examples  of  this. 
The  phenomenon  is 
shown  still  better 
in  firs  (see  fig.  105), 
in  which  the  twigs 
springing  from  the 
lowest  branches  fre- 
quently rise  almost 

vertically.  This  last  circumstance  is  also  of  interest  in  so  far  as  it  indicates 
fchat  it  is  not  only  the  weight  of  the  leaves  which  brings  about  the  altered 
direction  of  the  branching,  but  that  it  depends  also  on  other  conditions,  to  be 
discussed  later  on. 

In  the  terminal  twigs  of  the  lowest  branches,  which  are  again  turned  upwards, 
the  same  distribution  and  direction  of  the  leaf-blades  as  are  displayed  by  the 
erect  twigs  of  the  summit  will  naturally  be  resumed;  but  it  is  not  so  in  the  case 
of  those  twigs  which  retain  a  horizontal  direction,  or  whose  summits  are  even 
inclined  towards  the  ground.  Suppose  that  the  maple-twig,  which  is  illustrated 
here,  has  not  grown  from  a  central  bud  of  the  summit,  and  does  not  rise  vertically 
upwards,  but  that  it  has  been  developed  from  an  older,  lower  branch,  and  is  extended 


Fig.  106.— Erect  leafy  Twig  of  the  Norway  Maple  (Acer  platanoides) 


RELATION    BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES. 


417 


almost  horizontally.  If  the  surface  of  the  foliage-leaves  on  the  horizontal  twi«r 
retains  the  same  direction  as  those  on  the  erect  twig  here  represented,  this  will 
be  the  most  disadvantageous  position  imaginable  with  regard  to  the  incident  light. 
It  is  urgently  necessary  that  they  should  alter  this  position  and  again  arrange 
themselves  suitably.  This  rearrangement  of  the  leaf-surfaces  proceeding  from 
the  horizontal  twigs  is  carried  out,  and,  indeed,  in  four  different  ways.  Either 
an  adequate  twisting  of  the  internodes  is  effected;  or  a  twisting  of  the  leaf -stalks 
occurs;  or  the  leaf -stalks  do  not  undergo  actual  torsion,  but  their  inclination 
to  the  leaf -blade  becomes  altered;  or,  finally,  individual  leaf -stalks  lengthen  to  an 
extraordinary  extent;  so  that  the  blades  borne  by  them  are  carried  far  beyond 


Fig.  107.— Twisting  of  Internodes  and  Leaf-stalks. 

i  Erect  twig  of  the  large-flowered  Rock  Rose  (Belianthemum  grandiflorum).     «  Procumbent  twig  of  the  same  plant 

s  Erect  twig  of  Diervilla  Canadensis.     *  Twig  of  the  same  plant,  bent  downwards. 

their  neighbours.      It   naturally  very  frequently  happens  that  these  alterations 
are  also  combined  in  many  ways. 

The  first  instance,  the  twisting  of  the  internodes,  may  be  observed  in  haze 
beeches,  and  hornbeams,  and  especially  in  trees,  shrubs,  lianes,  and  bushes  with 
decussating  short-stalked  leaves,  as   for  example  in  Comus  and  Thunbergw,  in 
Lonicera  and  Diervilla,  in  Androscemum  and  Hypericum,  in  Thymus  and  Vim 
Ooriaria  myrtifolia,  Gentiana  asclepiadea,  and  innumerable  others, 
represents   an   erect   twig  of  Diervilla   Canadensis.      As  soon  as  such  a  twig 
develops  no  longer  upwards,  but  horizontally,  a  twisting  of  90°  takes  place  in  ea*l 
internode,  and  the  consequence  is  that  the  surfaces  of  all  the  pairs  of  leaves  tak 
up  the  same  position  towards  the  sun,  as  shown  in  fig.  107*. 
now  no  longer  arranged  in  four,  but  in  two,  rows. 

Very  often  twisting  of  the  petioles  goes  hand  in  hand  with  that  of  the  mter- 

VOL.  I. 


418 


RELATION   BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES. 


nodes.  The  torsion  of  the  leaf-stalks  of  the  Judas  Tree  (Cercis  Siliquastrum), 
where  this  alone  occurs,  i.e.  without  a  simultaneous  twisting  of  the  internodes,  is 
particularly  striking.  The  leaves  of  the  plant,  as  can  be  seen  on  the  erect  twigs, 
and  especially  well  in  the  suckers,  are  arranged  in  the  one-half  phyllotaxis,  i.e. 
in  two  rows.  The  leaf -blades  on  the  erect  branches  are  parallel  with  the  ground. 
If  a  sucker  be  cut  off  and  held  horizontally,  all  the  leaf -laminae  will  be  directed 
at  right  angles  to  the  earth.  One  might  perhaps  expect  that  they  would  also 
assume  this  direction  if  the  twig  had  grown  horizontally.  Anything  but  that, 
however.  The  stalks  of  all  the  leaves  twist  round  instead,  until  the  laminae, 
or  blades  they  bear,  are  again  placed  in  a  direction  parallel  with  the  ground  on 
the  horizontal  branches,  and  the  result  is  that  the  leaves  on  all  the  branches  of 


Fig.  108.— Horizontally  growing  leafy  twig  of  the  Paper  Mulberry-tree  (Broussonetia  papyri/era). 

the  Judas  Tree,  whether  erect,  oblique,  horizontal,  or  inclined  towards  the  earth, 
always  present  the  same  attitude  to  the  incident  light. 

The  third  case,  the  alteration  in  the  inclination  of  the  blade  to  the  leaf -stalk, 
which,  on  the  whole,  is  but  seldom  met  with,  is  represented  in  the  best  known 
example,  the  cursorily  mentioned  Japanese  Paper  Mulberry  (Broussonetia  papy- 
rifera)  in  fig.  108.  In  this  plant  the  leaves  are  decussate,  i.e.  arranged  in  four 
rows,  each  pair  of  leaves  being  inserted  at  the  same  level,  and  the  successive 
pairs  alternating  at  right  angles.  In  erect  twigs,  therefore,  they  display  the 
arrangement  seen  in  the  twigs  of  maple  (see  fig.  106)  or  of  Diervilla  (see 
fig.  107 3).  The  following  alteration,  however,  is  seen  to  occur  in  the  horizontal 
branches  of  the  lower  boughs  of  the  Paper  Mulberry.  In  each  pair  of  leaves  the 
leaf -stalk  of  one  leaf  becomes  parallel  to  the  surface  of  the  ground,  and  lies  in  the 
plane  of  the  blade  it  supports,  which  is  also  almost  horizontally  extended,  or  but 
slightly  inclined.  The  other  leaf -stalk,  however,  rises  vertically  from  the  horizontal 
twig;  the  leaf -blade  it  supports  is  bent  down  at  right  angles  from  it,  and  conse- 
quently is  here  again  parallel  to  the  surface  of  the  ground.  A  slight  torsion  of 
the  internodes,  a  shortening  of  the  erect  leaf -stalks,  and  a  diminution  of  the  leaves 


RELATION   BETWEEN   POSITION   AND   FORM   OF  GREEN   LEAVES.  419 

borne  by  them,  certainly  assist  in  the  completion  of  this  peculiar  arrangement  of 
the  leaves;  the  above  illustration  will  demonstrate  other  particulars  far  better  than 
the  most  detailed  description. 

The  elevation  of  individual  leaf-stalks  above  the  horizontal  branches  occurs, 
somewhat  more  often  in  low  semi-shrubs  and  herbs,  than  in  trees  and  shrubs, 
whose  shoots,  furnished  with  decussate  leaves,  come  to  lie  flat  on  the  ground, 
as  in  some  species  of  speedwell  (Veronica  officinalis  and  Chamcedrys),  and  in 
many  species  of  Rock  Rose  (Helianthemum).  In  the  large-flowered  Rock  Rose 
(Helianihemum  grandiflorum),  an  erect  branch  of  which  is  illustrated  in  fig.  107  \ 


Fig.  109.— Leafy  Twig  projecting  laterally  from  the  Stem  of  toe  Norway  Maple  (Acer  platanoida). 

the  leaves  are  arranged  in  pairs  and  placed  crosswise,  so  that  they  occur  on  the 
stem  in  four  rows.      If  such  a  shoot  bends  down  over  the  ground,  a  slight  twisting 
of  the  leaf -stalks  occurs  first  of  all,  so  that  their  leaf-blades  come  to  lie  parallel 
the  soil;  but  another  alteration  is  yet  to  be  noticed.     In  every  alternate  pair 
leaves  one  of  the  leaf-stalks  rises  up,  and  its  blade  is  bent  down  almost  at  a  right 
angle  and  lies  above  the  horizontal  stem  as  shown  in  fig.  107  2.     In  consequence 
this  alteration  of  position  the  leaves  no  longer  form  four  rows  as  on  the  . 
shoots,  nor  two  as  in  Diervilla,  but  three  rows,  the  middle  one,  however,  C( 
of  a  smaller  number  than  the  two  side  rows. 

The  fourth  case,  which  still  remains  to  be  discussed,  is  the  increase  m  length  o 
individual  leaf-stalks.     It  may  be  very  weU  seen  in  maple-trees,  especially  in  the 
Norway  Maple  (Acer  platanoides),  and  this  species  will  therefore  serve  us 
example.     Kg-  106  shows  an  erect  branch  of  this  maple.     The  stalks  of  every  pair 
of  opposite  leaves  are  of  equal  length  on  the  erect  branch     But  how  ent, 
different  in  respect  to  length  are  those  leaf-stalks  which  embelhsh  the  horizontally- 


420  RELATION   BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES. 


directed  branches  of  this  species.  Here  one  of  the  pair  always  appears  con 
siderably  longer  than  the  other;  and  it  is  not  a  rare  occurrence  for  it  to  be 
three  times  as  long  as  its  neighbour,  as  may  be  seen  in  figure  109.  And  why 
this  striking  dissimilarity?  The  reason  is  again  the  same  as  in  all  the  previous 
cases.  If  all  the  leaf -stalks  were  to  retain  the  same  length  on  the  horizontal  twigs 
which  they  have  on  the  erect  branches  (see  fig.  106),  then  one  of  the  leaves  of 
every  alternate  pair  would  come  to  be  very  unfavourably  situated  in  its  neighbour's 
shadow.  This  detrimental  condition  must  be  prevented,  and  this  may  be  effected 
most  simply  by  the  leaf-stalk  increasing  in  length  until  the  blade  it  carries  is 
projected  beyond  the  area  of  the  shadow. 


Fig.  110.— Leaf-mosaics  of  Unsymmetrical  Leaves. 
»  Begonia  Dregei  growing  in  front  of  a  vertical  walL     2  Ficus  scandens,  growing  on  a  vertical  wall 

It  may  be  expected  that  alterations  of  direction,  shortenings  and  elongations, 
similar  to  those  just  described  in  the  case  of  the  horizontal  leafy  twigs  of  the  lower 
boughs  of  trees,  shrubs,  and  bushes,  will  be  found  on  those  plants  which  are 
attached  to  a  steep  face  of  rock,  a  vertical  wall,  or  to  the  bark  of  an  upright  tree- 
trunk.  As  a  matter  of  fact  all  the  instances  discussed  here  are  again  met  with  in 
various  climbing  and  twining  growths,  as  well  as  in  those  whose  stem  is  parallel  to 
a  vertical  wall  without  being  attached  to  it,  e.g.  as  in  Rhamnus  pumila,  and  in 
many  begonias.  But  here  the  leaf -blades  do  not  place  themselves  parallel  to  the 
ground,  but  to  that  surface  on  which  the  plants  in  question  are  supported,  or  which 
they  adjoin.  In  these  plants  another  peculiarity  is  often  observed  which  it  will  be 
most  fitting  to  speak  of  here,  viz.  the  want  of  symmetry  of  the  leaves.  While  in  the 
majority  of  plants  each  foliage-leaf  is  divided  by  a  midrib,  running  from  the  apex 
to  the  leaf -stalk,  into  two  similar  or  almost  similar  halves,  in  the  begonias,  many 
climbing  figs,  in  Celtis  occidentalis,  elms,  and  numerous  other  plants,  the  two 


RELATION   BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES. 


421 


portions  of  the  leaf  separated  by  the  midrib  are  very  unlike.  The  dissimilarity  is 
seen  principally  at  the  base  of  the  leaf— it  looks  as  if  a  piece  had  been  taken  out 
of  one  side,  or  as  if  the  leaf  had  there  been  cut  off  obliquely  (see  fig.  110).  The 
correct  explanation  of  this  want  of  symmetry  will  perhaps  be  arrived  at  most 
easily  by  supposing  the  suppressed  portion  to  be  completed,  or  in  other  words, 
let  us  suppose  the  smaller  half  to  be  just  as  large  and  well-developed  as  the 
other.  It  is  then  evident  that  the  added  portions  would  be  covered  over  by  the 
neighbouring  leaves,  and  consequently  they  would  be  deprived  of  light,  and  that 
in  these  parts,  therefore,  the  chlorophyll-bodies,  if  present,  would  not  be  able  to 


Fig.  111.— Mosaic  of  Leaves  of  unequal  size. 

i  Projecting  branch  of  Deadly  Nightshade  (Atropa  Belladonna)  looked  at  from  above.     «  SelagineUa  Helvetica, 

seen  from  above. 

carry  on  their  activity.  These  portions  of  the  foliage -leaves  would  accordingly 
be  superfluous,  and  it  is  foreign  to  the  ways  of  plants  to  manufacture  so  much 
leaf-tissue  for  no  purpose  whatever.  Plants  never  form  anything  which  i 
superfluous  and  useless;  in  the  construction  of  all  the  organs  the  principle 
apparently  is  to  attain  the  greatest  possible  result  with  the  least  amount  of 
material,  and  to  utilize  the  given  conditions,  above  all,  the  existing  space,  as  far  a 

possible. 

Yet  another  phenomenon,  viz.  the  unequal  size  of  adjoining  leaves  on  ti 
plant,  must  be  considered  from  this  point  of  view.     It  must  strike  everyone  wh< 
looks   down   upon   a   horizontally-projecting   branch  of  the   Deadly  Nights 
(Atropa  Belladonna,  see  fig.  Ill 1),  that  larger  and  smaller  leaves  are  here 
in  quite  a  peculiar  manner.     The  larger  leaves  stand  in  two  rows,  and  in  vir 
of  their  shape  it  happens  that,  between  every  two,  gaps  are  left  nea 


422        RELATION  BETWEEN  POSITION  AND  FORM  OF  GREEN  LEAVES. 

These  cannot  be  of  use  as  apertures,  through  which  light  can  pass  to  leaves 
situated  below,  for  the  simple  reason  that,  as  a  rule,  no  other  leaves  requiring  light 
are  to  be  found  under  the  branches  in  question.  Smaller  green  leaves  are  now 
inserted  in  these  gaps,  which  serve  as  protective  leaves  for  the  flowers,  that  is, 
indirectly  for  the  fruits,  but  whose  function  also  coincides  entirely  with  that  of 
the  large  foliage-leaves.  The  small  leaves  twist  and  turn  until  each  comes  to  lie 
exactly  in  the  middle  of  a  gap,  where  they  neither  encroach  upon  the  large  leaves, 
nor  are  encroached  upon  by  them.  An  exactly  similar  insertion  of  smaller  leaves 
in  the  gaps  between  the  larger  can  also  be  observed  in  the  Thorn-apple  (Datura 


Fig.  112.— Mosaic  of  Unsymmetrical  Leaves  of  unequal  size. 
Leafy  horizontal  Twig  of  an  Elm  (Ulmus)  seen  from  above. 

Stramonium),  and  in  Impatiens  parviflora,  illustrated  respectively  in  fig.  1043, 
and  fig.  104 1.  This  mosaic-like  fitting  together  of  larger  and  smaller  blades 
appears  to  be  combined  with  the  want  of  symmetry  of  the  leaf -base  in  short- 
stalked  leaves,  as  e.g.  in  the  wall-climbing  stem  of  Ficus  scandens  (see  fig.  110 2), 
and  on  the  older  horizontal  branches  of  elms  (Ulmus),  one  of  which  is  illustrated 
in  fig.  112.  It  has  been  already  mentioned  that  the  blades  with  erect  petioles, 
arranged  in  the  central  rows  on  the  Paper  Mulberry,  are  considerably  smaller  than 
the  lateral  rows  of  leaves  with  horizontal  stalks  (see  fig.  108).  This  difference  in 
the  size  of  the  central  and  lateral  rows  of  leaves  on  horizontal  stems  is  very 
noticeable  also  in  the  dainty  selaginellas,  belonging  to  the  family  of  Lycopodiaceae, 
of  which  a  species  (Selaginella  Helvetica)  is  represented  in  fig.  Ill  2. 

It  is  worth  noticing  that  the  occurrence  of  leaves  of  two  sizes  on  the  same  stem, 
as  well  as  the  mosaic-like  arrangement  and  fitting  together  of  the  leaves  in  one 


RELATION   BETWEEN   POSITION   AND   FORM   OF   GREEN   LEAVES. 


4-2:5 


plane,  is  observed  especially  in  plants  growing  in  dark  or  half-shaded  places. 
There  they  do  not  require  to  protect  themselves  against  an  over-abundance  of  light, 
but  on  the  contrary  have  to  make  what  use  they  can  of  its  scanty  amount,  and 
this  is  best  effected  by  the  fitting  together  of  all  the  leaves  on  a  stem  in  one  plane, 
like  the  stones  of  a  mosaic.  It  is,  of  course,  not  so  easy  to  produce  a  mosaic  from 
symmetrically  circular  or  elliptical  leaves;  but  unsymmetrical,  or  rhomboidal, 
triangular,  pentagonal,  and,  generally,  polygonal  blades  lend  themselves  particularly 
well  to  this  arrangement.  Excellent  examples  of  this  are  furnished  in  the  leaf- 
mosaics  in  fig.  110,  as  well  as  in  the  elm  twig  represented  opposite.  The  leaf -mosaic 
formed  by  the  ivy  on  the  ground  of  shady  woods  is  particularly  instructive  in 
this  respect.  In  the  picture  below,  which  is  a  faithful  reproduction  of  a  piece 
of  ivy  carpeting  the  ground  of  a  wood,  it  is  seen  how  the  lobed,  five-pointed  leaves 


Fig.  113. — Leaf-mosaic. 
Ivy  on  the  ground  of  a  forest. 

have  in  the  course  of  time  fitted  into  one  another.     The  lobes  and  points  of  one  fit 
into  the  indentations  of  another,  and  thus  originates  a  layer  of  leaves  than  which 
one  better  fitted  to  the  given  external  conditions  could  hardly  be  imagined.    In  this 
mosaic,  indeed,  we  no  longer  see  two  rows  of  leaves  symmetrically  arranged  on  the 
horizontal  stem.      What  manifold  elevations  and  depressions,  torsions,  displace- 
ments, and  elongations  must   have  taken  place  in  order  to  produce  such  a  leaf- 
mosaic  from  the  regular  rows  of  leaves!    But  we  learn  from  the  consideration  of  all 
these  instances,  that  not  only  the  arrangement  and  distribution  of  the  foliage,  and 
the  direction  and  length  of  the  leaf-stalks,  but  the  size  and  even  the  shape 
the  leaf-blades  also,  and  the  resultant  mosaic-like  piecing  together,  stand  in  cau 
relation  to  the  conditions  of  illumination;  and  that  in  dimly-lighted  situations 
plants  endeavour  to  utilize,  and  turn  to  account,  the  sunlight  for  the  green  ti 
the  foliage-leaves  as  far  as  possible  by  the  means  at  their  disposal,  and 
to  the  given  conditions  of  space. 


424  AKRANGEMENTS   FOR   RETAINING   THE   POSITION   ASSUMED. 


ARRANGEMENTS  FOR  RETAINING  THE  POSITION  ASSUMED. 

When  the  green  tissues  of  plants  have  once  assumed  the  position  most  beneficial 
to  them,  they  must  be  kept  as  long  as  they  can  be  in  that  position,  and  any  further 
alteration  must  be  as  far  as  possible  avoided.  The  displacements,  curvatures,  and 
extensions  described  in  the  preceding  pages,  representing  a  struggle  for  the  best 
arrangement  of  the  green  tissue  for  light,  must  not  be  restricted;  whilst  distortion, 
folding,  and  rupturing  of  the  chlorophyll -containing  tissues,  which  would  be 
synonymous  with  the  destruction  of  the  portion  in  question,  must  obviously 
be  warded  off. 

In  the  depths  of  still  water,  at  the  bottom  of  pools,  ponds,  and  lakes,  an 
alteration  of  the  position  assumed  by  the  fully-developed  plants  in  consequence  of 
an  external  stimulus  occurs  but  seldom;  and  although  currents  and  eddies  are  set 
up  in  the  water  by  passing  aquatic  animals,  and  temporary  oscillations  caused  in  the 
water-plants,  these  quickly  subside,  and  the  agitated  portions  return  forthwith  to 
their  original  position,  having  suffered  no  injury.  In  aquatic  plants  of  this  kind 
there  are  no  special  contrivances  for  strengthening  the  individual  organs,  and  in 
particular  no  contrivances  for  protecting  the  green  tissue  from  rupture  and 
crushing.  The  small  amount  of  strength  and  elasticity  of  the  cell-walls  suffices  to 
withstand  the  thrusts,  and  pulls,  and  the  pressures  which  make  themselves  felt  in 
the  depths  of  the  water,  and  to  restore  the  temporarily  displaced  green  portions  to 
their  right  position.  Firm  woody  cells,  and  strands  of  elastic  bast-fibres,  which 
play  such  an  important  part  in  the  aerial  portions  of  plants,  are  wanting  here. 
Woody  plants  neither  grow  in  the  sea,  nor  in  fresh  water.  Aquatic  plants,  indeed, 
quickly  collapse,  in  consequence  of  the  absence  of  wood  and  last,  when  brought 
from  the  depths  into  the  air;  the  leaves  collapse  of  their  own  weight,  and  sink 
flaccidly  on  to  the  substratum.  They  are  able  to  retain  an  erect  position  in 
the  water,  because  a  portion  of  their  tissue  is  penetrated  by  comparatively  large  air 
spaces,  by  which  means  their  specific  gravity,  compared  with  that  of  the  water, 
becomes  much  diminished.  If  aquatic  plants  were  not  firmly  attached  to  the  sand 
and  slime,  or  submerged  rocks,  they  would  rise  to  the  surface  and  float  there. 
But  as  they  are  fixed  in  the  depths,  the  air  spaces  within  the  green  tissue  of  the 
leaves  or  stalks  bearing  the  leaves  cause  these  organs  to  remain  erect  as  if 
suspended  in  the  water. 

Plants  growing  in  running  water,  and  such  as  are  exposed  to  the  lapping  of 
the  waves  on  the  shore,  are  indeed  subjected  to  a  severer  proof  of  their  firmness 
and  tenacity.  Thus  many  of  them,  e.g.  sea-wracks  on  the  sea-coast,  the  long- 
leaved  pondweeds  in  the  quick-flowing  mountain  streams,  and  the  Podostemacese 
in  the  rushing  torrents  and  waterfalls  in  tropical  regions,  are  actually  swayed 
hither  and  thither  and  continually  shaken,  and  accordingly  due  allowance  must 
be  made  in  their  construction  for  this  circumstance  of  their  habitat.  The  tissue 
of  these  plants  is  much  tougher  than  that  of  the  Characese,  of  the  Naiadacese, 


ARRANGEMENTS   FOR   RETAINING   THE   POSITION   ASSUMED. 

of  Water  Milfoil,  and  of  various  others  which  lead  a  peaceful  life  in  the  depths 
of  calm  waters.  Their  tissues  are  not  feeble,  but  elastic  and  pliant,  and  many 
sea-wracks  look  just  like  leathern  straps  and  bands.  Many  of  these  sea-wrack^ 
are  periodically  left  lying  on  the  dry  ground  at  low  tide,  but  they  do  not  in  con- 
sequence shrivel,  or  at  least  not  if  the  water  soon  returns,  but  lie  with  their 
pliant  leaf -like  surfaces  flat  on  the  dry  sand  or  stone.  When  the  tide  return  >. 
they  are  again  gradually  raised  up,  and  assume  an  upright  position  in  the  sur- 
rounding water;  and  this  is  materially  assisted  in  the  sea- wracks  by  the  swollen 
bladder-like  cavities,  in  reality  swim-bladders,  which  they  contain  in  their  tissues. 
Many  species  of  Characese,  but  still  more  the  Lithothamnese  and  Coralline®, 
acquire  an  increased  capacity  of  resistance  against  the  force  of  the  waves  by 
the  deposition  of  lime  in  the  cell-membranes;  others  again  so  closely  apply  their 
large  surfaces  to  the  rocky  reefs  and  stones  of  the  shore,  that  they  look  like 
coloured  patches  on  them,  so  that  the  crushing  or  tossing  effect  of  the  surging 
waves  is  entirely  obviated.  This  applies,  for  example,  to  Hildebrandtia  rosea 
and  Hildebrandtia  Nardi,  which  cover  the  stones  with  blood-red  patches. 

Many  marsh-plants,  which  are  only  partially,  and  often  only  temporarily, 
submerged,  whose  floating  leaves  are  half  in  contact  with  water  and  half  with 
air,  or  whose  leaf -blades  are  wholly  raised  above  the  water,  behave  just  like 
these  water-plants.  The  alteration  of  the  water-level  brings  about,  of  course, 
a  higher  or  lower  position,  an  elevation  and  sinking  of  the  floating  leaves,  but 
this  is  effected  without  the  slightest  rupture  of  the  parts  in  question.  The  stem 
and  the  leaf-stalks,  which  proceed  from  a  stock  rooted  at  the  bottom  of  the  water, 
resemble  long  strings  and  threads  to  whose  upper  ends  the  leaf -blades  are  fastened. 
At  the  highest  water-level  the  floating  leaf -discs  stand  perpendicularly  above  the 
stock  to  which  they  belong,  which  is  rooted  in  the  depths.  If  the  water  then 
sinks,  the  leaves,  floating  on  its  surface,  fall  with  it,  and  at  the  same  time  separate 
from  one  another.  The  stalks  and  stem  proceeding  from  a  stock  perform  approxi- 
mately the  same  movement  as  that  seen  in  the  ribs  of  an  umbrella  held  downwards 
and  then  opened.  As  soon  as  the  level  of  the  water  rises  again,  the  reverse 
movement  naturally  occurs.  Many  of  these  marsh-plants,  as,  for  example,  the 
Water  Chestnut  (Trapa),  also  possess  air-bladders  in  the  floating  portions  of  their 
leaves,  having  the  same  function  as  those  of  the  sea-wracks.  Moreover,  usually 
two  kinds  of  green  foliage-leaves  are  noticed  in  these.  Submerged  leaves,  which 
are  constructed  like  those  of  aquatic  plants,  and  floating  leaves  which  display 
a  more  or  less  disc-like  form,  and  whose  under  side  is  in  contact  with  the  water, 
the  upper  with  the  air,  but  which  under  certain  circumstances  may  be  entirely 
surrounded  with  air  without  injury.  If  the  marsh  should  dry  up,  long  thin 
stems  and  leaf-stalks  would  be  anything  but  beneficial;  the  metre-long  leaf-stalk 
of  a  water-lily  would  not  be  able  to  support  the  leaves  in  an  erect  position,  b 
would  fall  and  become  bent.  Stretched  out  on  the  ground,  also,  such  long 
mentous  leaf-stalks  would  not  be  advantageous.  It  is  seen,  too,  that  marsh- 
plants  of  this  kind  immediately  become  modified  when  the  water  rece( 


426  ARRANGEMENTS   FOR  RETAINING  THE   POSITION   ASSUMED. 

fresh  leaves  have  only  short  stalks,  and  these  become  so  strong  and  elastic  that 
they  are  well  able  to  support  the  leaves.  Water-lilies  are  striking  examples  of 
this.  In  Polygonum  amphibium,  the  long  stems  of  the  aquatic  form,  bearing  at 
their  upper  ends  groups  of  floating  leaves,  are  much  thinner  than  the  short  stems. 
of  the  terrestrial  forms,  which  are  uniformly  beset  with  leaves  from  top  to  bottom. 

The  green  tissues  which  are  surrounded  with  air  are  much  more  exposed  to 
the  danger  of  being  torn,  bent,  and  broken  up  by  violent  gusts  of  wind  than  those 
which  live  either  wholly  or  partially  submerged  in  water. 

When  green  tissue  is  only  developed  in  the  cortex  of  the  branches,  as  in  the 
leafless  switch-plants,  the  branches  are  always  elastic  and  supple,  and  in  order  to 
produce  this  quality,  bundles  and  strands  of  hard  bast,  i.e.  elongated  spindle- 
shaped  thick-walled  cells  of  fibrous  appearance,  are  inserted  at  suitable  places. 
The  wood  in  these  branches  is  also  very  tough,  and  gusts  of  wind  can  consequently 
do  them  little  harm.  They  are  often  prostrated  by  storms;  but  when  the  wind 
subsides,  the  branches  forthwith  rise  up,  and  in  consequence  of  their  elasticity, 
resume  their  former  position  towards  the  light.  The  bundles  of  hard  bast-cells 
alternate  in  many  instances  regularly  with  the  green  tissue,  as,  for  example,  in 
Spartium  scoparium,  illustrated  in  fig.  81,  and,  generally,  very  manifold  con- 
trivances are  to  be  found  in  the  internal  construction  of  the  branches  for 
hindering  the  bending  up  and  crushing  of  the  green  tissue. 

Leaves,  as  well  as  stems  and  branches,  have  originally  a  tendency  to  grow  up 
perpendicularly  in  the  atmosphere,  and  there  are  many  plants  whose  foliage 
remains  throughout  life  in  this  position.  Obviously  these  leaves  are  no  less 
exposed  to  damage  by  storms  than  are  the  upright  branches  of  the  switch-plants. 
It  must  be  borne  in  mind  that  gusts  of  wind  rush  over  the  ground  in  waves  like 
a  powerful  torrent,  and  that  the  direction  of  the  air-current  is  usually  parallel 
to  the  surface  of  the  earth.  Plant-organs  which  grow  up  from  the  ground  are 
struck  at  right  angles  by  such  gusts,  and  are  thus  exposed  to  the  most  violent 
attacks  of  the  wind.  Leaves,  especially,  whose  blades  are  inclined  at  right  angles 
to  the  direction  of  the  storm,  are  much  more  easily  bent  and  crushed  than  those 
whose  blades  lie  parallel  to  the  current.  The  effect  of  the  attacks  of  wind  increases 
in  proportion  to  the  extent  of  the  surface  exposed  to  the  air-current,  and  a  large 
upright  projecting  leaf  will  be  bent  much  more  by  the  wind  than  a  small  leaflet 
which  lies  close  to  the  stem  like  a  scale. 

In  what  way  can  the  dangers  of  rupture  be  warded  off  from  a  green  leaf  which 
grows  towards  the  light  of  heaven,  is  surrounded  by  air,  and  is  exposed  on  all 
sides  to  the  attack  of  wind?  First  of  all,  at  any  rate,  by  the  same  developments 
as  those  mentioned  in  the  case  of  the  upright  green  branches  of  switch  plants, 
i.e.  a  suitable  placing  of  the  green  tissue  between  flexible,  elastic,  fibrous  bundles 
of  bast-cells,  by  support  from  thick- walled  woody  cells,  and  other  cellular  forma- 
tions; by  these  means  firmness  is  given  to  the  whole  structure  with  the  least 
possible  expenditure  of  material;  an  arrangement  which  the  thin- walled  green 
tissue  can,  by  itself,  never  have  on  account  of  its  special  function. 


ARRANGEMENTS   FOR   RETAINING   THE   POSITION   ASSUMED.  4'JT 

But.  the  whole  shape  and  position  of  the  leaf  must  also  be  adapted  to  the 
circumstances,  for  the  simple  reason  that  a  plant  constructed  unsuitably  with 
regard  to  the  prevalent  winds  would  suffer  injury,  perish,  and  sooner  or  later  be 
supplanted  by  other  species  better  adapted  to  the  given  conditions.  Therefore,  it 
may  so  far  be  looked  upon  as  an  adaptation  of  form  when  a  leaf  lies  with  'its 
surface  parallel  or  only  slightly  inclined  to  the  surface  of  the  earth,  and  therefore 
to  the  direction  of  the  wind,  so  that  the  moving  currents  strike  it  at  a  very 
oblique  angle,  and  rupture  of  the  blade  can  hardly  ensue.  Since  this  position 
of  the  green  leaves  is  also  very  favourable  for  most  plants  with  regard  to  light, 
it  is  not  surprising  that  it  occurs  so  generally.  In  such  flat  leaves,  a  rising  and 
falling,  and  occasionally  a  bending  of  the  blade  is  unavoidable,  especially  when 
the  gust  of  wind  comes  from  that  side  towards  which  the  tip  of  the  leaf  is  turned. 
But  such  an  attack  on  the  leaf -blade,  which  is  parallel  to  the  surface  of  the  ground, 
or  inclined  slightly  to  the  horizon,  is  rendered  as  little  injurious  as  possible  by  two 
arrangements. 

One  consists  in  the  fact  that  the  moderately  stiff  leaf -blades  can  turn  like 
weathercocks  on  the  stem  from  which  they  project;  this  occurs  in  many  reed- 
like  grasses,  particularly  in  Phalaris  arundinacea,  JEulalia  Japonica,  and 
in  the  widely-distributed  Phragmites  communis.  The  latter,  which  often  grows 
in  immense  quantity  in  the  marshy  lowlands,  in  the  depth  of  valleys,  and  on  the 
banks  of  rivers,  develops  lofty,  slender  culms  bearing  numerous  leaves.  These 
leaves,  like  all  grass-leaves,  consist  of  a  linear,  fairly  broad,  and  tapering  blade 
projecting  from  the  stem;  and  also  of  a  sheath  in  the  form  of  a  hollow  cylinder 
surrounding  the  haulm,  and  from  which  the  portion  of  the  haulm  in  question 
proceeds  as  from  a  tube.  As  long  as  the  haulms  and  leaves  are  not  fully  developed, 
the  leaf -blades  are  strongly  directed  in  a  line  parallel  to  the  culm;  later,  they 
decline,  project  horizontally,  and  finally  become  even  somewhat  depressed,  so  that 
their  apices  are  directed  ground  wards.  They  then  remain  flat,  and  are  so  stiff  that 
they  cannot  be  bent  by  light  winds.  Moreover,  if  a  stronger  gust  occurs,  they  do 
not  bend,  but  twist  round  like  the  weathercock  on  the  roof -gable  in  the  direction 
of  the  wind.  This  is  rendered  possible  only  by  the  fact  that  the  haulm  and  the 
tubular  leaf-sheath  surrounding  it  are  very  smooth  at  the  surfaces  in  contact 
with  each  other,  and  that  the  leaf -sheath  may  undergo  a  slight  splitting  without 
damage. 

This  development  is  found  in  the  reed-like  grasses  mentioned;  and  in  them 
there  is  a  further  contrivance,  an  interrupted  membrane  or  flap  inserted  at  the 
boundary  between  the  blade  and  sheath;  this  protects  the  sheath  from  the  entrance 
of  rain-water,  and  consequent  increase  of  friction,  rendering  the  twisting  difficult. 
The  common  Reed  (Phragmites  communis),  growing  in  quantities,  presents  a 
characteristic  appearance,  in  consequence  of  the  arrangement  here  described,  every 
time  a  breeze  passes  over  such  a  bed.  If  the  wind  blows  from  the  east,  all  the 
leaves  are  directed  to  the  west;  if  it  comes  from  the  west,  all  their  apices  are 
turned  to  the  east.  The  whole  mass  looks  as  if  it  had  been  combed,  as  if  all  the 


428  ARRANGEMENTS   FOR   RETAINING   THE   POSITION   ASSUMED. 

leafy  blades  had  been  stroked  like  the  hair  of  a  horse's  mane  in  the  direction  of 

the  wind. 

The  second  arrangement  for  protecting  broad  flat  leaves  against  crushing  is 
observed  in  fan-palms,  in  maples,  poplars,  birches,  in  pear  and  apple  trees,  and  in 
innumerable  other  woody  growths  of  all  regions.  It  consists  in  the  development 
of  long,  elastic  leaf-stalks.  The  Aspen  (Populus  tremula),  which  may  be  regarded 
as  the  best  example,  exhibits  leaves  on  the  branches  of  its  crown,  whose  circular 
blades  are  always  somewhat  shorter  than  the  stalks.  At  the  slightest  movement  of 
the  air  these  are  seen  to  tremble  and  sway  hither  and  thither,  and  this  phenomenon 
is  so  striking  that  it  has  furnished  the  nucleus  of  many  sayings,  such  as  to 
"tremble  like  an  Aspen  leaf".  But  even  in  the  most  severe  storms  it  is  only 
the  leaf-stalks  which  bend,  having  acquired  a  high  degree  of  elasticity  by  the 
development  of  bast  strands.  The  leaf-blades  borne  by  them  remain  flatly  ex- 
tended, stiff,  and  rigid.  They  are  not  bent  by  the  wind,  and,  therefore,  these 
elastic  leaf -stalks  ward  off  the  danger  of  fracture  from  the  blades  they  support. 

In  many  grasses — for  example,  in  the  most  widely-distributed  cereals,  wheat, 
rye,  and  barley — it  is  observed  that  the  first  green  leaves  developed  by  the 
seedlings  are  erect,  while  those  developed  later,  which  arise  from  the  slender 
haulm  which  springs  from  their  midst,  are  more  or  less  parallel  with  the  ground. 
In  many  other  plants  with  much  contracted  subterranean  stem-structures,  viz. 
in  the  Reed-mace  (Typha)  and  in  many  bulbous  plants,  all  the  foliage-leaves 
assume  an  erect  position  and  remain  so  until  they  fade  and  die.  Leaves,  when 
erect,  are  far  more  exposed  to  the  wind  passing  horizontally  over  the  ground,  and 
require  much  stronger  protections  against  bending  than  those  which  are  extended 
flatly  over  the  soil;  and  in  order  that  they  may  be  able  to  escape  fracture,  they 
must  be  provided  with  specially  effective  contrivances. 

The  fistular  leaf  is  to  be  regarded  as  one  of  the  most  striking  of  these 
contrivances.  Fistular  leaves  are  always  erect  at  the  lower  end,  where  they 
surround  the  stem  or  the  neighbouring  leaves,  like  the  equitant  leaves  of  irises; 
they  are  sheathing  and  hollow,  terminating  above  in  a  hollow  cone.  There  is  no 
conspicuous  midrib;  a  shallow  groove  is  frequently  seen  on  the  side  directed 
towards  the  central  axis  of  the  whole  plant;  otherwise  the  hollow  leaf  is  developed 
uniformly  all  round.  It  has  no  appearance  of  special  resisting  capacity,  and  those 
cellular  elements  which,  as  a  rule,  are  used  to  increase  strength,  are  absent;  and  yet, 
like  all  tubes,  it  possesses  a  relatively  great  resistance  to  flexion,  and  it  is  scarcely 
injured,  even  in  violent  storms.  On  the  whole,  this  striking  form  of  leaf  is  not 
common;  it  is  most  often  seen  in  bulbous  plants,  e.g.  in  Chives  and  in  the  Common 
and  Winter  Onions  (Allium  Schwnoprasum,  Cepa,  and  fistulosum).  Structures 
are  more  often  met  with  which  resemble  the  fistular  form  to  some  extent, 
since  their  long  green  blades  are  rolled  up  lengthwise,  sometimes  towards  the 
side  facing  the  central  axis  of  the  whole  plant,  and  sometimes  away  from  it. 
The  rolling  observed  in  leaves  of  crocuses  is  particularly  noticeable.  A  white 
central  strip  runs  the  whole  length  of  the  erect  leaf,  which  is  bordered  by  two 


ARRANGEMENTS   FOR   RETAINING  THE   POSITION   ASSUMED.  42',) 

green  bands.  At  first  sight  these  two  green  bands  appear  to  be  flat,  but  they  are 
not  really  so;  each  is  convolute,  and  thus  in  the  crocus-leaf  there  are  actually  two 
green  tubes  united  to  the  white  central  stripe,  which  is  destitute  of  chlorophyll. 
This  leaf  may  be  distinguished  by  its  erect  position  from  the  rolled  leaves  which 
have  been  described  in  detail,  and  which  are  similar  in  some  respects,  although  they 
differ  from  them  in  significance. 

The  spiral  leaf  furnishes  another  contrivance  of  this  kind.  It  is  frequently 
seen  in  the  leaves  of  bulbous  plants,  bur-reeds,  and  grasses,  principally  in  young 
plants — as,  for  example,  the  first  green  leaves  of  barley  and  rye.  Leaves  spirally 
twisted  like  this  are  always  long,  narrow,  and  erect.  Sometimes  but  a  single  spiral 
revolution  is  found,  or  even  only  a  half  revolution;  sometimes  two,  three,  often  even 
four  circuits  are  described.  The  leaves  of  the  New  Zealand  Flax  (Phormiwn 
tenax),  and  those  of  the  Asphodel  (Asphodelus  albus),  of  narcissus,  of  many  irises, 
and  of  some  pines,  exhibit  only  a  half,  or  at  most  only  a  single  spiral  twist;  those 
of  the  Lesser  Bulrush  (Typha  angustifolia),  and  numerous  species  of  Garlic  (Allium 
senescens,  rotundum,  obliquum)  present  two  to  three,  those  of  Stei^nbergia  Clusiana 
three  to  four,  and  the  Persian  Sternbergia  stipitata  five  to  six  revolutions.  Leaves 
of  this  kind  have,  consequently,  a  curled  appearance.  That  such  a  spiral  leaf 
resembles  the  fistular  leaves  in  its  mechanical  significance,  and  that  it  possesses  a 
greater  resistance  to  flexion  than  a  flat  leaf,  is  beyond  question. 

It  may  also  be  noticed  in  the  Reed-mace,  that  in  a  strong  wind  the  leaves  are 
not  only  bent,  but  are  also  somewhat  elongated,  i.e.  the  spiral  becomes  somewhat 
looser  in  the  bent  leaf.  But  as  soon  as  the  wind  subsides,  and  the  leaf  returns 
to  its  vertical  position,  the  previous  form  of  torsion  is  resumed.  The  advantage 
possessed  by  an  upright  spirally-twisted  leaf  over  an  erect  flat  one,  with  regard 
to  wind,  becomes  quite  obvious  when  one  imagines  the  two  forms  exposed  side 
by  side  to  the  same  wind-pressure.  When  the  gust  strikes  an  erect  flat  and  rigid 
leaf,  the  whole  of  its  surface  is  encountered  at  right  angles,  and  the  leaf  undergoes 
a  large  amount  of  bending,  and  possibly  fracture;  but  when  it  strikes  a  spirally- 
twisted  erect  leaf,  the  various  portions  of  the  blade  are  met  at  different  angles;  the 
air  current  becomes,  as  it  were,  diffused  into  innumerable  streams,  which,  passing 
along  the  revolutions  of  the  spiral,  effect  only  a  comparatively  small  curvature,  and 
scarcely  ever  cause  the  leaf  to  be  broken.  When  these  spiral  leaves  are  swayed  by 
the  wind,  from  a  distance  the  movement  has  a  very  peculiar  look,  much  more  like 
trembling,  tossing,  and  twisting  than  like  bending. 

The  arched  form  of  leaf  is  closely  allied  to  the  spiral.     It,  too,  is  found  in  long 
ribbon-shaped  leaves.     At  the  commencement  of  development  the  arched  leaf  is 
erect  and  lies  in  one  plane,  but  when  fully  developed  it  takes  the  form  of  a  bow, 
with  the  convex  side  directed  upwards.     It  may  spring  from  the  sides  of  erec 
lofty   stems,   or   may   originate   close   to  the   soil.      Arched  leaves  appear  vei 
noticeably  in  those  grasses  whose  habitat  is  on  the  ground  in,  and  at  the  margins 
of,  woods  and  on  steep  mountain  slopes,  e.g.  in  Milium  effusum,  Melica  altutsn 
Calamagrostis    Halleriana,    Brachypodium   silvaticum,   Avena  favescens,    and 


430  PROTECTION   OF  GREEN   LEAVES   AGAINST   ATTACKS  OF   ANIMALS. 

Triticum  caninum.  When  the  wind  sways  the  leaves  of  these  plants,  the  arch 
formed  by  them  is  either  narrowed  or  widened,  according  as  the  wind  comas  from 
this  or  that  side.  In  still  air  the  leaf  assumes  a  middle  position.  Although  the 
arch  may  be  widened  or  narrowed  by  the  wind,  in  no  case  does  the  bending  go 
so  far  as  to  break  the  blade.  Moreover,  these  leaves  are  rendered  so  elastic  by  a 
suitable  arrangement  of  bundles  of  bast,  that  even  violent  storms  cannot  do  them 
much  harm.  These  arched,  overhanging,  ribbon-like  leaves  are  often  further 
complicated  by  the  fact  that  all  the  leaves  are  turned  to  the  same  side  so  that  they 
present  a  combed  appearance — like  those  of  the  reed — although  their  sheaths  cannot 
twist  round  the  haulm.  This  is  seen  especially  when  the  plants  are  growing  on  the 
margin  of  a  wood  or  on  the  narrow  terraces  of  a  rock  face,  i.e.  on  places  where  they 
are  only  illumined  on  one  side.  The  one-sided  direction  of  the  leaves  is  connected 
with  the  illumination,  and  is  due  to  the  fact  that  a  semi-arched  leaf  turned  towards 
the  gloom  of  a  wood,  or  towards  a  shady  rock -wall,  would  not  obtain  sufficient 
light.  This  gives  rise,  indeed,  to  an  inversion  of  the  leaf -blade,  so  that  the  originally 
lower  side  of  the  leaf  becomes  the  upper. 

It  is  scarcely  necessary  to  state  that  the  relations  with  regard  to  light  exercise 
a  no  less  important  influence  in  the  determination  of  the  shape  of  the  spiral  and 
fistular  leaves  than  in  the  above-mentioned  grasses,  whose  leaves  are  arched,  over- 
hanging, aad  partially  twisted.  If  these  relations  are  not  taken  into  consideration, 
it  is  not  because  the  significance  of  light  in  these  special  instances  is  not  appre- 
ciated, but  only  because  a  clear  view  of  these  extremely  complicated  conditions  can 
only  be  obtained  by  a  rather  one-sided  treatment. 


PKOTECTIVE  ARRANGEMENTS  OF  GEEEN   LEAVES   AGAINST  THE 
ATTACKS  OF  ANIMALS. 

The  matrix  of  the  chlorophyll-granules  is  very  similar  in  composition  to  that  of 
protoplasm,  and,  like  it,  consists  of  nitrogenous  compounds;  by  the  activity  of  the 
chlorophyll-bearing  cells  sugar  and  starch  are  produced,  and  the  green  cells  contain 
not  only  albuminous  compounds,  but  also  carbohydrates,  and  these,  too,  in  a  form  in 
which  they  are  digested  with  comparative  ease.  What  wonder,  then,  that  these 
green  cells  furnish  a  very  desirable  food  for  innumerable  animals.  Many  animals, 
it  is  well-known,  live  exclusively  on  a  vegetable  diet,  and  principally  on  chlorophyll- 
bearing  tissues.  On  the  other  hand,  the  plants  in  question  would  perish  with  the 
loss  of  all  their  green  organs,  especially  if  the  store  of  reserve  food  in  them  were 
also  exhausted.  The  animal  and  vegetable  kingdoms  in  this  sense  are  at  war  with 
one  another.  The  instinct  of  self-preservation  forces  animals  living  on  green 
vegetables  to  seek  their  food  at  any  cost,  to  seize  the  plants  unsparingly,  and 
when  their  hunger  presses,  to  destroy  them  root  and  branch.  Herbivorous  animals 
cannot,  like  men,  foresee  that  in  the  consumption  of  the  means  of  subsistence  the 
plants  robbed  of  all  their  green  organs  must  perish,  that  consequently  in  the 
following  years  for  them  and  their  descendants  food  will  be  wanting,  and  that 


man 


PROTECTION    OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  431 

in  the  destruction  of  their  food-plants,  their  own  existence  is  imperilled.  If 
removes  a  portion  from  the  plants  serving  for  his  livelihood,  a  limit  is  always 
to  this  consumption  which  prudent  consideration  and  foresight  never  overstep.  He 
always  leaves  as  much  as  is  necessary  to  the  plant  in  order  that  it  may  maintain 
itself  and  multiply.  Indeed,  he  even  tries  to  assist  and  to  further  the  nourishment, 
growth,  and  multiplication  of  the  plants  useful  to  him,  and  is  at  considerable 
trouble  to  protect  and  to  save  serviceable  vegetation  from  the  ravages  of  animals. 
This  protection  of  man,  however,  is  limited  to  a  comparatively  small  section  of 
plant  species;  all  those  from  which  he  derives  no  benefit  remain  uncared  for,  and 
these  would  be  surrendered  to  the  overwhelming  onslaughts  of  animals,  and  final 
destruction,  if  means  were  not  at  their  disposal  by  which  they  could  protect  and 
maintain  themselves.  Of  course  these  means  are  not  adapted  to  offensive  attacks 
upon  the  animal  kingdom;  and  the  attitude  of  the  vegetable  world  towards  animals 
must  not  be  looked  upon  as  one  of  war,  but  rather  as  an  armed  peace. 

But  if  plants  have  only  at  their  disposal  means  of  defence,  these  are  none 
the  less  dangerous  to  offenders,  and  not  only  equipments  comparable  to  pointed 
weapons,  but  also  poisons  and  corrosive  fluids  are  abundantly  turned  to  account. 

First  of  all,  with  regard  to  poisons.  It  is  to  be  pointed  out  that  these  are  only 
developed  in  those  parts  and  to  an  extent  necessary  in  order  to  preserve  at  least  the 
greater  portion  of  the  foliage,  and  then  also  the  flowers  and  fruit.  Moreover,  it 
must  be  remembered  that  the  same  chemical  compound  does  not  act  as  a  poison  to 
an  equal  degree  in  all  animals.  The  foliage  of  the  Deadly  Nightshade  (Atropa 
Belladonna)  is  a  poison  to  the  larger  grazing  animals,  and  by  them  is  left  undis- 
turbed; but  the  leaves  of  this  plant  are  not  only  non-poisonous  to  a  small  beetle 
(Haltica  Atropce),  but  form  this  animal's  most  important  food.  The  larvae  of  this 
beetle  often  eat  numerous  holes  in  the  leaves,  which,  however,  by  no  means  prevent 
the  development  of  the  Deadly  Nightshade.  Accordingly  these  leaves  are  protected 
by  the  alkaloid  contained  in  them  only  against  wholesale  extermination  ;  limited 
portions  of  them  can  be  surrendered  and  sacrificed  with  impunity.  The  same 
thing  occurs  in  numerous  other  plants  which  contain  poisonous  alkaloids,  or  other 
materials  harmful  to  large  herbivorous  animals.  It  is  puzzling  how  grazing 
animals  find  out  the  materials  in  the  leaf  which  are  injurious  to  them.  In  many 
instances  the  plants  in  question  possess  characteristic  odours  which  act  offensively 
on  the  olfactory  nerves  of  men  at  any  rate,  as,  for  example,  the  Thorn-apple 
(Datura  Stramonium),  the  common  Henbane  (Hyoscyamus  niger),  the  Hemlock 
(Conium  maculatum),  the  common  Birth  wort  (Aristolochia  Clematitis),  the  Dwarf 
Elder  (Sambucus  Ebulus),  and  the  Sabin  (Juniperus  Sabina)',  many  other  poisonous 
species,  however,  which  are  likewise  avoided  by  grazing  animals,  bear  leaves  which 
to  men  are  odourless  as  long  as  they  are  intact  —  as,  for  example,  the  numerous 
species  of  Monkshood  (Aconitum),  Black  Hellebore  (Helleborus  niger),  the  White 
Hellebore  (Veratrum  album),  the  Meadow  Saffron  (Colchicum  autumnale),  the 
Mezereon  (Daphne  Mezereum),  species  of  Spurge  (Euphorbia)  and  Gentians 
(Gentiana),  which  are  never  disturbed  by  stags,  roes,  chamois,  hares,  and  just  as 


432  PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS. 

little  by  oxen,  horses,  and  sheep,  not  even  by  the  omnivorous  goat.  As  long  as  the 
plants  remain  undisturbed  in  wood  and  meadow,  their  characteristic  materials  have 
no  effect  on  the  olfactory  nerves  of  men,  but  they  must  make  themselves  known  to 
the  animals  mentioned  by  the  sense  of  smell,  and  this  even  before  the  plants  have 
been  bitten  and  injured.  The  fact  that  plants  which  contain  no  alkaloids,  and 
generally  are  not  poisonous  to  men,  are  at  the  same  time  carefully  avoided  by 
grazing  animals  makes  it  probable  that  to  eat  them  would  be  in  some  way  injurious 
to  these  animals.  This  remark  applies  particularly  to  mosses,  ferns,  succulent 
plants  (Sempervivum  and  Sedum),  many  cresses  (Lepidium  Draba,  perfoliatum, 
crassifolium),  Toadflax  (Linaria  vulgaris),  the  Greater  Plantain  (Plantago  major), 
and  many  oraches. 

That  horse-tails  (Equisetum),  the  green  leaves  of  the  Crowberry  and  Bear- 
berry  (JEmpetrum  and  Arctostaphylos),  the  Rhododendron  and  Cowberry  (Rhodo- 
dendron and  Vaccinium  Vitis-Idcea),  and  numerous  other  low  evergreen  shrubs, 
which  form  a  chief  constituent  of  the  vegetation  of  heaths  and  moors,  as  well  as 
the  declivities  of  high  mountains;  further,  that  the  Proteacese  and  Epacridese  which 
compose  the  bush  of  Australia  and  the  Cape,  are  avoided  by  the  animals  seeking 
their  food  there,  is  indeed  explained  by  the  fact  that  the  tissue  of  these  plants  is 
very  difficult  to  digest  in  consequence  of  strongly  developed  and  partially  silicified 
cuticular  strata.  It  is  certain,  therefore,  that  in  the  formation  of  a  very  thick,  firm 
cuticle,  and  in  the  deposition  of  silica  in  the  cell-wall,  a  protective  measure  is 
provided  against  the  attacks  of  grazing  animals;  though,  of  course,  it  must  not  be 
supposed  that  this  is  the  only  function  discharged  by  these  structures. 

In  many  plants  water  forms  an  excellent  protection  against  grazing  animals, 
including  that  which  falls  as  rain  and  dew  on  the  foliage -leaves,  and  then 
remains  for  days  and  even  weeks  collected  in  special  hollows.  In  the  morning 
when  the  plants  are  richly  bedewed,  the  ruminants  do  not  usually  graze;  they  wait 
until  the  cold  dewdrops  and  rain-drops  which  adhere  to  the  leaves  are  evaporated; 
and  later  also,  they  leave  on  one  side  those  plants  on  which  the  rain-drops  still 
remain.  In  this  respect  the  Lady 's-man  tie  (Alchemilla  vulgar  is),  known  also  in 
popular  language  by  the  name  of  Dew-cup  (illustrated  in  fig.  52 2),  is  a  very 
striking  instance.  Rain  and  dew  remain  collected  here  at  the  bottom  of  the  cup- 
shaped  leaves,  when  already,  in  the  meadow  round  about,  the  surfaces  of  other 
plants  have  become  quite  dry.  While  these  latter,  if  they  are  not  protected  in 
other  ways,  are  devoured  by  the  grazing  animals,  the  Dew-cups  remain  undis- 
turbed, and  are  evidently  avoided.  This  is  not  caused,  as  in  the  ferns,  by  the 
possession  of  certain  objectionable  materials — since  the  leaves  of  an  Alchemilla, 
from  which  the  water  has  been  shaken,  are  eagerly  taken  as  food  by  the  grazing 
animals,  which  must,  therefore,  in  some  way  dislike  to  feed  on  leaves  on  which 
water  is  standing. 

The  most  important  role  in  the  defence  against  food-seeking  animals  is 
performed  by  the  organs  terminating  in  strong,  sharp,  tapering  points,  which 
wound  offenders,  and  may  be  called  the  weapons  of  plants.  In  botanical 


PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  433 

terminology  they  are  known  as  spines  and  prickles.  A  structure  which  is  mainly 
composed  of  wood,  or  whose  interior  is  at  least  traversed  by  vascular  bundles 
springing  from  the  wood,  and  which,  therefore,  ends  in  a  firm  sharp  point  is  called 
a  spine  (spina).  On  the  other  hand,  a  prickle  (aculeus)  is  a  structure  which 
proceeds  from  the  epidermis  or  cortex  of  a  plant  member,  contains  no  vascular 
bundle  within,  may  be  multi-  or  unicellular,  but  always  terminates  in  a  point  which 
is  capable  of  wounding  the  skin  of  the  offender.  This  distinction  is  not  always  an 
easy  one  to  make,  and  botanists  have  never  laid  much  stress  upon  it. 

Spines  and  prickles  may  arise  from  all  the  plant  members  and  organs,  and 
appear  at  all  heights.  They  are  observed  most  usually  on  or  near  the  green  tissues 
to  be  protected,  but  often  even  the  road  to  the  green  organs,  passing  over  the  leaf- 
stalk, the  stem,  and  occasionally  also  over  aerial  roots,  is  provided  with  prickles 
and  spines  in  order  that  in  this  way  the  animals  which  feed  on  vegetables, 
particularly  snails,  which  creep  up  from  below,  may  be  kept  off.  Thus  very 
pronounced  spines  are  seen,  for  example,  on  the  aerial  roots  springing  from  the 
lower  part  of  the  stem  in  Trithrinax  aculeata.  The  lower  portions  of  the  main 
axes  up  which  these  animals  must  climb  in  order  to  reach  the  green  portions  are 
armed  with  spines  or  prickles,  in  many  Bombax  and  Pandanus,  in  Erythrynese, 
gleditschias  and  roses,  and  in  the  fan -palms  very  abundantly  on  the  leaf- 
stalks. 

The  size,  direction,  position,  and  distribution  of  the  weapons  depends  generally 
upon  the  nature  of  the  attack,  on  the  form  and  size  of  the  food-seeking  animals. 
and  on  the  nature  of  the  implements  at  their  disposal.  The  gigantic  floating  leaves 
of  the  Victoria  regia  are  only  armed  with  prickles  on  the  under  surface  and  on 
the  turned-up  margin,  i.e.  only  where  they  are  exposed  to  the  attacks  of  plant- 
eating  aquatic  animals.  It  is  also  an  interesting  fact  that  many  woody  plants  are 
only  protected  when  young,  i.e.  while  they  are  short  and  their  foliage  can  be 
reached  by  ruminants,  viz.  by  goats,  sheep,  and  oxen;  but  on  the  boughs  and 
branches  removed  beyond  the  reach  of  the  mouths  of  these  animals,  no  prickles 
and  spines  are  developed.  Young,  low  trees  of  the  Wild  Pear,  only  one  or  two 
metres  in  height,  bristle  with  the  spines  into  which  the  ends  of  the  woody  branches 
are  transformed;  while  the  branches  of  the  crown  of  trees  four  or  five  metres 
high  remain  without  spines.  The  same  thing  occurs  in  the  Chinese  Gleditschia 
(Gleditschia  Chinensis),  and  in  the  Holly  (Ilex  Aquifolium).  In  the  latter  it 
can  be  seen  that  the  leaves  of  the  crown  of  tall  trees  have  almost  entire  margins 
and  are  unarmed,  while  the  margin  of  the  leaves  in  shrubby  specimens  is  drawn 
out  into  bristle-like,  pointed  teeth. 

Plants  armed  with  weapons  for  warding  off  the  attacks  of  animals  may  be 
arranged  together  in  two  groups.  One  of  these  consists  of  those  forms  which 
protect  their  green  tissue  by  structures  actually  developed  on  the  organs  in 
question,  and  the  other  group  comprises  those  forms  which  have  no  such  capacity 
of  self-help,  where,  rather,  one  member  protects  another,  and  where  division 
of  labour  has  brought  it  about  that  certain  plant-organs  deprived  of  chlorophyll 


VOL.  I. 


434       PROTECTION  OF  GREEN  LEAVES  AGAINST  ATTACKS  OF  ANIMALS. 

and  metamorphosed  into  weapons  assume  the  protection  of  the  unarmed  adjoining 
chlorophyll-bearing  members. 

To  the  first  division  belong  chiefly  most  of  those  leafless  plants  which  have 
developed  green  tissue  in  the  cortex  of  their  branches  and  twigs.  Indeed,  the 
green  branches  of  these  plants  are,  as  a  rule,  so  firm  and  rigid  that  one  would 
imagine  they  would  scarcely  ever  tempt  animals  to  eat  them.  But  "  hunger  is  a 
hard  master",  and  in  cases  of  necessity,  as  shown  by  experience,  even  the  stiff 
switch-like  shrubs  of  the  Mediterranean  and  other  floral  districts  are  attacked.  In 
order  that  they  may  not  succumb  entirely  to  these  assaults,  many  of  the  leafless 
green-branched  plants  are  suitably  armed  by  the  possession  of  spines  at  the  ends  of 
their  green  branches,  which  confront  the  assailants.  Many  of  these  plants,  indeed, 
are  actually  built  up  entirely  of  much-branched  green  spines,  which  fact,  of  course, 
gives  them  a  very  peculiar  appearance.  The  spinose  flora  of  Spain  and  of  the 
opposite  coast  of  Africa  exhibits  a  whole  series  of  these  plants,  but  here  only  the 
Furze  (Ulex  nanus,  Gallii,  micranthus),  and  the  spring  Asparagus  (Asparagus 
horridus,  Broussonetia,  and  retrofractus)  need  be  cited  as  examples.  Also  the 
green  leaf -like  branches  of  plants  with  flattened  shoots,  which  are  not  protected  by 
poisonous  substances  like  those  of  Phyllanthus,  run  out  into  sharp  points,  as  may 
be  seen  in  the  European  Butcher's-broom  (Ruscus  aculeatus),  illustrated  in  fig.  82, 
and  in  the  South  American  Colletia  cruciata,  represented  in  fig.  83 x. 

The  weapons  developed  on  green  leaves  are  far  more  complicated  than  the 
implements  with  which  green  stems  are  furnished.  In  some  instances  points  which 
wound  aggressors  project  from  the  ends  of  the  ribs  and  veins  which  form  the  ground- 
work of  the  leaves,  rising  up  like  needles  above  the  green  tissue  of  the  foliage;  in 
other  cases  they  consist  of  cells  and  groups  of  cells  which  originate  from  the 
epidermis  of  the  green  leaf,  and  are  inserted,  sometimes  at  the  margin,  sometimes 
on  the  surface,  like  little  daggers.  In  the  first  instance  the  vascular  bundles, 
which  are  seen  traversing  the  leaf  as  ribs,  are  provided,  at  the  point  where  they 
project  beyond  the  green  tissue  and  terminate  as  spines,  with  a  covering  of  very 
hard  cells;  in  the  latter  case  the  cells  and  cell-groups  springing  from  the  epidermis 
and  rising  up  as  prickles,  bristles,  and  pointed  hairs,  exhibit  thickened  and  strongly  - 
silicified  walls.  The  following  equipment  appears  particularly  often  in  several 
pines,  many  grasses,  sedges,  and  rushes,  in  species  of  the  genus  Yucca,  in  several 
caryophyllaceous  plants  (Dry pis  and  Acanthophyllum),  in  Acantholimon, 
belonging  to  the  order  Plumbagineas,  and  in  some  saltworts  and  succulent  plants 
(Umbilicus  spinosus,  Sempervivum  acuminatum).  The  green  leaves  are 
numerous,  usually  crowded  thickly  together  to  form  a  tuft,  and  project  from 
the  axis  in  all  the  directions  of  the  compass;  they  are  rigid,  undivided,  linear, 
round  or  triangular  in  cross  section,  and  terminate  in  a  sharp,  strong,  piercing  spine. 
This  form  of  leaf  may  be  termed  acicular  (or  needle-shaped).  In  many  cases,  at  all 
events,  such  leaves  have  exactly  the  form  of  needles,  and  in  regions  where  the 
unaltered  products  of  nature  are  still  preferably  used  as  tools  and  utensils,  they 
actually  serve  as  needles.  That  plants  possessing  leaves  with  these  needle-like 


PKOTECT10N    OF   GREEN    LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  435 


436  PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF    ANIMALS. 

points  are  excellently  protected  against  the  attacks  of  animals,  scarcely  requires  to 
be  proved  more  in  detail;  however,  it  might  be  indicated  by  special  mention  of  two 
interesting  examples.  In  the  Southern  Alps,  in  the  neighbourhood  of  Monte  Baldo, 
and  on  the  opposite  mountains  behind  Vallarsa,  a  species  of  grass  (Festuca  alpestris, 
see  fig.  86  5)>  is  found  here  and  there,  whose  rigid  leaves,  projecting  in  all  directions, 


Fig.  115.— Group  of  Thistles  (Cirsium  nemorale). 

terminate  in  needle-shaped  points.  This  grass  is  the  plant  most  detested  in  the 
whole  district,  and  the  shepherds  try  to  destroy  it  by  burning,  wherever  it  appears 
in  quantity,  since  the  grazing  animals,  when  seeking  other  plants  growing  between 
the  patches  of  Festuca  alpestris,  cut  their  nostrils  so  severely  that  they  often  return 
from  their  grazing  in  a  bleeding  condition.  It  is  remarkable  that  when  these  grasses 
can  be  easily  uprooted,  the  grazing  animals  themselves  bring  about  this  destruction. 
The  Mat-grass  (Nardus  stricta),  when  growing  in  the  meadows,  is  seized  low  down 


PROTECTION   OF   GREEN    LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  437 

between  the  teeth  of  the  oxen,  torn  from  the  ground,  and  dropped,  so  that  it  forth- 
with dries  up  and  perishes.  I  saw  thousands  of  the  tufts,  which  had  been  rooted  up 
by  oxen,  lying,  dried  and  bleached  by  the  sun,  on  the  meadows  on  the  Almboden 
of  Oberiss,  in  the  Tyrolese  Stubaithal.  It  must  not  be  supposed  that  the  animals 
accomplish  this  clearance  of  the  meadow  deliberately;  but  it  may  indeed  be 
admitted  that  they  root  up  the  patches  of  Mat-grass  in  order  thus  to  obtain  the 


• 


Fig.  116.—  Acanthus  spinosissimus. 

enjoyment  of  the  other  plants  growing  between  them,  and  avoid  the  risk  in  doing 
so  of  wounding  their  mouths  with  the  pointed  Mat-grass  leaves. 

A  considerable  proportion  of  plants  with  sharp  acicular  leaves  inhabit  steppes 
specially  distinguished  by  the  great  dryness  of  their  summer,  particularly  the 
elevated  steppes  of  Persia,  where  they  form  a  remarkable  feature  of  the  landscape. 
This  applies  most  of  all  to  the  numerous  species  of  the  genus  Acantholimon,  a 
group  of  which,  intermixed  with  spiny  Tragacanth  bushes,  drawn  from  nature  by 
Stapf,  is  exhibited  in  fig.  114.  Like  gigantic  sea-urchins,  lying  strewn  in  groups 
on  the  sea-bottom,  these  plants,  growing  in  hemispherical  patches,  live  on  the  stony 


438  PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS. 

soil  of  the  elevated  steppes,  where  they  are  so  well  protected  by  their  needle-shaped 
leaves,  projecting  all  round  from  the  stem,  that  they  are  never  eaten  by  grazing 
animals. 

With  the  acicular  form  of  foliage-leaves  are  ranked  those  which  may  be  best 
compared  to  the  process  of  the  sword-fish.  The  outline  of  the  leaves  belonging 
to  this  form  is  linear,  or  linear-lanceolate,  generally  they  are  elongated,  and 
often  also  slightly  curved.  Many  of  them  are  thickened  and  fleshy,  but  at  the 
same  time  very  hard  and  rigid  on  the  outside.  The  points,  produced  by  the 
terminations  of  the  vascular  bundles,  spring  from  both  margins  of  the  leaf,  and 
in  the  majority  of  instances  stand  at  right  angles  to  the  margins;  more  rarely 
are  they  directed  forwards.  Each  leaf  either  ends  in  a  strong-pointed  thorn, 
as  in  the  agaves,  or  in  a  bundle  of  threads,  as  in  Bonapartea  and  Dasylirion. 
The  teeth  on  the  leaves  of  the  last-named  plants  remind  one  most  in  form, 
surface,  and  colour,  of  the  scales  of  a  shark,  and  can  inflict  frightful  wounds  on 
those  who  come  too  closely  into  contact  with  them.  The  table-land  of  Mexico 
is  particularly  rich  in  plants  with  leaves  armed  in  this  manner;  that  country  is 
especially  the  habitat  of  agaves  and  Bromeliaceae,  of  species  of  Dasylirion  and 
Bonapartea.  The  Cape  also  is  the  home  of  a  series  of  these  forms,  viz.  those 
belonging  to  the  genus  Aloe.  Species  of  Eryngium,  with  agave-like  leaves 
(Eryngium  bromelicefolium,  pandanifolium,  &c.)  belong  to  Mexico  and  South 
Brazil.  It  is  worthy  of  note  that  several  aquatic  plants,  such  as  Hydrilla, 
Naias,  and  the  Water  Soldier  (Stratiotes  aloides),  have  their  leaves  similarly 
armed,  and  are  thus  protected  from  the  attacks  of  plant-eating  aquatic  animals. 

A  third  form  of  foliage-leaf,  armed  with  spines,  is  that  of  the  thistle.  The 
word  thistle  is  here  used  in  its  widest  sense,  and  is  not  restricted  to  species 
of  the  genus  Carduus  and  Cirsium  (see  fig.  115).  By  the  term  thistle-leaves  are 
indicated  all  those  which  are  more  or  less  lobed  and  divided,  whilst  the  margins 
and  the  extremities  of  the  lobes  are  provided  with  stiff,  projecting  spines.  Such 
leaves  are  possessed  by  very  many  composites  of  the  genera  Carduus,  Cirsium, 
Chamcepeuce,  Onopordon,  Carlina,  Echinops,  Kentrophyllum,  Carduncellus, 
especially  also  in  many  Umbelliferae  (e.g.  Eryngium  amethystinum,  Echinophora 
spinosa,  Cachrys  spinosa),  some  nightshades  (e.g.  Solanum  argenteum,  pyra- 
canthos,  rigescens),  several  Cycadese  (Zamia,  Encephalartos),  and  are  very  strongly 
developed  in  Acanthus,  of  which  a  species,  Acanthus  spinosissimus,  growing 
in  the  Mediterranean  floral  district,  is  illustrated  in  fig.  116. 

Nowhere  in  the  whole  world  is  the  thistle-leaf  met  with  so  abundantly  and 
in  such  manifold  varieties  as  in  the  Mediterranean  flora;  Spain  and  Greece, 
Crete  and  Algeria,  are  particularly  rich  in  districts  covered  with  thistles.  Thistle- 
leaves  often  appear  divided  into  three,  four,  or  five  portions,  and  split  up  into 
numerous  points,  sections,  and  lobes.  The  ends  of  all  the  separate  portions 
being  transformed  into  stiff  points,  not  much  remains  of  the  green  tissue  of  the 
leaf;  only  a  small  narrow  green  lamina  is  seen,  from  which  radiate  out  yellow 
and  white  spines  on  all  sides,  like  lances  of  various  lengths 


PROTECTION  OP  GREEN   LEAVES  AGAINST  ATTACKS  OF  ANIMALS.  439 

Pridde  structures,  which  are  not  to  be  regarded  as  metamorphosed  terminations 
of  leaf-ribs,  but  which  originate  from  the  epidermis  of  the  leaf,  are  sometimes 
multicellular,  sometimes  unicellular.  The  former  are  termed  prickles  (acul,!) 
the  latter  bristles  (seta).  In  this  series  of  weapons,  barbs  are  particulnrlv 
worthy  of  notice.  These  are  formed  by  obliquely  directed  conical  cells,  which 
project  from  the  margin  of  the  leaf,  and  terminate  in  a  hard  silicified,  generally 
somewhat  bent,  apex  (figs.  117'  and  117 *).  Leaves,  whose  margins  are  thickly 
beset  with  these  cells,  exhibit,  under  the  microscope,  a  saw-like  appearance.  It  is 


Fig.  117.— Weapons  of  Plants. 

i  Barbed  bristles  of  Opuntia  Raflnesquii ;  x  25.  »  Upper  portion  of  this  barbed  bristle ;  x  180.  »  Vertical  section  through  a 
part  of  the  leaf,  based  with  stinging  hairs,  of  the  Stinging  Nettle  (ffrtica  dioica);  x86.  *  Capitate  termination  of  a 
stinging  hair;  x!50.  «  The  capitate  termination  broken  oft;  x!50.  'Pointed  bristles  of  Echium  Italicum;  x40. 
7  Margin  of  a  scabrous  leaf,  beset  with  barbs,  of  a  Sedge  (Carex  stricta);  x200.  •  Margin  of  a  scabrous  leaf,  beset  with 
barbs,  of  a  Grass  (Festuca  arundinacea) ;  x!80. 

to  be  noted  that  leaves  of  this  kind  can,  under  certain  circumstances,  really  act 
as  saws.  If  such  leaves  are  very  gently  stroked  in  the  direction  opposite  to 
that  of  the  points,  they  do  not,  of  course,  immediately  cut  the  hand,  but  they 
do  not  bend,  and  under  increasing  pressure,  the  lamina  of  the  leaf  becomes 
arched.  Since  the  leaf  is  also  well  stiffened,  a  resistance  is  encountered  which 
could  scarcely  have  been  expected  from  so  fragile  a  leaf.  If  a  surface  on  which 
portions  of  these  leaves  have  been  laid  be  shaken,  the  bits  move  in  a  direction 
opposed  to  that  of  the  points  of  the  barbs.  Movement  in  the  opposite  direction 
is  impossible,  because  it  is  opposed  by  these  apices.  When  such  leaf-portions 
get  into  the  mouths  of  ruminants,  they  can  easily  move  forward  to  a  particular 


440  PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS. 

side,  and  in  a  particular  manner,  such  as  does  not  suit  the  purposes  of  the 
grazing  animal,  and  is  by  no  means  welcome  to  it.  By  firmly  stroking  the 
margin  of  such  a  scabrous  leaf,  a  wound  is  produced,  the  silicified  points  on 
the  margin  acting  like  the  teeth  of  a  fine  saw.  It  is  readily  intelligible  that 
grazing  animals  will  shun  such  scabrous  leaves;  indeed,  it  is  a  matter  of  observa- 
tion that  they  seldom  (and  then  only  when  impelled  by  great  hunger)  eat  sedges 
(e.g.  Carex  stricta  and  C.  acuta),  and  those  grasses  which  possess  particularly 
sharp-edged  leaves. 

Still  worse  than  the  barbs  of  scabrous  leaves  are  the  barbed  bristles  (figs.  117  l 
and  1172),  which,  it  is  true,  but  rarely  occur  in  plants;  indeed,  almost  exclusively 
on  the  branches  of  opuntias.  They  are  always  found  surrounding  the  buds, 
which  rise  like,  warts,  with  fine  bristles  above  the  green  tissue  in  opuntias  or 
prickly  pears.  If  such  a  spot  be  ever  so  lightly  touched,  small  stiff  bristles 
will  certainly  remain  sticking  in  the  skin  of  the  hand,  and  will  produce  a  very 
unpleasant  itching  sensation.  On  trying  to  pull  out  these  small  brown  bristles 
the  matter  is  only  made  worse,  for  they  then  penetrate  much  deeper  into  the 
skin,  and  may  produce  violent  pain  and  inflammation.  The  reason  of  all  this 
is  at  once  evident  on  examining  one  of  the  bristles  under  the  microscope.  Each 
bristle  is  composed  of  numerous  rigid,  fusiform  cells,  arranged  in  spiral  rows; 
at  the  upper  end  each  of  these  cells  is  wedged  in  between  the  others,  but  the 
very  hard,  backwardly-directed,  pointed  end  is  free,  and  thus  the  whole  structure 
is  set  with  barbs.  When  once  the  point  of  the  bristle  has  penetrated  the  skin, 
it  is  held  there  by  the  barbed  cells.  With  the  slightest  pressure  they  are  easily 
moved  forward  in  one  direction,  but  on  trying  to  produce  a  movement  in  the 
opposite  direction,  the  free  ends  of  the  cells  resist  the  attempt,  and  it  is  unavoid- 
able that  the  forcible  extraction  of  one  of  these  bristles  should  injure  a  larger 
area  of  the  skin  than  would  have  been  thought  possible  from  the  small  size  of 
the  structure. 

Another  form  of  weapon  originating  from  the  epidermal  cells  consists  of 
stiff  hairs  or  bristles,  with  hard  silicified  cell-wall  and  sharp  apex,  which  prick 
and  wound  like  needles,  though  only  unicellular;  they  are  called  pointed  bristles. 
They  usually  project  from  the  surface  of  the  green  leaves,  closely  crowded  together, 
and  their  points  are  turned  in  the  direction  from  which  an  attack  might  be  expected. 
They  appear  gigantic  in  comparison  with  barbs,  for  even  the  smallest  are  much 
longer  than  these,  and  the  largest  resemble  pins  with  their  heads  imbedded 
in  the  leaf-blade.  This  comparison  becomes  the  more  fitting  since  the  pointed 
bristles  are  surrounded  at  their  base  by  very  regularly  arranged  cells  which 
rise  above  the  surface  like  a  cushion,  or  often  like  a  short  white  cone.  The 
bristle  itself  on  the  end  of  this  pedestal  is  formed  of  a  single  cell,  which,  when 
fully  developed,  loses  its  protoplasm  and  becomes  filled  with  air.  The  wall  of 
this  elongated  cell  is  hardened  by  the  deposition  of  silica,  and  is  usually  unequally 
thickened  by  small  knobs  (fig.  117 6).  Although  pointed  bristles  are  developed 
in  numerous  groups  of  the  vegetable  kingdom,  one  group  is  especially  so  armed. 


PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  441 

This  is  the  family  of  the  Boraginese,  which  has  been  thus  named,  indeed,  in 
consequence  of  its  characteristic  armour.  Examples  of  the  equipment  described 
are  furnished  in  abundance  particularly  by  species  of  the  Viper's  Bugloss  (Echium). 
from  which  the  pointed  bristles  in  fig.  117  6  are  taken,  and  of  the  genera  Onosma, 
Comfrey  (Symphytum),  and  Borage  (Borago). 

On  the  leaves  on  Nettles,  Loasaceae,  Hydrophyllese,  and  Euphorbiace®,  occurs 
a  very  peculiar  mode  of  protection  against  the  attacks  of  large  herbivorous 
animals,  in  the  formation  of  stinging  hairs  or  bristles.  These  stinging  hairs  are 
formed  of  single  large  cells  like  the  pointed  bristles  of  Boraginese.  They  expand 
like  a  club  at  the  lower  end,  and  are  much  elongated  above.  Only  in  Wigandia 
urens,  which  belongs  to  the  Hydrophylleaa,  is  the  upper  free  end  finely 
pointed;  in  the  species  of  the  genus  Jatropha,  in  Loasacese,  and  in  nettles,  the 
extremity  is  swollen  into  a  small  head,  which  is  bent  to  one  side.  At  the  knee- 
shaped  bend  the  cell -wall  of  the  stinging  hair  is  extremely  thin  (figs.  1173-4-6), 
so  that  the  slightest  contact  suffices  to  break  off  the  head.  As  the  head  is  broken 
off  obliquely,  a  very  sharp  point  is  produced,  and  the  opening  formed  by  the 
rupture  is  not  horizontal,  but  oblique,  so  that  the  broken  end  resembles  the 
poison-tooth  of  a  snake  or  the  nozzle  of  a  hypodermic  syringe.  The  breaking, 
independently  of  the  extreme  thinness  of  the  cell- wall  below  the  head,  is  helped 
by  the  brittleness  of  the  hairs,  and  this  is  caused  by  the  silicification,  sometimes 
by  the  calcification,  and  in  Jatropha  by  the  lignification,  of  the  cell-wall.  This 
modification  of  the  cell-wall,  however,  is  restricted  to  the  upper  part  of  the 
hairs.  The  cell-wall  of  the  club-like  swelling  at  the  base  of  the  stinging  hair 
is  neither  silicified  nor  calcified,  but  consists  of  unaltered  cellulose,  and  yields  to 
an  external  pressure,  so  that  by  such  a  pressure  the  outflow  of  the  cell-contents 
is  assisted.  By  these  means,  also,  the  stinging  hair  is  enabled  to  become  turgid, 
which  property  certainly  plays  a  very  important  part  in  the  outflow  or  outspurt 
of  the  cell-contents  from  the  silicified  or  calcified  funnel-shaped  apex  after  the 
head  has  been  broken  off.  When  by  a  pressure  from  above  the  brittle  end 
of  the  hair  is  splintered,  and  the  head  broken  off,  the  point  formed  at  the  place 
of  rupture  penetrates  into  the  body  causing  the  pressure,  provided  this  is  soft, 
as,  for  example,  the  skin  of  men  and  animals;  and  the  contents  are  injected 
into  the  wound  so  formed.  In  the  fluid  contents  of  the  stinging  hair  a  substance 
occurs  together  with  formic  acid,  resembling  the  unorganized  ferments  or  enzymes, 
and  it  is  this  which  produces  the  violent  inflammation  round  the  wound  formed 
by  the  puncture.  The  painful  sensation  felt  immediately  after  the  puncture, 
which  is  popularly  called  "burning",  on  account  of  its  resemblance  to  that 
produced  by  a  burn,  is  indeed  caused  by  the  formic  acid;  but  a  series  of  other 
phenomena  which  are  observed  after  the  puncture,  can  only  be  placed  to  the 
account  of  the  enzyme,  which  acts  like  a  poison.  When  numerous  stinging 
hairs  penetrate  the  skin  in  close  proximity,  a  wide  area  becomes  reddened,  and 
inflammatory  swellings,  with  violent  pain,  are  produced.  Even  the  European 
nettles,  viz.  Urtica  dioica  and  urens,  give  rise  to  unpleasant  burning  and 


442  PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS. 

itching,  and  very  severe  attacks,  tetanus,  &c.  are  produced,  as  by  snake-bites, 
by  the  Urtica  stimulans  of  Java,  the  Urtica  crenulata,  which  is  a  native  of 
India,  and  the  Urtica  mentissima,  growing  in  Timor.  Generally,  an  analogy 
between  stinging  hairs  and  the  hollow  poison-fangs  of  snakes  cannot  fail  to  be 
recognized. 

The  mass  of  tissue  in  which  the  stinging  hair  is  imbedded  consists  of  chloro- 
phyll-bearing cells,  and  is  elastic  and  flexible;  whenever  a  stinging  hair  is  pressed 
on  one  side  it  lies  close  to  the  leaf-surface,  so  that  the  point  does  not  penetrate 
the  skin  of  the  fingers  pressing  it,  and  does  not  form,  or  poison,  a  wound.  When 
the  pressure  is  removed,  the  hair  becomes  erect  again  in  virtue  of  the  elasticity 
of  its  knob-like  support,  and  directs  its  brittle  point  outwards.  Upon  this  fact 
depends  the  trick  of  stroking  a  nettle  with  the  hand  so  as  not  to  be  stung.  The 
lower,  unarmed  part  of  a  leafy  nettle,  whose  foliage  is  beset  with  innumerable 
projecting  stinging  hairs,  is  taken  in  one  hand,  and  the  other  hand  is  then  passed 
from  below  upwards  over  the  foliage,  and  in  this  way  the  hairs  touched  are 
pressed  on  to  the  leaf  surfaces  and  do  not  wound.  But  if  the  nettle  is  touched 
from  above,  the  heads  of  the  hairs  are  immediately  broken  off,  the  perforated 
points  penetrate  the  skm  and  discharge  their  poisonous  fluid  into  it.  Grazing 
animals  carefully  avoid  plants  furnished  with  stinging  hairs,  and  do  not  let 
their  nostrils,  nor  the  mucous  membrane  of  their  mouths,  get  poisoned  by  the 
corrosive  fluid.  The  nettle  is,  therefore,  well  protected  against  larger  animals. 
Their  foliage  is,  indeed,  eaten  by  the  larvae  of  Vanessa  Urticce  in  spite  of  the 
stinging  hairs,  but  this  injury  is  restricted  to  only  a  portion  of  the  leaves;  they 
can  always  develop  new  leafy  shoots  from  the  intact  stems  and  buds,  and,  at 
any  rate,  the  nettle  does  not  perish  on  account  of  the  ravages  of  these  larvae. 

This  is  also  the  most  suitable  place  for  the  consideration  of  a  form  of  plant- 
hair,  whose  cells,  indeed,  possess  no  stiff  silicified  walls,  and  which,  therefore, 
do  not  prick  and  wound,  but  which,  nevertheless,  keep  the  plants  they  clothe 
from  injury  by  grazing  animals,  and  which  thus  far  must  also  be  regarded  as 
agents  for  protecting  the  green  tissue.  These  hair-structures  have  already  been 
described  when  making  clear  the  protection  afforded  to  leaves  against  excessive 
transpiration.  Such  hairs,  as  we  saw,  are  particularly  well  shown  by  many 
species  of  the  genus  Mullein  (Verbascum).  These  branched,  radiating  hairs, 
reminding  one  of  tiny  fir-trees,  are  easily  detached  from  the  surface  of  the 
leaves  from  which  they  spring,  and  a  very  slight  pressure  of  the  hand  is  sufficient 
to  lift  off  numerous  flocks  of  this  hair-felt.  Although  the  cells  which  build  up 
the  hairs  of  the  leaf-felt  are  not  stiff  and  prickly,  and  do  not  penetrate  into 
the  skin,  they  very  readily  remain  hanging  to  the  smallest  inequalities  on  the 
surface  of  the  disturbing  body.  If  grazing  animals  bring  the  mucous  membrane 
of  their  mouths  into  contact  with  the  leaves  of  the  Mullein,  this  mucous  membrane 
immediately  become  covered  with  flocks  of  the  detached  hair-felt,  which  establish 
themselves  in  the  inequalities  of  the  surface,  and  they  certainly  produce  anything 
but  a  pleasant  sensation.  On  the  peculiar  adhesion  of  the  felt-hairs  of  the 


PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  443 

Mullein  to  the  mucous  membrane  rests  the  necessity  for  the  caution  which 
people  observe  in  the  preparation  of  Mullein.  The  flowers  of  the  Mull.in 
(Verbascum  thapsus)  have  been  used  from  time  immemorial  in  the  preparation 
of  a  tea.  When  hot  water  is  poured  over  the  flowers,  which  are  covered  on 
the  under  side,  just  like  the  foliage-leaves,  with  a  fine  felt  of  hair,  portions 
of  the  felt  are  detached,  and  remain  floating  in  the  infusion.  If  the  decoction 
is  not  filtered  through  a  piece  of  linen,  some  of  the  hairs  may  stick  to  the  mucous 
membrane  of  the  mouth,  and  there  produce  an  intolerable  irritation  and  itching. 
This  unpleasant  sensation  is  certainly  much  more  powerful  in  animals  when 
taking  the  leaves  of  Mullein  in  their  mouths  than  with  us  when  we  drink 
unfiltered  mullein  tea,  and  it  no  doubt  deters  animals  from  eating  the  foliage  of 
the  plants  in  question. 

The  protective  mechanisms  just  described  are  all  borne  directly  by  the  par- 
ticular organs  needing  protection.  But  there  are  many  plants  whose  foliage  is 
unequipped  with  armament  of  this  sort,  and  in  which  adjacent  parts  of  the  plants 
afford  the  protection.  One  may  instance  all  such  plants  as  have  soft,  unarmed 
leaves,  sheltered  from  attack  by  lateral  shoots  transformed  into  spines.  The  stem 
and  branches  of  these  plants  are  not  clad  with  foliage  entirely  to  their  summits. 
The  ends  are  usually  leafless,  and  look  as  if  their  leaves  had  been  stripped  off*. 
Generally  speaking,  if  leaves  are  present  on  the  summits  of  the  branches,  they 
are  stunted,  small,  indicated  only  by  scales  and  protuberances,  and  are  anything 
but  an  attractive  food.  Consequently,  the  end  of  the  woody  branch  appears 
tapering,  and  terminates  in  a  stiff,  sharp  spine.  In  a  bush  whose  branches  project 
out  in  all  directions  with  leafless  apices,  while  their  green  foliage -leaves  are 
collected  behind  the  apex,  a  most  efficient  system  of  defence  is  produced,  resting 
upon  division  of  labour.  The  green  leaves  can  carry  on  the  work  assigned  to 
them  undisturbed  under  the  protection  of  the  spines,  and  if  it  happens  now 
and  then  that  a  large  food-seeking  animal,  driven  perhaps  by  greed  or  hunger, 
pushes  his  mouth  carefully  between  the  confronting  spines,  and  knows  how  to 
procure  some  green  leaves  from  behind  the  spines,  the  existence  of  such  a  bush 
is  not  seriously  threatened.  The  Alhagi  shrubs  of  steppes,  as  well  as  several 
brooms  and  Cytisus  shrubs,  viz.  Alhagi  Kirgisorum,  Genista  horrida,  and  Cytisus 
spinosus  (fig.  118  5),  exhibit  the  protective  mechanisms  just  described  in  a  marked 
manner.  In  many  other  shrubs,  such  as  sloes,  sea  buckthorns,  and  buckthorns 
(Prunus  spinosa,  Hippophae  rhamnoides,  Rhamnus  saxatUia),  the  same  con- 
trivance is  indeed  met  with,  but  it  only  has  the  full  significance  while  the  foliage- 
leaves  are  quite  young.  Only  so  long  as  the  tender  leaves,  which  have  just  emerged 
from  the  buds,  are  overtopped  by  the  spiny  branches  are  they  protected  from  being 
devoured;  afterwards  when  they  have  developed,  those  only  are  protected  which 
clothe  the  base  of  the  spiny  branches.  On  the  long  axes  of  the  Hawthorn,  in 
the  axils  of  the  lower  foliage-leaves,  there  are  always  developed,  close  together, 
a  lung  spine  and  a  small  bud,  in  the  axils  of  the  upper  leaves  a  bud  only.  In 
the  following  year,  reduced  axes  develop  from  the  buds  situated  close  to  the 


444       PROTECTION  OF  GREEN  LEAVES  AGAINST  ATTACKS  OF  ANIMALS. 

long,  shiny,  brown  spines,  which  often  bear  flowers;  but  from  the  buds  on  the  upper 
half  of  the  shoot,  a  long  axis  arises,  which  repeats  the  development  just  described. 
Spines,  which  on  the  American  species  of  hawthorn,  become  in  Cratcegus  coccinea, 
4  cm.,  in  C.  rotundifolia,  6  cm.,  and  in  C.  Crus  galli,  7-8  cm.  long,  resemble 
sentinels  which  have  to  protect  these  developing  reduced  axes.  Most  of  these 
bushes  develop  horizontal  projecting  branches,  and  therefore  extend  as  far  trans- 
versely as  vertically,  and  since  the  spines  remain  for  many  years,  the  leaves  of 
all  these  axes,  which,  in  later  years,  spring  laterally  from  the  branches,  almost 
in  the  interior  of  the  bush,  behind  the  old  spines,  are  protected  by  them.  In 
several  Brazilian  mimosas,  the  spines  situated  on  the  branches  do  not  indeed 
project  beyond  the  outspread  leaves,  but  as  soon  as  animals  disturb  the  leaves, 
they  are  revealed  from  their  concealment  behind  the  protective  defence  of  spines, 
and  the  animals  retreat  before  the  sharp  points  now  confronting  them. 

A  very  peculiar  relation  is  observed  between  green  leaves  and  spines  in 
most  of  those  semi-shrubs  which  Theophrastus  in  olden  times  grouped  together 
under  the  name  of  "Phrygana".  In  these  semi-shrubs,  of  which  the  Vella 
spinosa,  represented  in  fig.  118 8,  may  be  selected  as  an  example,  each  shoot 
growing  out  from  the  winter  buds  develops  green  foliage-leaves  on  the  lower 
half  and  above  these,  and  frequently  also  in  the  region  of  the  inflorescence, 
green  lateral  twigs  transformed  into  sharp-pointed  spines.  These  spines,  which, 
in  many  instances,  as  when  they  appear  in  the  region  of  the  inflorescence,  may 
be  considered  as  metamorphosed  flower-stalks,  are  at  first  soft  and  succulent, 
contain  green  tissue  in  their  cortex,  and  function  at  first  exactly  like  the  narrow 
foliage-leaves  situated  near  them.  In  the  first  year  they  play  no  part  as  protective 
agents  on  account  of  their  softness;  in  the  autumn,  the  green  leaves  fall  from 
the  shoots;  the  spinous  tips  of  the  branches  are  also  dead  and  withered,  but 
they  still  remain,  and  do  not  fall  off.  During  the  summer,  having  become  hard 
and  stiff,  they  now  wound  anyone  who  seizes  them  roughly,  and  obviously  protect 
the  shoots  which  spring  from  the  lateral  buds  in  the  following  year  behind  their 
dried-up  ends,  in  which  the  development  just  described  is  repeated.  Thus  arise, 
in  time,  bristling  shrubs,  from  whose  periphery  radiate  out  a  quantity  of  dried-up 
spiny  branches,  and  which  often  look  as  if  the  branches  had  become  frozen 
and  shrivelled  in  the  winter,  and  as  if  the  whole  plant  were  in  a  dying  condition. 
This  "  Phrygian "  underwood  is  not  certainly  an  embellishment  of  that  region  in 
which  it  occurs  in  masses,  but  it  forms  a  highly  characteristic  feature  in  certain 
floral  districts.  The  Mediterranean  area  is  particularly  rich  in  these  "Phrygian" 
bushes,  and  species  belonging  to  the  most  diverse  families  develop  in  this  form. 
To  mention  only  a  few  examples,  of  Cruciferse,  Vella  spinosa  and  Koniga  spinosa, 
of  Rosaceae,  Poterium  spinosum,  of  papilionaceous  plants,  Genista  Hispanica  and 
Onobrychis  cornuta,  of  Composites,  Sonchus  cervicornis,  of  Euphorbiacese,  Eu- 
phorbia spinosa,  of  saltworts,  Noea  spinosissima,  and  of  Labiatese,  Teucrium 
subspinosum  and  Stachys  spinosa,  may  be  pointed  out.  The  elevated  steppes 
of  South-west  Asia  also  exhibit  Phrygian  forms,  and  indeed,  chiefly,  as  isolated, 


PROTECTION    OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  445 

prickly,  and  spine-stiffened,  low  bushes,  growing  together  with  thorns,  and  low 
Tragacanth-shrubs,  in  which  the  green  foliage  is  protected  differently.  In  northern 
regions  not  exposed  to  summer  drought,  where  grazing  animals  find  in  summer 
enough  green  fodder,  this  form  of  plant  is  almost  entirely  absent.  It  is  only 
met  with  in  the  heaths  and  pine  forests  of  Central  and  Western  Europe,  in  some 
species  of  broom  (Genista  German-tea  and  Genista  Anglica). 

In  these  regions,  certain  shrubs  and  young  trees,  which  do  not  possess  the 
spine  formation  described  above,  acquire  from  the  grazing  animals  themselves  a 
shape  which  strongly  resembles  the  Phrygian  form.  It  is  brought  about  in  the 
following  manner.  If  young  trees  of  beeches,  oaks,  and  larches,  or  bushes  of 
Ling  (Calluna  vulgaris),  are  accessible  to  goats,  sheep,  and  oxen,  these  bite  off 
the  ends  of  the  fresh  shoots,  together  with  the  leaves  attached  to  them.  The 
remaining  portion  of  the  mutilated  shoot  in  the  neighbourhood  of  the  wound 
dries  up,  but  the  part  behind  keeps  alive,  and  the  buds  on  it  develop  even  more 
vigorously  than  would  have  been  the  case  if  the  mutilation  had  not  occurred. 
The  shoots  which  in  the  following  year  arise  from  these  buds,  however,  may 
suffer  the  same  misfortune;  they  may  again  be  pruned  by  grazing  animals,  and 
when  this  is  repeated  every  year,  the  mutilated  beeches  and  larches  at  length 
come  to  resemble  the  beeches  and  larches  of  old  French  gardens,  which  have 
assumed  the  shape  of  pyramids  and  obelisks  in  consequence  of  the  continual 
clipping  of  the  gardener's  shears.  The  branches  of  these  small  mutilated  trees 
become  so  thick,  and  the  dry,  hard  twigs  on  the  periphery  of  the  crown  arc  so 
crowded  together,  that  even  the  greedy  goats  are  prevented  from  breaking  through 
the  armour,  and  abstain  from  pulling  out  the  green  shoots  from  behind  the  dry 
stumps.  Thus  at  length  the  unprotected  plants  obtain  a  defensive  armour  which 
is  capable  of  saving  them  entirely  from  the  further  attacks  of  grazing  animals. 
Many  of  these  young  mutilated  and  bitten  trees,  of  course,  never  develop  into 
strong  lofty  specimens;  but  in  some  species  the  rough  treatment  which  they 
undergo  in  their  youth  does  not  result  in  lasting  injury.  This  applies  especially 
to  larch-trees  growing  in  Alpine  valleys.  The  young  trees  gradually  form  thick 
branched  bushes  in  their  struggle  with  the  goats,  and  a  top  cannot  be  definitely 
distinguished  in  them,  since  the  central  shoots,  as  long  as  they  can  be  reached  by 
the  goats'  mouths,  are  not  spared.  But,  at  last,  after  a  number  of  years,  the 
bushy  larches  attain  to  such  a  height  and  circumference,  that  the  goats  can  no 
longer  reach  the  upper  shoots.  And  behold,  a  strong  shoot  arises  from  the  middle 
of  the  much-branched  bush,  develops  a  whorl  of  lateral  branches,  elongates  from 
year  to  year,  and  being  no  longer  harassed  by  the  grazing  animals,  grows  up 
into  a  lofty  larch-tree.  For  a  long  time,  at  the  lowest  portion  of  the  tree,  may 
be  seen  projecting  the  oldest  lateral  boughs,  which  have  become  abundantly 
branched  in  consequence  of  the  mutilation,  and  which  serve  to  protect  and  defend 
the  developing  central  stem.  But  gradually  these  lower  branches  decay,  and 
fall  crumbling  on  the  ground;  thus  the  last  reminiscence  of  their  severe  youth  ii 
obliterated. 


446       PROTECTION  OF  GREEN  LEAVES  AGAINST  ATTACKS  OF  ANIMALS. 

The  contrivance  for  protecting  the  green  tissues  of  the  cactuses,  depending 
upon  a  division  of  labour,  is  accomplished  in  a  very  peculiar  way.  Our  conception 
of  a  plant  is  a  stiff  grey  or  brown  stem  bearing  soft  green  leaves.  In  the 
cactus-like  plants,  however,  the  most  important  types  of  which  we  have  already 
recognized  in  the  Cactacese  of  the  New  World,  and  the  columnar  Euphorbiaceae 
of  Southern  Asia  and  Africa,  everything  is  reversed.  Here  the  stem  is  green 
and  succulent,  and  the  leaves  it  supports  are  transformed  into  stiff  grey  or 
brown  spines.  Food  is  conducted  to  the  green  transpiring  tissue  in  the  cortex 
of  the  stem,  in  which,  and  not  in  the  leaves,  new  organic  materials  are  produced. 
The  leaves  which  have  been  changed  into  spines,  on  the  other  hand,  have  to 
keep  guard  that  the  green  tissue  in  the  cortex  of  the  columnar  or  flattened 
stem  is  touched  no  more  than  is  necessary.  This  reversed  state  of  things  strikes 
us  as  most  strange  in  the  opuntias  (Cactacese),  because  here  the  portions  of  the 
stem  have  the  form  of  thick  elliptical  leaves,  and  consequently  are  usually 
held  by  non-botanists  to  be  leaves.  But  the  spines,  or,  strictly  speaking,  the 
leaves  transformed  into  spines,  occasionally  attain  to  an  extraordinary  length 
in  these  opuntias.  They  are  3-5  cm.  long  in  Opuntia  Tuna,  decumana,  and 
magacantha,  and  even  8  cm.  in  Opuntia  longispina.  It  has  already  been 
mentioned  that  the  buds  of  opuntias  are  based  with  very  small  barbed  bristles, 
and  consequently  these  plants  are  armed  with  a  twofold  defence  against  possible 
attacks  with  large  spines,  visible  from  afar,  and  with  these  horrible  small  incon- 
spicuous barbed  bristles.  In  the  cactus-like  plants  the  variety  of  weapons  is 
very  great,  and  if  all  the  various  forms  of  long  and  short,  thick  and  thin,  knotty 
and  smooth,  straight-pointed  and  barbed,  arched  and  wavy  spines  and  bristles 
were  placed  together  side  by  side,  quite  a  goodly  collection  of  arms  would  be  the 
result.  A  single  species  often  bears  three  or  four  kinds  of  weapons,  and  these 
are  arranged  and  distributed  in  a  great  variety  of  ways,  and  in  this  respect  a 
diversity  is  developed  which  has  a  fascinating  effect  on  anyone  who  has  an  inborn 
taste  for  such  changes  of  form,  and  we  can  understand  how  it  is  that  so  many 
lovers  of  flowers  have  devoted  themselves  to  the  study  and  culture  of  these 
curious  representatives  of  the  vegetable  kingdom.  Although  it  is  impossible 
to  show  the  connection  between  the  kind  of  armour  and  the  attacks  to  be 
warded  off  in  each  individual  instance,  even  the  most  cursory  glance  shows  us 
that  the  points  of  the  spines,  however  these  may  be  shaped  and  arranged,  are 
always  placed  in  front  of  that  portion  of  the  stem  which  is  best  furnished  with 
green  tissue.  In  the  columnar  euphorbias,  e.g.  in  Euphorbia  ccerulescens,  the 
stems  are  furnished  with  shallow  longitudinal  grooves  clothed  with  green  tissue. 
On  the  ridges  between  the  grooves  are  inserted  pairs  of  divergent  spines  with 
their  points  in  front  of  the  grooves,  and  thus  ward  off  every  assault  on  the  green 
tissue.  Exactly  the  same  thing  is  seen  in  the  columnar  Cereus,  and  in  the  cone- 
shaped  Echinocactus  and  Melocactus. 

On  looking  at  these  columnar,  flattened  and  spherical  cactuses,  the  question 
arises  whether  it  is  necessary  for  them  to  be  surrounded  with  such  a  complicated 


PROTECTION   OF   GREEN    LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  447 

system  of  spines.  According  to  the  ordinary  conception  of  the  method  of 
feeding  of  the  herbivorous  animals  referred  to,  it  would  be  thought  that  these 
green  clumps,  pillars,  and  balls,  even  without  their  terrible  equipment  would 
form  anything  but  choice  food.  But  when  they  are  seen  in  their  original  habitats 
it  is  easily  seen  that  they  have  every  need  to  protect  themselves  and  to  defend 
their  existence.  While  on  the  stony  and  sandy  plains  and  slopes  which  form 
the  habitat  of  cactuses,  all  other  plants  have  long  been  withered,  and  a  green 
leaf  can  no  longer  be  seen  for  far  and  wide,  when  all  the  springs  of  water  are 
dried  up,  and  not  a  drop  of  rain  has  moistened  the  ground  for  months— then 
the  cactuses  still  remain  always  fresh  and  green,  and  by  the  assistance  of  their 
central  aqueous  tissue,  they  are  able  to  survive  through  the  greatest  drought 
and  aridity  which  are  ever  observed  on  the  earth.  But  at  such  periods  of 
drought,  every  cactus-ball  appears  like  a  cordial  to  the  hungry  and  thirsty 
animals,  and  frequently  even  as  the  only  alternative  to  death.  In  spite  of  the 
frightful  spines  with  which  species  of  Melocactus  are  bristling,  these  are  sought 
by  the  wild  asses  in  the  plains  of  South  America  at  the  periods  of  greatest 
drought,  and  are  rooted  up  where  possible  by  their  hoofs  in  order  to  get  at  the 
juicy  tissue  of  the  unarmed  lower  parts;  or  the  animals  try  to  split  the  cacti 
with  their  hoofs,  and  in  this  way  to  get  at  the  interior,  in  which  proceeding  it 
very  often  happens  that  the  assailants  injure  themselves  by  the  spines  and  receive 
dangerous  wounds. 

Next  to  the  cactuses,  the  strangest  spine-formations  are  exhibited  by  the  low 
half -shrubby  tragacanth  bushes  (Astragali)  belonging  to  the  group  Tragacanthacei, 
which,  in  an  inexhaustible  variety  of  species  have  their  habitat  throughout  Southern 
Europe,  but  chiefly  in  the  east,  on  rocky  mountains  and  elevated  steppes.  We  will 
pick  out  one,  viz.  Astragalus  Tragacantha,  from  the  large  number  of  species,  and 
explain  by  words  and  picture  the  remarkable  protective  contrivance  of  its  green 
foliage-leaves  (fig.  118 x).  On  observing  this  plant  very  early  in  the  spring,  a  tuft 
of  numerous  long,  dry,  grey  spines,  whose  points  are  directed  upwards  and  outwards, 
is  seen  on  the  free  extremity  of  each  branch.  In  the  centre  of  this  tuft  of  spines 
lies  a  bud,  which  forms  the  top  and  termination  of  the  branch  in  question.  The 
warmth  of  spring  causes  this  bud  to  develop,  and  the  close-pressed  pinnate  leaflets 
become  loosened,  stretch  out,  and  unfold;  but  weeks  pass  by,  and  the  leaflets  are 
always  still  surrounded  by  the  bushy  garland  of  spines.  Their  green  colour  can 
only  be  seen  shining  through  from  behind  the  long  spines  as  from  behind  the  grey 
lattice  bars  of  a  cage.  When  they  are  fully  developed,  and  when  they  have  also 
somewhat  lengthened  the  branch  they  adorn,  the  uppermost  leaflets  at  length 
project  beyond  the  points  of  the  spines.  But  see— the  end  leaflet  which  had  been 
situated  on  the  rachis  of  the  pinnate  leaf  has  already  dropped  off,  and  often  a 
pair  of  the  lower  leaflets  with  it  (fig.  118 2),  and  all  that  now  projects  beyond 
last  year's  spines  has  also  become  changed  into  a  spine.  The  rachis  of  the  leaf  at 
the  point  where  the  terminal  leaflet  was  formerly  inserted  becomes  hardened  and 
transformed  into  a  pricking  point.  Then  comes  autumn,  the  period  of  the  leaf-fall. 


448       PROTECTION  OF  GREEN  LEAVES  AGAINST  ATTACKS  OF  ANIMALS. 

Most  deciduous  shrubs  now  throw  off  the  leaves  with  which  they  have  worked  all 
the  summer,  by  means  of  the  formation  of  a  layer  of  separation  at  the  place  where 
the  leaf  is  inserted  on  the  stem,  as  previously  described.  But  this  does  not  occur  in 
the  tragacanth  bushes.  Only  a  portion  of  the  long  grey  spines  by  which  this  year's 
leaves  were  surrounded  are  cast  off.  The  leaflets  of  the  present  year  are  now 
detached  from  the  leaves;  the  strong  midribs  or  axes  whose  ends  had  already 
become  changed  into  spines  during  the  summer  remain  firmly  joined  to  the  stem 
and  dry  up,  forming  thus  a  new  stiff  tuft  of  spines  which  is  as  like  the  one  thrown 
off  as  one  egg  is  to  another.  Accordingly  the  dried-up  remains  of  the  leaves  of  one 
year,  now  changed  into  spines,  become  an  apparatus  for  protecting  the  developing 
green  leaves  of  the  year  following.  Observation  in  the  natural  state  shows  that 
these  projecting  spines  are  capable  of  protecting  the  green  leaves  behind  them  from 
the  attacks  of  grazing  animals.  One  may  see  how  grazing  animals  stop  in  front  of 
the  shrubs  bristling  with  spines,  and  actually  abstain  from  further  attacks  after  the 
first  attempt,  although  the  foliage  of  the  tragacanth  named,  like  that  of  other 
papilionaceous  plants,  would  furnish  a  very  desirable  meal. 

Different  from  the  Tragacanth  is  the  Barberry  (Berberis).  On  looking  in 
summer  time  at  a  shoot  in  vigorous  growth,  it  will  be  seen  to  be  beset  with  two 
kinds  of  leaves:  first,  with  leaves  which  have  anything  but  the  appearance  of 
foliage,  being  transformed  entirely  into  spines  like  those  of  the  cactuses.  These  at 
the  base  of  the  shoot  are  drawn  out  into  from  five  to  seven,  and  further  upwards 
into  three,  needle-shaped  points,  as  shown  in  figs.  1186  and  1187.  Short  branches 
beset  with  ordinary  green  foliage-leaves  arise  simultaneously  in  the  axils  of  these 
metamorphosed  leaves.  These  short  branches  terminate  in  buds  which  develop 
early  in  the  following  year,  and  then  form  either  flowers,  or  long  branches.  The 
foliage-leaves  of  the  short  branches,  below  these  buds,  fall  off  in  autumn.  The 
three-pronged  spines  at  the  bases  of  the  short  branches,  i.e.  of  the  buds  which  have 
passed  through  the  winter,  remain  behind,  and  radiate  out  from  the  shoot  with 
their  three  needles  in  three  directions.  Now,  when  in  the  following  spring  the 
buds  at  the  end  of  the  short  branch  swell,  and  young  tender  foliage-leaves  burst 
from  them,  these  are  excellently  protected  against  being  devoured  as  long  as  the 
points  of  the  three-pronged  spine  still  project  beyond  them. 

In  Robinia  Pseudacacia,  popularly  known  by  the  name  of  Acacia,  and  also  in 
numerous  other  robinias  as  well  as  in  several  Siberian  caraganas  (Caragana 
microphylla  and  pygmcea),  the  stipules  are  transformed  into  prickles,  and  not,  as 
in  Berberis,  the  whole  leaf.  In  all  Leguminosae,  structures  arise  right  and  left  of 
the  place  of  insertion  of  the  leaf  on  the  stem,  known  as  stipules  (stipulce),  on 
account  of  their  position.  They  are  not  leaf -like  in  the  robinias  and  shrubs  named, 
but  are  transformed  into  brown  spines  drawn  out  into  a  sharp  point.  When  the 
foliage-leaf  becomes  detached  and  falls  off  in  autumn,  both  the  spinous  stipules 
remain  behind  and  persist  even  on  into  the  following  summer.  In  the  axil  of  the 
two  divergent  spine-stipules  is  situated  a  bud  which  unfolds  in  the  following  spring. 
Here  we  have  again  repeated  the  same  protective  mechanism  as  was  previously 


PROTECTION  OF  GREEN   LEAVES  AGAINST  ATTACKS  OF   ANIMALS.  449 

sketched  in  the  case  of  the  Barberry.     As  long  as  the  young  tender  foliage-lea 
remaln  m  this  station  between  the  two  spiny  stipules  (fig.  U8.)  they  235 

at  an  end 


. 

TSLS  £e™y  at  an  end  when  they  h-«  —  *«-  * 

Most  of  the  last-described  protective  contrivances  only  defend  the  green  foliage 


Fig.  118.-Weapons  of  Plants. 

'  Branch  of  the  Tragacanth  bush  (Astragalus  Tragaeantha)  in  spring.    »  A  single  leaf  of  this  Tragacanth  from  which  the 

upper  leaflets  have  fallen.  *  Leaf -axis  from  which  all  the  leaflets  have  fallen.  *  Portion  of  a  shoot  of  ftobinia  Pseudacacia 
in  spring.  «  The  spiny  Cytisus  (Cytisus  spinosus),  •,  1  Portions  of  branches  of  the  Barberry  (Berberit  vulgari$)  in  spring. 
8  Velio,  spinosa ;  the  end  of  last  year's  shoot  is  dried  up ;  this  year's  shoot  bears  flowers. 

whilst  it  is  young.  But  this  is  exactly  the  time  when  protection  is  most  needed. 
If,  later  on,  isolated  foliage-leaves,  which  have  grown  beyond  the  points  of  the 
prickles,  are  eaten,  this  does  not  so  much  matter,  as  part  of  the  foliage  still  certainly 
remains,  and  this  is  really  the  important  point. 

From  the  fact  that  the  protection  of  the  young  green  leaves  is  secured  by 
VOL.L  M 


450       PROTECTION  OF  GREEN  LEAVES  AGAINST  ATTACKS  OF  ANIMALS. 

a  portion  of  the  old  dead  leaves,  by  dried-up  structures  of  the  previous  year,  in 
tragacanth  bushes  and  also  in  many  caraganas,  and  generally  in  numerous  other 
plants,  two  things  may  be  learned:  first,  that  one  and  the  same  plant-member  may 
in  the  course  of  a  year  change  its  function;  and  secondly,  that  dead,  withered 
portions  are  often  called  upon  to  play  an  important  part  in  the  life  of  a  plant.  The 
same  thing  is  frequently  observed  in  flowers  and  fruits.  It  often  happens,  for 
example,  that  floral  leaves,  which  originally  served  to  allure  insects  and  to  protect 
the  pollen  from  moisture,  are  of  use  later  on  when  dried,  in  the  dissemination  of 
the  fruits  and  seeds.  In  foliage-leaves,  on  the  other  hand,  such  a  change  of  function 
is  comparatively  rare,  and  is  hardly  ever  observed  except  in  the  plants  of  steppes 
and  of  the  Mediterranean  flora. 

It  would  naturally  be  expected  that  the  protective  contrivances  required  by  the 
green  tissue  against  an  excessive  destruction  by  animals,  would  also  exercise  an 
influence  on  the  gregarious  growth  of  plants  as  well  as  on  the  dwelling  together  and 
distribution  of  plants  and  animals;  and  this  is  proved  by  numerous  observations. 
Let  us  suppose  ourselves  in  a  country  where  plants  of  a  hundred  different  kinds 
grow  up  side  by  side.  The  shrubs,  bushes,  and  herbs,  mixed  together,  contain  the 
most  diverse  substances.  Some  abound  in  milky  juices;  others  are  as  bitter  as  gall; 
whilst  others  again  taste  frightfully  sour  or  contain  in  their  sap  alkaloids,  the 
partaking  of  which  would  be  deadly  to  many  animals.  Here  is  a  plant  armed  with 
stinging  hairs;  there  from  a  bush  radiate  out  innumerable  spines;  and  again  in 
other  places  thistles  rear  their  prickly  leaves.  The  one  prevents  snails  from  eating 
the  foliage,  the  other  caterpillars  or  grasshoppers;  a  third,  goats;  a  fourth,  horses, 
•&c.  Let  it  be  supposed  that  the  country  producing  this  rich  vegetation  is 
temporarily  quite  shut  off  from  everything  which  creeps  or  flies.  But  now  arrives 
a  numerous  herd  of  some  species  of  animal  against  whose  attacks  one  portion  of  the 
plants  is  protected  as  completely  as  possible,  a  second  portion  only  partly,  a  third 
not  at  all.  What  will  be  the  consequence?  The  last  will  be  wholly  or  partly 
devoured,  while  the  first  will  remain  uninjured.  If  this  is  often  repeated,  at  last 
the  one  will  vanish  from  the  scene,  while  the  other  will  develop  in  overwhelming 
quantity.  But  in  this  manner  the  peculiar  composition  of  vegetation  in  places 
where  grazing  animals  regularly  appear  is  naturally  explained. 

It  must  strike  everyone  who  visits  the  Alps  that  in  the  neighbourhood  of  the 
cow-chalets  a  luxuriant  vegetation  springs  up  from  the  richly-manured  soil,  very 
tempting  in  appearance,  but  nevertheless  left  unattacked  by  the  grazing  animals. 
The  shepherds  do  not  prevent  the  animals  from  eating  of  this  luxuriant  growth;  it 
is  not  necessary,  for  instinctively  they  detest  these  plants.  The  bush  consists 
entirely  of  species  which  are  poisonous  or  disagreeable  to  the  animals,  or  which 
when  disturbed,  wound  them— viz.  of  Monkshood,  Good  King  Henry,  Nettle,  and 
Fuller's  Thistle  (Aconitum  Napellus,  Chenopodium  Bonus  Henricus,  Urtica 
dioica,  Cirsium  spinosissimum),  which  are  found  together  here,  and  have  developed 
so  much  the  more  vigorously,  since  the  other  species  originally  existing  (which  were 
innocuous  and  undefended)  have  been  long  ago  destroyed  by  the  grazing  animals. 


PROTECTION   OF   GREEN   LEAVES   AGAINST   ATTACKS   OF   ANIMALS.  451 

On  the  forest  pasture  of  the  Lower  Alps  often  all  that  is  to  be  seen  covering  the 
ground  are  mosses  and  ferns,  which  are  offensive  to  the  animals,  along  with  the 
bitter  Oentiana  asdepiadea  and  Aposeris  fcetida,  abounding  in  a  malodorous  milk, 
detested  by  all  ruminants.  In  some  meadows  in  the  Central  Alps  the  fern 
Allosorus  crispus,  and  with  it  the  Mat-grass  (Nardus  stricta),  are  so  prominent  that 
scarcely  any  other  species  of  plant  are  to  be  seen  there.  Again,  in  other  places,  the 
ground  is  overgrown  with  the  Bracken  fern  (Pteris  aquilina),  detested  by  grazing 
oxen,  and  also  with  prickly  juniper-bushes.  On  the  cultivated  grounds  near  Trieste 
the  stiff,  prickle -leaved  and  steel-blue  Eryngium  (Eryngium  amethystinum) 
impresses  one  by  its  profusion.  In  the  Hungarian  uplands  one  may  recognize  the 
spots  where  cattle  are  kept  by  the  abundant  occurrence  of  Xanthium  spvnosum  and 
Eryngium  campestre,  of  tall  thistles  and  of  Mullein,  of  Thorn-apples  and  Hen- 
bane, and  of  several  species  of  spurge,  which  are  only  eaten  by  the  animals  under 
the  greatest  stress.  It  is  thus  shown  by  a  hundred  examples  that  in  tracts  exposed 
to  the  pasturage  of  larger  animals,  those  plants  always  obtain  the  upper  hand  which 
are  not  attacked  by  the  animals,  in  consequence  of  their  poisonous  and  disagreeable 
properties,  or  because  of  their  defensive  spines  and  prickles. 

A  phenomenon  connected  with  the  conditions  here  described  deserves  mention. 
This  is  the  regular  occurrence  of  defenceless  plants  under  the  protection  of  those 
which  are  provided  with  abundant  means  of  defence.  Thus  certain  wild  vetches 
and  Umbellifers  (species  of  Vicia,  Lathyrus,  Anthriscus,  Myrrhis,  JZgopodium, 
Chcerophyllum,  &c.),  which  would  furnish  very  good  fodder  for  grazing  mammals, 
are  regularly  seen  in  the  prickly  hedges  along  the  roads,  and  under  spiny  bushes, 
which  form  a  belt  around  forests.  The  bushes  defend  not  only  their  own  foliage, 
but  also  that  of  the  delicate  vetches  and  Umbellifers  which  have  established  them- 
selves under  their  protection.  In  neighbourhoods  where  the  primeval  character  and 
distribution  of  the  vegetation  is  almost  entirely  lost,  the  companionship  of  certain 
plants  is  so  general  that  one  might  be  tempted  to  regard  it  as  a  symbiosis.  Here, 
however,  this  is  certainly  not  the  case,  for  the  advantage  is  all  on  one  side— that 
of  the  plants  protected;  while  the  bush,  armed  with  spines  against  the  assaults  of 
animals,  under  whose  branches  the  defenceless  plants  have  grown  up,  receives  no 
thanks,  no  profit,  and  no  return  from  them,  and  certainly  does  not  afford  the  prc 
tection  intentionally. 


METABOLISM  AND  TRANSPORT  OF 
MATERIALS. 


1.— THE  ORGANIC  COMPOUNDS  IN  PLANTS. 

Carbon  Compounds. — Metabolism  in  Living  Plants. 

CARBON   COMPOUNDS. 

It  is  naturally  to  be  expected,  from  analogous  relations  in  the  inorganic  world, 
that  the  variety  to  be  observed  in  the  vegetable  kingdom  as  to  colour,  taste,  and 
smell,  should  depend  upon  the  diversity  of  the  materials  manufactured  in  the  in- 
dividual species.  Numerous  characteristic  materials  have  been  shown  by  the  re- 
searches of  chemists  to  belong  to  certain  species,  and  in  the  names  given  to  many  of 
these  (as  in  the  terms  oxalic  acid,  benzoic  acid,  salicin,  amygdalin,  asparagin,  nicotin, 
strychnin,  atropin,  cocain,  &c.)  we  recognize  the  names  of  well-known  plants.  But 
it  would  be  erroneous  to  suppose  that  the  series  of  substances  belonging  to  the 
vegetable  kingdom  would  be  exhausted  by  the  sugars,  acids,  salts,  alkaloids,  oils, 
ethers,  and  pigments  which  are  already  known  to  us  by  their  varied  effects  on  our 
nerves  of  taste,  smell,  and  sight.  What  is  accurately  known,  indeed,  in  this  respect 
is  apparently  only  a  fraction  of  what  actually  exists.  In  the  meantime  we  cannot 
venture  on  even  an  approximate  estimate  of  all  the  substances  produced  by  plants. 
Only  this  much  can  be  affirmed  with  certainty,  that  their  number  is  far  greater 
than  that  of  inorganic  or  mineral  bodies.  This  is  the  more  remarkable,  since  the 
elements  of  which  the  inorganic  compounds  are  built  up  are  comparatively  so  many, 
whilst  the  elements  which  serve  as  building  materials  for  organic  compounds  are 
so  few.  The  fact  is  thus  explained,  that  carbon,  an  element  whose  chemical  nature 
admits  of  its  union  with  other  elements  in  inexhaustible  combinations,  appears 
as  the  centre  of  all  organic  compounds  in  plants. 

For  the  purpose  of  the  following  discussion  it  is  fitting,  first  of  all,  to  give  here 
a  brief  sketch  of  this  important  property  of  carbon.  Chemists  call  carbon  a  tetrad 
element,  by  which  is  meant  that  each  atom  of  carbon  enters  into  combination  with 
four  atoms  of  another  element,  and  can  form  a  mechanically  inseparable  group,  i.e. 
a  molecule.  It  can  be  shown  that  each  atom  of  a  tetrad  element  possesses  four 
centres  of  attraction,  i.e.  four  connection  points,  to  which  the  atoms  of  other 
elements  become  attached,  and  where  they  are  held  fast.  These  points  are  called 
bonds  of  union,  and  are  said  to  be  saturated  when  other  atoms  have  become 

452 


CARBON  COMPOUNDS.  453 

annexed  or  united  to  them,  or  free  when  this  is  not  the  case.     When,  for  exampl, 
four  atoms  of  hydrogen  unite  with  one  atom  of  carbon  (represented  graphically  in 
figure  119,  with  its  four  bonds  of  union),  its  four  bonds  are  thereby  saturated  and 
a  molecule  known  as  marsh  gas  is  produced.     Apart  from  its  tetravalency,  carbon 
also  has  this  remarkable  property,  that  its  atoms  can  also  combine  with   ' 
each  other,  and  to  a  much  higher  degree  than  the  atoms  of  any  other 
element.     Carbon  atoms  themselves,  and  not  the  atoms  of  other  elements, 
saturate  the  separate,  free  bonds  of  union  in  such  instances,  and  in  this 
way   are    produced   groups   of    atoms,  each  of   which  behaves  like  a      Ftg'119' 
chemical  unit.     Suppose  that  one  of  the  four  bonds  of  an  atom  of  carbon  has  united 
with  one  of  the  four  bonds  of   a  second  carbon  atom;  then  a  group  of  atoms 
like  that  shown  in  fig.  120  will   be  the  result.      Where  the  two  carbon  atoms 
have  become  connected   their  bonds  of   union  are  saturated;  but  in  each  atom 
there  are  still  three  unsatisfied  bonds,  and  accordingly  they  can  together 
annex  six  atoms  of  another  element.     The  pair  of   carbon  atoms  may 
now  be  considered  as  hexavalent,  and  if  they  annex  six  atoms  of  hy- 
drogen, a  compound  is  produced  which  is  called  ethane.     If  three  atoms 
of  carbon  combine   together,  so  that  one  bond  of  each  is  united  to  a 
bond  of  the  neighbouring  atom,  as  represented  graphically  in  fig.  121, 
four  bonds   are  saturated   and  eight   remain  free.     These  free  bonds      rig.uo. 
may  be  satisfied  with  atoms  of  other  elements,  for  example,  again  with 
hydrogen.      Thus  a  compound  arises  which  contains  three  atoms  of  carbon  and 
eight  of  hydrogen,  and  which  has  been  called  propane.     In  like  manner  four,  five, 
&c.  atoms  of  carbon  may  enter  into  combination  together,  in  which  case  the  remain- 
ing ten,  twelve,  &c.  bonds  of  union,  which  remain  free,  may  be  saturated  with 
atoms  of  other  elements.      If  we  suppose  that  all  the  free  bonds  are 
satisfied   by  hydrogen,  we  then  have  a  series  of  hydrocarbons  whose 
successive  members  differ  from  their  predecessors  by  the  increment  of 
one   atom  of  carbon   and   two   of  hydrogen,  but  which  must  each  be 
regarded   as   a  chemical  unit,  i.e.  as  a  chemical   individual   and   as  a 
particular   substance   with   peculiar  properties    not    possessed    by   the 
others. 

Parallel    with    this    series    of    hydrocarbons    run    two   comparable 
series,   whose   members   respectively   contain   two   and   four  atoms  of      rig.  in. 
hydrogen   less   than   the   corresponding  members  of   the   main   series; 
and  here  the  carbon  atoms,  from  which  the  atoms  of  hydrogen  have  been  removed, 
must  have  combined  with  one  another  by  the  bonds  thus  liberated. 

The  view  that  several  atoms  of  carbon  are  only  grouped  in  one  direction  in 
linear  series,  and  that  the  neighbouring  atoms  are  only  mutually  combined 
means  of  one  of  their  four  bonds,  as  shown  in  the  above  graphic  representations, 
is  not  always  confirmed.      In  many  instances  we  are  obliged  to  suppose  that 
carbon  atoms  are  distributed  in  several  directions  in  space,  and  are  combined  into 
net-work,  or  grouped  in  the  form  of  a  hexagon,  perhaps  in  the  manner  illust 


454  CARBON   COMPOUNDS. 

in  fig.  122.  Here  each  of  the  six  carbon  atoms  is  always  united  to  one  of  its 
neighbours  by  one,  and  to  the  other  by  two,  bonds,  and  thus  only  six  bonds  remain 
free.  When  these  are  saturated  by  atoms  of  hydrogen,  we  have  a  molecule  of  that 
important  compound  called  benzene.  In  all  the  special  instances  hitherto  mentioned 
the  free  bonds  of  the  carbon  atoms  have  been  satisfied  by  atoms  of  hydrogen,  and 
these  combinations  have  all  been  found  actually  realized  in  nature.  It  is  an 
extremely  important  property  of  carbon,  as  regards  the  chemistry  of  vegetable 
substances,  that  all  the  free  bonds  of  its  groups  of  atoms,  no  matter  how  numerous 
these  may  be,  can  be  satisfied  with  hydrogen.  Whilst  other  elements  can  only  form 
a  very  limited  number  of  hydrogen  compounds,  we  have  a  practically  unlimited 
quantity  of  hydrocarbons.  But  this  is  not  all.  These  hydrocarbons  form  the 
foundations  of  innumerable  other  compounds  which  are  produced  by  the  displace- 
ment, by  atoms  of  other  elements,  of  one  or  several 
atoms  of  hydrogen  in  each  member  of  the  hydrocarbon 
series.  Many  of  the  substances  occurring  in  plants  are 
hydrocarbons  in  which  a  part  of  the  hydrogen  has 
been  displaced  by  oxygen;  in  others  the  hydrogen  is 
partly  replaced  by  nitrogen;  or  for  the  hydrogen  may 
be  substituted  the  so-called  compound  radicles  (groups 
of  atoms  which  play  the  part  of  an  element  in  com- 
bination), as,  for  example,  cyanogen,  hydroxyl,  &c.  If 
Fig  122.  the  number  of  compounds  in  which  carbon  is  com- 

bined with  nitrogen  is  indeed  large,  the  number  of 

compounds  obtained  from  them  by  the  partial  replacement  of  the  hydrogen  by 
some  other  element,  and  known  as  derivatives  of  hydrocarbons,  becomes  almost 
beyond  conception. 

Finally,  the  astounding  variety  which  one  and  the  same  compound  can  exhibit 
in  its  outward  appearance,  in  form,  colour,  hardness,  and  transparency,  in  taste, 
and  in  smell,  is  due  to  the  inexhaustible  permutations  in  its  percentage  com- 
position, which  is  shown  by  the  hydrocarbons  as  well  as  by  their  derivatives. 
The  same  phenomenon  is  here  repeated  as  is  observed  in  pure  carbon  uncombined 
with  any  other  element.  It  is  known  that  carbon  appears  either  amorphous  as 
charcoal,  or  crystalline  as  diamond,  or  as  graphite — in  the  latter  case,  in  crystals 
which  belong  to  another  system  than  those  of  the  diamond,  and  differing  from 
them  in  colour,  hardness,  and  specific  gravity.  It  is  not  easy  to  imagine  a 
greater  contrast  as  regards  physical  properties  than  that  shown  by  these  three 
substances,  and  yet  it  is  beyond  question,  that,  chemically,  they  are  identical. 
The  same  thing  happens  in  some  of  the  compounds  of  carbon.  Dextrin, 
starch,  and  cellulose  all  have,  for  example,  the  same  percentage  composition; 
each  molecule  contains  six  atoms  of  carbon,  ten  of  hydrogen,  and  five  of  oxygen. 
And  yet  how  different  these  bodies  seem  to  our  senses;  how  different  is  their 
behaviour  to  heat  and  light,  to  various  solvents,  and  to  other  chemical  compounds! 
We  explain  this  remarkable  phenomenon  by  the  way  in  which  the  atoms  are 


METABOLISM   IN   LIVING   PLANTS.  455 

grouped,  and  imagine  that  the  varied  arrangement  of  the  atoms  building  up  a 
molecule  finds  expression  in  the  whole  mass  of  the  substance  in  question.  If 
six  black,  ten  blue,  and  five  red  balls  are  placed  close  together  in  a  frame,  they 
can  be  grouped  in  the  most  diverse  ways  into  beautiful  symmetrical  figures. 
They  are  always  the  same  balls,  they  always  take  up  the  same  space,  and  yet  the 
effect  of  the  figures  produced  by  the  different  arrangements  is  wholly  distinct. 
It  may  be  imagined,  similarly,  that  the  appearance  of  the  whole  mass  of  a 
carbon  compound  becomes  different  in  consequence  of  the  arrangement  of  its 
atoms,  and  that  not  only  the  appearance,  but  even  the  physical  properties 
undergo  very  striking  alterations. 

A  glance  back  at  the  history  of  the  development  of  carbon  compounds,  very 
briefly  stated  here,  will  render  sufficiently  clear  how  it  becomes  possible  that 
many  thousand  different  organic  substances  are  compounded  from  carbon  and 
a  few  other  elements,  viz.,  hydrogen,  oxygen,  and  nitrogen;  and  how  this  almost 
infinite  multiplicity  of  vegetable  organic  compounds  is  connected  with  the  re- 
markable chemical  nature  of  carbon.  The  materials  of  which  these  substances 
are  formed  are  extremely  simple,  and  the  changes  undergone  by  plant-substances 
depend  entirely  upon  the  insertion  and  rejection,  on  the  grouping  and  arrangement, 
of  the  atoms  of  a  few  elements. 


METABOLISM  IN  LIVING  PLANTS. 

In  the  living  plant  these  combinations,  decompositions,  and  rearrangements 
are  accomplished  with  great  ease,  and  multitudes  of  substances,  which  cannot 
be  manufactured,  either  directly  or  indirectly,  in  a  chemical  laboratory,  are  pro- 
duced in  plant  cells,  with  a  hand's  turn,  so  to  speak.  This  applies  principally 
to  those  organic  materials  already  generally  described  in  a  previous  section  of 
this  book,  which  have  been  formed  from  inorganic  food,  from  carbonic  acid 
and  water.  It  is  exactly  these,  however,  which  have  the  greatest  claim  upon 
our  interest.  They  are  of  the  utmost  importance  to  everything  which  lives 
and  moves  on  our  earth;  their  formation  is  the  adjustment  of  one  of  the  greatest 
contrasts  in  nature,  they  form  the  bridge  which  connects  the  inorganic  with  the 
organic  world,  the  dead  with  the  living.  As  a  matter  of  course,  these  primary 
organic  substances,  derived  from  carbonic  acid  and  water,  are  the  starting-points 
for  all  the  other  chemical  compounds  of  which  the  bodies  of  plants  and 
animals  are  composed;  or,  in  other  words,  they  form  the  commencement  of 
these  further  chemical  changes  in  living  cells  which  are  understood 

Metabolism.  . 

The   process  of   formation  of  these  primary  organic  compounds 
whole,   easily  comprehensible.      It  is  known  that  carbon  dioxide,  t.«.  e 
acid,  is  absorbed  by  plants,  and  that  oxygen  is  given  out;  it  is  also  known 
when  this  process  is  carried  on  in  a  plant  kept  in  a  confined  space,  a  vol 
of  oxygen  is  given  out  which  is  equal  to  the  amount  of  carbon  diox.de 


456  METABOLISM   IN   LIVING   PLANTS. 

up  and  consumed  by  the  plant.  In  this  way,  without  doubt,  a  reduction  of 
the  carbon  of  the  carbon  dioxide  occurs,  and  hand  in  hand  with  this  reduction 
a  union  of  carbon  with  water  must  take  place.  Thus  is  formed  some  one 
of  the  compounds  known  as  carbohydrates.  The  process  has  been  interpreted 
in  the  following  manner.  The  carbonic  acid  is  reduced  in  the  green  cells,  by 
the  separation  of  oxygen,  to  carbon  monoxide;  this  combines  with  hydrogen  to 
form  a  body  known  by  the  name  of  formic  aldehyde,  and  from  this  is  produced, 
by  the  action  of  alkaline  substances,  a  carbohydrate.  This  latter  process  is 
more  easily  understood,  from  the  fact  that  it  has  been  found  possible  to  produce 
a  sugar  from  the  formic  aldehyde  (which  consists  of  one  atom  of  carbon,  one 
of  oxygen,  and  two  of  hydrogen)  by  means  of  lime.  Thus  a  definite  carbohydrate 
would  be  established  as  the  first  organic  substance  formed  in  a  vegetable  cell. 
It  is  scarcely  probable,  however,  that  this  carbohydrate  alone  forms  the  starting- 
point  for  the  whole  of  the  other  organic  compounds  in  all  living  plants.  It  is 
much  more  likely  that  in  the  large,  fundamentally  different  series  of  plant-forms, 
in  Fucaceae,  Florideae,  mosses,  ferns,  conifers,  grasses,  palms,  &c.  different  carbo- 
hydrates are  produced  as  the  first  organic  derivatives  of  carbon  dioxide  and  water. 
It  must  not  be  forgotten  that  in  this  building  process  the  protoplasm  of  the 
green  tissue  plays  a  very  important  part,  that  this  is  actually  the  builder,  /  and 
that  the  structure  and  chemical  composition  of  the  constructor,  or,  in  other 
words,  the  specific  constitution  of  the  protoplasm,  will  not  be  without  influence 
on  the  arrangement  of  the  atoms  in  the  carbohydrate  formed.  The  whole  of 
this  process  has  been  termed  assimilation,  and  by  it  is  meant  that  the  protoplasm 
in  each  plant  forms  materials  from  the  inorganic  food  absorbed,  resembling  those 
of  which  the  protoplasm  itself  is  made  up.  Assimilating  protoplasm  thus  con- 
tinues to  organize  after  its  own  type,  and  in  this  matter  cannot  pass  beyond 
the  bounds  drawn  for  it  by  its  own  atomic  construction.  The  assumption  is 
now  justified  that  in  these  formative  processes  assimilation  takes  effect  from  the 
beginning,  and  that  protoplasm  which  exhibits  a  different  constitution,  and  which 
is  known  to  have  the  capacity  of  choosing  between  the  mineral  food-salts,  will 
form  different  carbohydrates.  However  this  may  be,  this  much  is  certain,  that 
the  first  organic  compound  arising  in  the  green  cells  is  a  kind  of  sugar  or  some 
other  dissolved,  undemonstrable  carbohydrate. 

Under  the  influence,  and  by  the  means  of  living  protoplasm,  and  in  accordance 
with  the  requirements  of  the  plant  species  in  question,  very  diverse  alterations 
and  the  most  varied  arrangements  and  connections,  insertions  and  separations 
of  the  atoms  are  carried  on  in  these  primary  carbohydrates,  and  as  long  as  the 
plant  is  alive,  a  continuous  transformation  of  the  materials  takes  place.  And 
this  transformation  is  carried  on  in  very  many  directions.  First,  compounds 
are  formed  indirectly  or  directly  from  the  primary  carbohydrates.  They  con- 
tribute to  the  extension  of  the  protoplasm  and  the  envelopes  produced  from  it. 
Without  them  no  increase  in  cells,  or  growth  of  the  plant,  would  be  possible. 
They  may  be  fitly  termed  the  building  materials. 


METABOLISM   IN   LIVING   PLANTS. 


467 


Of  the  various  sorts  of  building  materials,  the  albumens  must  first  be  con- 
sidered; they  are  to  be  reckoned  as  the  most  important  constituents  of  living 
protoplasm.  Although  their  chemical  composition  has  not  as  yet  been  ascertained 
with  complete  certainty,  it  is  known  that,  besides  the  carbohydrate  constituents, 
albumens  contain  nitrogen  and  0'8-17  per  cent  of  sulphur;  that  carbon  with 


Fig.  123.— Crystals  and  Crystalloids. 

leaf  of  the  Virginian  Creeper  (Ampelopsis  hederacea).    In  some  of  the  cells  are  clustered- 
n  one  cell  there  is  a  single  envelope-shaped  crystal 


Phalli  caninus. 


eedle  from  a  bundle  of  raphides.    •    ecion  o  a  P" 

^^ 


magnified. 


many   perhaps  with  more  than  a  hundred,  atoms  takes  part  in  the  construction 
of  a  molecule,  and   that  consequently  the  molecules  of  albumen  are  very  1 
In  order  that  a  carbohydrate  may  become  transformed  into  an  albuminous 
nitrogen  and  sulphur,  at  any  rate,  must  be  drawn  into  the  combination. 


458  METABOLISM   IN   LIVING   PLANTS. 

is  obtained  from  nitric  acid  and  ammonia,  and  sundry  of  their  compounds, 
especially  calcium  nitrate.  These  are  absorbed  by  the  plant  and  conveyed  by 
the  crude  sap  to  the  place  of  consumption.  The  nitric  acid  must,  of  course,  be 
liberated  from  this  salt,  and  this  is  brought  about  by  the  union  of  the  calcium 
with  oxalic  acid,  derived  from  a  portion  of  the  carbohydrates,  the  two  thus- 
forming  insoluble  crystals  and  crystalline  masses  of  oxalate  of  lime  (fig.  123). 
The  liberated  nitric  acid  is  now  reduced  in  a  manner  analogous  to  that  of  the 
carbonic  acid  in  the  formation  of  carbohydrates.  It  is  supposed  that  the  nitrogen 
of  the  nitric  acid  then  combines  with  a  hydrocarbon,  forming  an  amide 
(asparagin,  leucin,  tyrosin),  and  that  the  albumen  is  formed  by  the  union  of  this 
with  a  carbohydrate.  Sulphur  is  derived  from  the  calcium  sulphate,  or  from 
some  other  sulphate,  by  the  intervention  of  oxalic  acid,  in  the  same  manner 
as  just  described  for  nitrogen.  The  oxalic  acid  forms  an  insoluble  salt  with  the 
calcium  or  other  base  of  the  sulphate,  which  separates  out  in  the  cells  in  the 
form  of  small  crystals.  The  liberated  sulphuric  acid  must  then,  in  some  way, 
undergo  a  further  reduction,  in  order  that  sulphur  may  enter  into  the  molecule 
of  the  albumen.  Among  the  vegetable  albumens  are  to  be  distinguished  albumin, 
casein,  and  fibrin.  The  glutin  contained  in  corn,  is  a  mixture  of  a  casein  and  a 
fibrin.  All  these  albumens  appear  in  soluble  and  insoluble  forms.  Thus,  for 
example,  the  conglutin  contained  in  almonds  is  a  soluble  casein,  and  goes  into 
solution  when  milk  of  almonds  is  made  by  adding  water  to  the  almonds;  while 
the  legumin  contained  in  peas,  beans,  lentils,  and  other  pulse  seeds,  is  not  soluble 
in  water,  and  can  only  be  dissolved  by  pepsin  in  the  presence  of  an  acid.  Al- 
though all  these  albuminous  compounds  cannot  be  recognized  by  any  definite 
form,  the  aleurone  grains  and  the  so-called  crystalloids  have  perfectly  definite 
shapes.  The  crystalloids  are  formed  of  albuminous  substances,  and  have  exactly 
the  appearance  of  crystals  (fig.  1238"12). 

Next  to  the  albuminous  substances,  the  most  important  building  material 
to  be  noticed  is  cellulose.  This  is  a  carbohydrate  consisting  of  six  atoms  of 
carbon,  ten  of  hydrogen,  and  five  of  oxygen,  and  is  produced  from  the  primary 
sugar-like  carbohydrates.  The  transformation  is  effected  by  the  living  protoplasts, 
which  form  a  layer  of  cellulose  on  their  periphery,  called  the  cell-wall.  At  first 
this  cell-wall  is  mainly  composed  of  pure  cellulose;  then,  according  to  need,  the 
carbohydrate  is  changed  by  the  protoplasm,  either  wholly  or  partially,  into  some 
other  carbohydrate,  either  into  woody  material  (lignin)  or  cork  (suberin),  or  the 
cellulose  becomes  mucilaginous,  as,  for  example,  in  the  seed-coat  of  the  Quince. 
In  the  stems  and  branches  of  cherry,  plum,  almond,  apricot,  and  peach  trees,  the 
cellulose  is  generally  hardened  into  a  sticky,  shapeless,  amber-coloured  substance, 
which  exudes  from  the  fissures  of  the  bark,  and  is  known  by  the  name  of  cherry- 
gum  (cerasin).  In  like  manner  gum-arabic  (arabin)  is  formed  from  the  cellulose 
in  the  stems  of  some  acacias,  and  gum-tragacanth  in  several  tragacanth  shrubs 
(species  of  Astragalus). 

Protoplasm  forms  cellulose  from  a  portion  of  the  primary  sugar -like  carbo- 


METABOLISM   IN   LIVING   PLANTS. 


459 


hydrate  at  certain  points  in  the  interior  of  its  substance  as  well  as  at  its  periphery, 
in  addition  to  another  carbohydrate,  the  so-called  granulose.  Cellulose  and  grami- 
lose,  very  intimately  intermixed,  appear  in  the  form  of  grains,  and  the  mixture  is 
called  starch  or  amylum.  Starch-grains  are  among  the  commonest  of  cell -contents. 
They  appear  regularly  in  chlorophyll-bodies  and  are  conveyed  from  the  places  where 
they  are  first  formed  to  all  parts  of  the  plant.  This  of  course  is  only  effected  by 


Fig.  124.— Various  Forms  of  Starch-grains. 

*  From  the  seeds  of  the  Corn-cockle  (Agrostemma  Githago).    *  From  a  grain  of  Wheat    •  From  Spurge.    «  From  a  Bean  ieed. 
«  From  a  grain  of  Maize      «  From  the  root-stock  of  Canna.     »  From  a  Potato-tuber  (inclosed  in  cells).     •  From  a  Po 
tuber  (isolated  and  very  highly  magnified).     •  From  a  grain  of  Oats.     "  From  the  seed  of  Lolium  temtifer 
the  corm  of  the  Meadow  Saffron  (Colchicum  autumnale).    »  From  a  grain  of  Rice.    i«  From  a  grain  of  Millet    All  highlj 

magnified. 

the  solid  starch  bodies  being  made  fluid,  as  often  as  they  pass  from  one  cell  to 
another,  by  the  help  of  an  accessory  substance,  called  diastase,  which  has  yet  to  be 
described.  In  many  tissues  the  starch-grains  become  so  accumulated  that  the  cell 
appear  to  be  crammed  with  them  (see  fig.  124 7).  Starch  is  one  of  the  most  important 
of  reserve  materials,  i.e.  of  those  materials  which  are  not  consumed  immediately 
after  their  formation,  but  are  put  away  for  a  time  in  store-rooms  or  reservoirs,  and 
then  consumed  as  required  in  the  places  needing  them.  For  example,  they  i 


460  METABOLISM   IN   LIVING   PLANTS. 

remain  in  seeds  unaltered  for  years,  and  as  if  dead;  but  if  the  seed  germinates,  and 
the  seedling  begins  to  develop,  the  starch  is  dissolved,  that  is  to  say,  becomes  chan^ 
into  another  carbohydrate,  and  finally  is  made  use  of  in  the  construction  of  the  cell- 
walls  of  the  growing  seedling  by  a  fresh  transformation.  Starch-grains  in  varioi 
species  of  plants  differ  very  much  in  size  as  well  as  in  shape.  The  largest  grains 
exhibit  under  the  microscope  alternating  blue  and  red  zones,  which  are  accounl 
for  by  the  difference  in  the  amount  of  water  contained  in  the  several  zones.  The 
bluish  zones  contain  less,  and  the  red-tinted  more  water.  Many  starch-grains  exhibit 
a  "  nucleus  "  or  hilum  which  is  rich  in  water,  and  which  is  situated  excentricall; 
in  the  grains  of  the  Potato  and  of  Canna  (fig.  124 6);  centrally  in  those  of  the 
Wheat.  A  space  may  be  present  instead  of  the  hilum,  as  in  the  starch-grains  of 
beans  and  other  pulses  (fig.  1244),  in  consequence  of  the  drying  up  of  the  substance 
of  the  hilum.  In  most  plants  the  starch-grains  have  a  rounded  form;  but  those  of 
the  Corn-cockle  (Agrostemma  Githago)  are  fusiform  and  club-shaped  (fig.  124 1). 
Those  of  species  of  Euphorbia  resemble  tiny  bones  (fig.  1243),  and  others  again  are 
angular  and  cornered  like  crystalline  figures  (figs.  124  5  and  124 13).  This  last  form 
is  seen  especially  when  the  cells  which  serve  as  store-houses  are  densely  crowded 
with  starch-grains  so  that  growth  becomes  checked,  and  a  mutual  flattening  takes 
place.  In  the  Oat  and  Rice  many  small  angular  starch -granules  are  cemented 
together  to  form  ellipsoidal  grains  (figs.  124  9  and  12410),  and  in  the  starch  from  the 
corm  of  the  Meadow  Saffron,  regular  groups  of  four  rounded  grains,  each  exhibiting 
a  hollow  hilum,  are  found  (fig.  12411).  Granulose  forms  the  chief  of  the  two 
carbohydrates  which  are  intimately  mixed  to  form  starch.  It  is  soluble  in  saliva, 
and  is  therefore  extracted  by  it,  while  the  cellulose  remains  behind  insoluble,  a  fact 
which  is  of  great  importance  with  regard  to  the  digestibility  of  the  starch  present 
in  such  abundance  in  flour  and  bread. 

In  close  connection  with  these  essential  building  materials  are  other  substances 
which,  though  not  themselves  serving  as  building  materials,  take  an  active  part  in 
their  production.  These  furnish  the  conditions  under  which  the  manufacture  and 
transport  of  the  building  substances,  and  the  growth  and  propagation  of  the  plants 
can  occur.  They  avert  injurious  influences,  regulate  light  and  heat,  and  are  of  use 
to  the  plant  in  a  hundred  minor  directions. 

To  these  substances,  which  may  be  termed  accessory,  belong,  first  of  all,  the 
pigments  chlorophyll,  phycoerythrin,  and  anthocyanin,  which  are  so  important  on 
account  of  their  relations  to  light  and  heat,  and  whose  rdle  has  already  been  alluded 
to.  Then  we  have  those  compounds  whose  function  is  to  allure  animals  to  the 
plants  in  order  to  bring  about  fertilization  or  the  distribution  of  the  seeds  and 
spores,  or  whose  significance  lies  in  the  fact  that  they  frighten  and  ward  off  animals 
which  might  be  injurious  to  the  plants.  In  this  connection  are  of  course  to  be 
mentioned  colouring-matters  which  are  formed  in  flowers  and  fruits  in  order  that 
these  may  be  rendered  visible  at  a  distance  to  those  animals  whose  visits  benefit  the 
plant:  first  of  all,  anthocyanin,  which  in  the  presence  of  acids  is  red,  but  otherwise 
appears  violet  or  blue;  and  then  anthoxanthin,  to  which  most  yellow  flowers  and 


METABOLISM   IN   LIVING   PLANTS. 

fruits  owe  their  colour.  On  the  other  hand  must  here  be  mentioned  that  scarlet-red 
colouring-matter,  as  yet  little  known,  probably  belonging  to  the  anthracenes  and 
allied  to  the  madder-red,  which  perhaps  serves  to  frighten  animals,  and  which  is 
so  pronounced,  for  example,  in  the  accrescent  calyx  surrounding  the  fruit  of  the 
Winter  Cherry  (Physalis  Alkekengi). 

Besides  the  colouring -matters,  sweet-tasting  substances,  especially  cane-sugar, 
and  also  mannite  and  dulcite,  play  an  important  role  of  a  similar  nature.  Although 
their  function  can  only  be  discussed  in  detail  later  on,  it  is  nevertheless  well 
to  point  out  here  that  the  distribution  of  the  spores  of  Ergot  of  Rye  (Clavicepa 
purpurea),  for  example,  is  brought  about  by  means  of  a  sweet  fluid  excreted  by  the 
mycelium,  which  is  eagerly  sought  for  by  ants  and  other  insects.  The  insects  in 
sucking  and  licking  up  this  fluid  carry  off  the  spores  of  the  Ergot,  and  then  deposit 
them  on  other  plants.  Countless  plants  secrete  sweet  honey  in  certain  parts  of  their 
flowers,  which  serves  to  attract  bees,  humble-bees,  and  butterflies,  whose  task  is  to 
carry  the  pollen  from  flower  to  flower.  Certain  animals,  on  the  other  hand,  whose 
visits  would  be  injurious  to  the  flowers,  are  kept  away,  or  still  better,  diverted  from 
them  by  the  honey  secreted  at  the  base  of  the  foliage-leaves,  which  serves  as  a 
counter-attraction. 

The  numerous  ethereal  oils,  resins,  and  balsams  have  a  like  significance  for  the 
life  of  the  plant.  The  ethereal  oils  are  principally  hydrocarbons,  only  a  few  con- 
taining oxygen  in  addition;  oils  of  lavender,  cumin,  and  eucalyptus,  oil  of  turpentine 
and  camphor,  and  many  others  consist  of  ten  atoms  of  carbon  and  sixteen  of 
hydrogen.  In  spite  of  this  similar  percentage  they  differ  very  markedly  in  their 
optical  properties,  their  boiling  point,  and  particularly  in  their  smell,  as  can  indeed 
be  observed  from  the  few  examples  cited.  There  are  some  plants  whose  foliage, 
flowers,  and  fruit  contain  ethereal  oils  having  different  odours,  as,  for  example,  the 
Orange-tree,  whose  leaves  yield  "petit  grain",  the  flowers  neroli,  and  the  fruit  oil 
of  orange.  But  since  these  three  oils  contain  the  same  number  of  carbon  and 
hydrogen  atoms,  it  must  be  assumed  that  their  difference  depends  upon  the  varying 
arrangement  of  the  atoms  in  a  molecule.  The  majority  of  these  oils  are  trans- 
formed into  resin  by  the  addition  of  oxygen,  or  mixtures  of  volatile  oils  and  resins 
are  produced,  which  are  called  balsams.  Volatile  ethereal  oils,  which  are  perceived 
by  the  olfactory  nerves  even  at  a  distance,  function  in  part  as  means  for  alluring 
animals  which  benefit  the  plants  in  question  by  transferring  the  pollen  or 
disseminating  the  fruits,  seeds,  or  spores;  but  they  also  function  in  part  as  measures 
for  protecting  the  plant  against  attacks  from  the  animal  kingdom.  The  latter  is 
the  case  especially  in  foliage-leaves  with  powerful  odours,  and  in  resinous  fruits, 
and  these  are  not  used  by  animals  as  food.  Balsams,  which  cover  foliage-leaves 
issuing  from  the  buds  like  a  varnish,  form  a.  protection  against  excessive  trans- 
piration, and  also  render  material  help  in  the  absorption  of  water  by  the  leaves,  as 
has  been  already  described.  The  viscous  excretions,  formed  of  a  mixture  of  ream 
and  mucilage  on  the  stems  and  leaf-stalks,  which  are  formed  so  abundantly  : 
caryophyllaceous  plants,  keep  off  animals  which  try  to  climb  up  the  stem  i 


462  METABOLISM   IN   LIVING  PLANTS. 


the  flowers  in  order  to  obtain  the  honey,  but  which  would  not  be  welcome  guests 
to  the  plant. 

Fats  take  a  part  in  the  life  of  plants  similar  to  that  played  by  ethereal  oils. 
Fats  are  combinations  of  fatty  acids  with  glycerine,  and  may  be  divided  into  two 
groups;  in  one  group  the  members  dry  up  when  exposed  to  the  air  by  the  separation 
of  carbonic  acid,  as,  for  example,  in  poppy  oil  and  linseed  oil,  which  are  used  for  this 
very  reason  in  oil  painting.  In  the  other  group,  e.g.  in  almond  and  olive  oils,  the 
members  remain  fluid  when  exposed  to  the  air,  and  give  rise  to  malodorous  fatty 
acids,  and  when  this  change  occurs  the  body  is  said  to  become  rancid.  The  most 
abundant  production  of  facts  takes  place  in  fruits,  seeds,  and  spores,  where  they  are 
stored  up  as  reserve  materials,  but  in  many  instances  they  also  function  as  attrac- 
tive or  protective  agents.  Nor  must  we  forget  the  crystalline  or  amorphous  fatty 
excretions  formed  on  the  epidermis  of  foliage-leaves,  stems,  and  fruits,  which  are 
popularly  known  as  "  bloom  ".  These  are  very  like  wax,  and  have  a  very  manifold 
significance;  they  prevent  hurtful  moistening  by  water,  regulate  transpiration 
under  certain  circumstances,  and  can  also  ward  off  the  disadvantageous  attacks  of 
animals.  The  branches  of  many  willows  which  bear  honey-laden  flower-catkins,  as, 
for  example,  those  of  Salix  pruinosa  and  daphnoides,  are  provided  with  these  wax- 
like,  extremely  smooth  and  slippery  coverings  up  which  the  unwelcome  wingless 
ants,  scenting  the  honey  in  the  catkins,  in  vain  try  to  climb. 

Alkaloids  and  glucosides  are  developed  principally  as  means  for  protecting 
the  green  tissues  of  the  leaves  and  fruit,  and  the  underground  portions  of  the 
plants — the  roots,  rhizomes,  tubers,  and  bulbs — against  demolition  and  extinction 
by  animals.  The  alkaloids  are  distinguished  by  the  presence  of  nitrogen  in 
them.  Some  of  them  contain  no  oxygen,  and  are  volatile,  as,  for  example, 
trimethylamine,  which  occurs  in  the  leaves  of  many  oraches,  and  in  the  flowers 
of  Hawthorn  and  Pear,  as  well  as  in  the  American  Pachysandra.  Most,  however, 
are  non- volatile,  and  contain  oxygen.  To  this  latter  class  belong  the  well-known 
alkaloids — morphine,  nicotine,  atropine,  and  strychnine,  which  are  poisonous 
to  man  and  most  mammals;  also  the  well-known  drugs — quinine,  cocain,  and 
many  others.  Leaves  provided  with  these  materials  are  rejected  as  food  by 
grazing  animals,  and  consequently  they  at  least  may  be  regarded  as  efficient  in 
protecting  the  plants  from  being  devoured.  Only  the  volatile  trimethylamine  in 
flowers  can  serve  to  attract  insects.  Glucosides,  of  which  more  than  a  hundred 
are  already  known,  have  a  use  very  similar  to  that  of  the  alkaloids.  Saponin 
is  poisonous  to  man  and  mammals;  amygdalin  splits  up  into  the  poisonous  prussic 
acid,  oil  of  bitter  almonds  and  sugar;  and  many  others  behave  in  exactly  the 
same  way.  Tannin  has  an  extremely  bitter  taste,  and  therefore  protects  branches, 
cortex,  and  fruits  from  being  eaten.  It  is  interesting  to  see,  that  in  many  fruits 
which  are  distributed  by  means  of  animals,  the  pericarp  remains  acid  and  unwhole- 
some in  consequence  of  bitter  or  poisonous  glucosides,  until  the  seeds  hidden 
within  have  matured.  As  soon  as  they  can  germinate,  the  glucosides  become 
changed;  they  are  split  up  by  means  of  ferments,  which  will  be  described  later, 


METABOLISM  IN   LIVING  PLANTS.  463 

or  by  the  acids  which  are  present  in  such  abundance,  into  sugar  and  various  other 
harmless  materials,  and  the  pericarp,  which,  until  now  had  been  sharp,  acid,  and 
unwholesome,  becomes  sweet  and  luscious.  The  same  coat  which  formerly  served 
as  a  protection,  now  forms  an  attraction.  The  ripe  fruits,  with  the  seeds  they 
inclose,  are  now  sought  for  and  eaten  as  food,  especially  by  birds;  the  sweet 
covering  is  digested,  while  the  seeds,  excellently  protected  against  the  action  of 
digestive  juices,  are  excreted  with  the  waste  materials  of  the  food,  and  germinate 
in  the  places  where  they  are  deposited;  thus  the  widest  dissemination  of  the 
plants  is  brought  about.  All  this  will  be  discussed  in  detail  later  when  distributing 
agents  of  plants  are  being  considered;  but  it  seems  appropriate  to  mention  these 
processes  here  in  order  to  point  out  that  the  metabolism  of  materials  in  plants  keeps 
pace  with  the  requirements  for  the  time  being;  that  even  when  the  division  of 
labour  in  the  plants  is  as  much  developed  as  in  the  cases  just  mentioned,  the 
arrangements  and  displacements  of  the  atoms,  and  the  decomposition  and  formation 
of  chemical  compounds,  are  always  carried  on  in  the  right  place  and  at  the  right 
time,  i.e.  always  where  and  when  the  plant  is  benefited  thereby;  and  that  frequently 
the  reasons  for  all  these  changes  only  become  intelligible  when  we  consider  the 
inter-relations  of  animals  and  plants. 

The  significance  of  salts  of  sulphuric  and  nitric  acids,  as  well  as  the  relations 
of  these  to  oxalic  acid,  have  already  been  discussed  and  explained  —  how  by 
means  of  the  latter  the  sulphuric  and  nitric  acids  are  liberated,  yielding  sulphur 
and  nitrogen  for  the  construction  of  albuminous  substances.  If  oxalic  acid  ac- 
cordingly does  not  appear  to  be  a  necessary  plastic  constituent  of  the  framework 
of  the  plant,  it  is  nevertheless  quite  indispensable  as  an  accessory  to  metabolism. 
The  same  thing  applies  to  the  other  organic  acids  which  exist  in  plants.  They 
are  only  accessories  in  the  transformations,  or  intermediate  steps  between  the 
final  stages  of  the  compounds  formed  in  the  plants,  viz.  between  the  first  carbo- 
hydrate on  the  one  hand,  and  the  completed  substance  used  for  building  or  further 
purposes  on  the  other.  Under  these  conditions,  it  is  intelligible  that  the  organic 
acids  should  be  distributed  through  all  parts  of  the  plant,  and  that  the  juices 
in  living  plants  almost  universally  have  an  acid  reaction.  It  is  also  intelligible 
that  the  number  of  organic  salts  should  be  extremely  large.  Oxalic,  tartaric, 
citric,  and  malic  acids  may  be  cited  as  examples,  but  more  than  two  hundred 
such  acids  are  known  which  have  been  detected  in  various  plants.  Many  of 
them  are  also  found  in  animal  bodies,  viz.  isolated  members  of  the  series  of  the 
so-called  fatty  acids,  which  form  fats  when  combined  with  glycerine,  as,  for 
example,  butyric  and  formic  acids,  the  latter,  as  already  stated,  being  also  contained 
in  the  stinging  hairs  of  nettles.  It  has,  moreover,  been  already  pointed  , 
.glucosides  are  decomposed  by  organic  acids,  and  give  rise  to  various 
sugar.  It  is  interesting  with  respect  to  these  sugars  that  they  anse  as  t 
organic  products  (which  result  from  the  assimilation  of  carbonic  acid),  and  al* 
again  as  the  terminal  members  of  a  very  long  chain  of  transformations  and 
decompositions  of  glucosides  effected  by  the  action  of  organic  a*ids.  An  imp* 


464  METABOLISM    IN   LIVING   PLANTS. 

rdle  may  be  assigned  to  the  organic  acids  of  the  type  of  oxalic  and  formic  acids 
with  regard  to  the  turgescence  of  cells  in  living  plants,  since  they  suck  up  water 
with  great  energy  to  replace  that  lost  by  evaporation,  and  are  thus  able  to  main- 
tain the  turgidity. 

An  especial  function  is  also  assigned  to  the  so-called  amides,  by  which  are 
understood  asparagin,  tyrosin,  leucin,  glutamin,  &c.  These  are  produced  by  the 
splitting  up  of  albumens,  but  at  the  same  time  they  promote  the  reconstruction 
of  these  substances  in  the  living  protoplasm.  When  the  carbohydrate,  which  is 
derived  from  the  albumen,  together  with  the  amide,  is  used  up,  the  amide  again 
draws  to  itself  a  fresh  carbohydrate  (which  has  just  been  formed  in  the  green 
cells),  enters  into  combination  with  it,  and  in  this  way  again  forms  an  albumen. 
This  process  may  be  repeated  indefinitely,  and  will  be  referred  to  in  the  dis- 
cussion on  respiration.  Moreover,  when  albumens,  which,  in  their  usual  condition, 
cannot  pass  through  the  cell-wall,  are  to  be  transmitted  from  one  place  to 
another,  they  are  probably  first  transformed  into  asparagin,  or  a  similar  amide, 
which  again  becomes  a  complete  albuminous  compound  by  the  union  with  a 
carbohydrate  in  the  place  where  the  albumens  are  to  remain. 

Finally,  the  group  of  enzymes  or  ferments  comes  under  the  head  of  ac- 
cessories. These  substances,  so  extremely  important  to  the  life  of  plants,  have 
the  remarkable  property  of  being  able  to  decompose  other  substances  without 
themselves  being  split  up,  and  in  consequence  a  very  small  quantity  of  them 
can  produce  very  marked  results.  They  all  contain  nitrogen,  and  are  widely 
distributed  in  plants,  but  since  they  are  only  formed  in  minute  quantities  in  the 
places  where  they  are  required,  their  presence  is  not  always  easy  to  demonstrate. 
How  they  arise  is  still  a  problem;  perhaps  in  the  same  way  as  the  nitrogenous 
albumens.  They  are  to  be  found  wherever  solid  bodies  are  to  be  liquefied  or 
dissolved;  for  example,  when  the  stores  of  organic  food,  i.e.  the  so-called  reserve 
materials,  which  have  remained  resting  for  a  long  time  in  the  seeds,  tubers, 
and  roots,  and  have  been,  so  to  speak,  put  out  of  the  way,  are  to  be  liquefied 
and  again  brought  into  action;  further,  when  substances  which  cannot  pass 
through  the  cell- walls  are  to  be  brought  into  a  condition  suited  to  this  translation, 
in  which  case  they  act  like  the  amides  previously  described;  further  still,  as 
often  as  organic  compounds  are  to  be  absorbed  as  food,  insects  and  other  animals 
to  be  digested  by  insectivorous  plants,  the  dead  bodies  of  plants  to  be  broken 
up  by  saprophytes,  or  the  organized  portions  of  living  plants  to  be  consumed  by 
parasites.  When  the  sucking  cells  of  the  parasitic  plants  wish  to  obtain  the 
juices  of  the  host-plant;  when  the  hyphsB  issuing  from  the  spores  make  their 
way  through  the  epidermis  into  the  interior  of  the  plant  on  which  they  have 
fallen;  or  hyphal  threads  in  the  interior  of  a  many-celled  tissue  wish  to  pass 
from  one  chamber  into  another,  they  must  dissolve  the  cell- walls,  thus  creating 
an  open  passage  for  themselves.  Enzymes  also  appear  to  come  into  action  wherever 
those  remarkable  processes  are  carried  on  which  are  known  as  fermentations,  and 
which  will  be  considered  in  the  following  pages.  It  is  to  be  supposed  that  they 


MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO.  4(55 

form  a  constituent  of  the  protoplasm  of  the  fermentative  organism,  and  themselves 
affect  and  decompose  their  environment  through  the  cell-wall. 

The  most  important  enzymes  are,  first,  pepsin,  which  peptonizes  albumens 
in  the  presence  of  weak  acids,  i.e.  changing  them  into  a  soluble  condition,  whereby 
they  are  enabled  to  pass  through  the  partition-walls  from  one  cell-chamber  to 
another.  The  pepsin  contained  in  plants  does  not,  indeed,  differ  from  that  in  the 
gastric  juice  of  animals,  so  that  the  part  performed  in  both  cases  is  essentially 
the  same.  In  the  stomach  of  animals  it  has  to  perform  the  important  task  of 
bringing  the  albumens  taken  in  as  food  into  a  soluble  form,  so  that  they  can 
then  enter  the  blood.  The  presence  of  pepsin  in  insectivorous  plants  has  already 
been  alluded  to.  Another  enzyme  to  be  mentioned  is  diastase,  which  makes 
starch  grains  soluble,  since  it  decomposes  them  into  sugar  and  dextrin.  It  is 
found  wherever  starch-grains  have  been  stored  up  when  they  are  again  to  be 
utilized  and  to  be  assimilated.  Emulsin  and  myrosin  should  also  be  pointed 
out.  They  decompose  glucosides  in  the  manner  already  described,  and  thereby 
give  rise  to  sweet  sugar,  especially  in  fruits;  but  they  can  also  effect  various 
other  decompositions,  as,  for  example,  the  splitting  up  of  the  amygdalin  contained 
in  almonds  into  glucose,  prussic  acid,  and  oil  of  bitter  almonds,  which  is  effected  by 
emulsin.  Papain,  occurring  in  the  fruits  of  Carica  Papaya,  and  invertin,  which 
has  been  observed  in  Yeast,  are  to  be  regarded  as  enzymes.  All  substances  which 
have  a  decomposing  action  on  their  environment,  without  at  the  same  time  under- 
going any  chemical  change  themselves,  are  called  ferments,  and,  so  far,  all  enzymes 
are  to  be  considered  ferments.  It  has  been  demonstrated,  however,  that  under 
certain  conditions,  acids — for  example,  phosphoric  acid — and  even  water  at  a  high 
temperature,  exhibit  a  ferment  action,  and  for  this  reason  the  name  enzyme  has 
been  chosen  for  the  nitrogenous  compounds  detailed. 

We  have  now  enumerated  the  most  important  of  those  substances  whose 
building  up  and  breaking  down,  whose  transformations  and  interactions  constitute 
what  we  recognize  as  the  life  of  plants. 


2.  TRANSPORT  OF  SUBSTANCES  IN  LIVING  PLANTS. 

Mechanisms  for  Conveyance  to  and  fro.-Significance  of  Anthocyanin  in  the  Transportations  and 
Transformations  of  Materials.— Autumn  Colouring  of  Foliage. 

MECHANISMS  FOE  CONVEYANCE  TO  AND  FRO. 

It  has  already  been  explained  that  the  decomposition  of  carbonic  acid,  and 
the  formation  of  organic  matter  out  of  the  absorbed  gaseous  and  liquid  morgai 
food,  can  only  occur  in  cells  which  contain  chlorophyll-bodies.     The  shape  an 
arrangement  of  the  chlorophyll-corpuscles  in  individual  cells,  and  further ,  tl 
form   and   arrangement   of   these  green  cells  themselves,  have  also  been  ther 


VOL.  I. 


466  MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO. 

described.  It  has,  moreover,  been  stated  that  numerous  plants  exist  which 
consist  only  of  single  green  cells;  that  others,  which  are  multicellular,  exhibit 
in  all  their  cells  the  same  shape  and  grouping  of  the  chlorophyll-corpuscles; 
and  finally,  that  in  most  seed  plants,  a  division  of  labour  has  taken  place  in 
each  plant,  so  that  certain  cells  only  are  provided  with  chlorophyll,  while  others 
are  always  destitute  of  it.  Many  parasites  are  quite  free  from  chlorophyll,  and 
consequently  are  unable  to  decompose  carbonic  acid  and  to  manufacture  organic 
materials.  They  are  obliged  to  suck  these  up  from  their  hosts.  Closely  connected 
with  these  are  cases  of  symbiosis,  in  which  plants  possessing,  and  plants  devoid 
of,  chlorophyll  enter  into  partnership,  and  in  which  the  latter  receive  in  exchange 
certain  freshly-manufactured  organic  substances  from  the  former.  The  conclusion 
of  this  long  series  is  formed  by  the  saprophytes,  devoid  of  chlorophyll,  which 
derive  their  organic  materials  not  from  living  green  plants,  but  from  dead  animal 
or  vegetable  bodies,  and  from  the  lifeless  organic  substances  arising  out  of  plants 
or  animals.  In  the  green  unicellular  plants,  as,  for  example,  in  the  Desmidieae, 
two  species  of  which  are  illustrated  in  fig.  25A,  i,  k,  all  the  various  combinations, 
arrangements,  and  separations  of  the  atoms  which  lead  to  the  formation  of  sugar, 
starch,  cellulose,  chlorophyll,  albumen,  &c.  are  accomplished  within  a  single  cell; 
and  these  minute  plants  furnish  evidence  that  the  manifold  changes  of  the 
materials  connected  with  growth  and  construction  can  occur  side  by  side  at  the 
same  time  and  in  limited  compass.  In  order  to  be  able  to  demonstrate  this,  it 
must  be  assumed  that  each  tiny  protoplasmic  mass,  which  forms  the  living  body 
of  the  single  cells,  is  made  up  of  various  portions  to  which  are  assigned  different 
functions.  One  breaks  up  carbon  and  forms  carbohydrates;  another  takes  up 
these  carbohydrates  and  forms  albumen  from  them;  and  yet  another  transforms  the 
sugar  into  cellulose  and  builds  up  the  cell- wall. 

With  this  assumption,  however,  is  necessarily  connected  the  further  assumption 
of  a  transportation  of  materials.  In  unicellular  Desmidiese,  the  path  which  the 
sugar  produced  in  the  central  chlorophyll-bodies  has  to  travel  in  order  to  reach 
the  periphery  of  the  cells,  is  perhaps  only  two  or  three  thousandths  of  a  millimetre 
long;  it  is,  however,  a  measurable  distance,  and  therefore  there  is  such  a  thing  as 
conveyance  and  removal  of  sugar  in  cells  of  Desmidieae.  The  transportation  is 
without  doubt  again  carried  on  by  certain  portions  of  the  protoplasm,  and  perhaps 
the  manifold  strands  which  are  observed  in  the  substance  of  the  protoplasm  are 
associated  with  this.  In  multicellular  plants  the  road  which  the  materials 
have  to  follow,  in  order  to  reach  their  destinations,  though  frequently  limited 
to  the  space  of  a  single  tiny  cell,  is  often  represented  by  a  long  row  of  cells. 
This  is  especially  the  case  when  certain  functions  are  assigned  to  the  different 
cells  of  a  plant,  as  happens  in  many  spore-bearing  plants,  but  still  oftener  in 
seed-plants.  The  materials  formed  in  the  green  leaves  of  a  moss,  if  they  are 
to  be  employed  in  the  construction  of  the  spore-capsule  and  in  the  production 
of  spores,  must  be  transported  from  cell  to  cell,  to  the  archegonium  situated  on 
the  moss  stem — a  road  which  varies  according  to  the  species  from  some  millimetres 


MECHANISMS   FOR  CONVEYANCE  TO  AND   FRO.  467 

to  several  centimetres.  The  materials  which  serve  to  promote  the  growth  of  the 
branches  of  an  Aspen  are  manufactured  in  the  long-stalked,  green  leaf-blades 
of  this  plant.  That  they  may  reach  the  growing  branch,  they  must  pass  down 
the  long  leaf -stalk  and  travel  along  a  road  many  thousand  times  exc, 
the  length  of  those  cells  in  which  they  were  formed.  Let  us  glance  at  a  palm, 
whose  few  large  leaves,  forming  a  plume,  sway  about  at  the  summit  of  a  slender 
stem.  In  order  to  reach  the  growing  roots,  the  constructive  materials  formed 
in  the  green  leaves  have  to  travel  over  a  road  20  or  30  metres  long.  The  dis- 
tance is  still  greater  over  which  the  sap  prepared  in  the  foliage  of  tropical  vines 
is  conveyed  in  order  to  reach  the  roots,  where  it  serves  as  food  to  parasitic 
rafflesias  growing  thereon.  It  is  naturally  to  be  expected  that  in  such  instances 
the  routes  followed  by  the  travelling  materials,  and  also  the  starting  and  end 
stations,  should  exhibit  characteristic  features.  What  has  been  ascertained  on  this 
point  may  be  here  briefly  set  forth. 

The  green  tissue  which  is  developed  in  by  far  the  larger  number  of  cases  in  the 
cortex  of  green  stem-structures,  in  foliage-leaves,  young  fruits,  &c.,  more  rarely  in 
floral  leaves  and  roots,  must  be  regarded  as  the  first  or  departure  station.  In  the 
green  multicellular  thallophytes  and  in  mosses,  the  chlorophyll-containing  cells  also 
form  the  channels  of  removal  for  the  materials  which  have  been  formed  in  the  cells, 
and  these  are  always  extended  lengthwise  in  accordance  with  the  direction  of  the 
stream.  In  the  leaves  of  mosses  very  frequently  cell-rows  and  cell-bands  arise 
which  converge  towards  the  base  of  the  leaf,  and  in  the  vicinity  of  these  points  the 
cells  are  most  elongated  according  to  the  direction  of  the  current.  The  conducting 
cells  in  the  stem  are  also  much  elongated  in  the  direction  of  the  current.  But  here 
no  definite  line  can  be  drawn  between  the  forms  of  the  cells  at  the  departure 
station,  in  the  channel,  and  at  the  termination  of  the  current. 

It  is  different  in  those  plants  whose  leaves  and  stem  are  traversed  by  vascular 
bundles.     These  cells  devoid  of  chlorophyll,  and  peculiar  tubes  belonging  to  the 
bundles,  take  up  the  materials  proceeding  from  the  green  tissues  to  conduct  them 
to  the  places  of  consumption.     Division  of  labour  has  been  so  far  carried  out  in  all 
these  cases  that  a  portion  of  the  cells  undertakes  the  decomposition  of  carbonic  acid 
and  the  formation  of  the  first  organic  compounds,  and  another  the  conveying  away 
of  these  first  products;   but  obviously  this  does  not  preclude  the  possibility  that 
manifold  changes  may  still  take  place  during  the  transit.     In  such  a  division  of 
labour  it  is  important  that  the  organic  compounds  which  have  been  formed  in  the 
superficial  green  cells,  under  the  influence  of  light,  should  be  removed  as  quickly  as 
possible  from  the  places  where  they  are  produced,  so  that  the  important  process  of 
the  decomposition  of  carbonic  acid  should  suffer  no  kind  of  interruption.     It 
on  account  of   this  rapid  removal  by  the  shortest  path  that  the  green  eel 
elongated  in  the  direction  in  which  they  transport  their  products,  and 
neighbouring  green  cells  are  separated  as  much  as  possible  from  one  anc 
However  they  may  be  arranged  in  other  respects,  the  indicated 
isolation  are  always  observed  by  them  under  all  circumstances. 


468  MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO. 

The  isolation  is  brought  about  by  the  elongated  cells,  which  lie  parallel  side 
by  side,  assuming  a  cylindrical  form  in  consequence  of  which  they  merely  touch  one 
another,  leaving  large  air-spaces  between.  An  exchange  of  materials  between  these 
cylindrical  cells,  i.e.  a  passage  of  materials  transversely  across  their  elongated  sides 
is  wholly  prevented,  and  the  transport  of  the  materials  is  effected  only  in  that 
direction  in  which  the  cylindrical  cell  in  question  is  elongated.  The  organs  which 
convey  the  materials  away  from  the  green  cells  lie  within  the  strands  which  form 
the  veining  of  the  leaf,  which  traverse  the  leaf-stalk  and  stem  as  thick  bundles,  and 
when  densely  aggregated,  form  the  chief  part  of  the  trunks  of  woody  plants.  But 
it  would  be  erroneous  to  suppose  that  these  strands  (i.e.  the  vascular  bundles)  are 
composed  exclusively  of  structures  for  conveying  away  plastic  materials.  Adjoin- 
ing these,  and  connected  with  them,  are  regularly  found  woody  cells,  tubes,  and 
other  vessels,  which  conduct  the  mineral  food-substances  absorbed  by  the  roots,  and 
the  water  in  which  these  are  dissolved,  upwards  to  the  transpiring  tissues.  Finally, 
elastic  thread-like  bast-cells  are  always  added  to  these  structures,  which  serve  for 
the  two  kinds  of  transport,  by  which  means  the  whole  is  given  the  necessary  firm- 
ness and  elasticity.  In  these  strands,  therefore,  which  are  called  vascular  bundles, 
the  most  varied  structures  with  widely -differing  functions  are  found  crowded 
together  in  a  small  space,  and  it  happens  that  the  cells  and  vessels  which  serve  as 
the  passage  for  the  current  of  organic  materials  formed  in  the  green  tissues,  only 
occupy  a  very  moderate  share  of  the  space. 

Four  kinds  of  mechanisms  for  carrying  on  the  work  of  removal  have  been 
discovered.  First  of  all,  there  are  groups  of  parenchymatous  cells  which  adjoin  the 
other  elements  of  the  vascular  bundle,  especially  the  water-conducting  woody  cells 
and  vessels  which  they  usually  surround,  forming  an  actual  mantle  round  them, 
termed  the  vascular  bundle  sheath.  These  vascular  bundle  sheaths  are  particularly 
well  developed  in  the  foliage-leaves,  and  form  there  an  important  constituent  of 
the  leaf -ribs  and  veins  traversing  the  green  tissue  (see  fig.  1262).  In  the  finest 
and  most  delicate  veinlets,  representing  the  ultimate  terminations  of  the  vascular 
bundles,  the  few  water-conducting  cells,  stiffened  by  spiral  thickenings,  are 
surrounded  by  parenchymatous  cells.  These  are  continued  on  beyond  the  vascular 
bundle,  and  frequently  the  finest  veinlets  are  formed  to  such  a  large  extent  of  these 
parenchymatous  cells  that  they  have  been  distinguished  as  a  particular  form  of 
tissue  by  the  name  of  nerve-parenchyma. 

Next  to  the  vascular  bundle  sheaths,  medullary  rays  are  to  be  regarded  as 
organs  for  conveying  the  formed  materials  from  the  green  leaves.  These  consist 
also  of  parenchymatous  cells  with  lignified  walls  which  are  elongated  at  right 
angles  to  the  axis  of  the  stem-structure  to  which  they  belong.  They  form  layers  of 
tissue  which  are  situated  between  the  vascular  bundles,  and  connect  the  pith  in  the 
centre  of  the  stem  with  the  cortex.  Besides  these  medullary  rays,  which  are  known 
as  primary,  quite  similar  layers  are  formed  of  parenchymatous  cells  in  the  body  of 
the  vascular  bundles,  which,  however,  are  in  no  way  connected  with  the  pith  in  the 
centre  of  the  stem,  and  which  are  known  as  secondary  medullary  rays.  On  cutting 


MECHANISMS  FOR  CONVETANCE  TO  AND   FRO. 


409 


in 


the 


pith, 
rad^ateout  fro™ ,  the  ^'^^Z^Z^ZS^ 

Soft  bast  u,  to  be  considered  as  a  third  form  of  conducting  mechanism  fo  'the 
orgamc  compounds  formed  in  the  green  cell,  It  consists  partly  of  thin-wal  ed 
parenchymatous  cells  and  frequently  also  of  long,  narrow  cells  tapLng  at  the  ends 
(camb,form  cells),  which  are  elongated  in  the  direction  of  the  bundle  or  strand  to 


345678  9  10  11  12  IS 

Fig.  125.— Portion  cut  from  a  Branch  of  a  Dicotyledon;  x  about  200.    Diagrammatic. 

i  Superficial  coat  (Epidermis).  2  Cork  (Periderm).  *  Cortical  parenchyma.  <  Vascular  bundle  sheath.  •  Hard  bast  •  Bast 
parenchyma.  T  Sieve-tubes.  » Cambium.  » Pitted  vessel.  "Wood-parenchyma.  u  Scalarifonn  vessels.  "Medullary 
sheath,  is  Medulla  or  pith. 

which  they  belong,  and  form  a  tissue  called  the  bast  parenchyma  (see  fig.  1258). 
The  other  part  of  it  consists  of  tubes  which  contain  walls  separated  by  com- 
paratively large  intervals,  often  measuring  2  mm.,  usually  placed  horizontally,  but 
often  obliquely,  which  give  the  tubes  a  jointed  appearance.  Sharply -defined 
perforated  areas  appear  on  the  interpolated  horizontal  walls  as  well  as  on  the 
lateral  walls  of  the  tubes,  they  have  a  sieve-like  aspect,  and  are  called  sieve  plates, 
the  tubes  themselves  being  called  sieve-tubes,  bast- tubes,  or  bast- vessels  (fig.  1257). 
The  soft  bast  but  rarely  forms  isolated  strands,  as,  for  example,  in  some  Melasto- 
maceae;  as  a  rule,  strands  of  firm,  elastic,  string-like,  hard  bast  cells  adjoin  it,  but 
these  have  nothing  to  do  with  the  transportation  of  materials,  and  have  merely 
a  mechanical  significance  (see  fig.  1255). 

This  fibrous  or  hard  bast,  together  with  the  soft  bast,  forms  hi  very  many  plants 


470  MECHANISMS  FOR  CONVEYANCE  TO  AND  FRO. 

one-half  of  the  vascular  bundles,  the  so-called  bast  portion,  while  the  other  half,  the 
so-called  woody  portion,  consists  of  woody  cells  intermingled  with  lignified  tubes, 
and  other  water-conducting  elements  (see  figs.  125  9» 10'  n). 

Laticiferous  tubes  form  a  fourth  mechanism  for  conducting  away  the  products 
of  the  green  cells  (fig.  1261).  These  are  thin- walled,  much  branched,  frequently 
anastomizing,  tubular  structures  which  seem  to  penetrate  all  the  parts  of  the  plant, 
leaves,  stem,  and  roots,  without  much  regularity. 

They  may  be  divided,  according  to  their  development,  into  laticiferous  vessels 
and  laticiferous  cells.  The  former  are  produced  from  rows  of  cells,  whose  partition- 
walls  have  become  obliterated,  so  that  the  rows  of  cells  have  become  converted  into 
tubes;  the  latter  arise  from  isolated  cells,  at  first  very  small,  but  which  elongate 
enormously,  become  much  branched,  and  whose  branches  penetrate  between  the 
cells  of  other  tissues  just  as  the  hyphse  of  parasitic  fungi  grow  through  the  tissues 
of  their  host-plants.  Laticiferous  tubes  are  not  to  be  found  in  all  plants.  They 
are  particularly  abundant  in  species  of  Spurge,  some  thousand  species  of  Composite, 
for  example,  in  the  Salsify,  Lettuce,  and  Dandelion;  in  the  Oleander,  many 
Asclepiadeae,  Papaveracese,  and  Artocarpese.  In  the  gigantic  trunks  of  tropical  Fig- 
trees,  the  latex  often  wells  up  in  large  quantities  from  rifts  in  the  bark  which  have 
arisen  spontaneously,  and  thickens  into  long  strings  and  ropes  of  india-rubber 
hanging  down  like  a  mantle. 

The  Cow  Tree  of  Venezuela  (Galactodendron  utile)  is  especially  worth  noticing 
here;  when  pierced,  a  quantity  of  sweet,  delicious  milk  pours  out  from  it,  also 
Collophora  utilis  of  the  Amazon,  from  which  is  obtained  a  viscous  latex,  used  as  a 
medium  for  colouring  matters;  finally  the  poppy  (Papaver  somniferum),  whose 
dried  latex  is  known  as  opium.  In  the  majority  of  cases  the  latex  is  white,  but  in 
Papaveracese  other  colours  are  also  to  be  found;  thus  the  Celandine  (Chelidonium 
majus)  contains  an  orange,  and  the  Bloodwort  (Sanguinaria  Canadensis)  a  blood- 
red  latex.  The  milky  Agarics  (Lactarius)  contain  partly  white,  partly  sulphur- 
yellow,  partly  orange,  and  vermilion  latex. 

In  the  foliage-leaves  the  laticiferous  tubes  run  with  the  vascular  bundles,  and 
occasionally  replace  the  bundle  sheath;  at  least,  the  bundle  sheath  is  defective,  and 
only  very  incompletely  formed  where  the  laticiferous  tubes  adjoin  the  vascular 
bundle.  It  has  also  been  observed  that  in  the  stems  of  the  Asclepiadese,  where 
the  laticiferous  tubes  are  abundantly  developed,  the  sieve-tubes  are  much  reduced, 
and  it  is  therefore  supposed  that  the  various  mechanisms  for  conducting  away 
materials  are  sometimes  able  to  mutually  replace  one  another.  It  must,  moreover, 
be  expressly  noted  here,  that  the  laticiferous  tubes  do  not  serve  exclusively  to 
carry  away  the  materials  manufactured  in  the  green  cells;  they  are  used,  under 
certain  conditions,  and  at  certain  times,  as  receptacles  for  reserve  materials,  exactly 
as  the  medullary  rays,  sieve-tubes,  and  bundle  sheaths  which  in  the  winter,  when 
the  decomposition  of  carbonic  acid,  and  the  formation  of  carbohydrates  in  the  green 
cells  have  ceased,  and  when  generally  there  is  nothing  to  remove,  function  as 
reservoirs,  in  which  stores  are  deposited  until  the  following  spring.  The  parenchy- 


MECHANISMS   FOR   CONVEYANCE  TO   AND    FRO.  471 

matous  cells  of  the  vascular  bundle  sheaths  which,  in  summer,  had  been  „ 
conduc  mg  away  matenals,  are  then  crowded  with  starch-granules,  the  pores  of  ,1,, 
^eve-plates  are  closed  up  during  the  winter;  the  sieve-tubes,  laticiferous  tubes 
bundle  sheaths  and  medullary  rays  do  not  again  commence  their  activity  until  the 
next  period  of  vegetation,  when  everything  becomes  liquefied,  and  the  green  cells 
agam  form  fresh  carbohydrates.  These  structures  then  serve  again,  of  course 
chiefly  as  conducting  organs. 

With  regard  to  the  junction  of  the  conducting  organs  with  the  green  cells  we 
have  a  very  great  variety,  but  the  many  different  contrivances  may  be  grouped  into 


Fig.  126.— Organs  for  Removal  of  Substances. 

i  Laticiferous  tubes  from  the  leaf  of  Lactuca  virosa ;  x  250.    «  Vessels  with  spirally  thickened  walls,  surrounded  by  the 
bundle  sheath,  from  a  leaf  of  Ricinut  communit;  x  210. 

two  chief  forms,  viz.  where  the  junction  is  direct,  and  where  it  is  effected  by  means 
of  specially  interpolated  cells. 

In  the  first  group,  the  switch  shrubs  are  first  to  be  noted,  in  which  the  foliage  is 
entirely  or  almost  entirely  absent,  and  where  the  main  portion  of  the  green  tissue 
is  developed  in  the  cortex  of  the  rod-shaped  branches,  as,  for  example,  in  Cytisus 
radiatus  and  in  the  Broom  (see  figs.  69 8,  69  4,  81  \  and  81  2).  Here  the  ring  of 
vascular  bundles  forming  the  framework  of  the  whole  branch  is  surrounded  by  a 
common  bundle  sheath,  and  the  cells  of  the  green  tissue  in  the  cortex  are  connected 
on  one  side  with  the  epidermis,  and  on  the  other  with  this  bundle  sheath,  to  which 
the  organic  materials  produced  are  given  up  directly.  In  the  foliage-leaves  of 
many  liliaceous  plants,  especially  in  the  equitant  leaves  of  irises,  the  green  cells 
are  elongated  transversely,  forming  a  kind  of  bridge  stretched  between  the  vascular 


472  MECHANISMS   FOR   CONVEYANCE   TO   AND    FRO. 

bundles,  which  run  almost  parallel  from  the  base  to  the  apex  of  the  leaf.  Each  of 
the  bridge-like  green  cells  is  connected  at  either  end  with  a  vascular  bundle,  and 
delivers  the  materials  produced  to  the  conducting  portions  of  these  vascular 
bundles  on  both  sides,  i.e.  to  the  vascular  bundle  sheaths.  In  other  liliaceous 
plants,  especially  in  the  leaves  and  green  stems  of  species  of  onion,  the  green  cells 
are  palisade-shaped,  and  their  longer  diameter  is  perpendicular  to  the  surface  of  the 
part  to  which  they  belong.  Here  we  have  only  a  one-sided  connection  with  the 
conducting  cells  of  the  vascular  bundle,  but  the  junction  is  again  a  direct  one. 
Finally,  the  peculiar  connection  of  laticiferous  tubes  with  the  palisade-cells  in  the 
leaves  of  species  of  spurge  must  be  considered. 

Although  the  laticiferous  tubes  appear  to  be  very  much  branched  wherever  they 
occur  in  plants,  the  formation  of  branching  tubes  is  nowhere  else  so  abundant  as 
in  the  vicinity  of  the  palisade-cells.  Many  of  the  twigs  directly  adjoin  these  cells. 
It  also  happens  that  single  terminations  of  the  laticiferous  tubes  impinge  upon  the 
lower  ends  of  several  palisade-cells,  which  are  inclined  towards  one  another  (fig. 
25A,  s),  and  that  single  laticiferous  ramules  push  their  way  in  between  these 
cells.  In  all  these  examples  the  materials  manufactured  in  the  green  tissue 
are  taken  up  without  further  intervention  by  the  ultimate  terminations  of  the 
conducting  laticiferous  tubes. 

Of  the  second  group,  which  is  characterized  by  the  fact  that  the  junction  is 
brought  about  by  specially  intercalated  cells,  the  first  instance  to  be  considered  is 
that  often  observed  in  the  leaves  of  grass-like  plants,  where  the  intermediate  cells, 
which  are  also  called  conducting  cells,  are  more  or  less  extended  transversely,  and 
unbranched.  The  green  cells  lying  under  the  epidermis  are  palisade-shaped,  and 
at  right  angles  to  the  leaf -surf  ace;  the  longer  diameters  of  the  cells  lying  below 
these,  which  are  much  poorer  in  chlorophyll-corpuscles,  are,  on  the  other  hand, 
placed  obliquely  to  the  leaf -surf  ace,  or  even  parallel  to  it,  and  apparently  are 
directed  towards  the  large  cells  of  the  bundle  sheaths  in  the  middle  of  the  leaf. 
These  cells,  poor  in  chlorophyll,  therefore  connect  the  palisade-cells  with  the  con- 
ducting cells  of  the  bundle  sheath,  and  serve  as  agents  in  the  removal  of  the 
substances.  But  the  commonest  cases  are  those  in  which  the  conducting  cells 
are  much  branched.  The  foliage -leaves  which  possess  these  branched  cells  are 
differently  constructed  on  the  upper  and  under  sides  of  the  leaf.  Under  the 
epidermis  of  the  upper  side  is  seen  the  palisade-tissue,  consisting  of  cylindrical  or 
prismatic  cells,  whose  long  axis  is  directed  perpendicularly  to  the  plane  of  the  leaf 
(see  fig.  621  and  fig.  25A,  r).  Below  these  palisade-cells  come  the  branched  cells, 
which  are  connected  with  one  another  by  their  arm -like  processes,  forming 
a  large-meshed  tissue,  the  frequently-mentioned  spongy  parenchyma,  interrupted 
by  wide  interstices.  The  spongy  parenchyma  is  connected  with  the  palisade-tissue 
by  means  of  single  processes  bordering  the  lower,  that  is  to  say,  the  inner  ends  of 
the  palisade-cells;  very  often  a  single  process  is  connected  with  the  inner  ends  of 
several  palisade-cells,  in  which  case  these  have  a  clustered  arrangement.  As  with 
the  palisade-cells,  the  branched  cells  of  the  spongy  parenchyma  are  connected  with 


MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO.  47;} 

the  parenchyma  sheaths  of  the  veins.      Thus   the  branched  cells  of  the  sp,: 
parenchyma  become  agents  in  the  transportation  of  the  materials;  with  one  branch 
they  take  up  the  organic  substances  manufactured  in  the  palisade-cells,  and  with 
another  they  deliver  these  materials  up  to  the  bundle  sheath  for  further  translation 
to  the  places  of  consumption  or  storage. 

That  the  cells  of  the  spongy  parenchyma  serve  not  only  for  conduction,  but 
have  to  perform  several  other  functions,  does  not  need  to  be  confirmed  in  detail 
It  is  enough  to  point  out  that  they  contain  chlorophyll-corpuscles,  and  therefore 
are  capable  of  decomposing  carbonic  acid  and  of  forming  carbohydrates,  although 
to  a  much  less  extent  than  the  palisade-cells,  which  are  so  richly  supplied  with 
chlorophyll.  Moreover,  the  excretion  of  aqueous  vapour  occurs  in  the  spongy 
parenchyma  whose  lacunae  and  passages  communicate  with  the  outer  world  by  the 
stomata,  and  where  also  a  vigorous  inflow  and  outpouring  of  other  gases  takes 
place.  Then  the  part  which  the  conducting  structures  play  in  the  metabolism 
of  the  materials  must  not  be  overlooked.  All  these  structures  contain  active 
living  protoplasm,  in  all  there  is  a  protoplasmic  cell -body,  although  very 
often  it  is  only  in  the  form  of  a  delicate  parietal  layer,  and  in  all,  under  the 
influence  of  this  living  protoplasm,  we  have  not  merely  a  movement,  but  also  an 
inexhaustible  and  infinite  changing  of  the  materials,  corresponding  to  the  indi- 
viduality of  the  species  and  to  the  requirements  of  the  time  being.  These  structures 
must  then  be  regarded  not  only  as  simple  channels  for  the  fresh  carbohydrates 
produced  in  the  green  cells,  but  also  as  regions  for  transformations,  where  the  first 
organic  compounds  manufactured  in  the  green  cells  are  prepared  for  ultimate  con- 
sumption at  the  end  of  the  journey.  It  is  precisely  in  this  respect  that  they  differ 
essentially  from  that  conducting  apparatus,  whose  task  is  to  transmit  water  and 
mineral  salts  to  the  green  tissues,  and  which,  as  already  repeatedly  remarked,  is 
present  in  the  same  bundle  as  the  cells  and  vessels  which  take  away  the  organic 
materials.  When  once  the  water-conducting  tubes  and  cells  have  attained  their  full 
dimensions,  they  no  longer  contain  protoplasm,  and  no  transformation  of  the  trans- 
mitted raw  food-sap  is  carried  on  in  them;  the  water,  with  the  mineral  food-salts 
dissolved  in  it,  is  carried  through  them  unaltered  to  the  transpiring  cells.  To 
employ  the  simile,  often  used  before,  of  the  arrangements  of  a  well-conducted  house- 
hold, the  woody  cells  and  vessels  of  a  vascular  bundle  may  be  compared  to  an 
apparatus  for  delivering  water  and  salts  into  the  kitchen,  so  to  speak;  the  green 
tissue  forms  the  kitchen  in  which  the  raw  materials  are  worked  up  and  so  prepared 
that  they  can  be  brought  back  by  the  removing  cells  to  the  places  where  they  are 
required  and  consumed. 

That  these  two  fundamentally  different  kinds  of  conducting  apparatus  are 
universally  found  united  together  in  one  and  the  same  bundle  is  explained  by  t 
fact  that  the  places  which  form  the  goal  for  one  are  at  any  rate  to  some  e* 
the  starting-point  of  the  other;  besides,  of  course,  this  combination  economy 
building  materials.      All  conducting  apparatuses  must  be  strengthened  and  pro- 
tected in  their  position,  and  therefore  it  is  beneficial  and  saves  building  materials 


474  MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO. 

the  different  structures  taking  part  in  the  conduction  are  mutually  of  use  to  one 
another,  and  are  protected  and  saved  from  injurious  external  influences  by  the  same 
arrangement. 

The  vessels  and  cells  whose  task  is  to  conduct  water  and  salts  become  lignified, 
and  the  massive  bodies  of  wood  which  exist  in  the  trunks  of  old  woody  plants  form 
such  a  firm  support  that  the  thin- walled  soft  bast,  when  it  clings  to  these  and  runs 
parallel  with  them,  is  excellently  protected  from  breaking.  In  those  organs  which 
require  to  resist  bending,  however — in  leaf-ribs  and  leaf -stalks,  culms,  and  young 
branches, — hard  bast  is  put  in  as  an  accompaniment  of  the  cells  and  tubes  which 
conduct  up  and  down.  These  strands  of  thick- walled,  but  at  the  same  time  flexible 
and  elastic,  cells  of  hard  bast  prevent  the  organs  which  they  adjoin  from  being 
broken  and  ruptured  even  under  the  influence  of  a  considerable  push  and  strain. 
Let  us  look  at  the  haulms,  stems,  branches,  and  leaves  in  a  meadow  or  in  a  wood 
during  the  sultry  period  which  precedes  the  outburst  of  a  storm.  Not  a  leaflet 
stirs,  even  the  supple  haulms  are  still,  and  every  part  of  the  plant,  that  true  child 
of  light,  assumes  that  position  with  regard  to  the  light  most  beneficial  to  it.  The 
storm  bursts,  the  wind  whistles  over  the  meadows,  the  leaves  tremble,  sway,  and 
flutter,  the  leaf -stalks  shake,  the  culms  bow  and  bend,  the  stems  and  branches  are 
smitten  and  arched  so  that  their  tops  almost  touch  the  ground;  the  foliage  is 
pelted  with  the  rain,  and  shaken  and  displaced  by  every  drop  that  falls  on  it.  An 
hour  later  the  storm  is  over;  here  and  there  perhaps  may  be  still  seen  a  group  of 
stems  and  leaves  prostrate  under  the  weight  of  the  rain-drops,  and  part  of  some 
herbaceous  stem  which  has  been  shaken  by  the  storm  bent  like  a  bow,  but  the  rest 
stand  again  erect  and  motionless,  as  if  they  had  never  been  disturbed  by  a  breeze; 
finally,  even  the  plants  bent  by  the  shock  and  so  severely  prostrated  by  the  rain- 
drops raise  their  branches  and  foliage,  and  everything  again  resumes  the  same 
conditions  and  position  as  before  the  outbreak  of  the  tempest.  But  this  is  only 
rendered  possible  by  the  presence  everywhere  of  the  elastic  flexible  strands  of 
hard  bast  accompanying  the  sieve-tubes  and  the  other  soft  and  delicate  structures 
which  take  part  in  the  preparation  and  transportation  of  the  organic  materials.  It 
is  indeed  unavoidable  that  the  cross  section  of  the  cells  and  vessels  should  become 
narrowed  in  consequence  of  the  push  and  strain  caused  by  the  gusts  of  wind,  and 
that,  for  example,  the  cross  section  of  a  cylindrical  tube  should  become  elliptical  in 
consequence  of  the  curvature;  but  since  the  prostrated  stem  or  leaf  again  rebounds 
into  the  former  position  by  reason  of  the  elasticity  of  the  hard  bast,  the  alteration 
produced  by  the  push  and  strain  is  only  temporary,  necessitates  no  interruption 
of  function,  is  perhaps  even  beneficial  to  the  movement  of  the  materials,  and,  which 
is  the  main  point,  no  rupturing  and  no  permanent  bending  of  the  soft  delicate- 
walled  structures  ensues. 

These  delicate- walled  elements,  especially  those  of  the  soft  bast,  are  protected 
against  harm  from  lateral  pressure  by  the  deposition  of  various  tissues,  especially 
cork,  in  front  of  them  (fig.  1252),  which,  like  the  buffers  of  an  engine,  keep  off,  or 
considerably  weaken,  the  lateral  thrust  and  pressure.  Remarkable  contrivances  for 


MECHANISMS    FOR   CONVEYANCE  TO   AND   FRO.  475 

protection  against  lateral  pressure  are  also  found  in  creepers  and  climbing  plants 
with  perennial  woody  stems,  and  in  those  plants  which  are  commonly  called  lianes. 
In  order  to  comprehend  these  contrivances  rightly,  it  is  necessary  first  to  get  an 
idea  of  the  position  of  the  parts  requiring  protection  in  perennial  woody  plants, 


tig.  W.-Bhyncholia  plweolMts,  a  liana  with  ribbon-like  Stems. 


which  neither  climb  nor  creep,  and  which  possess  an  erect,  straight,  column-like 
trunk.    As  previously  stated,  in  these  plants  to  which  belong  the  firs,  oaks,  b 
elms,  limes,  apple-trees,  and,  generally,  the  majority  of  leafy  trees,  the  v» 
bundles  are  arranged  in  a  ring  round  the  central  pith,  and  consast  essentially 
of  the  woody  portion,  serving  to  conduct  the  raw  sap,  and  the  bast  portion,  * 
is  employed  in  the  transmission  and  transformations  of  the  orgamc  sub. 
formed  in  the  green  cells.     These  two  portions  are  separated  in  the  plants 


476  MECHANISMS  FOR  CONVEYANCE  TO  AND  FRO. 


tioned  by  a  layer  of  tissue  in  which  a  very  vigorous  formation  of  new  cells  is 
carried  on,  termed  the  cambium  (fig.  125  8).  From  this  cambium,  which  appears  as 
a  ring  in  the  circular  cross  section  of  erect  stems,  cells  develop  which  on  one  side 
abut  upon  the  wood  already  present  in  the  interior,  and  on  the  other  the  existing 
bast  portion  of  the  vascular  bundle  to  the  exterior.  In  this  way  both  portions,  and 
in  fact  the  whole  stem,  increase  in  dimensions;  and  in  the  wood,  in  particular,  arise 
the  annual  rings  which  are  visible  in  a  cross  section.  The  cambium  ring  also 
stretches;  it  becomes  larger  and  larger,  but  always  retains  the  same  position  and 
relation  to  the  wood  and  bast  of  the  vascular  bundle,  and  keeps  its  ring-like  form 
although  the  trunk  in  question  may  have  become  ever  so  old  and  thick,  and  may 
exhibit  hundreds  of  annual  rings.  Here,  therefore,  the  soft  bast  lies  outside  the 
cambium  ring,  and  is  screened  towards  the  exterior  by  various  tissues,  by  hard  bast 
and  corky  tissue  among  others,  and  the  latter  may  undergo  considerable  develop- 
ment in  trunks  of  many  years'  growth;  while  the  hard  bast,  on  the  contrary, 
diminishes  in  older  trunks,  because  it  is  no  longer  required  as  a  protection  against 
bending.  Accordingly  the  soft  bast  is  situated  fairly  near  the  surface.  Since  a 
strong  external  lateral  pressure  is  not  to  be  feared  in  them,  this  position  cannot  be 
characterized  as  unfavourable.  The  cork  and  other  external  portions  of  the  cortex 
comprehended  under  the  term  bark  afford  a  sufficient  protection  against  small 
pressures  in  old  stems.  In  lianes  it  is  very  different,  especially  in  those  which 
make  use  of  erect  stems  as  supports.  Apparatus  for  increasing  the  bearing  capacity 
and  elasticity  in  lianes  would  be  superfluous,  these  tasks  being  performed  by  the 
support;  on  the  other  hand,  a  protection  against  lateral  pressure  is  urgently 
required,  for  if  the  support  up  which  the  lianes  climb,  to  which  they  are  attached 
by  adventitious  roots,  or  which  they  encircle  and  entwine,  increases  in  thickness,  as 
is  usually  the  case,  then  a  lateral  pressure  on  the  adherent  liane  coils  is  unavoid- 
able. And  when,  as  a  result  of  such  pressure,  the  sieve-tubes  and  bast  parenchyma 
become  squashed  over  considerable  distances,  they  are  obviously  unable  to  perform 
their  functions  satisfactorily,  and  nutrition  will  certainly  be  impaired.  Lianes  are 
protected  by  the  most  varied  contrivances  against  this  source  of  injury,  and  some  of 
the  most  striking  will  be  here  briefly  indicated. 

In  Ehynchosia  phaseoloides,  the  young,  green,  twining  stem  is  circular  in  cross 
section,  and  exhibits  a  structure  which  does  not  differ  materially  from  that  of 
young  normal  stems.  In  the  centre  is  a  pith,  round  which  the  vascular  bundles 
form  a  ring — first  wood,  then  soft  bast,  further  out  hard  bast,  then  a  layer  of 
green  cells,  and,  finally,  the  epidermis,  which  envelops  the  whole.  It  might  be 
expected  that  in  the  second  year,  the  newly-formed  cells  and  tubes  would  deposit 
wood  on  wood  and  soft  bast  on  soft  bast,  and  that  the  cylindrical  stem  would 
increase  regularly  in  circumference  without  altering  its  shape.  But,  strangely 
enough,  this  does  not  happen.  New  cambiums  arise  at  two  points  near  the 
periphery  of  the  stem,  below  the  green  cells,  by  which  the  formation  of  wood 
proceeds  in  the  direction  of  the  first  year's  vascular  bundle  ring  (i.e.  inside), 
and  soft  bast  accompanied  by  hard  bast  on  the  opposite  side  (i.e.  outside).  At 


MECHANISMS   FOR   CONVEYANCE   TO   AND    FRO.  477 

the  end  of  the  second  year  the  stem  is  no  longer  circular,  as  at  the  first;  it 
has  added  two  rings,  as  it  were,  and  now  appears  elliptical  in  cross  section; 
and  since  new  portions  are  added  in  this  way  repeatedly  from  year  to  year, 
and  new  rings  are  always  becoming  annexed  to  those  already  existing,*  thfl 
stem  gradually  becomes  ribbon-like,  and  exhibits  a  cross  section  like  that  shown 
in  fig.  128 2.  The  soft  bast  has  thus  received  the  most  protected  position 
imaginable,  and  lateral  pressure  is  unable  to  interfere  with  its  functions.  When 
the  supporting  stem  round  which  the  Rhynchosia  has  twined  grows  enormously 
in  thickness,  the  liane  becomes  stretched,  and  experiences  a  lateral  strain,  but 
the  sap  can,  nevertheless,  continue  its  journeys  unhindered  in  the  soft  bast 

In  the  liane  Thunbergia  laurifolia,  a  cross  section  of  whose  stem  is  represented 
in  fig.  1281,  the  protection  is  obtained  in  quite  a  different  way.     Here  the  green 


Fig.  128.— Transverse  sections  of  Liane  Stems. 

i  Thunbergia  laurifolia.  2  Rhynchosia  phaseoloides.  »  Tecoma  radieans;  x  SO.  Diagrammatic.  The  rarious  tissues  are 
indicated  in  the  following  manner :  Soft  bast,  entirely  black ;  wood,  larger  and  smaller  white  dots  on  a  black  ground ; 
hard  bast  and  other  mechanical  tissues,  obliquely  shaded ;  cork  (periderm),  short  lines ;  pith,  reticulated. 

stem  is  hollow,  and  the  cavity  is  surrounded  by  an  enormous  pith.  In  the 
vascular  bundle  ring  which  surrounds  the  pith,  the  wood  and  hard  bast  are 
not  arranged  from  the  first  in  successive  concentric  circles,  as  is  usually  the  case, 
but  are  placed  side  by  side.  The  cambium  continues  to  form  soft  bast  in  some 
places,  and  wood  in  others,  towards  the  interior.  In  consequence  of  this,  the 
bundles  of  soft  bast  appear  to  be  walled  in  by  the  wood  ("bast-islands"),  and 
are  consequently  well  protected  against  pressure.  The  protection  is  increased 
by  the  fact  that  this  liane  is  hollow  in  the  centre,  and  can  "give",  an  un- 
common feature  in  twining  plants. 

Sometimes   the    delicate    soft  bast    is    protected  against   compress 
position  in  niches  and  grooves  at  the  periphery  of  the  hard  wood;  this 
seen   especially  in   several   twining  Asclepiadeae  and  Apocynaceae. 
most    remarkable   protective   arrangements    is    found    in    the  climbing    Tec 
radicans,  which  adheres  to  its  substratum  by  tufts  of  aerial  roots,  and 
leafless  branches  are  depicted  in  fig.  129.     A  cross  section  of  the'  stem  ui  shown 


478  MECHANISMS  FOR  CONVEYANCE  TO  AND  FRO. 


in  fig.  1283.  The  young  branches  rooted  to  the  wall  are  elliptical  in  transverse 
section,  being  always  somewhat  compressed  on  two  sides.  The  outer  portion 
is  composed  of  the  epidermis,  two  layers  of  elastic  parenchymatous  cells  below 
it,  and  a  layer  of  green  cells.  Then  comes  the  ring  of  soft  bast,  outside  which 
bundles  of  hard  bast  are  deposited;  then  the  rings  of  cambium  and  wood,  and 
in  the  centre  a  large  pith,  which  sends  out  single-  and  double-rowed  medullary 
rays  through  the  wood  ring.  So  far  the  arrangement  of  the  various  tissues 
exhibits  nothing  particularly  noticeable,  and  coincides  with  that  in  the  young 
branches  of  numerous  woody  plants.  But  tracts  of  cambium  cells  are  subse- 
quently formed  in  a  remarkable  manner  on  the  inner  side  of  the  ring  of  wood 
adjoining  the  pith;  these  develop  wood  towards  the  exterior  and  soft  bast  on  the 
interior.  The  constituents  of  the  soft  bast — sieve-tubes  and  bast  parenchyma — 
form  quite  conspicuous  bundles  which  project  into  the  pith,  and  being  here 
excellently  protected  against  lateral  pressure,  can  perform  their  duties  undis- 
turbed. Should  the  conducting  cells  and  sieve-tubes  of  the  outer  ring  of  bast 
not  perform  their  duty,  these  inner  ones  still  remain  for  the  transmission  of  the 
plastic  materials. 

Thus  the  various  arrangements  of  the  constituents  of  the  stem,  and  especially 
the  position  of  the  channels  for  the  streams  of  materials  formed  in  the  green 
tissues,  is  in  part  accounted  for  by  the  protection  gained  against  the  injurious 
action  of  external  pressures  and  strains,  and  these  act  in  the  most  varied  way 
on  the  exterior,  according  to  the  individual  mode  of  life  of  the  plant  and  the 
conditions  of  its  habitat. 

It  is  to  the  growing  parts  of  plants,  the  extremities  of  roots  and  branches 
especially,  that  organic  matter  is  conveyed;  also  to  places  where  the  cells  already 
present  become  stimulated  to  fresh  activity,  where  dead  and  dying  cells  are 
replaced  by  fresh  ones,  and  where  constructive  materials  in  sufficient  quantity 
must  be  at  hand.  Then  again,  the  travelling  substances  are  directed  to  those 
places  where  protective  and  attractive  agents  are  necessary  to  contribute  indirectly 
to  the  maintenance  and  multiplication  of  the  species,  and  where  a  consumption 
of  materials  is  connected  with  this  protection  or  allurement.  It  is  thus  of 
importance,  for  example,  that  the  honey  excreted  from  certain  parts  of  flowers, 
which  serves  as  food  to  the  insect  guests  which  effect  fertilization,  should  be 
always  present  in  sufficient  quantity,  and  that  in  case  of  its  removal  from  the 
receptacles  by  bees  or  butterflies,  it  should  be  immediately  replaced  by  fresh 
supplies.  Care  must  also  be  taken  that  pepsin  and  other  substances  necessary 
for  digesting  prey  should  be  abundantly  present  in  the  pitfalls  and  other 
mechanisms  which  serve  for  the  capture  of  animals,  and  that  a  sufficient  quantity 
of  alkaloids  and  bitter  substances,  which  prevent  ruminants  from  devouring 
foliage,  should  be  brought  to  the  right  places  at  the  right  time.  In  connection 
with  the  process  of  rejuvenescence  and  multiplication  also,  it  is  necessary  that 
those  cells  and  groups  of  cells,  which  become  detached  from  the  plant-shoot  and 
journey  out  into  the  wide  world  as  spores  and  seeds,  should  be  equipped  with 


MECHANISMS   FOR   CONVEYANCE   TO   AND    FRO. 


479 


a  store  of  materials,  so  that  they  may  be  nourished  until  they  can  manufacture 
for  themselves  the  necessary  food  from  the  air,  water,  and  soil.     The  places  where 
spores  and  seeds  are  produced,  therefore,  constitute  an  important  destination  for 
certain  journeying  materials.      Finally,  it  also  happens  that  in  regions  where  a 
temporary  standstill  of  the  vital  activity  of   the  plants  occurs,  and  where 
succulent  green  foliage  is  liable  to  be  dried  up  by  the  periodic  drought,  or  f r« 
by  the  winter  cold,  all  the  useful  substances  are  withdrawn  from  the  threatened 


Fig.  129.— Leafless  Branches  of  Tecoma  radicant,  rooted  on  a  walL 


leaves,  and  are  deposited  in  a  suitable  form  in  safe  places,  and  stored  up  for 
employment  later.  In  these  instances,  these  safe  places,  these  storehouses  or 
reservoirs,  form  the  destination  of  the  transported  materials. 

Not   only   are    there    channels   of    distribution    to    the  various    destinations 
enumerated,  but  we  find  even  distinct  routes  provided  for  the  different  substances 
transmitted.     Investigations  have  shown  that  the  conducting  mechanisms 
the  work  to  some  extent  between  them.      The  medullary  rays  and  wood  pa; 
chyma  chiefly  conduct  carbohydrates,  the  former  radially,  and  the  latter  longitudi 
nally  in  the  stem.     The  vascular  bundle  sheaths  of  the  leaf-veins  are  particularly 
rich  in  glucosides.     Certain  tracts  of  cells  in  the  parenchyma  accompanying  < 
vascular  bundles  in  the  stem  also  conduct  glucosides,  while  others  conduct  sugars 


480  MECHANISMS   FOR   CONVEYANCE  TO   AND   FRO. 

(sugar  sheaths),  and  oohers  again  are  the  route  for  the  transmitted  starch  (starch 
sheaths).  The  sieve -tubes  and  bast  parenchyma,  on  the  other  hand,  convey 
principally  albuminous  substances  which  are  employed  as  constructive  materials 
for  the  growing  and  enlarging  portions  of  the  plant. 

This  important  relation  of  the  soft  bast  to  the  growing  organs  explains 
many  remarkable  phenomena,  two  of  which  must  be  briefly  described  here. 
First,  the  surprising  increase  of  growth  in  certain  places  which  gardeners  produce 
by  the  operation  of  ringing.  If  two  parallel  circular  cuts  are  made  round  a 
growing  branch  of  a  tree  through  the  whole  thickness  of  cortex  down  to  the  wood, 
and  if  the  circular  piece  of  cortex,  together  with  the  soft  bast  lying  between 
the  two  cuts  is  removed,  the  sap  current  in  the  soft  bast  from  the  upper  portions 
to  the  base  of  the  branch  is  interrupted.  The  cut  surfaces  dry  up;  the  route 
down  the  soft  bast  is  therefore  closed,  and  the  part  of  the  branch  lying  below 
the  excision  can  no  longer  receive  from  the  soft  bast  the  materials  necessary 
to  its  further  growth.  The  same  result  is  obtained  by  passing  a  cord  tightly 
round  the  young  leafy  branch  of  a  tree  at  some  spot,  say  about  half-way  up. 
In  this  way  all  the  soft  tissues  which  lie  outside  the  wood,  including  the  soft 
bast,  are  compressed,  the  sieve-tubes  and  tracts  of  cells  of  the  bast  parenchyma 
tightly  squeezed  together,  and  the  conduction  of  sap  by  them  to  the  base  is 
rendered  impossible  by  the  strangling  cord.  The  ascending  current  of  water 
and  dissolved  food-salts,  in  the  deeper-lying  firm  wood,  flows  on  unimpeded  in 
either  case.  The  green  foliage -leaves  can  thus  decompose  carbonic  acid  and 
manufacture  organic  substances  above  the  circular  cut  or  ligature;  these  pro- 
ducts are  then  conducted  away;  the  albuminous  substances  enter  the  soft 
bast,  but  only  travel  as  far  as  the  place  where  the  cut  has  been  made  or  the 
ligature  been  tied.  They  can  no  longer  pass  these  places,  and  consequently 
the  plastic  albuminous  materials  become  dammed  up  above  the  "  ring  "  or  ligature, 
and  a  particularly  luxuriant  growth  of  all  these  parts  results.  Fruits  which 
develop  from  the  blocked-up  region  of  the  branch  sometimes  enlarge  to  an 
extraordinary  degree,  and  become  almost  twice  as  heavy  as  they  would  have  done 
had  the  operation  not  been  performed. 

The  following  phenomenon  is  also  explained  by  the  fact  that  the  passage  of 
plastic  albumins  takes  place  in  the  soft  bast.  If  a  branch  of  a  willow,  e.g.  of 
Salix  purpurea,  be  cut  off  and  the  entire  cortex  down  to  the  wood  be  removed 
from  the  lower  third  of  the  branch,  and  the  branch  so  treated  be  then  plunged 
half-way  into  a  vessel  of  water,  after  a  time  it  will  send  out  roots  into  the 
water.  But  these  differ  from  one  another  very  much  according  as  to  whether 
they  arise  from  the  lower  stripped  portion  of  the  branch  or  from  higher  up 
where  the  cortex  has  not  been  removed.  The  roots  developed  from  the  stripped 
portion  are  few  and  remain  very  short;  those  springing  from  the  upper  thickening 
portion  of  the  willow  branch,  where  the  cortex  is  intact,  are,  on  the  contrary, 
abundant  and  elongated,  since  they  can  utilize  the  plastic  juices  above  the  place 
where  the  cortex,  together  with  the  soft  bast,  has  been  removed. 


MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO.  481 

Both  of  the  experiments  described  only  exhibit  the  results  mentioned  whei 
performed  on  plants,  the  whole  of  whose  soft  bast  bundles  lie  outside  the  cambium 
ring  since  interruption  of  the  sap-current  by  ringing  only  takes  place  under  these 
conditions.  If  plants  are  experimented  on  which  possess  internal  bundles  of  soft 
bast  m  addition  to  those  lying  near  the  surface,  as  in  Tecoma,  Thunbergia,  and 
many  others,  the  ringing  does  not  have  the  result  described,  because  the  inner 
bundles  of  soft  bast  (being  protected  by  the  hard  wood)  are  not  cut  through  by  the 
knife,  and  cannot  be  compressed  by  the  ligature.  It  should,  however,  be  observed 
that  even  in  woody  plants,  whose  soft  bast  lies  entirely  outside  the  cambium  ring, 
this  result  is  restricted  to  the  year  in  which  the  ringing  or  ligaturing  was 
performed.  In  consequence  of  the  absence  of  supplies  of  albuminous  materials 
through  the  soft  bast,  the  portion  of  the  branch  below  the  cutting  or  ligature 
becomes  unhealthy,  its  cortex  dries  up  and  dies,  and  the  disparity  between  the  two 
portions  lying  above  and  below  the  ringed  cut  or  the  tight  ligature  usually 
occasions  the  death  of  the  whole  branch  tampered  with  in  the  following  year. 

In  the  tubular  conducting  mechanisms,  especially  in  the  laticiferous  tubes,  which 
are  entirely  free  from  transverse  walls,  and  also  in  sieve-tubes,  in  which  perforated 
horizontal  walls  are  inserted  here  and  there,  a  transport  of  substances  en  masse 
may  occur,  but  this  is  impossible  in  those  conducting  apparatuses  consisting  of  rows 
of  cells  whose  length  is  usually  only  three  or  four  times  their  widtL  In  these 
tracts  of  cells  the  numerous  non-perforated  partition-walls  of  the  adjoining  cell- 
chambers  are  interposed,  and  must  be  passed  through  by  the  travelling  materials. 
Whether  this  passage  through  the  walls  be  regarded  as  a  diosmosis  or  a  filtration, 
it  is  at  least  certain  that  solid  bodies  of  definite  form  cannot  traverse  the  walls. 
Even  starch-grains  of  the  smallest  diameter  are  always  much  larger  than  the 
interstices  which  we  imagine  to  exist  in  every  cell-wall  between  the  groups  of 
molecules;  and  therefore  it  follows  that  even  the  tiniest  visible  bodies  must  always 
remain  behind,  as  on  a  filter,  in  one  of  the  two  adjoining  cell-chambers,  that  is  to 
say,  on  one  side  or  the  other  of  the  dividing  partition- wall.  Accordingly,  only  fluid 
materials  travel  through  such  cell-tracts  as  serve  for  the  conduction  of  substances 
in  the  soft  bast,  parenchyma,  and  in  the  bundle  sheath.  If  solid  substances  take 
these  routes,  they  must  be  first  brought  into  a  soluble  condition.  This  applies 
especially  to  the  starch-grains  which  play  such  an  important  part  in  the  life  of 
plants,  and  not  only  share  in  the  formation  of  cellulose,  chlorophyll-corpuscles,  and 
fats,  but  are  also  heaped  up  in  the  storehouses  of  the  plants  as  materials  well  suited 
for  storage  during  the  summer  drought  or  through  the  winter,  for  use  in  the  next 
period  of  vegetation.  They  are  also  given  to  the  seeds  which  have  to  lead  an 
independent  existence,  as  the  first  food  for  the  journey  after  leaving  the  parent  plant. 
If  starch-granules  are  to  travel  through  the  cells  of  the  bundle  sheath,  composed  of 
hundreds  of  single  cells,  they  must  be  dissolved  a  hundred  times,  and  a  hundred 
times  reformed.  It  has  been  definitely  proved  that  this  transitory  starch  is  not 
liquefied  at  the  beginning  of  its  journey  and  again  formed  into  solid  only  when  it 
has  reached  its  destination,  but  that,  as  stated,  a  liquefaction,  and  after  it  has 


VOL.  I. 


482  MECHANISMS   FOR   CONVEYANCE   TO   AND   FRO. 

passed  through  the  dividing  wall,  a  solidification,  occurs  in  each  of  the  succeeding 
members  of  a  string  of  cells.  This  is  a  very  laborious  and  wearisome  process,  and 
the  question  involuntarily  arises,  after  observing  these  methods  of  transmission, 
why  these  numerous  partition  walls  in  the  rows  of  cells  are  not  done  away  with. 
The  wood  vessels  have  been  produced  from  rows  of  cells  by  the  solution  of  the 
dividing  partition  walls;  why  are  the  many  transverse  walls  retained  here  to 
complicate  and  retard  the  transportation  of  the  substances?  It  must  be  supposed 
that  these  cross  walls,  which  break  up  the  free  channel,  are  in  some  way  beneficial 
to  the  plant,  since  they  occur  so  generally  and  with  such  regularity.  It  might  be 
thought,  first  of  all,  that  these  walls  keep  open  the  road,  and  that  thereby  the 
delicate  walls  of  the  cells  forming  the  channel  are  protected  from  collapse.  Apart 
from  the  fact  that  the  cells  of  bast  parenchyma,  imbedded  in  niches  and  grooves  in 
the  periphery  of  the  hard  wood,  are  prevented  from  collapsing  by  their  sheltered 
position  and  nevertheless  exhibit  transverse  walls,  while  the  delicate-walled 
laticiferous  tubes,  which  are  anything  but  well-protected,  possess  none  and  yet  do 
not  collapse — apart  from  this,  such  a  delicate  wall  would  form  but  a  bad  stiffening 
agent,  and  the  support  would  be  obtained  much  better  by  band-like  circular 
thickenings.  It  has  also  been  surmised  that  the  cross  walls  inserted  in  the  channels 
might  be  of  use  in  that  they  prevent  an  excessive  accumulation  of  solid  bodies  at 
certain  places  on  the  road.  Where  the  cells  of  a  cell-row  stand  vertically  above  one 
another,  as,  for  example,  in  erect  stems,  it  is  found  that  the  small  starch-granules 
sink  to  the  bottom  of  the  cells  and  lie  on  the  lower  transverse  wall.  If  all  the 
solid  corpuscles  contained  in  the  sap  of  a  long  vertical  tube  were  to  sink  to  the 
bottom,  of  course  an  obstruction  might  arise  which  would  be  anything  but 
beneficial.  But  the  significance  of  the  partition  walls  most  probably  lies  in  the 
transformations  they  produce  in  the  substances.  It  may  be  safely  assumed  that 
those  materials  which  must  pass  through  not  merely  the  cellulose  transverse  wall, 
but  also  the  protoplasmic  parietal  layer  of  the  cell-chamber,  undergo  an  alteration 
thus  under  the  influence  of  the  living  protoplasm;  that  the  position  of  the  atoms 
becomes  different,  or  that  new  atoms  enter  into  combination  and  others  are 
displaced,  in  short,  that  re-arrangements  and  transformations  occur  from  which  it 
results  that  the  transmitted  materials  arrive  at  their  destinations  prepared  in  the 
best  possible  way.  With  this,  however,  we  return  to  the  important  theorem 
previously  stated,  that  these  rows  of  cells  have  not  merely  the  significance  of  a  road 
along  which  the  materials,  formed  at  the  starting-points,  are  conducted  unchanged 
to  the  terminal  stations;  but  that  they  also  form  places  for  the  continuous  trans- 
formation and  alteration  of  these  materials  for  subsequent  use. 


SIGNIFICANCE   OF   ANTHOCYANIN.  483 

SIGNIFICANCE  OF  ANTHOCYANIN  IN  THE  TRANSPORTATIONS  AND  TRANS- 
FORMATIONS OF  MATERIALS.    AUTUMNAL  COLOURING  OF  FOLIAGE. 

In  connection  with  the  foregoing  results  of  investigations  into  the  transmission 
of  substances,  the  fact  must  be  noted  that  those  agents  which  take  part  in  the 
transformations  of  carbohydrates  and  albuminous  substances  are  to  be  found  all 
along  the  road  which  these  follow  and  not  merely  at  the  beginning  and  end  of  the 
journey.  Diastase,  for  example,  is  to  be  found  everywhere  along  the  strands  of 
cells  forming  the  path  of  the  transitory  starch,  and  when  these  strands  run  near 
the  surface  that  colouring-matter  called  anthocyanin,  a  somewhat  detailed  descrip- 
tion of  which  must  be  given,  is  also  present. 

In  many  instances  the  route  of  the  travelling  substances  can  be  recognized  by 
the  naked  eye,  since  it  is  coloured  blue,  violet,  or  red.  Whether  all  these  tints 
actually  originate  from  one  colouring-matter,  which  is  red,  violet,  or  blue  according 
to  the  presence  or  absence  of  acids,  has  not  been  ascertained.  The  chemical 
composition  of  colouring-matters  is  yet  little  known,  and  it  is  possible  that  at 
present  a  whole  group  of  them  is  collected  together  under  the  name  anthocyanin. 
It  is  a  matter  of  indifference  with  regard  to  the  question  hi  hand,  as  also  is  the 
question  as  to  the  way  in  which  anthocyanin  originates  in  plants.  It  need  only  be 
mentioned  here  that  the  statement  according  to  which  anthocyanin  arises  from  the 
chlorophyll-corpuscles  present  in  young  plant  organs  cannot  be  correct  in  all  cases; 
since  this  pigment  occurs  regularly  in  parasites  entirely  devoid  of  chlorophyll,  in 
the  Balanophorese,  Rafflesiacese,  and  Hydnorese,  in  the  Toothwort,  in  Monotropa, 
and  numerous  other  plants  destitute  of  green  colour.  In  green-leaved  plants 
anthocyanin  is  most  usually  met  with  in  places  which  have  little  chlorophyll,  or 
which  have  never  possessed  any,  in  flowers  and  fruits,  along  the  ribs  of  leaves,  and 
principally  in  leaf -stalks  and  herbaceous  stems.  In  hundreds  of  species  belonging 
to  widely-differing  families  the  leaf -veins  and  ribs,  leaf -stalks  and  leaf -sheaths  are 
coloured  violet,  red,  or  blue,  and  this  colouring  is  co-extensive  with  the  vascular 
bundles  beneath  them. 

It  is  difficult  to  say  whether  anthocyanin  exercises  a  photochemical  effect  on  the 
travelling  substances  in  the  given  paths,  or  whether  it  is  only  of  use  in  that  it 
keeps  back  the  light  rays  which  would  decompose  the  travelling  materials.  In 
support  of  the  latter  view  we  have  the  fact  that  anthocyanin  is  much  more 
abundantly  deposited  in  paths  exposed  to  the  light  than  in  those  which  are  shaded, 
and  that  in  shaded  organs  the  same  changes  and  transmissions  of  materials  occur 
as  in  those  exposed  to  bright  light,  where  the  superficial  cells  are  coloured  with 
anthocyanin,  and  where  consequently  the  paths  of  the  transmitted  substances  below 
are  to  some  extent  screened.  It  is  noticeable  that  plant  organs  which  are  very 
thickly  covered  with  hairs  scarcely  ever  develop  anthocyanin.  From  all  this  it  may 
be  concluded  that  anthocyanin,  when  it  appears  in  places  directly  illumined  by  ligh 
rays,  serves  principally  as  a  screen,  i.e.  as  a  protective  agent  or  awning  agai 
injurious  light  rays. 


484  SIGNIFICANCE   OF   ANTHOCYANIN. 

Here  another  very  remarkable  phenomenon  may  be  considered.  If  the 
colourless  and  scaly  rhizome  of  Dentaria  bulbifera  be  dug  out  of  the  dark  forest 
soil,  it  appears  beautifully  white,  as  if  carved  out  of  ivory.  If  it  is  put  in  a  glass 
vessel  which  is  filled  up  with  water  and  placed  in  the  sun,  so  that  the  rhizome  is 
illumined  by  the  direct  rays,  the  white  scales  in  a  very  short  time  assume  a  slight 
violet  tint,  and  in  a  few  days  the  whole  of  the  scaly  rhizome  becomes  coloured 
a  deep  violet.  The  same  thing  happens  with  the  rhizomes  of  several  species  of 
Cuckoo-flower,  Violet,  Toothwort,  &c.,  but  in  these  it  is  a  little  longer  before  the 
violet  colour  appears.  The  tissues  brought  from  the  darkness  into  the  bright  light 
try  to  neutralize  the  influence  of  the  light  which  is  injurious  to  certain  substances, 
and  therefore  anthocyanin  must  not  be  regarded  merely  as  an  agent  for  protecting 
chlorophyll  alone,  but  other  chemical  compounds  also.  That  a  far  wider 
significance  in  the  life  of  plants  is  also  assigned  to  it  will  be  demonstrated  in  the 
following  section. 

Very  often  anthocyanin  only  appears  temporarily,  when  the  transmission  of 
food  substances  occurs  on  a  very  large  scale.  When  seeds  are  germinated,  and 
their  reserve  materials  are  conducted  into  the  rapidly  sprouting  seedlings,  such 
as  those  produced  from  the  starchy  seeds  of  polygonums,  oraches,  palms,  grasses, 
&c.,  anthocyanin  regularly  appears,  while  later  on  it  partly  or  wholly  vanishes. 
When  in  spring  the  foliage-buds  on  subterranean  root-stocks  or  branches  begin 
to  develop,  and  the  materials  stored  in  the  stem  structures  travel  into  the  young 
leaves,  to  be  employed  there  in  further  construction,  these  leaves  do  not  appear 
green  in  most  cases,  but  reddish-violet  or  reddish-brown  in  colour.  As  instances 
of  this  may  be  mentioned  the  well-known  Tree  of  Heaven  (Ailanthus  glandulosa), 
Walnut  (Juglans  regia),  Pistacia  (Pistacia  Terebinthus),  the  Sumachs  (Rhus 
Cotinus  and  Rhus  Typhinum),  the  Judas  Tree  (Cercis  Siliquastrum),  Berberi- 
deae  (Mahonia,  Podophyllum,  Epimedium),  Ampelideae  ( Vitis,  Cissus,  Ampelopsis), 
the  Trumpet  Tree  (Catalpa  syringcefolia),  the  red-berried  Elder  (Sambucus 
racemosa),  Cherry  (Prunus  avium),  Peony  and  Sea  Lavender  (Pceonia  and 
Statice),  and  Rhubarb  and  Dock  (Rheum  and  Rumex).  Later  on,  when  the 
transmission  is  effected,  when  the  foliage-leaves  are  developed  and  are  able  to 
act  independently,  the  green  chlorophyll  appears;  the  leaves  become  green, 
and  the  anthocyanin  either  vanishes  entirely  or  remains  only  in  those  places 
where  it  is  required  as  a  protection  to  the  chlorophyll,  or  for  another  important 
purpose  to  be  dealt  with  in  the  following  section,  viz.  the  transformation  of  light 
into  heat. 

In  many  plants,  anthocyanin  is  again  developed  in  great  abundance  when 
the  leaves  are  obliged  to  stop  their  activity  for  a  time  on  account  of  the  com- 
mencing dryness  of  the  soil,  or  still  more,  on  account  of  cold  and  the  consequent 
delay  of  supplies  of  crude  sap.  In  order  to  describe  this  formation  of  anthocyanin 
and  everything  connected  with  it,  it  is  necessary  to  go  back  a  little,  and  to  discuss, 
first  of  all,  the  metabolism  and  transport  of  materials  connected  with  the  stoppage 
of  activity  in  the  green  leaves  at  the  close  of  the  vegetative  period.  These 


AUTUMNAL  COLOURING.  435 

differ  essentially  according  as  the  leaves  of  the  plant  continue  active  through 
lone,  or  through  several  vegetative  periods,  i.e.  according  as  the  leaves  are  deciduous 
I  or  lasting  but  one  year,  or  evergreen,  that  is  to  say,  lasting  for  two  or  more 
years.  Evergreen  leaves  are  so  organized  in  all  those  regions  whose  climate 
necessitates  a  temporary  suspension  of  vital  activity,  that  they  may  be  able  to 
! survive  the  periods  of  drought  or  frost  of  one  or  even  of  several  years  without 
injury.  Before  they  enter  upon  their  summer  sleep  in  regions  of  summer 
drought,  or  their  winter  trance  in  regions  with  cold  winters,  alterations  occur 
in  their  cells,  which,  in  the  main,  terminate  in  the  diminution  of  the  water 
contents  and  the  formation  of  substances  which  will  not  be  altered  by  the  pre- 
vailing frost  or  dryness.  In  regions  where  we  have  a  winter  sleep,  the  chlorophyll- 
granules  assume  a  yellowish-brown  or  brownish-red  colour,  and  adhere  together 
in  clumps,  which  withdraw  as  far  as  possible  from  the  surface  of  the  leaf, 
travelling  down  to  the  floor  of  the  palisade-cells  and  occupying  their  lower  ends. 
These  alterations  are  only  slightly  apparent  outwardly  in  perennial  leaves  pre- 
paring for  their  winter  period  of  rest;  the  only  thing  one  notices  is  that  the 
leaves,  which  in  summer  are  a  vivid  green,  exhibit  a  darker  green,  or  incline 
to  brown  or  yellow;  which  change  of  colour  is  observed  to  the  greatest  extent 
in  Thuja,  Cryptomeria,  Sequoia,  Chamcecyparis,  Libocedrus,  and  generally  in  most 
evergreen  conifers. 

The  changes  which  are  accomplished  in  leaves  lasting  only  one  year,  at  the 
onset  of  the  summer  drought  or  winter  cold,  are  much  deeper  rooted  and  obvious. 
These  leaves  are  not  clad  so  as  to  be  able  to  defy  the  drought  or  frost,  and 
are  therefore  thrown  off  at  the  commencement  of  the  unfavourable  period. 
If  these  leaves  were  to  fall  without  further  ceremony,  all  the  substances  in  the 
tissues  of  the  leaves,  whose  production  entailed  a  considerable  amount  of  work, 
would  be  entirely  lost.  But  it  is  part  of  the  economy  of  plants  that  such  a 
waste  is  carefully  guarded  against.  Before  the  leaves  are  detached,  the  carbo- 
hydrates and  albuminous  materials,  in  general  everything  which  is  of  use  to  the 
plant,  is  conveyed  from  the  leaf -blades  into  the  woody  branches  or  subterranean 
root-stocks,  and  there  deposited  in  places  where  they  find  a  safe  resting-place, 
and  can  survive  the  drought  of  summer  or  cold  of  winter  unharmed.  In  this 
way  the  plant  suffers  only  the  slightest  loss  in  the  materials  manufactured  by 
it  in  the  preceding  vegetative  period;  for  the  leaves  from  which  everything 
useful  has  been  transported  into  the  stem-structures  now  form  nothing  more 
than  a  dead  framework,  and  their  cell-chambers  contain  only  small  yellow 
granules,  together  with  crystals  of  calcium  oxalate,  which  cannot  be  employed 
further,  and  are  of  no  more  use  (see  fig.  123  >).  The  shining  yellow  granules, 
which  are  found  in  the  cells  of  fallen  leaves,  and  to  which  is  due  the  yellow 
colouring  of  autumn  foliage,  are  to  be  regarded  as  the  ultimate  useless  resi 
after  the  withdrawal  of  the  transformed  chlorophyll-corpuscles, 
of  calcium  oxalate  have  arisen  in  the  foimation  of  albumens  by  the  decompo. 
of  nitric  and  sulphuric  acids.  Both  of  them  can  be  sacrificed.  As  a  matter  < 


486  AUTUMNAL   COLOURING. 

fact,  the  rejection  of  these  structures  is  no  sacrifice  in  reality,  since  they  are 
only  superfluous  ballast  by  which,  under  certain  conditions,  the  plants  may  be 
hampered  in  their  next  year's  work,  and  of  which  they  therefore  rid  themselves 
most  seasonably  and  suitably.  The  fall  of  the  leaf  may  be  looked  upon,  so  far, 
as  an  excretion  of  superfluous  matter,  which,  in  deciduous  plants,  occurs  only 
once  every  year,  but  is  then  carried  out  on  a  grand  scale.  To  the  benefits  which 
this  wholesome  excretion  of  waste,  formed  in  the  metabolism,  affords  to  individual 
plants  must  be  added  the  fact  that  the  fallen  leaf  reaches  the  ground  with  its 
abundance  of  lime,  decays  there,  contributes  to  the  formation  of  humus,  which 
contains  calcium  nitrate,  and  so  becomes  of  use  to  the  vegetable  kingdom  as  a 
whole,  as  already  described  in  detail. 

The  emigration  of  the  useful  materials  from  the  leaf-blades  to  the  store-rooms 
in  the  interior  of  the  branches  and  root-stocks  must,  as  a  rule,  be  accomplished 
fairly  quickly;  most  rapidly,  of  course,  where  the  period  of  vegetation  during 
which  the  foliage-leaves  can  be  active  is  short,  when  the  leaves  are  obliged  to 
make  use  of  the  "favourable  time  to  the  utmost,  and  where  the  change  of  seasons 
occurs  abruptly.  The  materials  withdrawn  travel  by  the  same  route  as  in 
general  is  taken  by  the  substances  normally  manufactured  in  the  leaves.  The 
accessories  by  which  the  carbohydrates  and  albumens  to  be  removed  are  prepared 
for  emigration,  might  (one  would  think)  be  the  same  in  every  case.  But,  just  as 
in  one  species  one  kind,  and  in  another  a  different  kind  are  developed  when  the 
leaves  are  most  active,  so  in  different  species  at  the  close  of  the  vegetative  period, 
when  the  great  emigration  takes  place,  we  have  again  various  accessories,  and 
various  despatching  and  protective  agents.  In  many  instances  the  accessories 
are  colourless,  and  are  not  recognizable  by  the  naked  eye  even  when  developed 
in  great  quantity.  It  can  only  be  seen  that  the  leaves  lose  their  fresh  green 
on  account  of  the  change  experienced  by  the  chlorophyll  bodies  for  the  purpose 
of  emigration,  and  that  a  yellowish  tint  appears  instead  of  the  green  colour,  which 
is  produced  by  the  already-mentioned  yellow  granules  remaining  behind  after 
the  departure  of  the  chlorophyll-corpuscles.  In  many  leaves  the  number  of 
these  yellow  granules  is  so  small  that  even  the  yellow  tint  is  hardly  apparent, 
and  these  leaves  then  are  a  dirty  yellowish-white,  shrivel  up  very  quickly,  and 
become  brown. 

Anthocyanin,  however,  is  produced  in  many  plants  during  the  emigration  of 
the  carbohydrates  and  albuminous  materials,  and  to  such  an  extent,  that  it  becomes 
plainly  visible  on  the  exterior.  It  appears  red  in  the  cell-sap  in  the  presence  of 
acids  which  occur  very  regularly  as  metabolic  accessories  in  the  autumn  leaves, 
blue  when  no  acids  are  present,  and  violet  when  the  amount  of  free  acids  is 
but  small.  If  there  is  an  abundance  of  yellow  granules  together  with  the  acid, 
red  anthocyanin,  the  leaf  assumes  an  orange  colour.  Thus  the  green  colour  of  the 
foliage  changes  at  the  period  of  the  great  autumal  emigration,  sometimes  into 
yellow,  or  brown,  or  red,  violet  or  orange,  and  in  this  way  we  have  a  play  of 
colour  exhibiting  the  greater  variety  the  more  numerous  are  the  plant  species 


AUTUMNAL   COLOURING.  487 


growing  associated  together  in  the  particular  spot.  If  the  leaves  are  thickly 
covered  with  silky  or  woolly  hairs,  or  if  the  hairs  are  felted  or  peltate,  anthocyanin 
is  scarcely  ever  developed;  but  when  the  green  tissue  of  these  leaves  becomes 
also  changed  in  colour,  the  new  tint  can  be  seen  as  little  as  was  the  green  previ- 
ously, on  account  of  the  hairy  coat  over  the  coloured  cells.  Accordingly,  such 
felted,  silky,  or  scale-covered  leaves  remain  grey  or  white  even  when  they  fall 
from  the  branches.  If  plants  of  this  kind  grow  among  others  whose  foliage  is 
bare,  their  grey  and  white  tints  considerably  increase  the  variety  of  the  entire 
collection.  But  the  greatest  amount  of  colour  is  seen  when  the  neighbourhood 
is  sprinkled  with  plants  having  evergreen  foliage;  it  may  then  happen  that  a 
relatively  small  space  of  meadow  or  wood  appears  decked  in  all  the  colours  of 
the  rainbow  in  the  most  manifold  variety. 

The  splendour  of  colours  exhibited  by  tropical  forests,  which  is  usually  repre- 
sented as   much  more  magnificent  than  it  really  is,  stands   no  comparison  with 
that  developed  in  autumn  in  the  north  temperate  zone.      The  forests  of  firs  and 
leafy  trees  on  the  mountain  slopes  along  the  Rhine  and  Danube  in  Europe,  and 
on  the  shores  of  the  Canadian  lakes  in  North  America  at  that  season  present  a 
scene  of  entrancing  beauty.     The  heights  along  the  middle  course  of  the  Danube, 
for  example,  the  region  known  as  the  Wachan,  below  the  town  of  Melk,  shows 
wide  expanses  of  forests,  in  which  beeches,  hornbeams,  evergreen  oaks,  common 
and  Norway  maples,  birches,  wild  cherries  and  pears,  mountain  ashes  and  wi] 
service-trees,  aspens,  limes,  spruces,  pines  and  firs  take  a  share  in  the  greatest 
variety.     Bushes  of  Barberry  (Berberis  vulgaris),  Dogwood  (Corny*  sanguin 
Cornel    (Cornus  mas),   Spindle    Tree    (Euonymus    Ewopceus    and    verrutosi 
Dwarf  Cherry  (Prunua  Chamcecerasus),  Sloe  (Prunus  spinosa),  Junior  (Ju 
perus  communis),  and  many  other  low  shrubs  arise  as  undergrowth,  and  spring 
up  on  the  margins  of  the  forests.     The  mountain  slopes  abutting  on  the  val 
are   planted   with  vines,  and   near  by  grow  peach  and   apricot  trees  in  gn 
abundance.      In  the  meadows  on  the  shore,  and  on  the  islands  of  the 
rise   huge   abeles   and  black  poplars,  elms,  willows,  alders,  and  ako  an  a 
dant  sprinkling  of   trees  of   the  bird  cherry  (Prunus  Padus).      The  mghts  are 
bitterly"  cold  Lre;    even  in  the  middle  of  October,  Jamp  =ts   hover   ove 
the  river,  and  hoar-frost  covers  the  grassy  regions  at  the  bottom  of      e  valley 
But  during  the  day  it  is  still  fairly  warm,  the  mormng  mists 
the  rays  o?  the  sun,  a  cloudless  sky  stretches  over  the  landscape,  and  ££™£ 

unfolded!     Th,  or.™,  «I  to  &*****  —  rf  btata 


-* 


488  AUTUMNAL   COLOURING. 

beeches  in  all  gradations  from  yellowish  to  brownish -red,  the  mountain  ashes, 
cherries  and  barberry  bushes  scarlet,  the  bird  cherry  and  wild  service  trees 
purple,  the  cornel  and  spindle-tree  violet,  aspens  orange,  abeles  and  silver  willows 
white  and  grey,  and  alders  a  dull  brownish-green.  And  all  these  colours  are 
distributed  in  the  most  varied  and  charming  manner.  Here  are  dark  patches 
traversed  by  broad  light  bands  and  narrow-twisted  stripes;  there  the  forest  is 
symmetrically  patterned;  there  again  the  Chinese  fire  of  an  isolated  cherry-tree 
or  tke  summit  of  a  single  birch,  with  its  lustrous  gold  springing  up  among  the 
pines,  illuminates  the  green  background.  To  be  sure  this  splendour  of  colour  lasts 
but  a  short  time.  At  the  end  of  October  the  first  frosts  set  in,  and  when  the 
north  wind  rages  over  the  mountain  tops,  all  the  red,  violet,  yellow,  and  brown 
foliage  is  shaken  from  the  branches,  tossed  in  a  gay  whirl  to  the  ground,  and 
drifted  together  along  the  banks  and  hedges.  After  a  few  days  the  mantle  of 
foliage  on  the  ground  takes  on  a  uniform  brown  tint,  and  in  a  few  more  days  is 
buried  under  the  winter  coat  of  snow. 

The  autumnal  colouring  of  the  foliage  in  those  parts  of  the  North  American 
forest  regions,  whose  vegetation  presents  the  greatest  analogy  to  that  of  the  Old 
World  just  described  (i.e.  in  the  neighbourhood  of  the  St.  Lawrence  and  from  the 
Canadian  lakes  to  the  Alleghany  Mountains),  lasts  much  longer  than  in  the  forest 
regions  of  Central  Europe.  There  also  evergreen  conifers  grow  side  by  side  with 
deciduous  trees,  and  there  again  a  rich  underwood  flourishes  in  the  forest  regions. 
To  some  extent  we  have  exactly  the  same  species  composing  the  woods — pines 
and  firs,  beeches  and  hornbeams,  oaks,  ashes,  limes,  birches,  alders,  poplars,  maples, 
elms,  hawthorn,  guelder-rose,  and  dogwood;  but  the  wealth  of  forms  is  far  greater 
than  in  Central  Europe.  In  the  neighbourhood  of  the  shores  of  Lake  Erie,  for 
instance,  a  district  pre-eminent  in  respect  of  the  glow  of  its  autumnal  tints, 
we  have  in  addition  to  the  trees  enumerated  the  Rhus  Toxicodendron  and 
R.  Typhinum,  the  Tulip-tree,  Western  Plane,  several  walnuts,  robinias,  Gymno- 
cladus,  Liquidamber,  and  especially  some  Ampelideae  which  climb  like  lianes  to  the 
highest  tree-top.  This  greater  variety  of  species  produces  an  even  richer  play  of 
colour  in  autumn  than  in  the  central  European  districts.  The  change  of  colour  of 
the  deciduous  trees  begins  in  some  species  always  at  the  commencement  of 
September,  and  stretches  over  a  whole  month,  so  that  -the  fall  of  the  last  leaves 
usually  does  not  occur  until  about  the  middle  of  October.  The  American  beech 
\Fagus  ferruginea)  changes  colour  exactly  like  the  European;  and  the  American 
birches  (Betula  nigra  and  B.  papyracea)  exhibit  in  their  autumn  foliage  the  same 
golden  yellow  as  do  their  European  allies;  but  the  autumn  foliage  of  oaks,  which 
flourish  with  an  extraordinary  number  of  species  south  of  the  Canadian  lakes, 
present  every  tint  from  yellow  to  orange  and  ruddy  brown;  the  Red  Maple  (Acer 
rubrum)  shrouds  itself  in  dark  red,  the  Tulip-tree  exhibits  the  lightest  yellow, 
the  large-spined  hawthorn  bushes,  the  Sheep-berry  (Viburnum  Lentago)  and 
the  Rhus  Toxicodendron  become  violet,  the  Sumach  (Rhus  Typhinum),  and  the 
wild  vines  (Vitis  and  Ampelopsis),  climbing  up  the  branches  of  the  trees,  clothe 


AUTUMNAL   COLOURING.  489 

them  in  flaming  scarlet.  With  this  gay  assemblage  of  vivid  colours  the  Canadian 
firs  mingle  their  deep,  dark  green,  and  the  Weymouth  Pines  the  dull  bluish-green  of 
their  needle-leaved  summits.  Where  such  a  wood  is  developed  with  all  its  wealth 
of  species,  and  where  there  is  an  opportunity  of  seeing  it  pass  slowly  under  view  in 
the  soft  light  of  a  September  day,  as,  for  example,  in  a  journey  along  the  southern 
shore  of  the  Canadian  lakes,  the  eye  revels  in  the  changing  pictures  of  scenery  and 
in  a  wealth  of  colour  such  as  it  meets  with  in  no  other  forest  country. 

Of  course  the  autumnal  colouring  is  not  limited  to  the  deciduous  foliage  of  the 
trees  and  shrubs  enumerated,  but  includes  the  perennial  low  shrubs  and  herbs.     In 
forest  regions,  however,  only  the  large  forms  of  the  greater  trees  stand  out,  and  the 
low  bush  only  rarely  forms  a  characteristic  feature  in  the  autumn  landscape.     But 
where  lofty  trees  are  absent,  and  where  the  clumps  of  low  plants  are  the  charac- 
teristic feature,  as  in  the  regions  of  the  Arctic  flora,  and  especially  in  the  mountain 
slopes  above  the  tree  limit,  the  matter  is  quite  different.     Of  these  latter  regions, 
however,  there  is  scarcely  one  which  can  rival  the  Alps  of  Central  Europe  in  respect 
of  the  autumnal  change  of  colour  of  the  vegetation.     It  is  especially  in  those  parts 
of  the  Central  Alps  characterized  by  the  great  variety  of  their  flora  and  their 
wealth  of  Ericaceae,  where  strata  of  slate  and  limestone  alternate  or  lie  side  by  side, 
that  the  spectacle  here  described  passes  with  a  splendour  of  which  the  ordinary 
summer  visitor  to  the  Alps  can  form  no  conception.     The  time  of  commencement  of 
the  display  cannot  be  definitely  given;  it  varies  from  year  to  year  according  to  the 
prevailing  conditions  of  temperature  and  moisture.     If  even  at  the  end  of  August 
fresh-fallen  snow  remains  for  several  days  on  the  slopes  above  the  tree  limit,  the 
colouring  may  make  its  appearance  as  early  as  this;  but  if,  as  is  usually  the  case, 
the   heights   do   not   assume   their   white   mantle   of    snow   until   the   middle  of 
September,  after  a  storm,  and  if  during  the  latter  half  of  the  month  the  fresh  snow 
melts  and  a  clear  sky  prevails  over  the  mountain  heights,  then  the  autumnal  change 
of  colour  is  retarded  so  much  longer.     Below,  in  the  depths  of  the  valley,  which  lie 
for  wide  expanses  already  in  the  shade  on  account  of  the  low  position  of  the  sun, 
the  ground  remains  continuously  whitened  by  the  frost,  while  up  above,  on  the 
southern  slopes  of  the  mountain  heights,  the  night's  frost  vanishes  with  the  first 
glimpses   of   the   sun,   and   soft   breezes   blow    over    them   throughout  the   day. 
Ptarmigans  and  swarms  of  birds  of  passage  journeying  over  the  Alpine  passes,  but 
stopping  here  for  a  short  rest,  are  busy  in  picking  off  the  berries  from  the  low 
bushes  which  cover  the  slopes  in  great  abundance;  but  the  butterflies  which  were 
so  active  in  the  summer  among  the  Alpine  flowers  have  vanished;  here  and  there 
isolated  scabiouses  and  the  dark  spikes  of  the  late-blooming  Gnaphalium  still 
linger,  but  everything  else  is  in  fruit,  and  the  procession  of  the  flowers  is  past. 
And   yet   the   slopes   have  all   the   brightness   of   summer   meadows,  which  are 
adorned  with  innumerable  flowers.     The  deciduous  foliage  of  the  low  shrubs  and 
herbs,  and  especially  that  of  the  stunted  thick-carpeting  bushes  (whose  materials 
are  conveyed  into  the  woody  branches  and  underground  stem-structures)  attains 
even  in  this  short  time  red,  violet,  and  yellow  tints,  which  are  in  no  wise  inferior 


490  AUTUMNAL   COLOURING. 

in  glow  and  brilliancy  to  the  most  vivid  colours  of  flowers.  The  deciduous 
whortleberries  and  a  species  of  bearberry  are  most  conspicuous.  While  the 
leaves  of  the  Bilberry  (Vaccinium  uliginosum)  assume  a  violet  colour,  the  red 
Whortleberries  (F.  vitis  idcea)  clothe  themselves  in  deep  red,  and  the  Bearberry 
(Arctostaphylos  alpina)  in  vivid  scarlet.  The  autumnal  leaves  of  these  plants 
exhibit  the  most  beautiful  red  observed  in  any  autumnal  foliage;  it  is  much 
more  fiery  than  in  the  North  American  vines  and  the  sumach  trees;  and  when  the 
foliage  of  this  Bearberry  is  illuminated  by  oblique  sunbeams  on  a  mountain  slope, 
the  observer  below  might  fancy  he  saw  flames  of  strontium  forking  up  out  of  the 
ground.  The  leaves  of  many  herbaceous  plants  also,  such  as  Alpine  geraniums  and 
Alpine  hawkweeds,  become  coloured  with  anthocyanin  at  the  margins,  and  along 
the  veins,  or  even  over  the  whole  surface,  before  withering;  and  seen  from  a 
distance,  look  like  red,  violet,  and  variegated  flowers.  The  Alpine  willows,  how- 
ever, especially  the  carpeting  Salix  retusa,  and  the  low  bush  of  Salix  hastata  and 
S.  arbuscula,  together  with  the  red-fruited  Sorbus  Chamce-mespilus,  take  a  golden 
yellow.  The  latter  chiefly  border  the  water-courses,  and  on  looking  down  from 
above  on  the  gullies  and  ravines  through  which  the  water  pursues  its  tortuous 
way,  interrupted  by  small  cascades,  these  bushes  are  recognized  as  a  twisted,  golden 
fretwork,  thrown  up  against  the  darker  background.  Between  the  low  under- 
growth of  whortleberries  and  bilberries,  but  principally  between  the  low-lying 
sprays  of  Alpine  bearberries,  spring  up  everywhere  white  and  grey  lichens, 
especially  the  Reindeer-moss  and  the  Iceland-moss,  and  some  rocky  ridges  and 
slopes  are  so  exclusively  covered  with  these  structures  that  they  look  from  a 
distance  like  white  patches  and  stripes  on  red,  violet,  and  yellow  grounds.  The 
display  of  colours  in  Alpine  regions  is  materially  heightened  by  the  fact  that  broad 
patches  of  dark  tints  are  not  wanting.  The  number  of  evergreen  plants  is  com- 
paratively large,  and  some  of  those  species  which  appear  in  clumps  retain  their 
green  foliage  under  the  long-continued  winter  coat  of  snow  until  the  vegetative 
period  of  the  next  year.  The  groups  of  mountain  pines  (Pinus  humilis,  Mughus, 
and  Pumilio),  the  rhododendron  bushes  (Rhododendron  hirsutum  and  ferru- 
gineum),  the  tufts  of  Crowberry  (Empetrum  nigrum),  and  the  glistening  carpet 
of  the  evergreen  Bearberry  (Arctostaphylos  Uva-Ursi),  with  their  dark-green 
tints,  bring  a  certain  calm  into  the  gay  confusion.  The  carpets  of  Azalea  pro- 
cwnbens,  which  in  the  autumn  becomes  brownish-green  in  colour,  in  consequence  of 
the  collection  of  the  chlorophyll-corpuscles  of  the  green  leaf-cells  into  balls,  also 
moderate  the  glare  of  the  picture  in  a  harmonious  manner. 

The  charming  spectacle  of  the  colouring  of  deciduous  foliage  in  Alpine  regions 
as  a  rule  only  lasts  for  about  a  fortnight.  If  the  slopes  still  remain  free  from  snow 
for  a  short  time,  all  the  red,  violet,  and  yellow  leaves  become  detached  from  the 
twigs  and  branches.  Whatever  useful  materials  were  still  present  in  the  foliage 
have  emigrated  during  this  time  to  the  stem-structures,  where  they  are  to  pass  the 
winter;  and  the  fallen  leaves  become  brown  and  blackened.  Soon  the  wintry  pall 
of  snow  is  spread  upon  the  mountains;  and  the  ridges,  slopes,  and  hollows,  from 


RESPIRATION.  491 


which  flamed  so  recently  tints  of  red  gold  between  the  dark  mountain  pines,  are 
now  covered  with  dazzling  white  from  the  winter  sky. 


3.— PROPELLING  FOECES   IN  THE   CONVERSION   AND 
DISTRIBUTION   OF   MATERIALS. 

Respiration.— Development  of  Light  and  Heat.— Fermentation. 

KESPIRATION. 

One  of  the  most  remarkable  things  about  metabolism  in  plants  is  that  every 
species  is  its  own  model  and  type,  that  the  compounds  which  are  manufactured 
in  various  species  always  remain  the  same  in  successive  generations,  and  that  from 
the  same  soil,  the  same  water,  and  the  same  air,  under  equal  illumination  and  under 
the  influence  of  the  same  temperature,  the  most  different  organic  compounds  are 
prepared  by  various  species  situated  in  close  proximity.  Within  an  area  of  a 
square  metre  spring  up  from  the  forest  soil  the  poisonous  Boletus  sanguineus,  the 
savoury  Mushroom,  and  the  latex-swollen  Russula',  and  if  the  seeds  of  Mustard, 
Corn-cockle,  and  Poppy  (Sinapis  nigra,  Agrostemma  Githago,  Papaver  Rhceas) 
are  strewn  on  a  garden  bed  of  uniform  soil,  so  that  the  plants  germinated  from 
these  seeds  grow  simultaneously  side  by  side,  their  seeds  will  indeed  exhibit 
materials  of  the  most  varied  composition,  but  every  mustard  seed,  every  seed  of 
the  corn-cockle,  and  every  poppy  seed  will  present  exactly  the  same  compounds 
as  were  possessed  by  the  seeds  sown,  compounds  which  the  seeds  of  these  species 
have  contained  for  thousands  of  years.  This  phenomenon  can  only  be  explained 
by  the  association  of  like  to  like  always  and  everywhere  in  the  plant,  and  by  the 
supposition  that  every  molecule  of  a  certain  material  not  only  operates  as  a 
centre  of  attraction  on  its  surroundings,  but  that  the  attracted  atoms  are  grouped 
according  to  the  special  type,  just  as  happens  in  the  crystallization  of  mineral 
substances. 

If  the  atoms  in  the  colourless  cells  of  a  seed  germinating  in  the  darkness  of 
the  soil  are  attracted  in  the  manner  indicated,  arranged  in  a  certain  way,  and 
connected  together  to  form  a  solid  body,  the  chemical  equilibrium  in  those  cells 
must  be  disturbed.  If  the  materials  thus  attracted  were  previously  dissolved  in 
the  sap  of  these  cells,  the  degree  of  concentration  of  their  sap  must  have  been 
diminished  in  consequence  of  their  withdrawal,  and  will  be  less  than  that  of  the 
neighbouring  cells.  But  this  dissimilarity  cannot  be  maintained,  and  therefore 
a  compensating  movement  occurs,  which  spreads  to  more  and  more  distant  cells: 
or,  in  other  words,  the  materials  stream  towards  the  places  of  consumption.  We 
return  to  this  process,  already  once  described,  in  order  to  review  the  propelling 
forces  which  are  concerned  in  the  metamorphoses  and  distribution  of  the  materials. 


492  RESPIRATION. 

The  process  of  the  combination  of  atoms  into  a  solid  body  which  we  are  now 
considering,  for  example,  the  formation  of  cellulose,  is  a  performance  of  work 
combined  with  the  fixation  of  sensible  heat  and  with  the  transformation  of  kinetic 
into  potential  energy.  But  whence  do  the  colourless  cells  derive  their  sensible  heat 
and  kinetic  energy?  When  carbonic  acid  is  decomposed  and  sugar  or  some  other 
carbohydrate  is  formed  in  a  green  cell,  a  sunbeam  becomes  imprisoned  and  fixed. 
But  this  is  not  the  case  in  cells  devoid  of  chlorophyll,  especially  in  those  working 
in  darkness  under  the  ground.  The  protoplasm  of  these  cells  derives  the  sensible 
heat  and  kinetic  energy  which  it  consumes  or  renders  latent  from  the  sun,  not 
directly,  but  by  very  indirect  methods.  It  obtains  them  by  a  portion  of  the 
material  conveyed  to  it  becoming  decomposed,  by  whose  synthesis  in  the  green 
cells  above-ground  the  kinetic  energy  of  the  sun's  ray  has  been  changed  into 
potential,  and  in  this  way  the  potential  energy  becomes  again  changed  into  kinetic, 
and  the  latent  heat  transformed  into  sensible  heat.  The  materials  which  the  green 
cells  manufacture  out  of  inorganic  food  would  be  merely  an  accumulated  dead 
capital  lying  unused  if  they  were  to  remain  in  the  condition  in  which  they  had 
been  formed.  They  must  be  turned  to  account,  dissolved,  transformed,  and  dis- 
tributed; the  impelling  forces  necessary  for  this  are  obtained  by  a  portion  of  the 
material  manufactured  in  the  green  cells  undergoing  a  process  which  is  exactly 
the  opposite  of  that  carried  out  in  their  formation.  At  the  very  time  when 
carbonic  acid  is  split  up,  oxygen  given  out,  a  carbohydrate  formed,  and  heat 
rendered  latent  thereby,  carbohydrates  are  being  decomposed,  oxygen  taken  up, 
carbonic  acid  excreted,  and  heat  liberated.  Of  course  this  process  of  decomposition 
cannot  extend  to  the  whole  mass  of  the  materials  manufactured  by  the  green  cells. 
It  would  indeed  be  absurd  if  in  one  part  of  the  plant  those  materials  became 
again  disorganized  and  changed  into  air  and  water  which  in  another  part  had 
been  compounded  of  these  same  elements.  As  a  matter  of  fact,  this  process  of 
decomposition  is  limited  to  but  a  part  of  the  materials  produced  in  the  green 
cells,  and  the  whole  process  may  be  most  correctly  represented  thus:  one  portion 
of  the  materials  formed  from  inorganic  food  in  the  green  cells  is  employed  in 
the  further  growth  of  the  plant  body;  but  this  further  growth  only  becomes 
possible  if  the  other  portion  supplies  the  forces  necessary  for  the  carrying  on  of 
the  building.  The  one  process  is  therefore  just  as  important  as  the  other;  they 
mutually  supplement  each  other,  and  this  supplementing  is  one  of  the  most  im- 
portant life-processes  of  plants. 

It  has  been  stated  that  in  order  to  obtain  the  necessary  impelling  forces 
oxygen  is  taken  in,  the  molecules  it  attacks  are  decomposed,  and  carbon  dioxide 
is  liberated.  This  process  is  therefore  an  oxidation,  a  burning  of  organic  material, 
and  is  to  be  placed  in  the  same  category  as  the  burning  of  carbohydrates,  which 
occurs  in  animal  bodies  in  respiration.  It  is  called  respiration  in  plants  also, 
although  here  we  do  not  find  special  localized  respiratory  organs  as  is  usually 
the  case  in  animals.  In  plants  all  the  living  parts  can  breathe,  and  to  them 
the  atmospheric  air,  that  is  to  say,  the  oxygen  contained  in  it,  obtains  access— 


RESPIRATION.  493 

to  roots  and  tubers,  stems  and  foliage,  fruits  and  seeds,  green  plants  and  parasites 
devoid  of  chlorophyll,  plants  with  and  without  stomata,  saprophytes  and  water 
plants.  All  these  breathe  as  long  as  they  are  alive,  and  in  plants  no  less  than 
in  animals  breathing  and  living  can  be  used  as  synonymous  terms  for  all  practical 
purposes.  The  first  fundamental  condition  of  respiration  is  naturally  the  presence 
of  free  atmospheric  oxygen.  When  this  is  absent,  plants,  like  animals,  are  suffo- 
cated and  die.  If  plants  are  put  under  the  receiver  of  an  air-pump,  from  which 
the  air  is  exhausted,  or  in  a  chamber  filled  with  hydrogen,  nitrogen,  or  coal-gas, 
the  streaming  movement  of  the  protoplasm  in  the  cells  ceases  in  a  short  time, 
foliage  and  floral-leaves,  if  they  exhibit  phenomena  of  movement  in  the  living 
plants,  become  rigid,  and  if  kept  for  a  longer  period  in  the  atmosphere  without 
oxygen,  the  plants  die.  Even  if  again  brought  into  air  containing  oxygen,  they 
can  no  longer  be  resuscitated,  but  remain  dead. 

The  parts  of  plants  surrounded  by  atmospheric  air  are  never  in  want  of 
oxygen,  but  roots  often  get  into  an  unfavourable  position  where  the  quantity  of 
oxygen  in  the  air  of  the  soil  is  very  small,  or  where  the  atmospheric  air  is  replaced 
by  other  gases.  This  explains  why  plants  do  not  prosper  in  so-called  "dead" 
earth,  and  why  the  roots  seek  principally  those  loose  places  of  the  upper  strata 
of  soil  which  are  porous  and  well-ventilated,  and  avoid  the  deeper-lying,  badly- 
ventilated,  dead  ground.  The  decay  of  trees  which  have  been  planted  in  towns 
and  parks  near  gas-pipes,  whose  roots  have  been  surrounded  with  coal-gas  for  some 
time  owing  to  a  leak  in  the  pipes,  is  also  explicable  in  this  way. 

Aquatic  plants  take  up  the  oxygen  of  the  atmospheric  air  absorbed  by  the 
water.  Where  there  is  none,  vegetable  life  under  water  becomes  impossible.  If 
anyone,  when  sending  off  water  plants,  tightly  corks  up  the  bottle  after  filling 
with  the  necessary  water,  under  the  impression  that  the  plants,  being  still  in  their 
element,  will  thus  bear  the  journey  well,  he  will  be  sadly  undeceived.  The  small 
quantity  of  oxygen  in  the  atmospheric  air  contained  by  the  water  is  soon 
exhausted,  and  the  aquatic  plants  are  suffocated  within  twenty-four  hours,  or  even 
in  a  much  shorter  time,  just  like  fishes  which  have  been  conveyed  in  a  tightly- 
corked  bottle  of  water. 

All  plants  do  not  breathe  with  the  same  energy,  and  in  any  plant  a  great 
difference  can  be  noticed  in  the  respiration  of  the  various  organs.  The  floral- 
leaves,  possessing  no  chlorophyll,  respire  much  more  vigorously  than  the  green 
foliage-leaves;  underground  root-stocks,  bulbs,  and  tubers,  also  without  chlorophyll, 
to  a  much  greater  extent  than  the  green  stem.  In  the  green  organs  of  plants 
exposed  to  sunlight  two  processes  are  carried  on,  the  formation  and  the  splitting 
up  of  carbohydrates.  The  latter  process,  however,  is  so  obscured  by  the  former, 
that  it  can  only  be  observed  with  difficulty.  It  has  been  estimated  that  in  a 
laurel  leaf  the  amount  of  carbohydrates  formed  in  any  given  time  is  thirty  times 
as  great  as  of  those  decomposed,  i.e.  respired. 

A  great  difference  is  also  exhibited  according  to  the  stage  of  development  of  the 
individual  plant  organa  Roots,  stems,  and  leaves  breathe  much  more  vigorously 


494 


RESPIRATION. 


when  young  than  when  fully  formed.  When  seeds  are  allowed  to  germinate  in 
damp  earth,  respiration  is  at  first  quite  inconsiderable,  but  when  the  parts  of 
the  seedling  begin  to  elongate,  and  when  the  stores  of  materials  furnished  by 
the  parent  plant  are  dissolved  and  used  up,  respiration  becomes  very  energetic. 
Later  on,  when  the  seedling  has  grown  up  so  far  that  it  can  itself  work  with 
the  help  of  its  leaves,  which  have  meanwhile  become  green,  respiration  again 
diminishes.  The  same  thing  occurs  in  the  development  of  buds;  there,  too,  the 
young  leaves  just  emerging  from  the  covering  of  the  bud  breathe  to  a  much 
greater  extent  than  the  fully-formed  green  foliage.  That  organs  which  have 
attained  their  full  size,  and  are  apparently  quite  inactive,  still  respire,  is  shown 
from  the  observation  that  roots  and  tubers  which  have  been  dug  up  in  the 
autumn  and  left  in  a  cellar  through  the  winter  exhale  carbonic  acid  without 
any  visible  outward  change.  In  beet-roots  which  have  been  dug  up  a  1-per-cent 
decrease  of  sugar,  and  an  exhalation  of  carbonic  acid  corresponding  to  this  decrease, 
have  been  observed  within  two  months,  a  proof  that  change  of  materials  and 
respiration  can  occur  even  in  structures  which  lie  dormant  during  the  winter. 

According  to  what  has  just  been  stated  about  the  significance  of  respiration 
to  the  life  of  plants,  it  is  quite  obvious  that  the  energy  of  respiration  which  is 
reckoned  by  the  amount  of  carbonic  acid  exhaled  from  a  certain  organized  mass, 
or  better,  by  the  amount  of  oxygen  absorbed,  becomes  greater  the  more  vigorously 
the  plant  grows,  and  the  quicker  it  builds  up  its  body,  just  as  a  machine  requires 
more  fuel  the  greater  the  results  required  from  it.  If  fuel  is  wanting  or  not 
present  in  sufficient  quantity  the  machine  stops,  or  does  not  perform  as  much 
work  as  it  should  be  capable  of  doing.  It  is  exactly  the  same  in  living  plants. 
If  the  respiratory  materials  are  absent,  respiration  is  discontinued  even  in  the 
presence  of  oxygen,  and  life  becomes  extinguished.  If  the  supply  of  these 
materials  is  insufficient  the  plants  only  prolong  their  existence  with  difficulty, 
and  their  growth  will  be  insignificant  in  consequence  of  the  insufficiency  of 
materials  for  carrying  on  the  work.  When  shoots  sprout  from  the  "eyes"  of  a 
potato,  it  is  at  the  cost  of  the  carbohydrates  and  other  materials  stored  up  in 
the  tuber.  If  this  sprouting  occurs  in  the  open  and  the  shoots  grow  up  into 
the  daylight,  their  leaves  become  green  and  manufacture  new  carbohydrates 
under  the  influence  of  the  sun's  rays;  and  of  these  a  portion  is  at  once  employed 
as  fuel  for  the  further  construction  of  the  potato  plants,  that  is  to  say,  it  is 
respired.  If,  on  the  other  hand,  the  development  of  shoots  from  potatoes  takes 
place  in  a  dark  cellar,  their  leaves  cannot  become  green,  and  consequently  no 
carbohydrates  can  be  manufactured.  The  shoots  then  only  continue  to  grow  so 
long  as  the  respiratory  materials  stored  up  in  the  tuber  last;  when  these  are 
exhausted,  respiration  comes  to  an  end,  and  the  shoots  die  off". 

An  approximate  idea  of  the  significance  of  respiration  as  an  impelling  force 
in  those  changes  of  materials  whose  end  is  the  further  growth  of  the  plant  may 
be  obtained  from  a  consideration  of  the  following  figures.  A  cubic  centimetre 
of  carbon  dioxide  contains  0'5376  milligramme  of  carbon,  whose  burning  furnishes 


RESPIRATION.  495 

4677  units  of  heat.  The  mechanical  equivalent  of  this  is  1,987,725  gramme- 
millimetres.  When  a  carbohydrate  is  respired,  for  every  cubic  centimetre  of 
carbon  dioxide  exhaled  a  store  of  energy  is  obtained  which  is  equal  to  1,987,725 
gramme-millimetres,  and  therefore  by  it  a  gramme  weight  can  be  raised  to  a 
height  of  1987  metres.  It  has,  however,  been  ascertained  that  seedlings  of  poppy 
(which,  when  subsequently  dried,  weighed  045  gramme)  exhaled  55  cubic  centi- 
metres of  carbon  dioxide  in  24  hours,  and  seedlings  of  mustard  (which,  when 
dried  later,  weighed  0'55  gramme)  32  cubic  centimetres  in  the  same  length  of  time. 
It  can  therefore  be  easily  imagined  what  a  large  store  of  energy  is  obtained  by 
respiration,  even  although  the  result,  in  consequence  of  various  interruptions  and 
obstructions,  should  fall  far  behind  this  estimate. 

In  comparing  the  living  plant  to  a  machine  heated  by  coal,  and  trying  to 
measure  the  work  performed  by  it  numerically,  we  are  justified  by  the  analogy 
of  the  transactions  in  the  two  cases,  which  are  obvious.  The  comparison  suggests 
itself  naturally  from  the  fact  that  in  both  cases  similar  impelling  forces  come  into 
play,  and  that  in  both  the  necessary  store  of  vital  force  is  obtained  by  the  com- 
bustion of  carbon.  Yet,  on  the  other  hand,  combustion  in  a  machine  and  respira- 
tion in  a  living  plant  are  widely  different.  The  peculiarity  of  plant  respiration 
lies  in  the  fact  that  materials  are  combined  with  the  oxygen  of  the  atmospheric 
air  which  would  not  enter  into  combustion  with  it  at  ordinary  temperatures 
outside  the  living  plant.  Neither  carbohydrates,  fats,  nor  albumins,  which  are 
either  directly  or  indirectly  affected  in  respiration  by  the  process  of  combustion, 
undergo,  outside  the  plant  cell,  the  alterations  and  decompositions  which  are 
carried  on  within  it,  and  it  may  be  taken  as  an  established  fact  that  oxygen  only 
operates  on  them  when  conveyed  to  them  by  means  of  the  living  protoplasm. 
The  effect  of  the  transmitted  oxygen  is  also  restricted  by  the  living  protoplasm 
to  the  carbohydrates  and  other  non-nitrogenous  compounds  which  it  incloses. 
Nitrogenous  compounds  are  not  respired  directly,  and  the  quantity  of  nitrogen  in 
breathing  plants  is  not  lessened.  We  can  only  imagine  these  remarkable  correla- 
tions as  occurring  in  the  following  manner.  The  starch  grains  and  droplets  of  oil 
are  first  rendered  soluble,  and  are  then  oxygenated  by  the  oxygen  brought  by  the 
protoplasm;  the  albumins,  on  the  other  hand,  are  first  split  up  into  asparagin  and 
a  carbohydrate.  The  latter  alone  becomes  oxidized,  for  the  nitrogenous  asparagin 
is  not  only  not  burnt,  but  is  reconstructed  into  albumin,  with  the  co-operation  of 
the  sun's  rays,  by  attracting  the  newly-formed  carbohydrates  of  the  green  cells 
and  combining  with  them. 

If  we  adhere  to  this  view,  it  at  once  becomes  evident  how  important  is  the 
co-operation  of  respiration  and  the  formation  of  fresh  carbohydrates  in  the  green 
cells.  If,  in  a  plant,  the  production  of  new  carbohydrates  should  be  suspended,  the 
reconstruction  of  albumins  cannot  ensue.  At  first  all  the  respirable  materials 
which  yet  remain  in  the  plant  are  used  up  for  the  continuance  of  action,  but  if 
the  formation  of  fresh  carbohydrates  remains  unaccomplished,  and  even  the  last 
reserves  are  consumed,  then  the  plant  becomes  exhausted,  and  ceases  to  breathe 


496  DEVELOPMENT   OF   LIGHT   AND   HEAT. 

and  live.  It  has  been  estimated  that  a  plant,  in  which  the  supplies  of  freshly- 
formed  carbohydrates  are  lacking,  can  consume  over  50  per  cent  of  its  substance 
by  respiration  before  it  perishes  from  exhaustion.  This  is  the  case,  for  example, 
in  the  potato-tubers  mentioned,  whose  stems,  developed  in  dark  chambers,  become 
overgrown,  i.e.  elongate  exceedingly,  while  their  rudimentary  foliage-leaves  remain 
very  small  and  destitute  of  chlorophyll.  Here,  in  the  dark,  no  new  formation  of 
carbohydrates  occurs,  but  respiration  continues  as  long  as  any  respirable  materials 
yet  remain.  At  length,  when  everything  that  can  be  made  use  of  in  this  way  is 
consumed,  the  shoots  die  off.  Their  dry  weight,  however,  is  only  half  as  much  as 
was  that  of  the  tuber  from  which  they  sprung;  the  other  half  has  been  completely 
burnt,  i.e.  changed  into  carbonic  acid  and  water,  which  have  rapidly  evaporated. 

Sunlight  is  not  necessary  to  respiration,  although  without  it  the  decomposition 
of  carbonic  acid  and  the  formation  of  carbohydrates  cannot  take  place.  Breathing 
can  be  carried  on  in  complete  darkness.  Underground  organs:  roots,  tubers, 
bulbs,  rhizomes,  runners,  likewise  the  mycelia  and  fruit-stalks  of  the  plants 
classed  together  as  fungi,  as  well  as  seeds  buried  in  the  earth — all  these  normally 
breathe  in  darkness.  Respiration  continues  throughout  the  darkest  night.  That 
growth,  the  most  important  of  all  the  processes  stimulated  by  respiration,  is 
restricted  by  the  influence  of  light,  will  be  discussed  when  describing  growth; 
concentrated  light  produces  a  rapid  oxidation  and  disorganization  of  the  organ 
exposed,  which,  however,  must  not  be  looked  upon  as  the  respiration  of  the  plant. 

DEVELOPMENT   OF   LIGHT  AND    HEAT. 

It  is  to  be  expected  that  respiration  will  be  more  vigorous  in  plants  the  higher 
the  temperature,  since  the  process  of  respiration  is  a  combustion  of  carbon  com- 
pounds, and  all  combustion  is  helped  by  a  rise  of  temperature.  As  a  matter  of 
fact,  it  has  been  observed  that  the  exhalation  of  carbonic  acid  (that  is  to  say, 
respiration)  also  increases  with  rise  of  temperature.  Of  course  this  is  true  only 
up  to  a  certain  point.  Respiration  may  commence  even  at  0°,  and  reaches  a 
maximum  between  15°  and  35°  C.  according  to  the  species,  but  beyond  that  it 
quickly  diminishes.  It  entirely  ceases  at  temperatures  which  produce  coagulation 
of  the  proteids,  and  which  are  followed  by  the  death  of  the  living  protoplasm. 

When  once  respiration  is  started,  the  oxygen  necessary  for  the  combustion  of 
carbohydrates  is  derived  from  the  surrounding  atmospheric  air.  But  the  first 
incitement  to  respiration  does  not  proceed  from  this,  or  in  other  words,  the 
absorbed  oxygen  does  not  furnish  the  first  stimulus  to  respiration.  Dead  plants 
into  which  oxygen  is  made  to  enter  do  not  breathe  any  more  than  do  butterflies 
which  have  been  suffocated  by  withdrawal  of  oxygen,  and  then  subsequently 
brought  into  the  fresh  air.  Oxygen  cannot  produce  those  movements  of  the 
atoms  which  are  peculiar  to  life  either  in  plants  or  animals  which  have  been 
completely  suffocated.  Since  only  living  plants  can  breathe,  respiration  must 
be  brought  about  by  a  force  which  is  liberated  in  the  living  protoplasm,  by  that 


DEVELOPMENT   OF   LIGHT   AND   HEAT.  497 

specific  force  which  must  be  designated  as  vital.  The  first  movement,  i.e.  the  first 
chemical  process  with  which  respiration  commences,  seems  to  be  a  splitting  up 
of  the  albuminous  molecules  in  the  living  protoplasm,  the  same  process  as  that 
by  which  albumen  is  separated  into  asparagin  and  a  carbohydrate,  perhaps  into 
asparagin,  a  carbohydrate,  and  carbon  dioxide.  The  next  thing,  of  course,  would 
be  a  withdrawal  of  oxygen  from  the  air,  but  it  should  be  noted  that  this  is  only 
for  the  purpose  of  continuing  the  metabolic  changes,  which  have  originated  spon- 
taneously in  the  living  protoplasm. 

Heat  likewise  is  liberated  in  all  combination  of  oxygen  with  other  substances, 
especially  in  every  combustion  of  carbon  compounds.  This  heat  is  not  always 
easily  demonstrable  in  the  plant  organ  in  which  it  is  set  free.  The  heating  of 
the  respiring  tissue  is  counteracted  by  the  evaporation  of  water  and  by  radiation 
in  organs  above-ground,  particularly  in  the  flattened,  outspread  foliage-leaves. 
Carbon  is  also  reduced  in  the  green  foliage  during  the  day  under  the  influence 
of  sunlight,  and  this  is  a  process  which  goes  hand  in  hand  with  the  fixing  of 
sensible  heat.  Now,  since  this  process  masks  the  respiration  in  the  green  leaves 
to  a  certain  extent,  it  is  intelligible  that  the  heat  liberated  by  respiration  in  these 
organs  is  but  seldom  perceptible,  and  that  as  a  rule  green  leaves  actually  feel 
cool.  It  is  even  probable  that  the  pleasant  coolness  under  a  shady  canopy  of 
leaves  is  not  solely  due  to  the  interception  of  the  sun's  rays,  but  that  the  imprison- 
ment of  these  rays  and  the  fixation  of  heat  during  the  manufacture  of  the 
primary  carbohydrates  also  shares  in  the  cooling  of  the  air  surrounding  the 
leaves.  But  where  these  conditions  are  out  of  the  question,  the  heat  liberated 
can  be  demonstrated  in  respiring  vegetable  organs  just  as  in  animal  bodies;  and 
if  respiring  green  leaves  could  neither  transpire,  nor  radiate  heat,  and  if,  moreover, 
a  supply  of  carbohydrates  were  stored  up  in  them,  the  heat  liberated  by  respira- 
tion would  make  itself  evident  in  the  immediate  neighbourhood.  This  applies 
still  more  to  subterranean  bulbs  and  tubers  in  which  transpiration  and  radiation 
are  not  only  partly  or  entirely  absent,  but  which  are  incapable  of  manufacturing 
carbohydrates  for  themselves  as  they  have  no  chlorophyll,  and  which,  conse- 
quently, render  no  heat  latent. 

Germinating  seeds,  and  seedlings  without  chlorophyll  behave  in  the  same  way 
as  these  respiring  underground  organs,  provided  that  they  are  protected  against 
evaporation  and  radiation.  Barley-corns  which  have  begun  to  germinate  and 
are  respiring  vigorously  raise  the  temperature  of  their  environment  quite  notice- 
ably if  they  lie  heaped  together  so  that  the  heat  liberated  becomes  thus  concen- 
trated. It  is  well  known  that  malt  is  germinated  barley,  and  in  the  preparation 
of  malt  heaped-up  barley-corns  are  caused  to  germinate.  In  this  process  the 
temperature  of  the  immediate  neighbourhood  rises  5-10°  C.  above  the  temperature 
of  the  air  which  surrounds  the  piles  of  barley-corns  outside.  The  liberation  of 
heat  in  fungi  is  also  very  instructive.  These  derive  the  organic  compounds  from 
which  they  build  up  their  mycelia  and  fruit  bodies  from  other  living  organisms, 
or  from  the  decaying  remains  of  dead  plants  and  animals.  The  receptacles  often 

VOL.  I. 


498  DEVELOPMENT   OF   LIGHT   AND   HEAT. 

develop  very  rapidly  to  a  considerable  size,  and  connected  with  this  rapid  growth 
there  is  always  a  rapid  movement  of  the  food  absorbed  by  the  mycelium,  com- 
bined with  an  energetic  respiration.  Respiration  is  carried  on  chiefly  at  the 
periphery  of  the  receptacle — in  the  mushrooms  especially  in  the  hymenial  layer, 
which  is  very  well  protected  from  evaporation  and  radiation  by  its  position  on 
the  lower  side  of  the  cap.  Transmission  of  the  food,  and  in  particular  of  a  large 
amount  of  water,  takes  place  through  the  stalk  which  bears  the  cap.  Numerous 
observations  of  fungi  growing  in  their  natural  free  condition,  and  rising  but 
little  above  the  soil,  have  invariably  shown  this  result:  the  rise  of  temperature  in 
the  tissue  of  the  cap  is  most  pronounced  where  respiration  is  carried  on  most 
actively,  i.e.  in  the  hymenial  layer.  It  is  less  in  the  central  portion  of  the  cap, 
and  least  in  the  stalk,  through  which  the  watery  fluid  travels  at  a  temperature 
which  differs  but  slightly  from  that  of  the  surrounding  earth.  Respiration,  of 
course,  cannot  be  considerable  here.  For  example,  in  Boletus  edulis,  from  its  size 
and  shape  particularly  well  suited  for  these  investigations,  the  following  results 
were  obtained  while  the  temperature  of  the  surrounding  earth  was  about  13°  C. : 
temperature  of  the  stalk,  14'2-15*6°;  temperature  of  the  body  of  the  cap,  15'2-16'8°; 
of  the  hymenial  layer,  16'7-18*1°.  Further  developed  (but  still  quite  fresh)  fructi- 
fications exhibit  higher  temperatures  than  younger  ones  which  have  just  appeared 
above  the  ground.  Observations  on  other  fungi  of  the  Hymenomycetes  yield  like 
results.  When  the  temperature  of  the  surrounding  earth  was  12'2°  Lactarius 
scrobiculatus  exhibited  in  its  stalk  a  temperature  of  14'8°,  and  in  its  cap  of  16'0°; 
Agaricus  muscarius  in  its  stalk  14*2°,  and  in  its  cap  15*2°,  while  the  temperature 
of  the  surrounding  soil  was  13'0°;  Hydnum  imbricatum,  13'0°  in  the  stalk  and 
14'5°  in  the  cap,  while  the  surrounding  earth  showed  a  temperature  of  12 '2°.  The 
peculiar  shape  of  the  cap  in  these  last-named  fungi  is  not  well  adapted  to  a 
separate  measurement  of  the  temperature  in  the  body  of  the  cap  and  in  the 
hymenial  layer,  but  it  is  probable  that  a  slight  difference  exists  between  them, 
similar  to  that  found  in  Boletus.  The  puff-balls  belonging  to  the  Gasteromycetes 
also  exhibit  a  considerable  rise  of  temperature  above  that  of  their  surroundings 
in  the  respiring  portions  of  their  fructifications.  Thus  in  Lycoperdon  ccelatum  a 
temperature  of  15'8°  was  observed  in  the  spherical  receptacle  shortly  before  dehis- 
cence,  while  the  temperature  of  the  surrounding  soil  was  only  12 '2°. 

The  liberation  of  heat  appears  especially  noticeable,  too,  in  respiring  flower-buds 
and  in  the  rapidly-growing  stalks  which  bear  them,  as  well  as  in  opened  flowers. 
If  the  flowers  are  small,  and  if  there  are  but  few  of  them  at  the  end  of  the  stem, 
or  if  only  a  single  small  flower  is  borne  at  the  end  of  a  delicate  stalk,  the  heat 
liberated  may  easily  escape  observation;  but  under  very  favourable  conditions  it 
makes  itself  readily  manifest,  and  gives  rise  to  a  phenomenon  so  strange  and 
unintelligible  that  everyone  on  observing  it  for  the  first  time  is  surprised  and 
puzzled.  I  refer  to  the  fact  that  small  and  delicate  flowers  grow  buried  beneath 
the  snow,  and  obtain  the  space  they  require  by  melting  the  hardened  snow.  The 
Alpine  Soldanella  is  a  very  marked  instance  in  point.  As  the  snow  melts  and  the 


DEVELOPMENT   OF  LIGHT   AND   HEAT.  499 

tricklings  therefrom  moisten  the  earth  below,  the  Soldanella  plants  are  aroused 
from  their  winter's  rest.  Their  little  arched  flower-stalks  begin  to  elongate  and 
come  into  contact  with  the  hard  under  surface  of  the  snow,  though  the  temperature 
here  is  zero.  Growth  is  carried  on  at  the  expense  of  the  supplies  of  materials 
obtained  by  the  Soldanellas  in  the  previous  summer,  which  had  been  stored  up 
partly  in  the  evergreen  leathery  leaves  lying  flat  on  the  ground,  and  partly  in  the 
short  root-stock  embedded  in  the  soil.  The  reserves  are  employed  as  substances  for 
building,  and  a  portion  of  them  is  respired,  in  order  that  it  may  be  possible  to  dis- 
solve the  rest,  to  bring  them  to  the  places  where  they  are  required,  and  to  obtain 
the  force  necessary  for  the  work.  The  heat  liberated  by  this  respiration  melts  the 
granular  ice  covering  in  the  immediate  neighbourhood  of  the  flower-buds.  In  con- 
sequence of  this  a  cavity  is  formed  in  the  ice  above  each  bud,  or  rather,  each  bud 
becomes  overarched  as  if  by  a  tiny  dome  of  ice.  But  the  stem  continues  to  grow  in 
height;  and  the  flower-bud  borne  on  it,  which  is  respiring  and  giving  out  heat,  is 
accordingly  raised  up  in  the  dome-shaped  hollow  space  and  pushed  into  it.  There 
it  promotes  afresh  the  melting  of  the  ice  and  an  extension  of  the  cavity,  and  thus 
actually  bores  a  path  for  itself  upwards  through  the  ice-strata.  This  goes  on  until  at 
length  the  respiring  and  heat-producing  Soldanella  bud  has  melted  an  actual  canal 
through  the  covering  of  hardened  snow,  and  makes  its  appearance  above,  the  stem 
looking  as  if  it  had  been  stuck  into  the  snow.  The  flower-buds  now  open  and  the 
pretty  violet  bells  sway  about  in  the  wind.  Naturally  the  snow  will  be  penetrated 
first  where  it  is  thinnest,  i.e.  near  the  margin,  where  also  the  melting  from  above 
proceeds  most  rapidly.  Consequently  it  is  the  edge  of  the  snow-field  mainly  which 
is  riddled,  the  Soldanellas  growing  up  through  the  holes.  It  is  not  at  all  uncommon 
to  find  places  where  10-20  flowers  appear  on  the  border  within  a  stretch  a  metre 
long.  On  looking  closer  and  making  cuttings  through  the  ice,  all  the  stages  of  de- 
velopment described  may  be  seen  side  by  side.  Two  other  phenomena,  however,  are 
not  a  little  surprising.  Here  and  there  are  to  be  found  single  Soldanellas  whose 
buds  have  already  opened  before  they  have  emerged  above  the  ice-covering.  These 
Soldanellas  actually  blossom  in  a  small  cavity  of  the  ice,  and  remind  one  of  plant- 
organs  or  insects  inclosed  in  amber  or  small  coloured  splinters  which  have  been 
fused  inside  glass  balls.  This  sub-glacial  blossoming  of  the  Soldanellas  is  not 
limited,  strangely  enough,  to  the  opening  of  the  corolla;  the  anthers  actually 
dehisce  and  liberate  their  pollen. 

What  also  surprises  us  very  much  on  closer  inspection  is  the  fact  that  the  holes 
in  which  the  flower-stalks  are  situated  narrow  like  a  funnel  towards  the  base,  so 
that  there  the  ice  touches  the  stem,  or,  in  other  words,  that  the  canal  down  below  is 
completely  filled  by  the  stem.  When  it  is  remembered  that  the  flower-bud  which 
melted  the  ice  and  formed  the  canal  had  a  diameter  at  least  three  times  as  large  as 
that  of  the  stem,  it  would  be  expected  that  the  stem  would  be  placed  in  the  centre  of 
a  comparatively  wide  hole.  But,  as  stated,  this  is  not  the  case,  and  the  phenomenon 
can  only  be  explained  by  supposing  that  the  porous  granular  ice  forms  a  plastic 
mass,  and  that  the  granules  displaced  by  the  melting  sink  down  in  accordance 


500  DEVELOPMENT   OF   LIGHT   AND   HEAT. 

with  the  law  of  gravitation,  unite  together  where  a  boring  has  taken  place,  and 
again  form  a  compact  mass  in  consequence  of  the  regelation  of  the  lower  strata. 
It  has  still  to  be  mentioned  that  the  green  leaves  of  the  Soldanellas,  which  lie  flat 
below  the  snow  and  ice,  becoming  flaccid  during  the  growth  of  the  flowers,  and  that 
the  reserve  materials  stored  up  in  them  are  completely  used  up  by  the  growing  stem 
and  flowers.  The  green  leaves  then  become  wrinkled  and  perish,  while  new  leaves 
develop  after  the  snow  has  melted.  These  provide  themselves  with  reserve  food  in 
order  that  in  the  next  period  of  vegetation  the  growing  stem  and  flowers  may 
be  efficiently  nourished. 

Here  and  there  with  the  flowers  of  Soldanellas  are  found  the  young,  but  never- 
theless yellowish-red,  foliage-leaves  of  Polygonum  viviparum,  which  grow  up  from 
below  into  the  ice,  and  occasionally  melt  holes  in  it  close  to  the  edge  of  the  snow- 
field.  The  white  flowers  of  Ranunculus  alpestris  growing  in  company  with  the 
Soldanellas  in  the  same  habitat  have,  on  the  other  hand,  not  attained  to  the  capacity 
of  growing  through  the  ice,  and  need  as  an  incitement  to  growth  a  temperature 
which  is  rather  above  0°  C.;  in  consequence  of  which  they  always  open  their  flowers 
first  in  places  from  which  the  snow  has  vanished  a  short  time  before. 

The  amount  of  the  heat  set  free  by  the  small  flower-buds  of  Soldanellas  might 
be  estimated  by  the  quantity  of  ice  melted,  but  so  many  sources  of  error  enter  into 
a  calculation  of  this  kind  that  the  numbers  obtained  cannot  lay  claim  to  much 
accuracy,  and  we  must  be  satisfied  with  the  fact,  even  although  it  is  not  verified  by 
figures  based  on  a  calorimetric  experiment. 

The  melting  of  the  ice  by  the  heat  liberated  in  the  respiration  of  Soldanellas  is 
also  of  the  greatest  interest,  since  it  furnishes  a  proof  that  single,  small,  extremely 
delicate  flowers  warm  not  only  their  own  tissues  but  also  their  environment,  and 
that  the  heat  liberated  in  them  does  not  become  perceptible  only  because,  as  already 
remarked,  it  is  counteracted  by  evaporation  and  radiation  which  are  carried  on  at 
the  same  time,  and  because  the  respiring  flowers  are  usually  surrounded  by  atmo- 
spheric air,  i.e.  by  a  medium  which  is  movable,  fluctuating,  and  unstable.  The 
air  which  in  one  moment  is  warmed  by  the  respiring  leaves  is  carried  far  away  in 
the  next  instant,  and  is  replaced  by  other  air.  This  is  the  case  especially  in  shallow 
flowers  with  recurved  leaves,  or  in  salver-shaped  corollas  widely  opened  above,  round 
which  there  cannot  be  said  to  be  any  stagnation  of  air.  But  if  the  flower  has 
the  form  of  an  inverted  bell,  as  in  the  Foxglove,  Gloxinias,  and  most  campanulate 
flowers;  if  the  floral-leaves  bend  upwards  like  a  helmet,  as  in  the  Monkshood;  if 
the  flowers  are  tubular,  inflated  at  the  base  like  a  flask,  or  pitcher-like  as  in 
Aristolochias,  or  form  deep  goblets  as  in  the  Cactacese  and  many  gourds — then  the 
air  in  the  inclosed  space  is  scarcely  at  all  disturbed,  there  is  stillness  within  the 
flower,  the  air  there  collected  and  warmed  is  retained  almost  unaltered  in  its  quiet 
corner,  and  is  not  very  easily  replaced  by  other  air. 

On  cool  days  a  rise  of  temperature  above  that  of  the  surrounding  air  can  be 
usually  perceived  in  the  interior  of  such  flowers,  even  when  they  stand  quite  alone. 
In  an  Alpine  meadow  the  interior  of  a  flower  of  Gentiana  acaulis  in  the  morning 


DEVELOPMENT   OF   LIGHT   AXD   HEAT.  501 

shortly  before  sunrise  exhibited  a  temperature  of  10'6°  C.  when  the  temperature  of 
the  surrounding  air  was  8'4°.  On  a  mountain  meadow  under  a  cloudy  sky  and  in 
calm  air  the  interior  of  a  flower  of  Campanula  barbata  showed  a  temperature  of 
16'6°,  and  not  far  off'  on  the  borders  of  a  forest  the  interior  of  the  helmet-shaped 
sepal  of  Aconitum  paniculatum,  14'6°,  while  the  temperature  of  the  outside  air  in 
both  instances  did  not  exceed  13'2°.  The  temperature  of  the  air  in  the  neighbour- 
hood of  a  respiring  plant  shows  a  much  greater  rise  if  numerous,  small,  thickly- 
crowded  flowers  are  inclosed  in  a  common  sheath,  and  especially  when  the  space 
inclosed  is  undisturbed.  In  the  same  mountain  meadow  in  which  the  temperature 
of  the  interior  of  the  ball  in  the  above-mentioned  campanula  (Campanula  barbata) 
was  tested,  the  Carline  Thistle  (Carlina  acaulis)  was  also  in  full  bloom.  As  the 
sky  was  cloudy,  the  capitula  were  closed,  i.e.  the  apices  of  the  stiff,  involucral  leaves 
were  bent  together,  and  formed  a  hollow  inverted  cone  over  the  flowers.  A  ther- 
mometer placed  between  these  bracts  and  pushed  down  as  far  as  the  flowers,  showed 
a  temperature  of  20*4°,  the  temperature  of  the  surrounding  air  being  13'2°,  the  dif- 
ference, therefore,  was  more  than  7°  C, 

In  palms,  whose  numerous  small  crowded  flowers  are  covered  by  large  floral 
sheaths  or  spathes,  the  air  within  these  coverings  exhibits  a  rise  of  temperature 
which  is  so  noticeable  that  it  can  be  felt  by  placing  the  bare  hand  inside.  The 
same  thing  occurs  in  the  aroids.  Here  numerous  small  flowers  are  united  into  a 
spike  on  a  thick  fleshy  axis,  forming  the  so-called  spadix,  and  each  spadix  is 
surrounded  by  a  bract  which  at  first  is  twisted  together  like  a  conical  paper  bag, 
being  often  distended  like  a  barrel  or  inflated  like  a  bladder.  It  is  soon  formed 
into  the  characteristic  shape,  but  always  incloses  a  cavity  whose  air  is  hardly  ever 
disturbed  by  the  influence  of  other  air  currents.  With  care  a  thermometer  may  be 
introduced  into  this  cavity,  and  the  temperature  shown  by  it  may  be  compared 
with  that  of  the  surroundings.  For  example,  it  was  found  when  the  temperature 
of  the  outer  air  was  25°,  that  in  the  interior  of  the  spathe  of  the  Brazilian  Tomelia 
fragrans  was  almost  38°.  At  the  same  air-temperature  the  interior  of  the  spathe 
of  Arum  cordifolium,  in  the  island  of  Bourbon,  exhibited  a  temperature  of  35-39°. 
But  the  highest  temperature  has  been  noticed  in  the  Italian  Arum  (Arum  Italicum). 
This  plant  is  very  common  in  the  region  of  the  Mediterranean  flora,  and  is  fre- 
quently to  be  met  with  in  vineyards  under  bushes,  and  even  in  hedges  and  road- 
sides. Its  spadices,  surrounded  by  large  pale-green  spathes,  push  their  way  in  the 
spring  through  the  soil  like  inverted  conical  bags;  the  spathe  begins  to  open 
between  4  and  6  o'clock  in  the  afternoon,  and  at  the  same  time  a  peculiar  fragrance, 
like  wine,  becomes  noticeable  in  the  neighbourhood  of  the  plant.  If  a  thermometer 
is  introduced  into  the  cavity  of  this  spathe,  it  is  shown  that  while  the  temperature 
of  the  outside  air  is  about  15°,  that  in  the  interior  has  risen  to  40°,  sometimes  even 
to  44°.  These  Aroidese  therefore  exhibit  a  temperature  in  the  neighbourhood  of 
their  respiring  flowers  which  exceeds  that  of  blood-heat. 

In  proportion  as  the  energy  of  respiration  increases  with  the  rising  temperature 
of  the  surrounding  air  from  morning  till  afternoon,  the  temperature  in  the  interior 


502  DEVELOPMENT   OF    LIGHT   AND   HEAT. 

of  the  flowers  also  rises,  as  shown  by  the  following  observations  which  were  con- 
ducted in  a  place  in  the  garden  shaded  from  the  direct  influence  of  the  sun's  rays: 

8-8°    ...  15-2°  ...  17'7°  ...  20-0°  ...  21'2° 


of  the  Eed  Foxglove, 

Corresponding  temperature  of  the  surrounding  air,     8'7°     ...  15'0°  ...  17'2°  ...  19'1°  ...  19'5° 
Difference, 0'1°    ...     0'2°  ...     0'5°  ...     0'9°  ...     17° 

While  the  liberation  of  heat  occurs  in  all  living  plants,  and  is  a  natural  con- 
sequence of  respiration,  i.e.  of  the  combustion  of  carbon  compounds,  the  development 
of  light,  which  in  other  respects  appears  to  be  in  many  ways  connected  with  the 
processes  of  combustion,  is  observed  in  living  plants  but  seldom.  It  is  only  recog- 
nized with  certainty  in  the  Hymenomycetes,  a  group  of  fungi  in  which  the  rise  of 
temperature  during  respiration  has  already  been  described.  But  even  of  these 
Hymenomycetes  only  relatively  few  are  luminous,  and  these  few  only  in  certain 
stages  of  development.  Most  frequently  the  luminosity  occurs  in  the  mycelium  of 
mushroom-like  forms  (Agaricinece),  which  permeate  the  wood  of  old  tree- trunks 
and  the  creeping  roots  of  trees  on  the  surface  of  the  damp  forest  ground.  This 
mycelium  forms  thicker  dark  strands,  frequently  joined  together  by  cross-connections, 
which  penetrate  principally  between  the  wood  and  the  cortex,  and  these  form  most 
characteristic  nets  and  lattice- works;  it  also  consists  of  very  slender  dark  threads, 
which  take  up  their  position  in  the  wood  usually  at  right  angles  to  the  long  axis  of 
the  trunk;  and,  finally,  there  are  extremely  delicate  colourless  threads  which  grow 
through  the  woody  cells  in  the  manner  shown  in  fig.  32.  These  actually  per- 
meate the  entire  wood,  and  are  only  perceptible  to  the  naked  eye  when  they  are 
woven  into  net- works,  and  then  are  seen  as  whitish  fringes  and  membranes  situated 
on  the  sides  of  the  holes  formed  in  the  disorganized  wood. 

It  is  these  fine  threads  and  webs  of  the  mycelium  which  exhibit  the  remark- 
able illumination.  Where  they  completely  invest  the  wood-cells,  it  looks  as  if 
the  wood  itself  were  luminous,  and  we  commonly  speak  of  luminous  wood  and 
the  luminous  decay  of  tree-trunks.  There  is  no  doubt  that  the  luminosity  is  ex- 
hibited by  the  mycelia  of  various  agarics,  which  destroy  the  wood  of  firs  and 
other  foliage-trees.  Usually  the  Rhizomorph  (Agaricus  melleus)  is  alone  pointed 
out  as  the  cause  of  the  luminosity  in  wood,  since  this  species  is  widely  distributed; 
and  where  it  has  established  itself  sends  up  every  year  many  receptacles,  so  that 
there  is  no  difficulty  in  determining  the  species.  But  since  luminous  wood  is  also 
observed  in  the  pine  forests  of  higher  mountain  districts  where  the  Rhizomorph 
is  no  longer  found,  it  must  be  concluded  that  the  mycelia  of  various  other  agarics, 
whose  species  cannot  be  determined  on  account  of  the  absence  of  fructifications, 
exhibit  the  same  phenomenon.  The  light  is  best  seen  in  the  open,  in  midsummer 
and  autumn,  after  many  days  of  wet  weather,  when  the  wood  permeated  by  the 
mycelium  has  been  moistened  by  the  rain.  But  the  moisture  absorbed  by  the 
wood  must  not  exceed  a  certain  amount.  Too  much  saturation  prevents  the 
phenomenon  of  luminosity  just  as  much  as  excessive  dryness.  If  the  wood  is 
removed  from  the  place  where  it  shines  so  well,  the  luminosity  rapidly  diminishes, 


DEVELOPMENT   OF   LIGHT   AND   HEAT.  503 

and  ultimately  entirely  vanishes,  although  apparently  the  relations  and  conditions 
of  life  are  exactly  the  same  as  before.  I  have  repeatedly  taken  up  luminous  wood 
at  night,  and  having  brought  it  home,  have  tried  to  reproduce  as  far  as  possible 
the  conditions  under  which  the  luminosity  existed  in  the  open;  in  the  first  night 
the  light  was  unweakened,  but  after  twenty-four  hours  it  had  usually  disappeared 
entirely.  If  the  luminous  wood  is  placed  in  a  closed  space  where  the  renewal  of 
the  air,  i.e.  of  oxygen,  is  not  carried  on  to  a  sufficient  extent,  the  luminosity  soon 
ceases.  A  rise  of  temperature  is  not  favourable  to  its  continuance,  principally  from 
the  fact  that  a  higher  temperature  brings  about  an  alteration  in  the  hygrometric 
condition  of  the  wood.  In  pure  oxygen  the  wood  shows  a  decrease  rather  than  an 
increase  of  light.  In  the  depths  of  the  forest  the  luminosity  may  be  observed  day 
after  day  for  more  than  a  week  on  the  same  trunk,  if  the  conditions  of  humidity 
remain  the  same. 

It  is  difficult  to  compare  the  light  emitted  from  the  mycelium  with  any  other. 
It  is  not  so  green  as  that  of  glowworms,  and  has  not  the  brilliancy  of  the  phos- 
phorescence of  the  sea;  it  is  a  dull  white  light.  It  most  resembles  that  of  pure  phos- 
phorus held  under  water.  In  the  gloom  of  the  forest  it  has  a  strange  and  therefore 
uncanny  appearance,  and  the  "  will-o'-the-wisp  "  may,  in  part  at  any  rate,  be  attri- 
buted to  luminous  wood.  If  a  decayed  tree-trunk  penetrated  by  the  light-giving 
mycelium  is  vigorously  struck,  so  as  to  split  it  into  hundreds  of  fragments,  which 
fly  out  in  all  directions  and  fall  scattered  on  the  ground,  each  splinter  becomes 
luminous,  and  the  dark  forest  ground  seems  to  be  strewn  with  dots  of  light.  The 
luminosity  of  these  fragments,  however,  comes  to  an  end  before  the  next  night. 

The  Rhizomorph  and  other  allied  agarics  only  exhibit  the  luminosity  in  their 
mycelium,  their  fructifications  remaining  dark  under  all  circumstances.  In  a  series 
of  other  agarics,  viz.  in  the  Brazilian  Agaricus  Gardneri,  in  Agaricus  igneus,  a 
native  of  Amboina,  in  Agaricus  noctilucens,  living  in  Manila,  and  in  Agaricus  olear- 
iua,  which  is  widely  distributed  through  the  Mediterranean  floral  district,  the  actual 
fructifications  emit  light,  usually  from  the  hymenium  developed  on  the  under  side 
of  the  cap,  but  more  rarely  the  stipe  also  which  bears  the  cap.  The  light  produced 
by  these  fungi  is  like  that  from  the  mycelium  of  the  agarics  described  previously, 
and  the  external  conditions  under  which  it  occurs  are  also  similar,  except  that  the 
hygrometric  state  has  not  such  a  noticeable  effect  on  it  as  on  the  luminous  wood 
permeated  by  mycelial  threads.  At  least  in  Agaricus  olearius,  a  mushroom  which 
grows  among  the  roots  of  olive  trees  and  forms  its  golden-yellow  fructification  in 
late  autumn,  the  luminosity  is  to  be  seen  equally  well  in  dry  and  wet  weather.  As 
soon  as  the  temperature  falls  below  +3°,  the  light  immediately  ceases;  it  is  best  at 
8-10°,  and  under  higher  temperatures  it  does  not  increase  but  gradually  diminishes. 
If  oxygen  be  kept  away  or  withdrawn  from  the  air,  the  luminosity  immediately 
vanishes,  but  as  soon  as  the  atmospheric  air  is  again  restored,  the  phenomenon 
reappears.  Dying  agarics  become  less  and  less  luminous,  and  their  light  is  ex- 
tinguished at  their  death.  It  is  to  be  noted  that  not  only  agarics  with  luminous 
hymenia,  but  also  those  with  luminous  mycelia,  emit  light  both  by  day  and  night. 


504 


FERMENTATION. 


On  fine  days  in  the  open  the  light  is  not  seen,  but  as  soon  as  these  structures  are 
brought  into  a  dark  room,  the  phenomenon  of  light  is  to  be  seen,  even  during  the 
day.  The  luminosity  of  the  night  is  not  increased  by  sun-illumination  during  the 
day,  and  consequently  the  phenomenon  has  nothing  in  common  with  that  peculiar 
phosphorescence  exhibited  during  the  night  by  fluor-spar,  which  has  previously 
been  exposed  to  sunlight. 

There  are  certain  organic  substances  which  shine  in  alkaline  solutions  when 
oxygen  is  present.  It  seems  natural  to  suppose  that  such  materials  are  formed  in 
the  agarics  mentioned,  and  that  oxygen  is  conveyed  to  them  in  respiration,  thus 
producing  the  phenomenon  of  light.  At  any  rate  this  would  be  the  simplest  way 
of  explaining  the  luminosity.  As  to  the  advantage  accruing  to  the  plant  itself,  we 
can  only  form  surmises.  It  seems  most  probable  that  the  fungus-flies  and  beetles 
which  deposit  their  eggs  in  the  mycelia  and  fructifications  of  Hymenomycetes, 
and  which  are  connected  with  the  distribution  of  their  spores  in  a  manner  to  be 
described  in  detail  later,  are  thereby  guided  to  the  fungi  in  the  dark  of  night. 
Many  of  these  flies  and  beetles  only  fly  at  night,  and,  like  so  many  winged 
nocturnal  animals,  direct  their  path  towards  a  luminous  object.  It  may  be,  there- 
fore, that  the  light  proceeding  from  the  agarics  cited  serves  as  an  allurement  and 
guide  to  the  night-flying  insects,  just  as  the  odour  and  brilliant  colouring  of  other 
Hymenomycetes  attracts  the  fungus-flies  and  beetles  which  swarm  in  broad  day- 
light. 

FERMENTATION. 

About  thirty  years  ago  the  difference  between  plants  and  animals  was  formu- 
lated as  follows: — Plants  transform  kinetic  into  potential  energy,  and  form  organic 
compounds  by  the  reduction  of  inorganic  food,  especially  from  carbonic  acid,  nitric 
acid,  and  water;  animals  transform  potential  into  kinetic  energy,  and  decompose 
and  burn  by  respiration  the  organic  compounds,  formed  by  green  plants,  which 
serve  them  as  food.  This  distinction,  however,  only  holds  good  in  part.  On  the 
one  hand,  plants  devoid  of  chlorophyll  are  not  taken  into  consideration,  and  on 
the  other,  it  has  been  established  that  green  plants  also  breathe,  and  therefore 
transform  potential  into  kinetic  energy.  The  respiration  of  plants  does  not  differ 
either  in  its  method  or  in  its  object  and  significance  from  that  of  animals.  In  both 
cases  the  living  protoplasm  withdraws  oxygen  from  the  air  in  order  to  convey  it 
to  certain  expressly  prepared  carbon  compounds  which  have  been  rendered  com- 
bustible, and  in  both  cases  these  carbon  compounds  are  burnt  in  order  that  the 
necessary  impelling  forces  may  be  obtained  for  further  life  and  growth.  But  the 
analogy  between  plants  and  animals  holds  still  further  in  this  respect.  When 
animals  which  are  tenacious  of  life,  e.g.  frogs,  are  placed  in  an  atmosphere  contain- 
ing no  oxygen,  they  do  not  immediately  perish,  and  do  not  at  once  cease  to  exhale 
carbon  dioxide,  consequently  they  still  convert  a  certain  amount  of  potential 
energy  for  a  short  time,  by  the  combustion  of  carbon  compounds  in  their  bodies. 
They  cannot  derive  the  oxygen  necessary  for  this  from  the  surrounding  air;  there 


FERMENTATION.  505 

is  nothing  left  for  them  except  to  obtain  it  from  the  organic  compounds  of  their 
own  bodies.  This  cannot  be  carried  on  permanently,  and  if  the  frog  is  kept  for 
a  long  time  in  an  atmosphere  without  oxygen,  it  will  at  length  die.  For  a  short 
period,  however,  it  is  able  to  prolong  its  life  in  the  way  indicated.  Exactly  the 
same  thing  is  seen  in  plants.  When  placed  in  a  chamber  from  which  free  oxygen 
is  absent,  they  do  not  immediately  die,  but  endeavour  for  a  short  time  to  retain 
their  life  by  utilizing  combined  oxygen,  by  withdrawing  it  from  nitrates  which 
have  been  absorbed  with  food,  or  from  the  organic  compounds  of  their  own 
bodies,  richly  furnished  with  oxygen.  The  oxygen  obtained  in  this  way  is  able 
to  replace  that  usually  derived  from  the  environment,  and  can  also  bring  about  a 
combustion  of  carbon  compounds;  it  can  therefore  provide  the  kinetic  energy  neces- 
sary for  the  continuance  of  life.  Carbon  dioxide  is  then  exhaled  from  plants,  even 
in  an  atmosphere  without  oxygen,  and  heat  is  liberated  just  as  in  normal  respira- 
tion. But  this  abnormal  source  of  energy  does  not  last  very  long.  If  free  atmos- 
pheric oxygen  continues  lacking,  the  plants  exposed  to  such  unaccustomed 
conditions  at  length  perish  from  exhaustion  and  suffocation. 

But  it  is  also  possible  that  living  plants  may  exist  in  a  region  which  is  indeed 
devoid  of  free  oxygen,  but  in  which  combined  oxygen  is  present.  Let  us  suppose 
that  a  plant,  hitherto  surrounded  by  atmospheric  air  from  which  it  obtained  free 
oxygen  for  use  in  respiration,  has  been  plunged  into  a  sugar  solution,  in  which, 
of  course,  free  oxygen  is  absent,  but  which  contains  a  large  quantity  in  combina- 
tion with  carbon  and  hydrogen  in  the  form  of  sugar.  Would  such  a  plant  be  able 
to  wrest  the  oxygen  from  the  sugar  and  to  utilize  it  for  itself?  In  most  cases 
certainly  not.  But  in  a  few  instances  the  living  protoplasm  has  the  power  of 
splitting  up  the  fluid  oxygen -containing  compounds  with  which  it  comes  into 
contact,  and  can  so  obtain  the  oxygen  necessary  for  the  continuance  of  its  life. 
It  can  also  make  use  of  other  materials  liberated  from  combination  in  the  decom- 
position. This  process  has  the  greatest  resemblance  to  respiration,  carbon  com- 
pounds are  actually  burnt  with  the  help  of  the  derived  oxygen;  carbon  dioxide  is 
exhaled,  and  heat  is  liberated.  The  plant,  the  living  protoplasm  of  which  accom- 
plishes all  this,  maintains  itself  alive,  prospers,  and  even  grows  and  multiplies  in 
a,  surprising  manner.  This  process,  however,  is  not  called  respiration,  but  is  known 
as  fermentation. 

Of  course  the  plants  producing  fermentation  must  not  be  supposed  to  include 
large  leafy  structures.  On  the  contrary,  they  are  all  very  insignificant  and  belong 
exclusively  to  spore-plants  which  are  devoid  of  chlorophyll,  and  which  are  gener- 
ally classed  together  under  the  name  of  fungi.  In  particular  there  are  the  four 
allied  families,  Bacteria,  Yeasts,  Moulds,  and  Basidiomycetes,  of  which  many  species 
in  certain  stages  of  development  are  capable  of  inducing  fermentation. 

Bacteria,  which  are  also  called  Fission-Fungi  or  Schizomycetes,  are  the  smallest 
of  all  living  organisms,  and  the  question  has  repeatedly  arisen  as  to  whether  they 
are  to  be  regarded  as  independent  organisms,  or  as  organized  portions  of  dead,  de- 
composing protoplasm.  The  discussion  of  this  question  will  be  left  to  the  second 


5()(j  FERMENTATION. 

volume.  Here  it  is  sufficient  to  remark  that  bacteria  appear  as  spherical,  oval,  or 
rod-like  cells,  which  develop  by  repeated  transverse  division  into  chain-like  or 
filamentous  structures,  very  much  resembling  hyphal  threads.  These  chains  of 
cells  break  up,  however,  sooner  or  later,  into  their  individual  members,  and  then 
look  as  if  they  had  been  split  into  fragments,  this  appearance  accounting  for  their 
name  of  "  Fission-Fungi ".  In  this  way  arise  colonies  of  irregularly  accumulated 
cells  which  are  frequently  embedded  in  a  mucilaginous  matrix.  Many  bacteria 
can  live  and  multiply  without  taking  free  oxygen  from  the  air.  They  obtain  the 
materials  necessary  for  this  by  setting  up  a  fermentation  in  their  immediate 
neighbourhood,  i.e.  a  splitting  up  of  carbohydrates  and  nitrogenous  compounds. 
Fermentation  gives  rise  to  very  different  products,  and  makes  itself  evident  in  very 
different  ways,  according  to  the  composition  of  the  body  attacked  by  the  bacteria, 
and  according  to  the  species  to  which  the  bacteria,  which  are  commencing  their 
destructive  activity,  belong.  In  many  instances  pigments  are  produced,  in  conse- 
quence of  the  decomposition,  which  colour  the  attacked  body  yellow,  red,  violet,  or 
blue;  at  another  time,  as,  for  example,  in  the  souring  of  milk,  a  molecule  of  milk- 
sugar  is  decomposed  into  two  molecules  of  lactic  acid;  or,  by  the  ferment  action 
of  the  Bacterium  aceti,  acetic  acid  is  produced  from  alcohol;  again  in  another 
instance,  sugar  is  split  up  into  dextrin,  mannite,  and  carbonic  acid,  by  a  species 
of  Bacterium,  in  the  so-called  viscous  fermentations.  One  of  the  commonest  fer- 
mentations is  that  to  which  albuminous  compounds  succumb,  known  as  putre- 
faction. The  albumens  are  decomposed  by  the  action  of  one  or  perhaps  several 
different  species  of  bacteria  into  tyrosin,  leucin,  various  amines,  volatile  fatty 
acids,  ammonia,  carbon  dioxide,  sulphuretted  hydrogen,  hydrogen,  and  water; 
and  some  of  these  make  themselves  evident  by  their  offensive  odour  in  a  most 
unpleasant  manner.  To  this  class,  too,  belong  the  most  notorious  of  all  bacteria, 
which  give  rise  to  a  decomposition  of  the  liquids  in  living  human  and  animal 
bodies,  which  deprive  the  blood  of  oxygen  and  bring  about  in  it  various  other 
decompositions  of  organic  compounds,  and  which  are  regarded  as  the  cause  of 
epidemic  and  endemic  diseases.  Contagions  and  miasmas  are  indeed  for  the  greater 
part,  if  not  wholly,  bacterial,  and  the  species  which  produce  splenic  fever  in  rumi- 
nants, diphtheria,  small-pox,  and  cholera  in  man,  are  of  such  great  interest  that  a 
whole  section  will  be  dedicated  to  them  in  the  next  volume. 

The  various  species  of  yeast,  which  are  called  Saccharomyces,  consist  of 
spherical  or  ellipsoidal  cells,  which  are  much  larger  than  the  cells  of  bacteria, 
and  also  multiply  in  quite  another  way.  They  increase  by  sprouting,  i.e.  knob- 
like  outgrowths  arise  on  the  surface  of  the  multiplying-cells  which  rapidly  enlarge, 
so  that  each  outgrowth  in  a  very  short  time  is  equal  in  size  to  the  cell  from  which 
it  originated.  The  daughter-cell  thus  formed  is  detached  from  the  parent-cell, 
and  may  now  itself  produce  daughter-cells  by  sprouting.  Occasionally  several 
successive  buddings  remain  joined  together,  and  then  form  colonies  which  some- 
what resemble  the  prickly  pears  or  opuntias  in  miniature.  Yeast  produces 
alcoholic  fermentation.  It  causes  grape-sugar  to  split  up  into  alcohol  and  carbon 


FERMENTATION.  507 

dioxide,  the  process  also  giving  rise  to  a  small  quantity  of  succinic  acid  and 
glycerine.  This  fermentation  is  never  very  noticeable  in  living  plants  in  free 
nature;  there  it  is  at  any  rate  only  carried  on  to  a  small  extent.  It  is  very 
important  in  the  extensive  artificial  production  of  alcoholic  beverages,  for  example, 
of  wine,  cider,  beer,  brandy,  "  pulque  ",  rum,  and  many  more,  from  grapes  and  other 
fruit,  and  from  grape-sugar,  obtained  from  starchy  seeds,  tubers,  and  roots. 

Moulds  consist  of  colourless,  elongated,  thin-walled  cells,  which  appear  to  the 
naked  eye  like  extremely  delicate  threads.  These  divide  up  by  the  intercalation 
of  transverse  walls,  but  they  do  not  separate  into  their  individual  elements  like 
the  bacteria.  The  threads  multiply  very  rapidly,  and  frequently  numerous  threads 
are  crossed  and  intertwined  like  the  threads  of  a  cobweb,  forming  a  loose,  white 
net-work.  They  generally  dwell  on  damp  or  fluid  substrata,  and  closely  invest 
them  with  their  crowded  threads.  They  also  penetrate  into  the  interior  of  these 
substrata.  The  cells  which  make  their  way  into  sugary  solutions  assume  another 
form;  they  remain  short,  and  increase  by  sprouting.  The  bud-forms  of  the  mould 
are  often  so  like  Yeast  that  they  are  mistaken  for  it.  Only  the  parts  of  a  mould 
which  respire  and  are  in  contact  with  the  oxygen  of  the  air  develop  spores,  these 
being  usually  distributed  by  currents  of  air;  the  parts  submerged  in  a  fluid  to 
which  the  free  oxygen  of  the  air  has  no  access  do  not  form  spores,  but  they 
multiply  with  incredible  rapidity,  just  like  Yeast  and  bacteria.  This  multiplication 
is  carried  on  at  the  expense  of  the  organic  compounds  contained  in  the  liquids  or 
succulent  bodies  attacked  by  the  mould.  The  changes  in  the  objects  attacked 
are  not  limited  to  the  acquirement  by  the  mould  of  as  many  organic  compounds 
as  it  requires  for  food,  but  the  whole  mass  becomes  decomposed  and  destroyed, 
and  finally  is  wholly  converted  into  carbon  dioxide,  water,  sulphuretted  hydrogen, 
ammonia,  and  other  volatile  substances — a  process  which  has  already  been 
described.  This  decomposition  brought  about  in  the  absence  of  oxygen  must  be 
termed  fermentation.  If  the  fluids  and  succulent  bodies  attacked  by  the  moulds 
contain  nitrogenous  compounds,  they  make  their  presence  known  by  the  unpleasant 
odour  they  give  off  when  undergoing  fermentation,  i.e.  putrefaction.  If,  on  the 
other  hand,  non-nitrogenous  compounds  are  fermented  by  a  mould,  alcohol  may 
be  produced.  In  sweet,  fresh  fruit  which  has  been  attacked  by  moulds,  the  cells 
of  the  mould  which  permeate  the  succulent  tissue  produce  a  fermentation  of  the 
juices  by  which  alcohol  and  ethereal  oils  first  arise  as  products  of  decomposition, 
and  by  whieh  the  characteristic  smell  of  putrescent  fruit  is  produced.  It  is 
ascertained  that  one  species  of  mould,  Aspergillus  niger,  when  on  the  surface 
of  a  tannin  solution,  consumes  the  tannin  in  the  presence  of  atmospheric  air,  by 
which  means  carbon  dioxide  is  formed.  When  this  same  species  is  submerged 
in  the  fluid,  and  has  no  free  oxygen  at  disposal,  it  splits  up  the  tannin  completely 
into  glucose  and  gallic  acid.  It  has  also  been  shown  that  mould  cells  which  get 
into  the  blood  of  living  men  and  animals  cause  it  to  decompose  as  do  bacteria, 
i.e.  they  produce  severe  diseases,  sometimes  ending  in  death.  Many  species  of 
mould  not  only  bear  the  high  temperature  of  the  blood  without  injury,  but 


508  FERMENTATION. 

even  develop  very  luxuriantly  there.  The  principal  genera  whose  species  cause 
fermentation  are  Mucor,  Aspergillus,  Penicillium,  Botrytis,  and  Eurotium. 

Finally,  in  addition  to  bacteria,  yeasts,  and  moulds,  the  mycelia  of  thosi  fungi, 
which  are  called  Basidiomycetes  (in  reference  to  their  characteristic  reproduction, 
which  will  be  described  in  the  next  volume),  can  induce  fermentation.  The 
thread-like  cell-chains  of  these  mycelia  look  like  mould-structures;  they  grow 
through  and  permeate  the  dead  bodies  of  plants  and  animals,  dung  and  refuse, 
and  black  meadow-soil,  the  humus  of  the  forest,  and  especially  the  trunks  of 
dead  trees.  But  living  plants  also,  especially  the  wood  of  living  trees,  may  be 
penetrated  by  these  mycelia,  and  the  tree  ultimately  killed  in  consequence.  When 
the  mycelial  threads  penetrate  into  the  wood  of  a  living  or  dead  tree  (see  fig.  32 3), 
they  are  not  satisfied  with  merely  piercing  the  cell-walls,  and  destroying  those 
places  only  with  which  they  come  immediately  into  contact,  and  absorbing  the 
results  of  the  destruction  as  food;  on  the  contrary,  we  have  an  extensive  decom- 
position, with  which  is  associated  a  liberation  of  carbon  dioxide,  water,  and  various 
volatile  materials,  not  well  known,  which  give  rise  to  a  peculiar  musty  smell.  The 
wood  loses  weight,  becomes  rotten,  and  wholly  transformed  into  a  mass  which 
on  drying  crumbles  to  powder,  or  into  a  fibrous  asbestos-like  substance.  Finally, 
it  disintegrates  into  dust.  In  popular  language  this  fermentation  produced  by 
the  mycelium  is  called  "rotting".  By  many  basidiomycetous  mycelia  the  wood 
is  not  only  changed  into  a  powdery,  but  even  into  a  liquid  mass,  as,  for  example, 
by  the  mycelium  of  the  detested  Dry-rot,  or  Wine-cask  Fungus. 

All  these  fermentations,  whether  caused  by  the  mycelia  of  Basidiomycetes,  the 
bud-forms  of  mould,  by  yeast,  or  by  bacteria,  have  one  thing  in  common,  that  they 
have  been  set  up  by  ferment-causing  cells,  i.e.  by  the  active  living  protoplasm 
within  them  without  the  excretion  of  any  special  chemically-active  materials 
which  would  come  directly  into  contact  with  their  surroundings.  The  living 
protoplasm  of  the  mycelia  named,  of  bacteria,  yeast,  and  mould,  itself  remains 
chemically  unaltered;  it  acts  most  energetically  in  the  immediate  neighbourhood, 
less  vigorously  further  off,  and  its  effect  diminishes  with  increasing  distance.  The 
effect  proceeding  from  the  ferment-cells  might  be  compared  with  the  concentric 
waves  produced  on  the  surface  of  water  into  which  a  stone  has  been  thrown. 
A  hypothesis  has  been  formulated,  according  to  which  the  groups  of  atoms  in  the 
ferment-protoplasm  are  supposed  to  be  oscillating  as  long  as  it  is  alive,. and  it  is 
imagined  that  these  oscillations  are  propagated  and  conveyed  to  the  'environment 
after  the  manner  of  a  wave-motion.  Alterations  in  the  construction  of  the  shaken 
molecules,  displacement  of  the  atoms,  and  decomposition  of  the  compounds  in 
question,  would  thus  result  from  the  shaking  so  produced.  It  has  even  been 
estimated  that  the  vibrations  which  proceed  from  the  living  protoplasm  of,  e.g. 
Yeast  cells,  are  propagated  to  a  distance  of  -gV  mm.  from  the  surface  of  the  cells, 
and  that  they  shake  and  alter  the  arrangement  of  the  molecules  of  sugar  even  at 
this  distance.  The  shaking  would  of  course  vary  according  to  the  specific  consti- 
tution of  the  protoplasm.  It  may  be  assumed  that  vibrations  differing  in  quality 


FERMENTATION.  509 

proceed  from  different  fermentative  agents,  and  that  consequently  diverse  decom- 
positions are  produced  by  different  bacteria. 

This  much  is  certain,  that  in  fermentation,  as  in  respiration,  a  certain  amount 
of  kinetic  energy  is  set  free  by  the  living  protoplasm  and  transmitted  to  the 
environment,  and  that  in  this  respect  fermentation  and  respiration  behave  alike. 
Thus  it  also  becomes  evident  that  fermentation  and  respiration  can  replace  and 
supplant  one  another.  In  many  moulds,  as,  for  example,  in  Mucor  racemo&us, 
this  substitution  is  very  noticeable.  If  the  mycelial  threads  rise  up  from  the 
liquid,  which  serves  for  its  substratum,  into  the  air,  and  if  they  can  draw  oxygen 
from  the  surrounding  atmosphere,  then  respiration  takes  place;  but  if  this  mould 
is  submerged  in  the  liquid,  so  that  it  can  no  longer  obtain  free  atmospheric 
oxygen,  then  the  cells  become  altered,  pass  into  the  sprout  form,  and  instead  of 
respiration  we  observe  in  them  the  ferment-action  described.  Submersion  may 
be  regarded  as  an  abnormal  condition  for  these  moulds,  and  perhaps  for  Yeast 
also,  but  for  bacteria  it  is  scarcely  so,  and  for  them  respiration  must  be  regarded 
rather  as  the  abnormal  condition. 

I  cannot  close  these  speculations  without  again  repeating  that  fermentation 
and  respiration  are  only  carried  on  by  living  protoplasm,  that  the  movements 
which  thus  proceed  from  the  protoplasm  cease  immediately  life  is  extinguished, 
and  that  these  movements  must  be  assigned  to  that  force  of  nature  acting  in  the 
protoplasm  for  which  I  claim  the  old  term  "  vital  force  ". 


GROWTH  AND  CONSTRUCTION  OF  PLANTS. 


1.— THEORY  OF  GROWTH. 

Conditions  and  Mechanics  of  Growth. — Influence  of  Growing  Cells  on  their  Environment. 

CONDITIONS  AND  MECHANICS  OF  GROWTH. 

Whoever  wishes  to  germinate  seeds  must  moisten  the  earth  selected  as  soil, 
or  else  must  supply  water  to  the  seeds  in  some  other  way.  The  seeds  absorb 
the  water;  the  embryo  bursts  its  covering,  sends  out  rootlets  into  the  ground,  and 
its  stem  and  leaves  grow  up  towards  the  light.  The  young  seedlings  must  now 
be  diligently  watered  if  they  are  to  flourish  and  increase  in  bulk,  for  they  require 
for  their  growth  an  astonishingly  large  amount  of  water.  Other  plant  organs 
which  it  is  desired  should  grow  or  be  kept  in  vigorous  development  are  like  the 
seeds,  and  the  suitable  watering  of  cultivated  land  is,  and  always  has  been,  one 
of  the  fundamental  conditions  of  plant  culture.  In  uncultivated  districts  the 
dependence  of  growth  on  the  water  supply  appears  no  less  remarkable.  Where 
vegetative  activity  is  brought  to  a  standstill  not  by  the  cold  of  winter,  but  by 
heat,  the  commencement  of  the  rainy  season  is,  each  year,  the  signal  for  the 
revival  of  growth.  The  amount  and  duration  of  the  rainfall  govern  in  a  most 
striking  way  the  whole  progress  of  plant  development.  As  soon  as  the  first 
moisture  soaks  through  the  soil,  after  a  long  drought,  the  plants  wake  up  from 
their  lethargy,  the  dry,  sunburnt  landscape  becomes  adorned  with  vivid  green, 
and  the  luxuriance  of  the  shoots  and  leaves  arising  from  the  seeds  and  buds 
stands  in  strict  proportion  to  the  quantity  of  water  daily  supplied  to  the  growing 
plants. 

Why  do  plants  require  these  quantities  of  water?  The  answer  to  this  question 
has  already  been  partly  given  in  a  previous  section  of  this  book,  when  it  was 
shown  how  the  absolutely  necessary  mineral  food-salts  were  taken  up  by  means 
of  water;  how  the  water  in  which  the  food-salts  are  dissolved  is  conveyed  by 
root-pressure  and  by  suction  to  the  place  of  consumption.  But  this  is  certainly 
not  the  only  significance  water  has  for  plants,  for  it  would  leave  unexplained 
why  growing  seedlings  which  cannot  yet  absorb  mineral  food  from  the  earth,  and 
which  do  not  even  require  it,  still  consume  so  much  water.  It  must  also  be 
remembered  that  those  chemical  processes  in  vegetable  cells  in  which  mineral  food- 
salts  are  worked  up  do  not  yet  themselves  constitute  growth,  but  only  a  prepara- 
tion for  growth.  Mineral  salts  play  an  important  rdle  in  the  transformations  going 


510 


CONDITIONS   AND   MECHANICS   OF   GROWTH.  511 

on  in  the  living  cells,  and  the  manifold  changes  in  the  production  of  organic  com- 
pounds of  the  food  taken  in  from  outside,  and  in  the  preparation  of  these  com- 
pounds for  building  materials.  But  they  are  not  directly  concerned  in  the  insertion 
and  fixing  of  the  building  materials  in  the  living  cell-body,  in  the  further  growth 
of  protoplasm,  and  in  the  increasing  dimensions  of  the  growing  cells,  which  last- 
named  processes  alone  may  be  looked  upon  as  growth.  Exactly  how  far  water  is 
concerned  in  growth  will  be  described  in  the  following  lines. 

Although  only  very  little  is  known  with  regard  to  the  minute  structure  of  the 
protoplasm  of  the  cell,  yet  this  much  is  beyond  question,  that  it  consists  of  firmer 
and  softer  parts,  which  form  an  extremely  complicated  net-work,  ever  varying  in 
structure  from  species  to  species,  and  with  meshes  filled  with  very  many  different 
substances,  with  water,  fluid  carbohydrates,  albuminous  compounds,  dissolved  salts, 
&c.  It  may  also  be  imagined  that  fluid  substances  can  be  interpolated  in  the  net- 
work, resembling  it  in  structure  and  in  consistency  at  the  moment  of  insertion; 
that  is  to  say,  which  receive  the  same  molecular  arrangement,  and  so  become  an 
organized  portion  of  the  cell-body.  The  cell-wall  also,  at  the  periphery  of  the 
protoplasm,  must  possess  a  structure  which  renders  it  possible  that  between  the 
already- formed  firm  portions  fluid  molecules  can  be  inserted,  which  then  assume 
the  properties  of  those  established  portions.  This  insertion,  however,  presupposes 
an  extension  of  the  firm  parts  already  present,  a  separation  of  the  groups  of 
molecules  of  the  organized  structures,  and  a  place  for  the  particles  to  be  inserted, 
and,  on  the  other  hand,  repelling  and  attractive  forces  which  control  the  portions 
to  be  introduced. 

We  are  now  prepared  to  admit  that  here  a  very  important  part  must  be 
assigned  to  the  turgidity  of  cells.  As  has  been  shown,  the  cell- sap  of  growing 
cells  is  acid,  and  the  acids  and  acid  salts  contained  in  it  attract  water  from  their 
surroundings  with  considerable  energy.  The  water  thus  brought  into  the  vacuoles 
of  the  protoplasm  exercises  a  strong  pressure  on  the  peripheral  layer,  and  indeed 
on  the  cell- wall  as  well  as  on  the  protoplasm,  which  pressure  first  of  all  causes  an 
extension  of  these  layers  beyond  the  normal  cohesive  limit.  By  the  elasticity  of 
the  extended  layers  obviously  a  pressure  is  exercised  on  the  fluid  in  the  interior, 
and  this  condition  of  mutual  tension  is  called  turgidity.  In  order  to  explain  the 
existence  of  this  turgidity,  it  must  be  taken  for  granted  that  the  water  conveyed 
into  the  vacuoles  of  the  protoplasm  by  the  attraction  of  the  acids  and  acid  salts 
does  not  go  back  again,  in  spite  of  the  pressure  it  exercises  on  the  surrounding 
layers;  that  it  rather  is  held  fast  by  the  molecules  of  sugar  and  albumin  in  the 
protoplasm.  Experience  confirms  this  supposition,  and  it  is  evident  that  water 
penetrates  with  great  energy  from  the  surroundings  into  the  cells,  that  the  cell 
swells,  the  peripheral  cell-layers  experience  a  tension,  and  that  yet  no  water  pro- 
ceeds through  them.  When  protoplasm  forces  out  water  in  consequence  of  a 
stimulation,  or  when  the  strained  layers  are  artificially  punctured,  only  then  does 
the  fluid  come  out  of  the  rent  formed  like  a  tiny  spring.  But  this  again  only 
shows  that  the  fluid  in  the  interior  is  subject  to  a  strong  centra-pressure  from  the 


519  CONDITIONS   AND   MECHANICS   OF   GROWTH. 

peripheral  layers.  This  pressure  is  obviously  stronger  the  more  elastic  and  the 
firmer  are  the  peripheral  layers;  and  the  elastic  outermost  parietal  layer  of  the 
cell  is  of  course  adapted  to  exercise  an  especial  reaction  on  the  fluid  in  the  interior 
of  the  cell.  But  that  a  pressure  exists  both  towards  the  interior  and  in  the  reverse 
direction,  in  structures  which  have  no  cell-wall  and  consist  only  of  protoplasm, 
is  shown  by  the  fact  that  if  rents  are  made  in  the  outermost  layer  of  myxomycetous 
plasmodia,  fluid  substances  immediately  pour  out. 

It  is  indeed  a  matter  of  course  that  in  a  swollen,  turgid  cell  the  molecules  of 
the  peripheral  extended  layers  will  be  separated  beyond  the  usual  limit  of  cohesion, 
and  this  is  near  to  assuming  that  in  the  widened  interstices  so  formed  fluid 
materials  are  forced  which  become  firm  the  moment  they  are  deposited,  and  then 
resemble  in  every  way  the  molecules  they  have  driven  apart.  This  intercalation 
and  hardening  of  constructive  materials,  which  indicates  an  accompanying  increase 
in  the  bulk  of  the  organized  substances,  is  to  be  regarded  as  growth.  We  thus 
obtain  a  conception  of  the  mechanism  of  growth,  which,  though  only  hypothetical, 
is  in  harmony  with  the  external  visible  phenomena.  We  are  led  to  it  especially 
by  the  fact  that  only  cells  which  are  turgid  grow,  whilst  cells  stop  growing,  even 
although  the  necessary  amount  of  fluid  building  material  is  present,  as  soon  as 
their  turgidity  diminishes. 

The  turgidity  of  cells,  that  is,  the  presence  in  them  of  water  necessary  for  their 
swelling,  is,  however,  only  one  condition  of  growth;  a  second  condition,  no  less 
important,  is  warmth.  Without  heat  there  is  no  growth.  When  in  the  temperate 
zones,  where  the  year  is  divided  into  summer  and  autumn,  winter  and  spring,  the 
summer  draws  towards  a  close,  and  the  days  become  shorter  and  shorter,  when 
during  the  long  nights  the  soil  loses  more  warmth  by  radiation  than  it  gains  during 
the  day,  and  when,  too,  plants  become  very  much  cooled,  growth  above-ground 
entirely  ceases,  and  the  whole  energy  of  the  plants  is  concentrated,  as  we  have 
shown  in  previous  sections,  in  changing  itself  into  a  chrysalis  for  the  winter,  in 
withdrawing  from  the  deciduous  foliage  the  materials  which  can  be  employed 
in  the  ensuing  period  of  vegetation,  and  in  lodging  them  in  protected  store-rooms. 
During  the  winter,  then,  the  cooled  portions,  unprotected  against  frost,  rest,  and 
their  growth  is  completely  interrupted.  At  length  winter  is  past,  the  last  snow 
has  vanished  under  the  breath  of  mild  spring  breezes;  the  hard  frozen  earth  is 
liberated  from  the  bondage  of  the  frost;  everywhere  new  life  stirs,  buds  swell, 
trees  adorn  themselves  with  flowers  and  fresh  foliage,  the  meadows  become  green, 
seeds  germinate,  and  the  crops  in  the  fields  spring  up  vigorously,  to  the  joy  of  the 
farmer.  On  warm,  sunny  spring  days  everything  grows  with  astounding  rapidity: 
on  cool,  dull  days  the  increase  is  only  small.  If  occasionally  a  relapse  occurs,  and 
the  temperature  again  sinks  low,  then  the  growth  is  wholly  arrested.  It  has 
been  found  that  the  increase  of  young  herbaceous  plants  on  two  successive  days  had 
sunk  in  consequence  of  a  sudden  storm  and  visitation  of  cold  from  8  cm.  to  J  cm. 
There  is  no  doubt  that  such  a  decrease  of  growth  stands  in  causal  connection  with 
the  fall  of  temperature,  and  also  that  quick  growth  is  to  be  laid  to  the  account  of 


EFFECTS   OF   GROWING   CELLS   ON    ENVIRONMENT.  513 

a  rapid  increase  of  heat,  provided,  of  course,  that  the  other  factor  of  growth  pre- 
viously indicated,  viz.  water,  is  present  in  sufficient  quantity. 

It  has  been  shown  in  a  previous  section  that  the  mineral  salts,  which  plants 
require  for  the  production  of  building  materials,  are  brought  by  means  of  water 
to  the  place  of  need,  and  that  this  transporting  water  is  raised  from  below  by 
evaporation  from  the  surface  of  organs  exposed  to  the  air  and  sun.  This  evapo- 
ration, however,  requires  much  warmth,  and  there  can  be  no  doubt  that  the 
hastened  or  retarded  development  of  vegetation  is  partly  dependent  on  quickened 
or  retarded  transpiration;  that  is  to  say,  on  the  greater  or  less  amount  of  heat 
supplied.  The  conduction  of  food-salts  by  means  of  water  from  the  soil  is,  how- 
ever, not  by  any  means  growth;  it  is  only  a  preparatory  process,  as  also  is  the 
formation  of  organic  materials  in  green  cells,  and  the  complicated  transformations 
and  distribution  of  materials  which  follow  the  elevation  of  the  water  from  the 
ground.  Warmth  is  an  essential  condition  of  that  process  which  is  being  here 
discussed,  that  is  of  growth  in  its  narrow  sense,  as  well  as  of  all  these  preparatory 
processes. 

The  part  taken  by  heat  in  actual  growth,  that  is,  in  the  transformation  of  fluid 
building  materials  into  firm,  organized  portions  of  the  plant-body,  and  increase 
of  bulk  of  the  cells,  cannot  be  essentially  different  from  that  which  occurs  in 
other  molecular  re-arrangements  and  chemical  changes.  Heat,  according  to  pre- 
valent theories,  is  the  expression  of  vibration  of  ultimate  particles.  Those  vibra- 
tions of  ether  which  are  known  as  free  heat  can  induce  a  corresponding  motility 
of  the  molecules  in  any  ponderable  body.  Similarly,  heat  induces  a  state  of 
motility  amongst  the  molecules  of  living  protoplasm.  We  must  imagine  that 
work  is  done  upon  the  organic  bodies  which  constitute  the  building  materials 
of  plants,  that  they  are  led  in  a  fluid  state  to  the  regions  where  they  are  required, 
and  there  transformed  into  solid  organized  matter.  In  this  way  free  heat  is 
transformed  into  latent  heat,  and  in  this  sense  we  may  regard  growth  as  a  con- 
sumption of  free  heat.  Accompanying  this  organizing  action  of  heat  there  is  an 
insertion  of  new  molecules  between  the  pre-existing  ones.  The  separation  of  these 
latter  is  of  course  brought  about,  as  already  described,  by  turgidity.  Thus,  by 
the  co-operation  of  heat  and  turgidity,  fluid  organic  materials  are  changed  into 
firm,  solid,  organized  substances,  and  in  this  way  the  organized  portions  increase 
in  bulk,  in  other  words,  they  grow. 


EFFECTS   OF  GEOWING  CELLS  ON  ENVIRONMENT. 

Work  is  not  only  performed  in  the  interior  of  cells,  but  pressures  also  come 
into  action  which  operate  on  the  surroundings  with  irresistible  power.  What 
the  cells,  apparently  so  delicate,  are  able  to  perform,  borders  almost  on  the  in- 
credible. 

Where  the  filamentous  hyphal  threads  of  crustaceous  lichens  have  penetrated 

VOL.  I. 


514  EFFECTS   OF   GROWING   CELLS   ON    ENVIRONMENT. 

into  the  tiny  crevices  of  stone,  they  crack  and  crumble  the  permeated  substratum 
not  only  by  lateral  pressure,  but  they  act  also  lever -wise,  and  vigorously  press 
up  the  shattered  particles.  The  absorbent  cells  or  rhizoids  of  mosses  and  liver- 
worts also  exercise  a  like  action  on  their  substratum,  and  this  is  maintained,  as  in 
the  lichens,  essentially  by  the  fact  that  substances  are  excreted  from  the  growing 
cells  by  which  the  substratum  is  partially  converted  into  soluble  compounds. 
Moreover,  the  pressure  which  these  delicate  cells  exert  on  the  substratum  by 
their  growth  may  be  demonstrated  by  experiment.  If  liverworts  are  laid  upon 
damp  folded  filter -paper  in  a  space  saturated  with  vapour,  in  forty -eight  hours 
they  will  send  out  rhizoids  which  grow  through  the  paper.  The  holes  in  which 
the  cells  of  the  rhizoids  are  now  seen  certainly  did  not  previously  exist  in  the 
paper.  The  felt  of  threads  in  the  filter-paper  is  so  dense  that  starch-grains  of 
maize,  having  a  diameter  of  only  about  2  thousandths  of  a  millimetre,  cannot  find 
sufficient  space  to  slip  through,  and  thus  still  less  can  the  rhizoids  of  liverworts 
penetrate  the  felt,  as  these  have  a  diameter  of  from  10  to  35  thousandths.  The 
holes  must,  therefore,  be  first  formed  by  the  growing  cells  of  the  rhizoids.  The 
threads  of  the  felt  must  be  powerfully  driven  asunder,  and  this  requires  at  any 
rate  a  comparatively  large  expenditure  of  force. 

The  hyphal  threads  of  a  mushroom,  which  unite  to  form  dense  fructifications 
and  grow  up  from  the  subterranean  mycelium  in  a  comparatively  short  time,  often 
raise  considerable  pieces  of  earth,  and  the  cap-shaped  fructifications  of  Lactarius 
scrobiculatus,  Agaricus  vellereus,  and  Hydnum  repandum  are  indeed  frequently 
thickly  covered  with  larger  and  smaller  fragments  of  earth,  raised  by  them  during 
their  upward  growth.  An  instance  is  also  known  in  which  a  stone  of  160  kg. 
was  raised  and  shifted  by  the  growing  fructification  of  a  fungus  of  the  mushroom 
tribe. 

Nor  is  the  pressure  which  the  growing  cells  of  flowering  plants  exert  on  their 
environment  less  considerable.  The  absorbent  cells  of  roots  embedded  in  the  earth, 
which  are  called  root-hairs,  appear  fairly  straight,  although  the  spaces  between  the 
particles  of  soil  filled  with  air  and  water  are  certainly  not  rectilinear.  It  cannot  be 
doubted,  therefore,  that  the  root-hairs  in  spite  of  their  delicacy,  nevertheless  push 
the  small  particles  of  earth  on  one  side,  and  in  their  growth  follow,  as  nearly  as 
possible,  a  straight  course.  The  apices  of  the  main  roots  of  flowering  plants,  when 
they  grow  downwards,  form  actual  channels  by  pressure  on  their  environment, 
pushing  the  portions  of  soil  powerfully  asunder,  and  penetrating  into  the  ground 
like  a  gimlet.  And  it  would  be  a  mistake  to  suppose  that  they  are  only  drawn 
downwards  by  gravity.  The  roots  of  bean-seeds  which  have  been  germinated  in  a 
layer  of  water  spread  above  quicksilver  actually  penetrate  into  the  quicksilver.  It 
has  often  been  noticed  that  the  roots  of  trees  which  have  reached  fissures  in  walls 
or  clefts  of  rocks  are  able  to  shatter  the  walls  and  to  crack  the  stone  by  their 
further  thickening.  One  at  least  out  of  the  great  number  of  instances  may  here  be 
noticed.  On  either  side  of  the  little  Tyrolese  Gschnitz-thal  are  terraces  strewn  with 
large  blocks  of  stone,  which  are  considered  ancient  diluvial  moraines.  The  blocks 


EFFECTS   OF   GROWING   CELLS   ON   ENVIRONMENT. 


515 


of  stone  are  composed  for  the  most  part  of  crystalline  schist,  especially  of  gneiss,  in 
which  mica  is  arranged  in  almost  parallel  streaks.  On  one  of  these  blocks  (repre- 
sented in  fig.  130),  at  a  height  of  2  metres,  a  larch  has  long  ago  established  itself 
and  rooted  firmly,  so  that  the  strongest  of  its  roots  grow  downwards  in  a  cleft 
parallel  to  the  direction  of  the  mica  streaks.  By  the  thickening  of  this  root  the 
crevice  became  widened;  half  of  the  upper  block  was  separated  from  the  lower  and 
was  raised  about  30  cms.  It  is  estimated  that  the  weight  of  this  raised  portion 
amounts  to  1400  kg.,  and  the  root  which  was  able  to  raise  this  burden  exhibits  in 


Fig.  130. — Elevation  of  a  Block  of  Stone  in  consequence  of  the  growth  in  thickness  of  a  Larch  Root. 

its  thickest  part  a  diameter  of  only  30  cm.  Moreover,  the  burden  overcome  by  this 
larch  root  is  small  in  comparison  with  that  raised  by  the  roots  of  old  trees.  The 
large  superficial  roots  which  creep  over  the  ground  of  the  forest  like  gigantic  snakes 
were  not  always  situated  in  this  position.  As  long  as  the  trees  were  young  their 
roots  extended  under  the  ground.  Only  with  increasing  thickness  did  these  roots, 
pressing  against  the  firmly  compacted  earth  lying  beneath  them,  become  visible  above 
ground,  since  they  burst  through  the  layer  of  earth  situated  above  them.  But  with 
this  must  also  be  connected  the  elevation  of  the  whole  trunk  with  its  boughs,  which 
all  bear  upon  the  roots,  and  often  weigh  several  thousand  kilogrammes. 

It  is  a  matter  of  course  that  growing  stem-structures  also  exercise  a  considerable 
pressure  on  their  environment.  Those  underground  stems  which  are  called  runners 
do  not  in  this  respect  differ  materially  from  roots,  and  are  similarly  able  to  shift 


516  EFFECTS   OF   GROWING   CELLS   ON   ENVIRONMENT. 

and  press  asunder  small  stones  and  clumps  of  earth.  In  many  plants  the  growing 
points  of  the  runners  are  covered  with  hard  scales,  which  produce  exactly  the  same 
effect  as  the  points  of  an  auger.  This  applies  especially  to  several  grasses  (e.g. 
Calamagrostis,  Lasiagrostis,  and  Agropyrum).  The  runners  of  the  common  creep- 
ing Couch-grass  (Agropyrum  repens)  bore  through  the  roots  of  trees,  and  not  only 
through  old  and  rotten  but  also  through  young  vigorous  specimens.  The  runners 
of  the  Couch-grass  are  often  found  penetrating  through  the  centre  of  potato- 
tubers,  and  it  has  been  confirmed  experimentally  that  these  runners  in  their  growth 
are  capable  of  even  boring  through  discs  of  tin-foil.  Very  instructive  also  is  the 
penetration  of  old  tree-trunks  by  the  stems  of  various  small  shrubs  and  shrubby 
trees  whose  growing  points  are  comparatively  delicate  and  soft  in  texture,  and  are 
not  beset,  like  those  of  the  Couch-grass,  with  hard  pointed  scales.  Almost  every- 
where in  our  mountain  regions  are  to  be  seen,  in  places  where  not  very  long  before 
a  forest  has  been  cleared,  dead  stumps  of  fir-trees,  rising  perhaps  a  metre  above 
the  ground,  and  overgrown  with  cranberry  and  bilberry  bushes.  The  surface 
where  the  saw  has  cut  through  the  huge  trunk  is  partly  overgrown  with  the  same 
plants  as  those  growing  in  the  soil  round  about,  and  it  has  a  very  peculiar  appear- 
ance when  on  these  decayed  stumps,  as  if  on  the  platform  of  the  base  of  a  pillar, 
small  colonies  of  cranberry  bushes  are  seen  to  flourish  luxuriantly — a  story 
higher  than  on  the  surrounding  ground.  Without  closer  investigation  anyone 
would  think  that  these  bushes  had  germinated  from  seeds  which  had  previously 
fallen  into  the  cracks  of  the  stem  section,  and  it  is  not  a  little  surprising  therefore, 
on  splitting  such  old  tree-stumps,  to  find  that  this  is  not  the  case,  but  that  rather 
the  cranberry  bushes  of  the  surrounding  forest-ground  have  sent  out  their  runners 
into  the  lower  portion  of  the  tree-trunk,  and  that  these  have  then  grown  up  through 
the  rotten  wood  of  the  stump — especially  through  the  decayed  part  between  the  wood 
and  bark — until  they  have  again  reached  the  daylight  above  on  the  exposed  section, 
showing  at  any  rate  that  they  must  have  exerted  a  very  considerable  pressure  on 
their  surroundings.  The  thin  stems,  also,  of  plants  growing  on  boulders  have 
frequently  to  make  a  new  pathway  for  themselves  when  their  habitat  has  been 
covered  by  a  torrent  with  sand  and  stones  a  span  high,  and  thus  have  to  push  out 
of  the  way  obstacles  of  comparatively  large  dimensions.  On  a  forest  soil  covered 
with  sand  and  boulders  I  saw  indeed  how  the  delicate  thread-like  stem  of  a  winter- 
green  (Pyrola  secunda)  had  grown  up  more  than  60  cm.,  and  in  doing  so  had  pushed 
on  one  side  stones  of  a  gramme  weight.  If  peas,  beans,  and  other  large  seeds  are 
buried  in  the  earth  and  allowed  to  germinate,  it  may  be  seen  how  by  the  growth 
of  the  seedling  small  clods  of  earth  and  stones  are  raised,  and  the  earth  in  which 
pine-seeds,  oats,  and  beech-nuts  have  been  embedded,  looks  when  the  seeds  are 
germinating  as  if  it  had  been  rummaged  and  thrown  up  by  mice.  A  fine  example 
of  external  work  done  by  growing  stems  must  yet  be  instanced  in  the  growth  in 
height  of  the  forest-trees  which  we  have  daily  before  our  eyes,  but  only  too  easily 
overlook  on  account  of  its  commonness.  A  young  beech  trunk  50  cm.  thick  will 
raise  each  year  a  crown  which  has  a  weight  of  two  thousand  kilogrammes 


SOURCES    OF    HEAT.      TRANSFORMATION    OF   LIGHT   INTO   HEAT.  517 

through  a  metre,  and  in  still  larger  forest-trees  the  figures  become  even  more 
impressive. 

And  all  this  is  accomplished  by  the  invisible  atoms  of  the  living  protoplasm, 
which,  set  in  motion  by  heat,  alter  their  position,  attract  and  repel  one  another, 
displace  and  travel  between  one  another,  assume  new  groupings,  and  in  these  new 
arrangements  appear  outwardly  to  our  senses  in  altered  form  and  increased  volume. 

On  glancing  over  these  effects  of  growing  cells  and  groups  of  cells,  one  is 
reminded  involuntarily  of  the  analogous  phenomena  of  ice  crystallization.  When 
ice  is  formed  in  a  glass  bottle  filled  with  water,  it  bursts  the  vessel  with  irresistible 
force,  and  the  splitting  of  masses  of  rock  in  high  mountains  and  in  all  those  regions 
where  the  temperature  in  winter  sinks  below  freezing-point  depends  in  no  small 
degree  on  the  freezing  of  the  water  which  has  penetrated  into  the  smallest  crevices 
and  rocky  clefts.  And  yet  there  is  an  essential  difference  between  growth  and 
crystallization.  Crystals  are  formed  spontaneously  from  fluid  substances,  and 
grow  from  the  depositions  of  small  atoms  on  their  surface.  Vegetable  cells,  on 
the  other  hand,  never  arise  spontaneously  from  fluid  materials,  but  always  only  by 
means  of  an  already  present  organized  and  living  mass  of  protoplasm.  Thus  all 
growth  in  living  things  is  really  only  a  further  development  of  what  already  exists. 
The  crystal  can  again  be  transformed  into  a  formless  fluid  mass,  can  be  reconstructed 
from  this  fluid,  and  this  alternation  may  be  repeated  innumerable  times.  In  plants, 
on  the  other  hand,  the  passage  from  the  formed,  organized,  to  the  formless,  fluid 
condition  is  synonymous  with  death,  and  from  the  gases  and  fluids  which  are 
derived  from  the  decomposition  of  a  vegetable-cell,  a  plant-cell  never  again  forms 
itself  spontaneously,  that  is,  without  the  interposition  of  a  living  agent.  While,  as 
above  remarked,  crystals  grow  by  the  deposition  of  small  particles  on  their  surface, 
growth  of  protoplasm  takes  place  by  the  interpolation  of  new  molecules  between 
those  already  present;  these  are  separated  from  one  another,  and  only  subsequently 
can  parts  of  the  cell  increase  by  deposition  brought  about  by  living  protoplasm. 


2.    GEOWTH   AND   HEAT. 

Sources  of  Heat — Transformation  of  Light  into  Heat. — Influence  of  Heat  on  the  Configuration  and 
Distribution  of  Plants. — Measures  which  protect  Growing  Plants  from  Loss  of  Heat. — Freezing 
and  Burning. — Estimation  of  the  Heat  necessary  for  Growth. 

SOURCES  OF  HEAT.     TRANSFORMATION  OF  LIGHT  INTO  HEAT. 

Whence  do  plants  derive  the  heat  necessary  for  their  growth  ?  With  regard  to 
this  question  one  may  first  of  all  think  of  that  heat  which  is  liberated  by  the  plant 
itself  in  respiration,  and  which  can  again  find  employment  immediately  after  its 
release,  not  only  in  metabolism  and  transport  of  materials,  but  also  in  growth. 
Further,  we  may  be  reminded  of  that  heat  which  is  liberated  by  the  breathing  of 


518  SOURCES   OF   HEAT.      TRANSFORMATION   OF   LIGHT   INTO   HEAT. 

animals  and  in  various  other  instances  of  slow  and  quick  combustion  of  organic 
bodies,  which  the  growing  plants  can  now  and  then  directly  utilize.  These,  how- 
ever, are  only  derived  sources  of  heat.  Heat  which  is  liberated  in  respiration  is 
really  only  the  sun's  rays  which  the  plants  have  absorbed  on  a  previous  occasion, 
and  ultimately,  so  far  as  it  comes  under  consideration  for  the  life  of  plants,  all  heat 
is  derived  from  the  sun.  The  heat  which  is  conducted  to  plants  from  the  soil,  from 
water,  and  from  air,  also  takes  its  origin  from  the  sun,  which  is  therefore  to  be 
looked  upon  as  the  fountain-head  of  all  the  heat  utilized  by  plants. 

It  has  been  found  that  the  sun  sends  out  three  kinds  of  rays  distinguished  by 
their  different  periods  of  vibration,  and  known  respectively  as  heat  rays,  light  rays, 
and  chemical  rays.  These  three  undulating  movements  of  the  ether  interfere  with 
each  other  in  their  course  as  little  as  the  wave-circles  which  intersect  on  the  surface 
of  water.  We  recognize  and  measure  them  by  their  effects.  As  soon  as  they  strike 
a  body,  work  is  performed  by  the  active  force  of  these  ether  waves  which  we  picture 
to  ourselves  as  movements  of  the  molecules  and  atoms  of  the  body  affected;  and  this 
work  appears  either  as  heat,  or  light,  or  chemical  change.  But  it  is  exceedingly 
remarkable  that  only  that  movement  which  we  regard  as  heat  can  produce  that 
transformation  of  building  substances  into  organized  materials,  which  is,  in  other 
words,  growth.  The  vibrations  which  constitute  light,  and  whose  great  importance 
in  the  formation  of  the  constructive  materials,  and  generally,  of  organic  compounds 
from  inorganic  food,  has  been  previously  described  in  detail,  are  not  able  to  cause 
such  an  effect,  at  least  directly.  There  are  even  instances  which  justify  the  opinion 
that  growth  is  actually  restricted  and  hindered  by  light.  This  much  is  certain, 
that  growth  can  proceed  in  the  deepest  gloom,  if  only  the  two  earlier-mentioned 
factors — turgidity  and  heat — are  undiminished.  Seeds  and  the  majority  of  spores 
germinate  in  darkness.  The  cells  of  underground  stems  and  scale  leaves,  those  of 
roots  embedded  deep  under  the  soil,  as  well  as  the  mycelia  of  fungi,  grow  in  regions 
wholly  deprived  of  light.  Moreover,  plant  organs  which  are  brought  from  the  light 
into  darkness  continue  to  grow  there,  provided  always  that  the  necessary  amount 
of  moisture  and  heat  be  supplied  to  them. 

Nevertheless  very  numerous  experiments  tend  to  prove  that  growth  can  be 
assisted  by  light.  The  following  is  one  of  the  most  remarkable.  If  plants  are 
cultivated  in  two  places,  identical  as  to  the  amount  of  heat  affecting  them  during 
growth,  but  differing  in  the  intensity  and  duration  of  the  flow  of  light,  they  will 
exhibit  a  quicker  growth  in  the  place  where  the  light  can  act  on  them  more  power- 
fully and  for  a  longer  time.  Thus  plants  grow  much  more  quickly  in  the  far  north, 
where  they  are  daily  illuminated  for  twenty  hours,  than  in  southern  latitudes 
where  they  are  exposed  to  the  light  for  only  twelve  hours,  even  although  in  the 
same  space  of  time  comparatively  less  heat  reaches  them  in  their  northern  habitat. 
From  the  small  table  inserted  opposite,  giving  the  commencements  of  the  flowering 
periods  best  adapted  for  the  comparison  of  a  definite  amount  of  growth  in 
several  widely-distributed  plants,  at  Athens,  Vienna,  and  Christiania,  it  may  be 
seen  that  Athens  is  about  forty-six  days  earlier  than  Vienna,  but  Vienna  only 


SOURCES   OF   HEAT.      TRANSFORMATION   OF   LIGHT   INTO   HEAT. 


519 


Commencement  of  Flowering. 

Athens. 
37"  58'  North  Lat. 

Vienna. 
48'  11'  North  Lat. 

Christiania. 
69*  55'  North  Lat. 

Hepatica  (Hepatica  triloba),  .  .  . 
Sloe  (Prunus  spinosa\  

22nd  January 
5th  February 

llth  March 

1  Rrh   A  rvril 

2nd  April 

IftfVi   Mav 

Gean  (Prunus  avium),  

1st  March 

IQth  Anril 

IQth  Mav 

Wild  Pear  (Pyrus  communis),  . 
Barberry  (Berberis  vulgaris),  ... 
Elder  (tiambucus  nigTa),  

20th  March 
10th  April 
15th  April 

23rd  April 
9th  May 
SfitVi  TVTav 

22nd  May 
6th  June 

Privet  (Ligustrum  vulgare\  .... 
White  Lily  (Lilium  candidum\ 

20th  April 
1st  May 

4th  June 
24th  June 

6th  July 
16th  July 

about  twenty-nine  days  before  Christiania.  And  yet  the  difference  of  the  geo- 
graphical latitude  between  Athens  and  Vienna  amounts  to  10°  13',  and  that 
between  Vienna  and  Christiania  to  11°  43';  from  which  it  would  be  expected  that 
Vienna  would  have  a  start  of  fifty-one  days  in  advance  of  Christiania. 

One  is  tempted  to  think  at  first,  in  explanation  of  this  phenomenon,  that 
growth  depends  upon  the  formation  of  constructive  materials  from  inorganic  food; 
that  this  latter  process  can  only  be  accomplished  under  the  influence  of  light;  and 
that  therefore  light  so  far  is  important  for  growth.  On  the  other  hand,  it  is 
difficult  to  imagine  that  the  light  enjoyed  by  plants  growing  in  Athens  should 
not  be  sufficient  for  the  formation  of  organic  compounds  in  the  green  cells,  and 
for  the  production  of  a  sufficient  quantity  of  building  materials,  since,  as  a  matter 
of  fact,  the  species  in  question  do  not  appear  in  any  worse  condition  in  Athens 
than  in  Christiania,  which,  however,  it  must  be  supposed  would  be  the  case  if 
there  were  a  disparity  between  the  food  absorbed,  metabolism,  and  growth.  This 
phenomenon  suggests  rather  that  the  light  in  the  north  is  able  to  take  the  place 
of  heat.  And  herein  lies  also  the  solution  of  the  problem.  Not  only  is  there 
compensation  alone;  but  the  light  is  changed  into  heat  before  it  acts  on  the  build- 
ing materials.  A  portion  of  the  light  falling  on  the  plants  is  reflected,  another 
portion  penetrates  into  the  plants,  and  of  these  latter  rays  part  bring  about  the 
transformation  of  carbonic  acid  into  carbohydrates,  and  increases  the  store  of 
chemical  energy,  while  another  portion  is  changed  into  heat.  This  applies  par- 
ticularly to  those  light-rays  which  are  most  vigorously  absorbed  by  chlorophyll 
and  anthocyanin,  and  which  also  cause  the  fluorescence  of  these  colouring-matters; 
and  among  the  tasks  assigned  to  chlorophyll  and  anthocyanin,  the  transformation 
of  light  into  heat  is  certainly  not  the  least  important. 

But  with  this  we  come  back  once  more  to  anthocyanin— that  remarkable 
colouring-matter  which  has  repeatedly  been  spoken  of  in  detail.  It  has  been 
mentioned  that  anthocyanin  frequently  occurs  only  on  the  under  side  of  foliage- 
leaves.  This  is  observed  especially  among  plants  in  the  depths  of  shady  forests, 
which,  although  belonging  to  widely-differing  families,  agree  in  a  remarkable 
manner  in  this  one  point.  One  group  of  these  plants  has  thick,  almost  leathery, 
evergreen  leaves  lying  on  the  ground,  which  arise  from  subterranean  tubers,  or 
root-stocks,  or  from  procumbent  stems.  The  widely -distributed  Cyclamen 
europceum  may  serve  as  a  type  of  this  group.  A  vertical  section  of  a  similar 


520  SOURCES   OF   HEAT.      TRANSFORMATION   OF   LIGHT   INTO   HEAT. 

leaf  is  given  in  figure  25A,  q.  Amongst  other  species  belonging  to  this  group 
may  be  mentioned  Cyclamen  repandum  and  C.  hederifolium,  Cardamine  trifolia, 
Soldanella  montana,  Hepatica  triloba,  and  Saxifraga  Geum  and  cuneifolia. 
Growing  in  habitats  similar  to  these  are  to  be  met  biennial,  occasionally 
perennial,  plants  which  in  autumn  form  a  rosette  of  leaves  on  their  erect  stems 
which  survive  the  winter;  these  are  always  coloured  violet  on  the  side  turned 
towards  the  ground,  while  the  leaves  which  develop  in  the  following  warm  summer 
on  the  elongated  flower-stalks  usually  appear  green  below.  To  this  group  belong, 
especially,  numerous  Cruciferae  (e.g.  Peltaria  alliacea,  Turritis  glabra,  Arabis 
brassicceformis);  species  of  spurge  (e.g.  Euphorbia  amygdaloides),  bell-flowers 
(e.g.  Campanula  persicifolia),  and  hawkweeds  (e.g.  Hieracium  tenuifolium). 
Finally,  deciduous  shrubs  are  to  be  found  in  the  depths  and  on  the  margins  of 
forests  whose  leaves  do  not  survive  the  winter,  but  which  produce  on  the  stems 
developing  in  the  summer  flat  leaves  whose  under  side  contains  abundant  antho- 
cyanin,  as,  for  example,  Senecio  nemorensis  and  nebrodensis,  Valeriana  montana 
and  tripteris,  Epilobium  montanum,  Lactuca  muralis,  and  many  others. 
Amongst  non-European  species  may  be  noticed  many  Flowering  Rushes,  Trades- 
cantias,  Begonias,  and  Cypripediums,  as  well  as  the  Japanese  Saxifrages  (Saxi- 
fraga sarmentosa  and  cortuscefolia),  which  are  coloured  deep  violet  on  the  lower 
side  of  the  leaf  with  anthocyanin,  and  are  only  found  in  shady  spots  in  forests. 

Since  anthocyanin  has  been  already  indicated  as  one  of  the  means  of  protecting 
chlorophyll,  the  question  must  first  of  all  be  considered  as  to  whether  such  a 
relation  does  not  exist  in  the  instances  just  enumerated.  It  might  even  be 
possible  that  the  violet  side  of  the  foliage-leaves  now  turned  earthwards  was 
originally  turned  towards  the  incident  rays  of  light,  while  the  leaves  were  still 
very  young,  and  that  the  anthocyanin  remains  in  the  position  once  assumed  in 
consequence  of  the  twisting  of  the  leaves,  without  being  assigned  any  particular 
function  on  that  account.  Opposed  to  this  idea,  however,  are  the  facts  that  in 
the  majority  of  the  plants  cited,  anthocyanin  is  only  first  developed  when  the 
side  of  the  leaf  in  question  has  already  been  turned  towards  the  ground;  that  in 
many  species  the  violet  side  is  never  turned  upwards  at  any  period  of  develop- 
ment; and  especially  that  in  all  these  plants  which  grow  in  the  shade,  no  protec- 
tion of  chlorophyll  against  an  over-abundance  of  light  appears  necessary;  that, 
on  the  contrary,  it  is  important  for  these  shaded  growths  that  the  scanty  light 
and  heat  should  be  appropriated  and  utilized  to  the  utmost  extent. 

We  cannot  therefore  assign  to  the  anthocyanin  on  the  under  side  of  foliage- 
leaves  any  protective  influence  upon  chlorophyll.  On  the  other  hand,  everything 
goes  to  show  that  the  anthocyanin  developed  here  absorbs  light  and  changes 
it  into  heat.  Light  which,  passing  through  the  leaf,  would  reaxih  fallen  dead 
and  dry  foliage,  or  the  ground  itself  in  the  depth  of  the  forest,  would  be  wasted 
and  useless  there.  When  absorbed  by  the  anthocyanin  and  changed  into  heat,  it 
becomes  serviceable  to  the  plants,  and  can  exert  a  helpful  influence  on  the  growth 
of  neighbouring  cells,  and  to  a  less  extent  apparently  also  on  the  metabolism  and 


SOURCES   OF   HEAT.      TRANSFORMATION   OF   LIGHT   INTO   HEAT.  521 

transportations  of  the  substances.  In  the  evergreen  leaves  of  those  plants  in  the 
depths  of  the  forest  which  are  natives  of  inclement  regions,  this  advantage  is 
obtained  from  the  layer  of  anthocyanin  developed  on  the  lower  leaf -surface,  that 
every  sunbeam,  even  in  the  cooler  seasons,  can  be  utilized  to  the  utmost.  It  is 
in  harmony  with  this  explanation  that  foliage-leaves  of  trees,  shrubs,  and  high 
bushes  which  grow  a  considerable  distance  above  the  ground,  and  have  below 
them  other  green  foliage-leaves,  are  never  violet-coloured  on  their  earthward  side, 
and  that  in  richly-leaved  bushes  whose  lowest  leaves  lie  on  the  soil,  these  only 
are  provided  with  anthocyanin.  That  portion  of  the  light  not  turned  to  account 
in  the  highest  green  leaves,  and  which  is  allowed  to  pass  through  them,  can  still 
be  utilized  by  the  lower  ones;  only  that  light  which  would  pass  through  the 
lowest  leaves  would  be  lost  to  the  plants,  and  therefore  we  have  a  violet  absorbent 
layer  only  on  that  side  which  lies  on  the  ground. 

That  which  occurs  in  plants  of  the  forest  shade  occurs  similarly  in  those  marsh 
plants  whose  leaf-like  stems  or  flat,  disc-like  leaves  float  on  the  surface  of  the 
water.  The  green  discs  of  duckweeds  (e.g.  Lemna  polyrrhiza),  of  the  Frogbit 
(Hydrocharis  morsus-rance),  of  the  Villarsia  (Villarsia  nymphoides),  of  water 
lilies  (Nymphcea  Lotus  and  thermalis),  and  of  the  magnificent  Victoria  regia, 
are  strikingly  bi-coloured,  being  light-green  above  and  deep  violet  below.  Here 
again  it  cannot  be  said  that  the  anthocyanin  forms  a  protection  for  chlorophyll, 
but  the  violet  colouring-matter  can  retain  light  in  the  cells  on  the  lower  surface 
of  the  leaf,  and  can  change  it  into  heat  and  so  make  it  useful  to  the  plants.  The 
rays  which  penetrate  the  green  leaf-discs  and  shine  through  the  water  would 
otherwise  be  lost,  to  the  plants  in  question,  for  none  of  the  species  enumerated 
have  submerged  leaves,  but  possess  only  these  floating  discs,  green  on  the  upper 
and  violet  on  the  lower  side. 

If  anthocyanin  were  found,  not  only  on  the  under  but  also  on  the  upper  side 
of  the  foliage-leaves,  then  indeed  the  significance  would  primarily  be  assigned  to 
it  of  a  means  of  protection  for  chlorophyll,  and  of  assisting  the  metabolism  and 
transport  of  materials;  but  obviously  the  blue  colouring-matter  would  not,  on  the 
upper  side  of  the  leaf,  behave  essentially  otherwise  as  regards  its  capacity  of 
changing  light  into  heat,  than  on  the  lower  side.  It  is  even  probable  that  the 
importance  of  anthocyanin  lies,  not  only  in  its  retention  of  the  rays  injurious  to 
metabolism,  but  also  in  the  transformation  of  light  waves  into  heat.  In  support 
of  this  view  there  is  at  least  the  fact  that  anthocyanin  is  also  richly  deposited 
on  the  upper  side  of  the  foliage-leaves  at  times  when,  and  in  places  where,  other 
sources  of  heat  are  deficient,  and  that  generally  leaves  and  stems  of  many  plants 
growing  in  such  places  are  entirely  overspread  with  red  or  violet.  A  number  of 
small  annuals  which  grow  very  early  in  the  spring  at  a  low  temperature  (e.g.  Saxi- 
fraga  tridactylites,  Hutchinsia  petrcea,  Veronica  prcecox,  and  Androsace  maxima) 
are  usually  coloured  with  anthocyanin  on  all  sides  of  their  growing  organs.  More- 
over, seedlings  which  spring  up  from  the  earth  at  low  temperatures,  and  above 
all  high  Alpine  forms  in  the  neighbourhood  of  the  snow -line,  are  abundantly 


522  SOURCES   OF   HEAT.      TRANSFORMATION   OF   LIGHT   INTO   HEAT. 

provided  with  anthocyanin  on  both  leaf -surfaces.  The  leaflets  and  stem  of  the 
Alpine  Sedum  atratum,  those  of  Bartsia  alpina,  and,  above  all,  numerous  species 
of  Pedicularis  (e.g.  Pedicularis  incarnata,  rostrata,  recutita)  are  coloured  wholly 
purple  or  dark  violet,  and  this  in  habitats  where  the  colouring  could  not  possibly 
be  regarded  as  a  protection  for  chlorophyll.  It  is  also  a  very  striking  phenomenon 
that  widely-distributed  grasses  (e.g.  Aira  ccespitosa,  Briza  media,  Festuca  nigres- 
cens,  Milium  effusum,  Poa  annua  and  nemoralis),  which  in  the  valley  possess 
pale-green  glumes,  develop  anthocyanin  in  them  on  lofty  mountains,  so  that  there 
the  spikes  and  panicles  exhibit  a  deep  violet  tint,  and  on  this  account  the  regions 
in  which  grasses  of  this  kind  grow  in  great  quantities  receive  a  peculiar  dark 
colouring.  Indeed,  this  tint  becomes  the  more  intense  the  nearer  the  habitat 
of  the  plants  in  question  is  to  the  snow-line,  and  the  more  intense  the  action  of 
the  sunlight  becomes.  In  this  case  anthocyanin  can  certainly  not  be  looked  upon 
as  a  means  of  protecting  chlorophyll,  as  the  glumes  generally  contain  but  little 
of  that  substance,  and  take  so  little  part  in  the  formation  of  organic  materials, 
that  the  few  chlorophyll-grains  might  be  entirely  absent  without  the  plant  suffer- 
ing any  damage.  On  the  other  hand,  it  may  be  supposed  that  the  intense  light 
of  the  elevated  region  is  changed  into  heat  by  the  abundant  anthocyanin  of  these 
glumes,  that  this  heat  reaches  the  germs  hidden  under  the  glumes,  and  there 
favourably  influences  the  growth  of  the  seeds  as  well  as  the  transformations  of 
materials.  The  same  occurs  in  the  numerous  sedges  and  rushes  growing  in  the 
Alps,  which  have  dark-violet,  almost  black,  scales  covering  the  flowers  (e.g.  Carex 
nigra,  atrata,  aterrima,  Juncus  Jacquinii,  trifidus,  castaneus),  and  probably 
some  of  the  varieties  of  tint  observed  in  the  corollas  of  Alpine  plants  are  also  to 
be  explained  in  the  manner  indicated. 

It  is  known  that  the  floral-leaves  of  many  plants  growing  on  lofty  mountains, 
and  in  the  far  north,  are  coloured  blue  or  red  by  anthocyanin,  whilst  in  the  same 
species,  growing  in  the  warm  lowlands  and  in  southern  districts,  they  appear  white. 
Particularly  noticeable  in  this  respect  are  the  Gypsophyllas  (Gypsophylla  repens), 
the  Carline  Thistle  (Carlina  acaulis),  the  large-flowered  Bitter-cress  (Cardamine 
amara),  the  Milfoil  (Achillea  Millefolium),  and  many  of  those  Umbelliferse  which 
have  a  very  wide  distribution,  and  occur  all  the  way  from  the  lowlands  up  to  a 
height  of  2500  metres  in  the  Alps,  such  as;  Pimpinella  magna,  Libanotis  montana, 
Chcerophyllum  Cicutaria,  and  Laserpitium  latifolium.  Since  it  has  been  proved 
that  the  colours  of  flowers  are  eminently  important  as  a  means  of  attracting 
insects,  it  might  be  thought  that  the  above  cases  are  in  some  way  connected  with 
insect-visits.  Without  wishing  altogether  to  deny  such  a  relation,  the  possibility, 
on  the  other  hand,  must  not  be  excluded  that  anthocyanin  plays  the  same  part 
here  in  the  flowers  as  in  the  glumes  of  grasses,  and  in  the  clothing  scales  of  sedges 
and  rushes;  and  that  in  the  cold  Alpine  regions,  that  which  is  deficient  in  the 
amount  of  heat  directly  absorbed  as  such,  is  compensated  for  by  such  as  is 
obtained  from  light-rays  by  means  of  anthocyanin.  In  support  of  this  view  there 
is  also  the  phenomenon  that  many  plants  which  develop  white  flowers  in  the  warm 


INFLUENCE   OF   HEAT   ON   CONFIGURATION   AND  DISTRIBUTION   OF   PLANTS.       523 

summer,  as,  for  example,  Lamium  album,  produce  late  in  the  autumn,  under  a 
very  low  temperature  (if  they  bloom  a  second  time),  corollas  whose  upper  side  is 
tinged  with  red;  and  that  in  the  winter,  and  in  frosty  habitats,  the  ray-florets 
also  of  many  Compositse,  as,  for  example,  of  the  well-known  Daisy  (Bellis  perennis), 
are  coloured  red  on  that  side  which  is  turned  towards  the  sky  when  the  capitulum 
is  closed,  and  towards  the  ground  when  the  capitulum  is  open. 

INFLUENCE   OF   HEAT   ON   THE  CONFIGURATION   AND   DISTRIBUTION 

OF    PLANTS. 

On  high  mountains  near  the  snow-line,  and  generally  in  all  those  districts 
where  the  heat  supplied  to  the  plants  is  extremely  scant,  there  occurs,  together 
with  a  production  of  anthocyanin,  a  dwarf  and  tufted  habit.  Usually  this 
phenomenon  is  explained  by  the  large  amount  of  snow,  which  must  have  a  great 
effect  in  these  frosty  heights  during  the  long  winter,  and  it  is  believed  that  high 
Alpine  plants  are  protected  by  this  form  and  position  of  their  stems  and  leaves  from 
injury  by  the  pressure  of  snow.  It  cannot  indeed  be  denied  that  the  pressure  of 
the  snow  has  some  influence  on  the  form  and  direction  of  the  stem-structures,  and 
this  influence  will  be  explained  fully  in  the  following  pages  in  a  particularly 
instructive  example,  viz.  in  the  mountain  pines.  But  this  nestling  on  the  ground 
of  plants  growing  in  the  high  Alps  can  only  be  partially  referred  to  this  cause. 

It  is  a  mistake  to  suppose  that  the  annual  snow-fall  increases  with  the  height. 
The  amount  of  snow  which  falls  attains  a  maximum  at  about  2500  metres  above 
the  sea-level.  This  height  marks  only  the  upper  limit  of  mountain  pines,  dwarf 
junipers,  alders,  and  rhododendrons.  Above  this  the  fall  diminishes,  and  at  a  height 
of  3000  metres  the  snow  is  no  deeper  than  far  down  in  the  valleys.  Even  where 
the  maximum  fall  occurs  trees  are  still  met  with;  there  are  yet  larches  and  Arolla 
pines,  which,  on  account  of  the  great  elasticity  of  their  branches  and  the  downward 
direction  of  their  older  boughs,  can  bear  very  heavy  weights  of  snow  without 
becoming  broken  or  crushed.  The  willows  of  mountain  regions,  characterized  by 
the  way  in  which  their  elongated  stems  and  branches  are  pressed  to  the  earth 
(Salix  serpyllifolia,  S.  retusa,  Jacquiniana,  reticulata),  and  which  are  represented 
in  fig.  131,  grow,  however,  far  above  the  tree  limit,  at  a  height  above  the  sea 
where  the  depth  of  snow,  already  beginning  to  diminish,  is  in  no  case  greater  than 
in  the  valleys,  where  Purple  and  Sweet  Willows,  and  other  species  of  large-leaved 
willows  raise  their  straight  stems  several  metres  high  above  the  ground  on  the 
banks  of  streams.  It  must  also  be  remembered  that  the  woody  growths  close  to 
the  ground  in  high  Alpine  regions  are  very  often  established  on  steep  places,  where 
the  snow  could  not  easily  lie,  could  in  no  instance  be  deeply  piled  up,  and  could  not 
exert  a  pressure  on  the  stems  and  branches.  The  delicate  Thyme-leaved  Willow 
(Salix  serpyllifolia)  nestles  with  an  especial  predilection  to  the  surfaces  of  rocks, 
and  covers  them  with  an  actual  carpet,  and  the  Buckthorn  (Rhamnus  pumila)  is 
found  exclusively  on  steep  declivities,  where  it  roots  in  the  crevices  of  'the  narrow 


524       INFLUENCE  OF  HEAT  ON  CONFIGURATION   AND   DISTRIBUTION  OF  PLANTS. 

rock  galleys,  and  growing  out  from  them  overspreads  like  ivy  the  vertical  rock- 
faces. 

In  all  these  cases  it  is  certain  that  the  weight  of  snow  cannot  have  any  c 
mining  influence  upon  the  form  of  the  plants,  and  some  other  explanation  must  be 
sought.     May  it  not  be  perhaps  that  strong  winds  render  it  impossible  for  woody 


Fig.  131.— Alpine  Willows  with  stems  and  branches  clinging  to  the  ground  in  the  Tyrol 

plants  with  erect  stems  to  grow  in  high  Alpine  regions?  Observing  the  mist  and 
volumes  of  clouds  rushing  across  the  tops  of  the  mountains,  one  gets  some  idea  of 
the  strength  of  the  air  currents  which  operate  there,  and  whoever  has  experienced 
the  effects  of  a  storm  on  a  high  mountain  ridge  can  estimate  the  force  of  the  power- 
ful gusts  of  wind.  And  yet  it  would  be  erroneous  to  suppose  that  the  force  of  the 
storms  on  lofty  mountain  heights  is  greater  than  in  mere  hill  regions.  In  the  case 
of  many  winds  it  is  even  certain  that  they  increase  in  violence  as  they  rush  down 


INFLUENCE  OF  HEAT  ON   CONFIGURATION  AND  DISTRIBUTION  OF  PLANTS.       525 

from  the  mountain  ridge  deeper  into  the  valley.  The  Fohn-wind  in  the  Alps  often 
appears  on  the  heights  as  only  a  slight  breeze,  but  accelerates  its  velocity  as  it  enters 
the  valley,  and  when  it  arrives  there  may  be  as  destructive  as  a  hurricane.  There- 
fore if  the  woody  plants  on  the  slopes  of  high  mountains  were  unable  to  exhibit 
erect  growth  on  account  of  storms,  then  the  neighbouring  valleys  must  also  be 
deprived  of  upright  trees,  which,  however,  is  known  not  to  be  the  case. 

The  clinging  of  woody  plants  to  the  ground  in  high  Alpine  regions  must  not  be 
regarded  either  as  an  adaptation  to  snow  pressure  or  to  storms;  it  is  due  rather  to 
the  fact  that  in  the  high  Alps  the  ground  is  relatively  much  warmer  than  the  air, 
and  that  plants  lying  on  the  soil  profit  by  this  higher  temperature.  I  have  ascer- 
tained through  numerous  observations  at  different  heights  in  the  Central  Tyrolese 
Alps  that  the  mean  temperature  of  the  soil  exceeds  that  of  the  air  by  the  following 
amounts : — 

At  a  height  of  1000  metres,  about  1'5°C. 

1300  „  1-7°  C. 

1600  „          2-4°  C. 

1900  „          3-0°  C. 

2200  „          3-6°  C. 

Thus  the  soil,  in  comparison  with  the  air,  becomes  warmer  the  higher  one 
ascends  the  mountain.  Everywhere  the  earth  absorbs  the  sun's  rays  to  a  much 
greater  degree  than  the  air  does;  but  that  the  excess  of  the  heat  of  the  soil  above 
that  of  the  air  increases  so  remarkably  with  the  increasing  altitude,  is  due  to  the 
fact  that  the  intensity  of  the  sun's  rays  increases  as  we  ascend. 

This  is  further  explained  by  the  fact  that  the  layers  of  air  which  absorb  the 
sun's  rays  are  less  dense  the  greater  the  elevation  above  the  sea-level,  or,  to  use  a 
current  expression,  that  the  air  is  thinner  on  the  heights  than  in  the  valleys.  As  is 
well  known,  the  water  vapour  of  the  air  also  absorbs  the  sun's  rays,  and  since  this 
aqueous  vapour  diminishes  rapidly  with  the  height,  as  might  be  concluded  from  the 
lessening  of  the  pressure,  the  intensity  of  the  sun's  rays  consequently  increases  with 
the  increasing  altitude.  It  has  been  estimated  that  the  force  of  the  sun's  rays  on 
the  top  of  Mont  Blanc  (4810  metres)  is  26  per  cent  greater  than  at  the  level  of 
Paris,  and  that  at  an  altitude  of  2600  metres  the  chemical  activity  of  the  sun's  rays 
is  11  per  cent  greater  than  at  the  sea-level.  Everything  which  is  benefited  by  the 
sun  has,  in  consequence,  a  relatively  striking  appearance  in  the  higher  regions  of 
the  mountains,  and  the  illuminated  soil  especially  exhibits  a  temperature  of  sur- 
prising height.  On  the  Pic  du  Midi  in  the  Pyrenees  (2885  metres)  the  temperature 
of  the  illuminated  soil  rose  on  a  clear  September  day  to  33'8°  C.,  while  the  air  only 
registered  101°  C.,  and  in  point  of  fact  the  temperature  of  the  soil  on  this  summit 
was  almost  twice  as  great  as  at  the  Bagneres  situated  2326  metres  below.  On  the 
Diavoiezza  (Switzerland)  the  black  bulb  thermometer  registered  59*5°  in  the  sun, 
and  at  the  same  time  in  the  shade  a  temperature  of  6'0°.  In  the  Himalayas  the 
blackened  thermometer  at  a  height  of  over  3000  metres  showed  in  the  sun  40°- 50° 
above  the  temperature  of  the  shade,  and  once  stood  at  55*5°  while  the  temperature 
on  the  snow,  in  the  shade  close  by,  amounted  to  only  — 5*6°.  In  Leh  (Kashmir)  at 


526       INFLUENCE  OF  HEAT  ON   CONFIGURATION   AND  DISTRIBUTION   OF  PLANTS. 

3517  metres,  a   blackened  thermometer  in   vacuo   rose   to   even    101'7°,  that   is 
almost  14°  higher  than  the  boiling  point  of  water,  which  at  that  height  is  only 

88°  C. 

It  is  readily  intelligible  that  under  such  conditions  growing  plants  which  require 
heat  should  nestle  to  the  ground  in  high  mountain  regions,  or  more  correctly,  that 
only  such  plants  are  capable  of  living  at  these  heights  which  make  the  best  possible 
use  of  the  most  abundant  of  all  sources  of  heat;  which,  so  to  speak,  seek  a  warm 
situation  and  settle  themselves  against  the  sunny  stones  and  the  black  humus, 
occupying  and  covering  the  rocky  crevices.  Plants  whose  nature  is  to  grow  erect 
with  their  woody  stems  in  the  air  would  not  succeed  well  in  Alpine  regions,  and 
ultimately  would  be  crowded  out  by  species  which  thrive  better  by  clinging  to  the 
relatively  warm  soil. 

The  increase  in  the  excess  of  the  ground  temperature  above  that  of  the  air  with 
the  increasing  altitude  is  also  manifested  in  another  phenomenon  which,  though  it 
has  been  frequently  observed  and  discussed,  has  not  always  been  correctly  inter- 
preted. The  Ling  (Calluna  vulgaris),  which  extends  from  the  lowlands  at  the  foot 
of  the  Alps  up  to  high  Alpine  regions,  blossoms  on  the  sea-coast  in  Istria  usually  at 
the  end  of  July;  in  Alpine  valleys,  which  lie  1000  metres  above  the  sea-level,  it 
opens  its  first  flowers  at  the  end  of  August,  and  therefore  the  retardation  of  flower- 
ing at  1000  metres  amounts  to  something  over  a  month.  From  this  it  might  be 
expected  that  the  Ling  would  first  blossom  at  an  altitude  of  2000  metres  at  the 
end  of  September,  but  this  is  not  so,  for  on  mountains  of  the  Central  Alps  at  2000 
metres  the  Ling  nestling  on  the  ground  is  seen  to  be  in  full  bloom  before  the 
middle  of  September.  By  comparing  the  time  of  blossoming  of  high  Alpine  plants 
cultivated  in  the  botanic  gardens  at  Innsbruck,  with  the  time  at  which  the  same 
species  open  their  flowers  at  various  altitudes  on  the  neighbouring  mountains,  it 
was  shown  that  the  retardation  of  the  blossoming  amounted  to  a  mean  of  25  days 
at  an  altitude  of  500-1000  metres;  an  average  of  18  days  at  1500-2000  metres; 
and  14  days  at  2500-3000  metres;  and  this  can  only  be  explained  by  the  much 
greater  intensity  of  the  sun's  rays  in  the  high  regions,  and  the  consequent  elevation 
of  the  temperature  of  the  ground  above  that  of  the  air.  It  must  yet  be  mentioned, 
for  the  completion  of  the  observations  here  detailed,  that  all  plants  in  the  valleys 
develop  larger  leaves  and  taller  stems  than  those  on  lofty  mountain  sites.  While 
the  Ling  forms  considerable  bushes  with  erect  branches  on  the  coast  of  Istria, 
plants  of  the  same  species  on  the  slopes  of  high  mountains  2000  metres  above  the 
sea,  appear  as  dwarf  shrubs,  whose  woody  stems  lie  on  the  ground  and  are  par- 
tially imbedded  in  the  dark  humus. 

The  great  contrast  which  vegetation  on  a  mountain  exhibits  in  different  parts  of 
the  world  may  be  explained  by  the  action  of  the  sun's  rays.  On  slopes  illuminated 
directly  by  the  sun,  the  temperature  of  the  soil,  and  indirectly  that  of  the  layer  of 
air  in  contact  with  it,  rises  far  higher  than  on  shady  declivities,  and  in  consequence 
of  this  very  remarkable  differences  may  occur  even  in  the  closest  proximity.  Ob- 
servations of  the  temperature  of  the  soil  at  a  depth  of  80  centimetres,  spread  over 


INFLUENCE  OF   HEAT   ON   CONFIGURATION   AND  DISTRIBUTION  OF   PLANTS.       527 

three  years,  at  Innsbruck  in  the  Tyrol,  and  in  the  eight  points  of  the  compass  round 
an  isolated  conical  sand-hill,  have  shown  the  following  mean  temperatures: 

North.      North-east.       East.       South-east.       South.      South-west.       West       North-west, 
15-3°.        17-0°.         187°.        20-0°.        19-3°.         18'3°.          18«5°.        15'0°. 

The  difference  between  the  south-east  and  north-west  amounts,  according  to  this,  to 
not  less  than  5°,  and  it  is  probable  that  at  higher  altitudes  it  would  show  even  a 
more  marked  increase.  And  herewith  is  connected  the  rising  and  falling  of  the 
upper  limit  of  vegetation  on  the  different  sides  of  a  mountain.  On  slopes  long 
exposed  to  the  sun  the  plants  advance  much  further  upwards  than  on  the  shaded 
sides  of  a  mountain,  or  those  which  are  warmed  by  the  sun's  rays  during  only  a 
short  time;  and  the  difference  of  the  upper  limit  on  the  north  and  south  sides 
oscillates  in  high  mountain  regions  between  200  and  300  metres.  It  frequently 
happens  that  species  reach  their  upper  limit  on  the  north  side  at  2000  metres, 
while  on  the  south  side  not  until  2400  metres  is  reached.  From  this  it  strikes  us 
that  the  contrast  between  the  upper  limit  of  plants  on  the  north  and  south  sides 
becomes  greater  the  higher  we  climb  up  into  the  mountain.  In  this  respect  a  com- 
parison of  beeches  and  firs  is  very  interesting.  Beech  trees  (Fagus  sylvatica)  find 
their  upper  limit  in  the  Limestone  Alps  of  the  North  Tyrol  on  an  average  at  an 
altitude  of  1430  metres;  on  the  sunny  side  of  the  mountains  the  beech  limit  rises 
to  149  metres  above  this  average,  while  on  the  shady  side  it  falls  short  of  the 
average  by  112  metres;  thus  the  difference  between  the  sunny  and  shady  side  for 
beeches  amounts  to  261  metres.  Norway  spruces  (Abies  excelsa)  find  their  upper 
limit  in  the  same  region,  on  an  average,  at  an  altitude  of  1777  metres;  on  the  sunny 
side  of  the  mountain  the  spruce  limit  rises  to  185  metres  over,  while  on  the  shady 
side  it  remains  125  metres  below  the  average,  and  thus  the  difference  between  the 
sunny  and  shady  side  amounts  for  spruce  to  310  metres.  Thus  whilst  in  the  zone 
stretching  from  1300-1600  metres,  the  difference  between  the  shady  and  sunny 
sides  amounts  only  to  261  metres,  it  rises  to  310  metres  in  a  zone  from  1600- 
1900  metres,  which  again  can  only  be  accounted  for  by  the  rising  intensity  of  the 
sun's  rays  with  the  increasing  altitude. 

From  all  this  it  may  be  seen  how  vegetation  adapts  itself  to  the  given  heat 
conditions;  how  the  smallest  advantage  offered  in  any  spot  is  made  use  of;  and 
how  much  the  form  of  the  plant  depends  upon  the  conditions  of  warmth  in  the 
habitat. 

The  above  statements  also  demonstrate  that  the  distribution  of  plants  on  the 
earth  stands  in  the  closest  connection  with  the  distribution  of  heat.  In  another 
volume  of  this  work  an  opportunity  will  be  taken  of  discussing  this  connection 
fully;  here  it  is  sufficient  to  mention  that  from  the  local  conditions  of  warmth,  viz., 
from  the  elevation  of  the  temperature  of  the  soil  effected  in  circumscribed  spots  in 
mountainous  districts  by  the  sun's  rays,  the  preservation  of  colonies  of  plants,  from 
earlier,  warmer  periods  is  explained.  The  largest  part  of  the  central  European 
uplands,  especially  the  Northern  Limestone  Alps,  exhibit  colonies  of  plant-species 
on  limited  areas,  which  are  entirely  absent  in  the  immediate  neighbourhood,  and 


528  MEASURES   FOE   PROTECTING   GROWING    PLANTS    FROM    LOSS   OF    HEAT. 

which  now  no  longer  spread  beyond  the  narrow  circle  of  their  confined  habitat, 
although  they  ripen  seeds  capable  of  germinating,  and  are  met  with  again  in  great 
quantities  one  or  two  degrees  farther  south.  We  may  conclude  that  these  plants  were 
first  brought  to  their  isolated  habitats  within  the  historical  period  by  wind  or  other 
distributive  agents,  and  everything  tends  to  show  that  they  represent  the  remnant 
of  a  vegetation  which  was  distributed  very  widely  over  adjacent  districts  in  ages 
long  past,  but  have  withdrawn  thence  in  consequence  of  the  severe  climate  which 
has  intervened;  that  is  to  say,  have  died  and  been  replaced  by  other  vegetation.  That 
such  foundlings  on  isolated  mountain  slopes,  often  only  a  small  steep  ravine,  or  on 
a  single  rocky  face,  could  maintain  themselves  even  in  the  later  cold  periods,  is 
explained  by  the  fact  that  conditions  of  warmth  can  prevail  over  very  restricted 
areas  on  the  mountains  which  differ  in  toto  from  those  of  the  environment,  and  are 
only  found  generally  prevailing  quite  a  degree  further  south.  The  southern  slope 
of  the  Solstein  range,  between  Hall  and  Zirl,  produces  in  limited  areas  Hop-horn- 
beams and  Bladder-senna  (Ostrya  carpinifolia  and  Colutea  arbor  escens);  from 
the  boulders  an  umbellifer,  the  curious  Tommasinia  verticillaris,  rises  to  the 
height  of  a  man;  the  rock  terraces  are  overgrown  with  Stipa  pennata,  Lasiagrostis 
Calamagrostis,  Saponaria  ocymoides,  Dorycnium  decumbens,  and  here  and  there 
one  might  imagine  one's  self  a  degree  further  south  on  the  other  side  of  the  Alps. 
It  is  beyond  question  that  the  plant  forms  named  on  the  warmest  and  most  pro- 
tected of  the  Solstein  range  are  remnants  from  a  primeval  warmer  period,  and 
were  formerly  distributed  generally  over  the  adjoining  mountain  ranges.  These 
cursory  remarks  should  show  that  the  accurate  knowledge  of  the  relation  of  heat 
to  individual  species  of  plants  may  render  important  help  in  speculations  about  the 
history  of  our  vegetation. 

MEASUKES  FOR  PROTECTING  GROWING  PLANTS   FROM  LOSS  OF  HEAT. 

Since  certain  developments  in  plants  have  assigned  to  them  the  task  of  utilizing 
external  circumstances  as  far  as  possible  so  that  heat  may  reach  the  growing 
organs  to  the  extent  actually  necessary,  it  is  naturally  to  be  expected  that  contri- 
vances will  not  be  wanting  to  protect  them  from  an  excess  of  heat,  and  also  that 
care  will  be  taken  that  the  heat  once  obtained  is  not  again  lost.  It  would  not  be 
in  harmony  with  what  we  know  of  the  economy  of  vegetation  that  a  plant  exposed 
to  the  sun  should  lose  by  radiation  in  the  following  night  all  the  heat  which 
it  had  gained  during  the  day.  It  is  known  that  growth  is  carried  on  during 
the  night,  and,  indeed,  that  certain  organs  grow  more  in  the  night  than  in  the 
day,  and  in  these  an  excessive  loss  of  heat  would  be  most  disadvantageous. 

As  a  matter  of  fact  arrangements  exist  for  protecting  plants  from  an  excessive 
loss  of  heat.  These  contrivances  coincide  in  great  part  with  those  which  regulate 
transpiration,  and  have  already  been  fully  described  in  the  discussion  on  that 
subject,  to  which  therefore  we  may  refer.  But  those  developments  which  claim  a 
particular  interest  as  measures  of  protection  against  the  danger  of  excessive  loss 


MEASURES   FOR   PROTECTING   GROWING    PLANTS    FROM    LOSS   OF   HEAT.  529 

of  heat  which   are  not  at  all  connected   with   transpiration,  or  only  to  a  slight 
degree,  are  brought  together  here  in  the  form  of  a  general  sketch. 

First  of  all  in  this  respect  are  to  be  considered  flowers  of  comparatively  rapid 
growth,  whose  parts  therefore  require  much  warmth,  but  for  which  many  of  the  con- 
trivances suited  to  foliage-leaves  are  not  well  adapted  as  protective  measures  against 
loss  of  heat,  since  other  functions  might  be  encroached  upon.  And  yet  these  flowers 
especially  require  an  abundant  protection  against  loss  of  heat,  even  more  than 
other  plants  on  account  of  their  great  sensitiveness.  If  in  the  spring  a  blossoming 
snowdrop,  having  already  penetrated  the  soil,  is  surprised  by  a  frost,  the  flower-stalk 
and  the  leaves  sink  down  as  if  withered,  while  the  flowers  outwardly  are  not 
at  all  altered.  Anyone  observing  this  might  think  that  the  green  stem  and  leaves 
had  been  injured,  but  that  the  flowers,  on  the  contrary,  had  survived  the  catas- 
trophe without  harm.  But  exactly  the  opposite  is  the  case.  The  stem  and  leaves 
become  erect  with  returning  warmth  and  continue  to  grow,  but  the  pollen  in  the 
anthers  of  the  flower  is  dead;  also  the  ovules,  styles,  and  stigmas  are  affected  so 
that  they  wither  and  shrivel  up:  obviously  the  production  of  ripe  seeds  is  then  out 
of  the  question.  It  is  also  observed  that  the  pollen  in  the  anthers  is  best  formed 
when  the  flower-buds  in  question  are  warmed  through  by  the  sun,  and  when  the 
blossoming  plants  grow  on  a  free  open  space  which  the  sun's  rays  can  reach. 
Moreover,  the  floral  envelopes  develop  much  better  in  such  spots  than  in  cool  shady 
places ;  they  become  larger,  exhibit  brighter  colours,  and  consequently  are  more  often 
visited  by  insects  than  those  which  receive  relatively  little  light  and  heat.  But 
the  danger  that  the  flowers  and  flower-buds  will  again  lose  by  radiation,  through 
the  night,  the  heat  which  they  have  gained  during  the  day  is  most  likely  to  be 
felt  in  open,  unshaded  habitats,  that  in  consequence  of  the  great  loss  of  heat 
the  formation  of  the  pollen  in  the  yet  closed  anthers  will  be  injured,  and  finally, 
that  the  petals  will  also  be  disturbed  in  their  growth  and  function.  In  order  to 
avoid  this,  in  many  cases  the  flower-buds  and  also  the  open  flowers  are  pendulous, 
bell-shaped,  and  tubular,  or  leaves  become  arched  in  the  shape  of  a  dome,  cap, 
or  umbrella  above  the  stamens  and  pistils,  in  which  case  the  inner  portions  of 
such  flowers  are  hidden  as  in  a  niche  or  groove.  In  these  hidden  nooks  they  are 
comparatively  well -protected  against  loss  of  heat,  and  at  least  no  radiation  of 
warmth  towards  the  night  sky  proceeds  from  the  anthers  and  stigmas.  Only  the 
coverings  spread  over  the  stamens  and  pistil,  as  a  protecting  roof,  lose  during  the 
night  a  great  part  of  the  heat  obtained  in  the  day.  These,  however,  are  not  so 
much  endangered,  since  they  have  already  obtained  their  normal  size  and  have  no 
need  of  heat  for  further  growth;  besides,  they  are  usually  clothed  with  air-contain- 
ing, hairy  structures,  surrounded  by  dry  membraneous  edges  or  entirely  transformed 
into  dry  parchment  or  paper-like  scales,  in  which  case  they  can  suffer  no  further 
damage  from  loss  of  heat.  The  air  in  the  pendulous  bell-flowers  is  1-2  degrees 
warmer,  in  the  morning  before  sunrise,  than  the  surrounding  air;  here,  closed  in, 
it  remains  practically  unaltered  during  the  night;  and  this  of  course  is  exceedingly 

useful  to  the  warmth-loving  anthers  and  stigmas  there  hidden. 

VOL.  I. 


530  MEASURES   FOR   PROTECTING   GROWING    PLANTS    FROM   LOSS   OF    HEAT. 

In  many  instances  the  flower-buds  and  young  flowers  only  assume  an  inverted 
position  periodically,  i.e.  only  when  a  cold  night  is  to  be  expected.  Many  umbel- 
liferous plants  are  particularly  noticeable  in  this  respect,  especially  Falcaria 
Rivini  and  the  Burnet  Saxifrage  (e.g.  Pimpinella  magna,  and  saxifraga)  and 
Carrot  (e.g.  Daucus  Carota  and  maximus).  The  sun  has  scarcely  set  when  in  all 
these  species  the  stalks  which  bear  young  flower-umbels  bend  downwards,  crook- 
like,  so  that  the  flower-buds,  which  during  the  day  have  been  turned  towards  the 
sun,  now  face  the  earth,  and  the  finely-divided  involucral  leaves  spread  out  like  an 
umbrella  over  the  nodding  umbel.  These  finely-divided  coverings  radiate  out 
heat  in  the  night  without  injury;  the  flower-buds  below  them,  on  the  other  hand, 
are  protected  in  the  manner  described  against  the  nocturnal  radiation  so  fatal  to 
them;  whilst  the  heat  they  absorb  during  the  day  is  thus  in  great  measure,  if  not 
entirely,  retained.  With  the  next  sunrise  the  young  umbels  rapidly  become  erect; 
their  bent  stalks  rise  up  stiffly;  and  the  flower-buds  are  again  exposed  to  the 
sun,  as  may  be  seen  in  the  illustration  of  the  Common  Carrot  (Daucus  Carota) 
inserted  opposite  (Fig.  132  x> 2).  Later,  when  fertilization  has  taken  place,  and  the 
young  fruits  are  developing,  the  necessity  for  protecting  the  stamens  and  pistils 
from  radiation  no  longer  exists,  and  the  periodic  bending  down  of  the  umbel  is 
discontinued.  Young  flower -heads  of  several  scabiouses  (e.g.  Scabiosa  lucida 
and  Columbaria)  behave  like  the  umbelliferous  plants  named,  as  also  do  the  single 
flowers  of  pansies  (Viola  tricolor),  represented  in  fig.  132  3»  4>  in  day  and  night 
position  next  the  umbels  of  the  carrot.  In  numerous  Composite,  Labiatae,  and 
plantains  (e.g.  Leontodon  hastile,  Mentha  sylvestris,  Plantago  media,  recurvata  and 
maritima)  there  are  no  such  regular  periodic  movements;  in  these  the  capitula  and 
spikes  are  always  pendulous  while  the  flowers  are  still  in  bud,  and  they  remain  in 
this  position  as  long  as  it  is  advantageous  to  them.  Afterwards,  when  the  nocturnal 
loss  of  heat  can  no  longer  be  injurious  to  the  anthers  and  stigmas,  or  if  other  protec- 
tive measures  have  been  developed  meanwhile,  the  axis  of  the  inflorescence  becomes 
stiffly  erect.  In  many  Composite  the  involucres  of  the  capitula  or  the  peripheral 
ligulate  florets,  and  in  other  families  the  sepals  and  petals,  bend  up  after  sunset 
over  the  stamens  and  pistils.  They  thus  form  a  protecting  roof  under  which 
the  temperature  of  the  air  alters  comparatively  slowly,  and  the  delicate  anthers 
and  stigmas  are  secured  from  radiation. 

A  very  striking  contrivance  for  protecting  against  loss  of  heat  by  nocturnal 
radiation  is  also  observed  in  the  seedlings  of  flowering  plants,  in  those  which 
possess  two  seed-leaves  or  cotyledons.  As  long  as  the  embryo  surrounded  by 
protecting  coats  remains  quiescent  in  the  seed,  the  two  seed-leaves  are  situated 
with  their  upper  surfaces  in  contact;  later,  when  germination  has  taken  place 
when  the  radicle  has  penetrated  into  the  earth  and  the  seed-coat  is  thrown  ofl, 
the  two  seed-leaves  become  separated,  turn  their  upper  sides  towards  the  sky,  so 
that  the  seedling  above-ground  resembles  an  open  book.  In  this  position  the 
broad  surfaces  are  exposed  to  the  sun's  rays;  they  are  also  illuminated  and  warmed 
as  much  as  possible,  and  if  they  are  coloured  green,  the  formation  of  organic 


MEASURES   FOR   PROTECTING   GROWING   PLANTS   FROM   LOSS   OF   HEA1.  531 

substances  from  inorganic  food  can  be  carried  on  in  them.  These  cotyledons  are 
frequently  seen  to  increase  in  extent,  and  to  grow  and  function  exactly  like  foliage- 
leaves.  It  would  certainly  be  a  great  disadvantage  to  green  cotyledons  of  this 
kind  if  they  were  obliged  to  give  up  either  partially  or  perhaps  entirely  in  the 
following  night  the  heat  received  during  the  day.  In  neighbourhoods  where  the 
greater  part  of  the  seeds  germinate  at  a  low  temperature,  at  the  close  of  winter 
at  a  time  when  the  nights  are  still  long,  warmth  must  be  economized  as  far  as 
practicable,  and  especially  must  the  loss  of  heat  from  the  cotyledons  by  nocturnal 


Fig.  132.— Periodic  bending  of  Flowers  and  Inflorescences. 

i  The  umbel  of  the  Carrot,  day  position.      2  The  same  umbel,  night  position.     »  Flower  of  Pansy,  day  position. 

*  The  same  flower,  night  position. 

radiation  be  prevented.  This  is  accomplished  by  the  cotyledons,  which  are 
separated  from  one  another  like  the  leaves  of  a  book,  and  whose  broad  surfaces  are 
turned  towards  the  sky,  closing  together  when  the  sun  sets,  and  again  assuming 
that  position  which  they  occupied  in  the  quiescent  seed.  In  this  way  the  two 
cotyledons  now  have  their  narrow  edges  turned  skywards,  and  the  loss  of  heat 
in  the  night  is  as  much  as  possible  minimized.  This  movement  of  the  cotyledons, 
which  on  cloudless  evenings  and  in  exposed  spaces  occurs  more  quickly  than  under 
cloudy  skies  and  in  places  which  are  overshaded  by  trees,  has  also  the  advantage 
that  the  small  foliage -leaves,  which  are  visible  on  the  rudiments  of  the  shoot 
between  the  cotyledons,  are  covered  over  through  the  night.  When  the  morning 
breaks,  and  the  danger  of  excessive  loss  of  heat  is  passed,  the  cotyledons  again  open 
out  in  order  to  sun  themselves  afresh  to  their  full. 


532  MEASURES   FOR   PROTECTING   GROWING   PLANTS   FROM   LOSS   OF   HEAT. 

This  opening  and  closing  of  the  cotyledons  is  seen  particularly  well  in  species  of 
clover  and  Bird's-foot  Trefoil  (Trifolium  and  Lotus),  in  all  mimosas  and  bauhinias, 
and  numerous  other  leguminous  plants;  also  in  species  of  wood  sorrel  (e.g.  Oxalis 
Valdiviana,  rosea,  sensitiva),  in  the  gourds,  cucumbers,  and  melons,  in  the  Sun- 
flower (Helianthus  annuus)  and  in  the  Tomato  (Solanum  Lycopersicum),  in 
species  of  Mimulus  and  Mirabilis,  the  Corn-cockle  (Agrostemma  Githago),  the 
Chickweed  (Stellaria  media),  and  many  others. 

By  alterations  of  position,  similar  to  those  exhibited  by  cotyledons,  the  so-called 
compound  leaves  are  also  in  many  instances  protected  against  nocturnal  radiation. 
By  compound  leaves  are  understood  those  which  bear  either  pinnate  or  radiating 
leaflets  on  a  common  stalk. 

These  compound  leaves  in  some  cases,  which  have  already  been  alluded  to,  are 
spread  out  during  the  mild  night,  but  are,  on  the  contrary,  folded  together  under  the 
burning  noonday  sun.  In  by  far  the  greater  number  of  cases,  however,  especially 
in  species  whose  habitat  is  exposed  to  great  cooling  in  the  night,  the  reverse  is 
observed.  In  sunshine  the  surfaces  of  the  leaflets  are  arranged  more  or  less 
parallel  to  the  ground,  the  upper  side  is  turned  to  the  sky,  and  is  fully  and 
completely  flooded  by  the  sun's  rays.  If  this  position  were  retained  after  sunset, 
the  surfaces  of  the  leaflets  would  be  forced  to  give  up  much  heat  by  radiation 
towards  the  night  sky.  In  order  to  avoid  this  the  leaflets  fold  together  either 
upwards  or  downwards,  and  place  themselves,  so  to  speak,  on  edge.  In  this  way 
their  broad  sides  become  vertical,  in  which  position  they  are  protected  from 
radiation  as  much  as  possible. 

There  are  provided  for  the  accomplishment  of  these  movements  certain  swollen 
cushions  of  succulent  tissue  at  the  bases  of  the  several  leaflets,  and  often  at  the 
base  of  the  common  petiole.  These  are  known  as  pulvini  and  each  consists  of 
parenchymatous  thin-walled  cells  surrounding  a  strand  of  compressed  vascular 
bundles,  which  further  up  becomes  the  midrib  of  the  leaflet,  which  is  inserted  on 
the  pulvinus.  The  parts  of  this  strand  where  surrounded  by  the  pulvinus  are 
supple  and  very  flexible,  but  above  the  pulvinus  they  become  stiff  and  firm, 
forming  as  it  were  the  main  support  of  the  whole  leaflet,  so  that  indeed  alterations 
of  position  of  the  midrib  are  participated  in  by  the  whole. 

In  order  to  represent  clearly  how  a  movement  is  brought  about  in  the  leaflet  by 
means  of  its  supporting  pulvinus,  let  us  imagine  a  straight  rod  which  is  only  flexible 
at  the  base,  and  is  there  held  fast  between  two  springs.  The  pressures  proceeding 
from  the  two  springs  is  equally  strong,  and  the  rod  is  therefore  maintained  in  an 
upright  position.  If  the  pressure  of  the  spring  relaxes  on  one  side,  the  stick  must 
bend  over  in  the  direction  of  the  diminished  pressure.  If  the  pressure  of  the  two 
springs  be  afterwards  equalized,  the  rod  will  again  assume  its  original  erect  position. 
If  for  the  rod  we  now  substitute  a  leaflet  traversed  by  a  rod-like  midrib,  i.e.  by  the 
vascular  bundle-strand  mentioned  above,  and  imagine  two  opposed  halves  of  a 
turgid  cell-tissue  instead  of  the  two  springs,  then  the  leaflet  will  be  kept  upright 
by  the  equal  tension  of  the  pulvinus  situated  at  the  base  of  the  strand;  but  as  soon 


MEASURES   FOR   PROTECTING   GROWING   PLANTS   FROM   LOSS   OF   HEAT.  533 

as  the  turgidity  of  the  cells  increases  in  one  of  the  halves  of  the  pulvinus,  in  con- 
sequence of  an  increased  afflux  of  water,  this  half  elongates,  bulges  out,  becomes 
convex,  and  exerts  a  stronger  pressure  than  the  opposite  half,  so  that  the  latter 
becomes  concave  and  much  contracted.  The  supple  portion  of  the  bundle-strand 
between  the  two  halves  of  the  cushion  becomes  bent,  and  the  leaflet,  whose  stiff 


Fig.  133.— Alteration  of  Position  of  Leaflets  in  Compound  Leaves. 

*  Leaf  of  Mimosa  Lindheimeri,  seen  from  above,  in  day  position,  a  The  same  in  night  position.  »  Leaf  of  Amorpha  fruticosa 
in  day  position.  *  The  same  in  night  position.  8  Leaf  of  Coronilla  varia  in  day  position.  •  The  same  in  night  position. 
*  Leaf  of  Tetragonolobtis  siliquosus  in  day  position.  »  The  same  in  night  position. 

midrib  is  formed  by  the  continuation  of  the  bent  bundle-strand,  is  inclined  over 
in  the  direction  of  the  concave  half  of  the  pulvinus.  If  the  increase  of  turgidity 
occurs  alternately  first  in  the  one  and  then  in  the  other  half  of  the  pulvinus,  the 
leaflet  will  also  be  bent  now  to  the  one,  then  to  the  other  side;  and  if  the  leaf- 
support  has  a  horizontal  position,  an  alternate  rising  and  sinking  of  the  leaflet  will 
occur.  It  is  to  be  noticed  here  that  the  leaflet  itself  remains  actually  quite  passive, 
and  that  the  pressures  which  have  come  into  play  only  operate  in  the  pulvinus. 
The  commonest  stimulation  to  periodic  alteration  of  the  turgidity  in  the  pulvini 


534  MEASURES   FOK  PROTECTING   GROWING   PLANTS   FROM   LOSS   OF   HEAT. 

is  the  diminution  of  light  and  heat  after  sunset,  and  since  the  rising  and  sinking  of 
the  leaflets  effected  thereby  coincides  with  the  nocturnal  sleep  of  birds  and  other 
animals,  the  phenomenon  described  has  been  interpreted  in  this  sense,  and  termed 
the  sleep  of  plants. 

The  rapidity  with  which  the  movement  of  the  leaflets  is  accomplished  varies 
very  much  in  different  plants,  and  even  in  the  same  species  is  sometimes  quicker, 
sometimes  slower,  according  to  external  influences.  All  the  circumstances  which 
increase  the  turgidity  of  vegetable-cells  have  also  an  accelerating  effect  on  these 
movements.  It  is  still  an  unsolved  problem  how  far  light  and  darkness  influence 
the  turgidity  of  the  pulvini.  It  is  supposed  that  the  darkening  produces  an 
increased  afflux  of  water  and  an  increase  of  turgidity  in  the  whole  pulvinus,  but 
more  rapidly  in  one  half  than  in  the  other;  while  the  protoplasm  in  one  half  of 
the  cells  of  the  pulvinus  is  stimulated  by  light  to  give  up  a  portion  of  the  watery 
sap,  lying  at  the  time  within  the  sphere  of  its  influence,  to  the  surroundings — by 
which  indeed  not  very  much  is  explained. 

In  one  section  of  plants  whose  leaflets  assume  a  sleep  position  when  darkness 
sets  in  after  sunset,  the  leaflets  provided  at  the  base  with  pulvini  move  upwards, 
and  in  the  other  section  downwards.  The  movement  is  upwards  as  a  rule  in 
ternate  leaves,  of  which  the  clover  may  serve  as  a  type.  When  the  elevation  has 
been  accomplished,  the  leaflets  are  directed  either  all  three  almost  at  a  right  angle 
to  the  horizon,  or  the  terminal  leaflet  is  bent  up  rather  more  than  the  two  lateral 
ones.  A  very  pretty  example  of  this  is  furnished  by  Tetragonolobus  siliquosus, 
represented  in  figs.  133  7  and  133 8,  and  also  by  Desmodium  penduliflorum  as  well 
as  by  various  species  of  Lotus,  Trifolium,  Melilotus,  Medicago.  Pinnate  leaves, 
whose  leaflets  rise  up  and  arrange  themselves  next  one  another  like  the  leaves  of  a 
closed  book,  are  found  on  numerous  small  scrubby  mimosa  bushes  of  Peru,  of  which 
a  species,  viz.  Mimosa  Lindheimeri  is  represented  in  figs.  133  *  and  133  2,  in  the 
day  and  night  positions.  In  the  Australian  Acacia  lophantha  and  several  other 
true  acacias,  in  Schrankia  aculeata  and  species  of  JZschynomene,  in  the  American 
gleditschias,  further  in  the  Australian  Clianthus  Dampieri  and  in  the  widespread 
European  Coronilla  varia.  In  fig.  133 6  is  shown  how  the  erect  leaflets  of  the 
Coronilla  lie  against  one  another  very  regularly  in  pairs.  Just  as  often,  instances 
are  observed  in  which  the  leaflets  of  the  pinnate  or  digitate  leaves  fall  downwards 
after  sunset.  An  example  of  this  group  is  afforded  by  the  leaf  of  one  of  numerous 
American  amorphas  (Amorpha  fruticosa),  which  is  illustrated  in  figs.  1333  and 
1334.  These  leaflets  which  droop  at  night  are  also  very  noticeable  in  the  Indian 
Averrhoa  Carambola,  in  various  species  of  indigo  and  liquorice  (Indigofera  and 
Glycyrrhiza),  in  the  sophoras  (e.g.  Sophora  alopecuroides),  in  the  American  tree, 
Gymnocladus  Canadensis,  and  in  robinias,  of  which  Eobinia  Pseudacacia  (popu- 
larly called  acacia)  is  planted  everywhere  as  a  decorative  tree.  In  like  manner  in 
the  widely-spread  common  Wood  Sorrel  (Oxalis  Acetosella),  cf.  fig.  90  8,  in  the 
Indian  pinnate-leaved  Oxalis  sensitiva,  and  in  numerous  American  sorrels. 

With  respect  to  the  radiation,  it  is  immaterial  whether  the  leaflets  rise  up  or 


MEASURES   FOR   PROTECTING   GROWING   PLANTS   FROM    LOSS   OF   HEAT.  535 

sink  down;  the  main  point  is  that  they  turn  their  profile  towards  the  night  sky, 
and  this  occurs  in  all  the  above-mentioned  cases.  But  it  should  be  noticed  here 
that  besides  the  protection  against  excessive  loss  of  heat,  other  advantages  are 
gained  by  the  periodic  alteration  of  the  position  of  the  leaflets,  and  in  this  respect 
it  is  anything  but  a  matter  of  indifference  whether  the  leaflets  fold  together 
above  or  below.  Since  the  vertical  position  of  the  leaf -surfaces  also  furnishes  an 
important  protection  against  excessive  transpiration,  various  conditions  of  the  leaf 
construction  connected  with  this  protection  are  also  significant.  For  example,  the 
question  whether  the  stomata  are  developed  on  the  upper  or  under  side  of  the 
leaflet  is  determined,  inasmuch  as  the  sides  provided  with  stomata,  as  a  rule,  come 
in  contact  with  one  another.  Finally,  it  must  net  be  denied  that  bedewing  also  has 
an  influence  on  the  alteration  of  position  of  the  delicate  leaflet. 

A  great  number  of  plants  whose  leaflets  assume  the  sleep  position  at  nightfall 
also  exhibit  this  phenomenon  on  bright  days  when  shaken  or  touched,  and  indeed 
more  rapidly  under  these  circumstances  than  at  the  on-coming  of  darkness.  The 
slightest  touch  of  the  finger,  even  shaking  by  a  moderate  wind,  suffices  to  cause 
the  leaflets  of  these  plants  to  fold  together.  In  the  Oocalis  sensitiva  of  tropical 
India  even  the  disturbances  of  the  air  caused  by  the  approach  of  man  is  enough  to  . 
cause  the  pinnate  leaflets  to  fall  together  rapidly,  and  the  same  thing  occurs  in 
several  papilionaceous  plants  (e.g.  Smithia  sensitiva  and  JZschynomene  Indica),  as 
well  as  in  several  mimosas.  When  we  move  away  from  the  immediate  vicinity  of 
these  plants,  and  complete  stillness  once  more  reigns  in  the  air,  the  folded  leaflets 
again  spread  out  and  turn  their  upper  surfaces  skyward.  The  phenomenon  that 
the  plants  close  up,  frightened  at  the  approach  of  man,  as  if  they  felt  or  in  some 
way  became  aware  of  his  approach,  caused  the  older  botanists  to  name  them 
Sensitive  Plants. 

From  a  cursory  examination  it  appears  that  the  folding  of  the  leaflets  in  these 
sensitive  plants  caused  by  shaking,  and  the  assumption  of  the  sleep  position  at  the 
setting  in  of  darkness,  are  the  same  processes;  but  closer  investigation  shows  that 
there  is  an  essential  difference  between  them.  Outwardly  this  difference  is  recogniz- 
able by  the  fact  that  in  the  sleep  position,  brought  about  by  darkness,  the  pulvinus 
below  a  leaflet  remains  quite  rigid,  while  in  the  folding  of  the  leaflets  produced  by 
shaking  a  relaxation  of  one  half  of  the  pulvinus  occurs.  In  sections  through  the 
pulvinus  of  sensitive  plants,  it  is  seen  that  numerous  intercellular  spaces  are 
contained  in  that  portion  of  the  parenchyma  which  adjoins  the  flexible  strand  of 
vascular  bundles.  It  is  also  seen  in  these  sections  that  the  thickness  of  the  cell- 
walls  in  one  half  of  the  pulvinus  is  three  times  as  great  as  in  the  opposed  half,  and 
that  all  these  cells  communicate  with  each  other  by  extremely  fine  canals.  If  the 
thick-walled  portion  of  a  pulvinus  is  disturbed  with  a  bristle,  no  alteration  is  pro- 
duced; but  as  soon  as  that  side  characterized  by  its  delicate  cell-walls  is  touched 
ever  so  lightly,  it  changes  colour.  It  now  appears  a  darker  green,  due  to  the  fact 
that  watery  sap  has  been  pressed  out  from  the  cells  into  the  intercellular  spaces. 
The  slightest  pressure  is  felt,  accordingly,  as  a  stimulus  by  the  protoplasm  in  thos 


536  MEASURES   FOR   PROTECTING   GROWING   PLANTS    FROM   LOSS    OF   HEAT. 

cells,  and  causes  them  to  discharge  a  portion  of  their  water  into  the  adjacent  spaces. 
By  this  means  the  turgescence  in  this  part  of  the  cushion  is  very  much  diminished, 
the  tissue  becomes  flaccid,  and  in  proportion  as  this  flaccidity  obtains  the  turgidity 
in  the  tissues  of  the  opposite  half  of  the  leaf -cushion  increases.  It  seems  that  a 
portion  of  the  water  given  up  by  the  stimulated  protoplasm  is  forced  into  the 
opposite  tissue,  and  that  thus  the  turgidity  there  is  augmented.  Such  a  contrast  in 
the  turgidity  of  the  two  halves  of  the  pulvinus  cannot  be  without  influence  on  the 
strand  of  vascular  bundles  lying  in  its  centre;  it  becomes  bent  in  the  direction  of 
the  diminished  turgidity,  and  the  leaflet,  whose  midrib  is  formed  by  a  continuation 
of  the  said  vascular  bundle-strand,  follows  this  movement. 

In  nature,  of  course,  stimulation  of  the  protoplasm  by  contact  of  a  solid  body 
only  occurs  exceptionally.  There  the  process  described  above  is  brought  about  by 
currents  of  air,  and  principally  by  falling  rain-drops.  Few  phenomena  have  such  a 
peculiar  appearance  as  the  movements  which  occur  in  the  foliage  of  the  already 
mentioned  Oxalis  sensitiva  when  rain  comes  on.  Not  only  do  the  leaflets  on  which 
the  first  rain-drops  fall,  fold  together  in  a  downward  direction,  but  all  the  neigh- 
bouring ones  perform  the  same  movement,  although  they  have  not  themselves  been 
shaken  by  the  impact  of  the  falling  drops,  and  one  is  involuntarily  reminded  of  the 
children's  game  in  which  sloped  cards  are  placed  behind  one  another  lengthwise  in 
a  long  series,  and  the  fall  of  the  outermost  card,  produced  by  the  touch  of  a  finger, 
causes  in  a  moment  the  collapse  of  all  the  others.  But  it  is  not  enough  that  the 
opposite  leaflets,  until  now  flatly  outspread,  are  depressed  by  the  shaking.  The 
movement  is  continued  to  the  common  leaf-stalk  bearing  the  numerous  pinnae. 
This  also  bends  towards  the  ground,  and  hangs  down  apparently  prostrated,  in 
consequence  of  the  alteration  of  turgidity  in  the  pulvinus  at  its  base.  The  rain- 
drops now  slide  over  the  bent  leaf -stalk,  whose  point  is  turned  towards  the  ground, 
and  down  over  the  depressed  leaflets,  and  not  a  drop  remains  behind  on  their 
delicate  surfaces. 

The  transmission  of  the  stimulus,  at  first  received  only  by  a  single  leaflet,  to  the 
neighbouring  leaflets  and  common  leaf-stalk,  and  finally  even  to  the  whole  plant, 
reminds  one  strongly  of  the  like  process  in  the  leaves  of  the  Sundew  and  of  the 
Venus's  Fly-trap.  It  also  recalls  the  transmission  of  the  stimulus  in  the  protoplasm 
of  lower  animals,  and  is  indeed  to  be  explained  in  a  similar  manner.  Probably  the 
protoplasmic  masses  of  the  sensitive  groups  of  cells  in  all  pulvini  are  connected 
together  by  continuous  delicate  protoplasmic  threads  penetrating  the  cell-walls,  and 
the  molecular  disturbance  of  the  protoplasm,  produced  by  the  stimulus,  although  at 
first  it  comprehends  only  a  single  cell,  is  transmitted  like  an  electric  current  in 
telegraph  wires  over  the  masses  of  protoplasm,  strung  together  in  close  connection, 
and  linked  by  the  delicate  plasma-threads;  thus  the  same  phenomenon  is  produced 
in  all,  viz.  contraction  of  the  cells  and  a  forcing  out  of  cell-sap  into  the  intercellular 
spaces. 

The  other  sensitive  plants  behave  like  the  above-described  Oxalis  sensitiva, 
except  that  there  is  a  difference  in  the  direction  in  which  the  leaves  fold  together. 


MEASURES   FOR   PROTECTING   GROWING   PLANTS   FROM   LOSS   OF   HEAT. 


537 


JZschynomene  Indica,  an  elegant  herb-like  plant  with  papilionaceous  flowers  and 
extremely  delicate  doubly-pinnate  leaves,  as  well  as  the  Indian  Smithia  sensitiva 
(which  likewise  belongs  to  the  Papilionacese),  fold  their  leaflets  together  above,  and 


rK.X.A. 


Fig.  134.—  Mimosa  pudica  in  day  and  night  positions. 


•depress  the  common  leaf -stalk  directly  the  first  rain-drop  has  produced  a  shaking. 
The  same  applies  to  several  mimosas  (Mimosa  pudica,  sensitiva,  casta,  dormiens, 
humilis,  viva),  of  which  the  first,  a  species  native  in  Brazil,  is  represented  in  fig. 
134.  In  these  mimosas  there  is  really  to  be  noted  a  threefold  movement;  first  of 
all  the  tiny  leaflets  fold  together  above,  and  at  the  same  time  direct  themselves  a 


538  MEASURES  FOR  PROTECTING  GROWING   PLANTS   FROM  LOSS   OF  HEAT. 

little  forward,  so  that  each  in  front  is  partly  covered  over  by  the  one  immediately 
behind  it;  then  the  four  ribs  or  axes,  beset  with  the  tiny  leaflets,  move  towards  one 
another  like  fingers  which  had  been  outspread  and  are  now  closed  together;  and 
thirdly,  the  common  leaf -stalk,  bearing  in  front  the  four  axes,  droops  downwards. 
The  leaflets  of  several  species  of  wood  sorrel  which  have  clover-like  or  fan-like 
leaves,  and  not  pinnate  leaves  like  the  above-mentioned  Oxalis  sensitiva,  also  fold 
their  leaflets  together  when  shaken  by  rain-drops.  In  these  species  of  wood  sorrel, 
however,  we  have  again  a  way  of  diverting  water,  essentially  differing  from  that 
above  described.  The  common  leaf -stalks  do  not  bend  towards  the  ground,  but 
remain  erect;  on  the  other  hand,  the  drooping  leaflets  fold  slightly  along  the  midrib, 
each  of  them  forming  a  shallow  groove,  and  the  rain  water  trickles  on  to  the 
delicate  leaves,  and  then  flows  through  these  channels  to  the  ground.  (Cf.  fig.  90 8, 
the  lowest  leaf,  whose  three  leaflets  are  beginning  to  droop  and  to  fold.) 

From  the  above  it  is  indeed  evident  that  one  benefit  which  the  sensitive  plants 
obtain  by  the  sudden  folding  together  of  their  leaflets  lies  in  the  rapid  diversion 
of  the  falling  rain -drops  thereby  rendered  possible.  By  this  we  do  not  imply 
that  this  is  the  only  advantage  which  ensues  from  the  movements  described.  It 
frequently  happens  that  dry,  gusty  winds  and  drifting  sand  and  extraordinary 
noon-tide  heat  cause  the  folding  of  the  leaflets.  In  the  last-mentioned  instances 
it  is  rather  the  danger  of  excessive  transpiration  which  causes  the  plants  to  place 
the  broad  surfaces  of  their  leaflets  vertically,  and  all  observations  go  to  show  that 
the  leaflets  can  escape  very  various  dangers  by  the  assumption  of  the  so-called 
sleep  position — in  the  clear  night,  the  loss  of  heat  by  radiation  towards  the  starry 
sky;  in  the  hot  mid-day,  drying  up  in  consequence  of  rapid  evaporation;  in  rainy 
weather,  the  breaking  up  of  the  tender  leaves  and  their  inclination  towards  the 
ground,  as  well  as  the  collapse  of  the  whole  plant  under  the  weight  of  the  falling 
drops  in  a  sudden  severe  downpour  of  rain.  It  is  possible  that  yet  a  fourth 
advantage  is  obtained  by  these  movements.  Grazing  animals  which  sniff  the 
delicate  leaves  of  the  sensitive  plants  and  disturb  them  with  their  mouths  are 
perhaps  astonished  and  frightened  at  the  sudden  movements  of  the  leaflets,  and 
abstain  from  eating  these  uncanny  plants,  especially  when  between  the  descending 
leaflets  pointed  rigid  spines  become  visible,  as  is  especially  the  case  in  many 
mimosas. 

It  cannot  be  too  often  insisted  that  the  same  and  similar  contrivances,  as  well 
as  the  same  and  similar  processes,  may  have  a  very  different  significance  according 
as  they  occur  in  this  or  that  plant,  in  this  or  that  habitat,  and  under  these  or 
those  climatic  conditions;  just  as,  on  the  other  hand,  several  advantages  can  be 
simultaneously  obtained  by  one  and  the  same  contrivance.  Thus  for  instance, 
the  position  which  the  leaves  emerging  from  the  buds  in  spring  assume  is  very 
instructive.  When  the  vegetative  activity  is  interrupted  by  a  cold  winter,  and 
when,  moreover,  the  temperature  occasionally  in  clear  spring  nights  sinks  below 
zero,  the  surfaces  of  the  leaves  just  escaping  from  the  buds  are  regularly  placed 
vertically  (cf.  fig.  90).  During  the  day  excessive  transpiration  from  the  still  thin- 


FREEZING   AND   BURNING.  539 

walled  tissues  is  prevented  by  this  position,  and  during  the  night  the  vertical 
position  of  the  young  leaves  has  this  advantage,  that  by  it  radiation,  that  is  to 
say,  excessive  loss  of  heat,  is  hindered.  The  young  not  yet  completely  developed 
foliage  is  in  both  these  respects  very  sensitive,  much  more  so  than  adult  foliage, 
and  this  depends  upon  the  fact  that  the  latter  is  comparatively  poor  in  watery 
contents,  and  the  composition  of  the  protoplasm  has  become  altered.  It  may 
happen  that  in  the  same  plant,  under  the  same  conditions  of  habitat  and  like 
conditions  of  temperature  of  air  and  soil,  while  the  young  leaves  perish  after 
bright  nights  in  consequence  of  too  great  loss  of  heat,  the  fully-developed  leaves 
suffer  no  injury.  This  brings  us  to  the  question,  Wherein  the  damage  to  plants 
caused  by  great  loss  of  heat  actually  consists? 

FREEZING  AND  BURNING. 

Pancratius,  Servatius,  and  Bonifacius,  whose  names  stand  in  the  calendar 
against  the  12th,  13th,  and  14th  of  May,  have  popularly  been  called  "  Eismanner  " 
in  southern  Germany  and  Austria.  They  have  received  this  nickname  on  account 
of  the  fall  of  temperature  which  takes  place  every  year  about  the  middle  of  May, 
the  cause  of  which  is  not  yet  fully  explained.  Later  in  the  summer  such  falls  in 
temperature,  connected  with  cooling  of  the  atmosphere  on  a  large  scale,  occur  on 
certain  days  with  some  regularity;  but  these  have  not  received  so  much  notice 
because  they  are  not  so  dangerous  to  field  products,  fruit  and  wine,  as  the  relapses 
about  the  middle  of  the  month  of  May.  Moreover,  although  really  cold  days  occur 
in  June  or  July,  they  are  never  followed  by  a  frost,  while  the  three  "  Eismanner  " 
of  May  usually  bring  with  them  severe  frosts  at  night,  even  in  the  mildest  regions 
of  Central  Europe,  thus  doing  incalculable  mischief  to  vegetation. 

What  first  of  all  strikes  us  in  a  frozen  plant-organ  is  that  it  has  completely 
lost  its  elasticity.  On  bending  and  pressing  back  with  the  finger  the  frozen, 
stiffened  foliage-leaf,  a  permanent  fold  is  immediately  produced;  the  leaf  is  broken 
along  this  fold,  and  can  no  longer  resume  its  former  position.  At  the  time  of 
breaking  a  noise  is  heard  like  the  crushing  of  pounded  ice,  and  as  a  matter  of 
fact  it  is  actually  crystallized  ice  formed  in  the  interior  of  the  leaf  which  is 
broken  by  the  pressure  and  causes  this  crunching  to  be  heard.  As  the  tempera- 
ture rises  during  the  day,  the  frozen  plants  become  thawed,  but  most  of  them 
retain  no  longer  the  elasticity  which  they  possessed  before  the  frost.  The  leaves 
hang  down  flaccidly,  are  of  a  different  green,  and  are  more  transparent  than 
formerly.  The  surface  is  damp,  and  the  epidermis  is  easily  detached  from  the 
deeper  tissue -layers.  Gradually  the  languid  leaves  shrivel  up,  become  dried, 
and  assume  a  brown  or  black  colour.  They  entirely  resemble  burnt  or  charred 
leaves,  and  the  farmer  says  that  the  frost  has  burnt  them. 

What  takes  place  in  the  interior  of  the  plant  on  account  of  this  freezing? 
The  idea  which  botanists  once  held  is  as  follows:  the  watery  cell-sap  of  the 
plants  stiffens  to  ice;  but  the  ice  takes  up  a  larger  space  than  was  occupied  by 


540  FREEZING   AND   BURNING. 

the  fluid  cell-sap,  and  consequently  the  walls  of  the  cells  are  torn  and  burst  like 
a  glass  bottle  in  which  water  has  frozen.  A  tissue  whose  cells  are  rent  can, 
however,  no  longer  perform  its  functions.  Moreover,  although  the  ice  melts  by 
and  by,  the  damage  to  the  ruptured  cell- walls  is  irreparable.  Besides,  the  cell-sap 
streams  from  the  cell-chambers  of  a  thawing  plant,  and  the  leaves  and  stem  which 
have  thawed  after  freezing  are  seen  to  be  not  only  blackened,  soft,  and  pulpy,  but 
are  also  covered  over  with  a  watery  film  which  is  never  absorbed  again  into  the 
interior. 

Renewed  investigation  has  shown  that  this  idea  of  the  freezing  of  plants  needs 
revision.  First,  in  that  no  rupturing  and  bursting  of  the  cell- walls  occurs  by  the 
pressure  of  the  ice  formed  in  the  interior  of  the  cells.  In  the  tissues  of  plant- 
organs  surrounded  by  air  the  formation  of  ice  does  not  usually  commence  in  the 
interior  of  the  cells,  but  in  the  intercellular  spaces;  and  the  ice-crystals  are  first 
formed  in  the  interior  of  the  cells  only  in  those  aquatic  plants  in  which  intercellular 
spaces  are  absent. 

If  Nitella  syncarpa,  belonging  to  the  Characeae,  which  is  met  with  in  the  clear 
water  of  lakes  and  pools  in  Central  Europe,  is  exposed  to  a  temperature  of  0°  C., 
its  vital  activity  is  not  disturbed.  Even  the  streaming  of  the  protoplasm  in  the 
cells  is  still  very  active,  and  even  if  by  further  cooling  of  the  surrounding  water 
to  —2°  needles  of  ice  are  formed,  the  streaming  of  the  protoplasm  may  still  be 
recognized.  The  cells  are  indeed  somewhat  compressed  by  the  ice-needles,  but  the 
protoplasm  is  never  killed,  even  at  —  3°.  It  first  begins  to  shrivel  up  between  —  3° 
and  —4°,  gives  up  a  portion  of  its  water,  shrinks  away  from  its  cell-wall,  forms  a 
folded,  contracted  sac  in  the  middle  of  the  cell,  whilst  the  water  excreted  stiffens 
into  ice  between  this  sac  and  the  cell-wall.  If  this  Nitella  be  again  exposed  to 
a  higher  temperature  the  ice  melts,  the  protoplasm  expands,  and  lies  close  to  the 
cell- wall;  but  it  is  incapable  of  again  producing  the  streaming  movement;  it  has 
ceased  to  live;  its  molecular  constitution  has  evidently  become  so  fundamentally 
altered  by  the  separation  of  water  from  it  that  a  reconstruction  is  no  longer 
possible. 

In  the  stems  and  leaves  of  plants  surrounded  by  air,  the  ice  always  first  arises, 
as  remarked  above,  in  the  intercellular  spaces.  But  since  usually  air,  and  not 
water,  is  contained  in  the  intercellular  spaces,  the  water  stiffening  into  ice  in  them 
must  have  been  first  excreted  from  the  neighbouring  cells  shortly  before  the 
freezing.  And  this  is  what  actually  happens.  The  structure  of  the  ice-crystals 
plainly  shows  that  the  water  has  come  from  within  through  the  cell-wall  to  the 
exterior,  and  that  not  once,  but  over  and  over  again;  for  on  the  outer  walls  of 
the  cells  which  face  towards  the  intercellular  spaces  the  ice  is  seen  in  the  form 
of  small  discs  placed  above  one  another  and  combined  into  pillars,  and  these  discs 
can  have  been  only  gradually  formed  one  after  the  other.  This  observation, 
however,  raises  the  questions,  What  portions  of  the  cell  give  up  the  water?  and 
why  does  the  water  freeze  in  the  intercellular  spaces  and  not  in  those  places 
which  it  occupied  before  the  frost?  In  answer  to  these  questions  it  must  first 


FREEZING   AND   BURNING.  541 

be  remarked  that  the  water  absorbed  by  the  plants  only  partly  enters  into 
chemical  combination  with  the  materials  of  the  cell-body  and  cell-wall;  that 
another  part,  which  we  have  called  the  water  of  imbibition,  is  not  chemically 
combined.  The  cell-wall  and  cell-body  are  saturated  with  this  latter,  and  the 
cell-sap  in  the  vacuoles  of  the  protoplasm  also  contains  a  large  quantity  of  such 
water.  In  the  cell-sap  it  appears  as  the  solvent  of  the  acids,  salts,  and  other 
materials  there  present.  The  water  by  which  the  protoplasm  and  cell-walls  are 
saturated,  and  which  we  must  imagine  filling  the  interstices,  like  capillary  spaces, 
between  the  groups  of  molecules,  is  indeed  held  fast  by  the  molecules  of  the 
protoplasm  and  cell-wall,  and  the  water  in  the  cell-sap  by  the  molecules  of  acids 
and  salts,  but  yet  certainly  not  so  energetically  as  the  chemically-combined  water 
in  the  albuminous  substances  of  the  protoplasm. 

What  happens  now  in  a  body  which  holds  fast  the  water  in  its  smallest 
interstices,  like  paste,  for  example,  or  in  which  the  water  appears  as  a  solvent 
as  in  an  alum  solution,  when  warmth  is  withdrawn,  and  when  it  is  cooled  down 
to  the  freezing  point  of  water?  It  is  very  remarkable  that  the  water  does  not 
immediately  stiffen  into  ice  as  long  as  it  is  retained  in  the  capillary  spaces,  or 
as  a  solvent,  and  many  salt-solutions  can  be  cooled  down  to  5°  C.,  some  even  to 
10°,  below  zero  without  freezing.  When  at  length  under  the  influence  of  still 
lower  temperatures  a  stiffening  occurs,  a  separation  has  always  taken  place 
previously;  the  water  has  run  together  from  the  finest  interstices  of  the  paste 
into  its  larger  spaces;  it  is  first  changed  into  ice  in  these  cavities,  and  the  water 
of  the  salt  solution  has  separated  from  the  molecules  of  salt,  and  is  then  first 
changed  into  ice-crystals. 

The  same  thing  occurs,  however,  with  the  water  saturating  the  cell-wall  and 
protoplasm,  and  serving  as  solvent  of  certain  materials  contained  in  the  cells.  The 
formation  of  ice  occurs  in  a  very  few  species  only  on  cooling  the  plant-tissues  down 
to  —  1°;  in  most  instances  the  temperature  must  sink  to  —  2°  or  —3°  in  order 
that  ice  may  be  formed  in  the  cooled  tissue.  And  indeed  the  water  here  has  sepa- 
rated from  the  molecules  by  which  it  was  hitherto  held  fast  before  it  congealed, 
and  it  does  not  freeze  in  the  interior  of  the  cells,  but  outside  them  in  the  inter- 
cellular spaces.  In  order  that  the  water  should  get  from  the  interior  of  the  cell 
into  the  adjoining  intercellular  spaces,  a  pressing  and  squeezing  is  necessary,  and 
this  pressure  can  only  proceed  from  the  living  protoplasm  in  the  cell-chambers; 
consequently  the  process  of  freezing  can  be  most  correctly  represented  in  this  way, 
viz.  that  the  protoplasm  becomes  stimulated  and  roused  by  the  lowering  of  the 
temperature  to  transport  a  portion  of  the  water  from  the  interior  to  the  exterior  of 
the  cell,  by  means  of  contraction  and  pressure.  What  happens  there  is  accordingly 
not  unlike  the  excretion  of  watery  sap  into  the  intercellular  spaces  in  the  stimulated 
pulvini  on  the  leaf -stalk  of  Mimosa]  but  the  advantage  obtained  by  the  excretion 
of  water  in  the  two  cases  is  very  different.  In  the  cooled  leaves  the  benefit  of 
course  is  to  be  sought  for  in  the  fact  that  the  living  portion  of  the  cells  is  protected 
from  destruction  as  long  as  possible  by  the  formation  of  ice-crystals  in  the  inter- 


542  FREEZING   AND   BURNING. 

cellular  spaces.  If  the  water  were  forthwith  frozen  inside  the  cells,  between  the 
groups  of  molecules  of  the  living  cell-body  and  its  wall  by  a  few  degrees  of  cold, 
fundamental  displacements  and  disorganizations  of  the  groups  of  molecules  would 
be  unavoidable.  On  the  other  hand,  the  ice-crystals  on  the  exterior  of  the  cells  do 
not  produce  such  destruction.  In  the  intercellular  spaces  they  can  form  large 
clusters,  the  spaces  may  be  even  enlarged  by  them,  and  the  adjoining  portions  of 
tissue  may  be  compressed  and  split,  without  a  disorganization  of  the  molecular 
structure  of  the  living  cells  occurring  simultaneously. 

It  is  shown  by  numerous  other  phenomena  that  the  excretion  of  water  described 
does  not  connote  the  death  of  the  living  cells.  It  is  also  beyond  doubt  that  the 
excreted  water  can  be  again  received  back  subsequently  under  favourable  conditions; 
and  that  by  slow  thawing  of  the  ice  formed  in  the  intercellular  spaces,  the  water 
again  returns  to  the  places  it  previously  occupied  within  the  cell.  If,  on  the  other 
hand,  the  cells  are  no  longer  able  to  take  back  the  separated  water,  or  if  the  cold 
becomes  so  severe  that  finally  the  water  retained  by  the  protoplasm  and  indispens- 
ably necessary  to  its  existence,  becomes  changed  into  ice,  then  a  disorganization  of 
the  molecular  structure  is  the  natural  consequence;  or,  in  other  words,  the  proto- 
plasm of  the  cells  in  question  has  been  killed  by  the  loss  of  heat.  Then  we  say 
the  plants  are  frozen  dead. 

Thus  the  difference  between  mere  freezing  and  freezing  to  death  is  made  clear; 
and  at  the  same  time  the  experience  of  gardeners  is  confirmed,  that  the  former  is 
not  necessarily  attended  by  the  latter. 

At  what  degree  of  cold  freezing  occurs,  and  at  what  freezing  to  death,  depends 
first  of  all  on  the  specific  constitution  of  the  protoplasm  of  the  various  species,  but 
also,  in  each  individual  species,  upon  the  stage  of  development  arrived  at  by  the 
organs  exposed  to  the  cold.  Just  as  the  water  in  various  salt  solutions  becomes 
changed  into  ice  at  various  temperatures,  so  the  protoplasm  of  one  species  exhibits 
a  different  behaviour  to  that  of  another.  It  has  been  mentioned  above  that  the 
hydrophyte  Nitella  syncarpa  is  frozen  at  a  temperature  of  —  4°  C.  Other  aquatic 
plants  bear  a  much  greater  degree  of  cold  without  their  protoplasm  being  killed. 
Sphcerella  nivalis,  which  produces  the  red  colour  of  snow,  is  exposed  in  the  winter 
for  months  to  a  temperature  of  —  20°  C.  in  Arctic  regions,  and  is  not  destroyed 
thereby.  This  Sphcerella  is  also  frequently  exposed  to  very  severe  cold  on  the 
snow-fields  of  the  Alps  during  winter  nights,  and  the  same  remark  applies  to 
various  species  of  the  genera  Epithemia  and  Navicula  and  to  other  Diatomacese 
which  are  to  be  found  together  with  Sphcerella  nivalis  living  on  the  glaciers.  It 
may  be  mentioned  here  in  passing  that  there  are  also  animals  which  live  with  these 
unicellular  plants  in  the  ice  regions,  and  are  not  killed  although  they  remain  frozen 
for  months.  As  soon  as  they  thaw,  these  Rotifers  bring  their  cilia  into  action 
again;  the  black  Poduras,  known  by  the  name  of  glacier-fleas,  take  their  flying 
leaps,  and  the  spotted  spiders  again  stride  with  their  long  legs  over  the  sun-illu- 
mined ice-fields;  while,  on  the  other  hand,  the  insects  driven  by  the  wind  to  these 
glaciers  are  in  a  very  short  time  killed  by  the  frost 


FREEZING    AND    BURNING.  543 

The  same  thing  occurs  in  land-plants  and  lithophytes  as  with  animals  and 
aquatic  plants.  Plants  which  closely  resemble  each  other  externally  and  show 
great  similarity  in  their  anatomy  may  yet  behave  quite  differently  in  the  matter  of 
freezing.  While  the  Stone  Pine  and  the  Shore  Pine  (Pinus  Pinea  and  Halepensis) 
cannot  bear  the  frost  of  winter,  the  Arolla  Pine  and  Bhotan  Pine  (Pinus  Gembra 
and  excelsa)  flourish  in  regions  where  the  trunks  and  acicular  leaves  of  all  the 
trees  are  cooled  dovrn  for  weeks  to  -20°.  Rhododendron  Ponticum  freezes  at 
-2°,  but  Rhododendron  Lapponicum  survives  the  severest  cold  of  the  northern 
winter.  If  Echeverias  are  brought  out  of  the  greenhouse  on  a  cold  autumn  night 
into  an  open  place  where  the  temperature  falls  to  —1°,  they  will  be  irretrievably 
lost;  while  most  of  the  European  succulent  plants  closely  allied  to  the  Echeverias, 
and  agreeing  with  them  in  the  structure  of  the  fleshy  leaves,  endure  the  same 
degree  of  cold  without  injury — not  only  for  a  night  but  even  for  weeks.  The 
northern  Sedum  Rhodiola  and  several  Alpine  species  of  house-leek  growing  on  the 
narrow  ledges  of  rock  faces  in  the  high  Alps  (e.g.  Sempervivum  montanum  and 
Wulfenii)  are  exposed  for  weeks  to  a  temperature  of  — 10°,  and  yet  the  protoplasm 
of  their  fleshy  leaves  does  not  freeze.  There  are  also  a  number  of  biennial  and 
perennial  plants  which  cannot  actually  be  called  succulents,  but  which  nevertheless 
form  smooth,  turgid  leaves  in  the  autumn  arranged  in  rosettes  lying  on  the  ground, 
outwardly  in  no  way  protected  against  loss  of  heat.  The  leaves  of  these  rosettes 
are  exposed  to  the  greatest  cold  in  regions  where  the  winter  is  severe,  especially 
when  little  or  no  snow  has  fallen,  and  the  temperature  of  the  succulent  tissue  is 
often  cooled  down  to  —20°,  and  yet  the  protoplasm  is  not  killed.  The  Scurvy 
Grass  (Cochlearia  officinalis)  is,  in  this  matter,  particularly  worthy  of  notice. 
It  would  naturally  be  expected  that  its  smooth,  turgid,  dark-green  leaves  would 
be  killed  with  the  first  hoar-frost,  while  in  reality  they  endure  a  very  consider- 
able cold  without  tha  slightest  injury.  There  are  few  places  on  the  earth  where 
such  a  severe  -winter  climate  prevails  as  on  the  shores  of  Pitlekaj  on  the  northern 
coast  of  Siberia,  where  the  Vega  expedition  passed  the  winter  of  1878-79.  In 
November  the  mean  temperature  amounted  to  -16'58°,  in  December  to  -22'80°, 
in  January  to  -26'06°,  in  February  to  -25'09°,  in  March  to  -21'65°,  in  April  to 
-18-93°.  But  these  were  only  the  averages;  on  many  days  the  temperature  fell  to 
—  30°,  and -40°,  and  once  the  minimum  even  reached  —46°  C.  On  the  summit  of 
a  fairly  high  sand-hill  over  which  the  icy  north  and  north-east  wind  swept  almost 
uninterruptedly,  a  plant  of  Scurvy  Grass  (Cochlearia  fenestralis)  was  observed. 
This  plant  had  begun  to  bloom  in  the  summer  of  1878,  and  had  also  partly 
developed  fruit.  When  the  winter  began,  however,  this  Cochlearia  still  possessed 
unripe  fruits,  flowers,  and  flower-buds  as  well  as  succulent  green  foliage-leaves; 
and  it  was  to  be  expected  that  the  delicate  succulent  tissue  would  be  completely 
destroyed  during  the  long  winter  under  the  influence  of  the  continuous  cold.  But 
in  the  summer  of  1879  the  plant,  whose  tissue  had  undoubtedly  been  cooled  down 
for  a  long  time  to  -30°,  and  frozen,  began  again  to  grow,  and  continued  its  growth 
where  it  had  been  interrupted  at  the  beginning  of  winter.  The  leaves  resumed 


544  FREEZING   AND   BURNING. 

their  functions  as  in  the  previous  summer,  the  flower-buds  opened,  and  new  inflor- 
escences sprang  from  the  axils  of  the  leaves,  proving  that  the  protoplasm  of  this 
plant  had  not  been  killed  even  by  a  temperature  of  —  46°. 

Myrtle  and  orange  trees  freeze  dead  from  —2°  to  —4°,  cypresses  and  fig-trees 
from  —7°  to  —9°;  vines  at  —21°,  oaks  and  beeches  at  —25°,  plums  and  cherries 
at  —31°,  and  apple  and  pear  trees  at  —33°,  and  this  can  only  be  explained  by  the 
specific  constitution  of  the  protoplasm.  We  are  forced  to  assume  that  the  cell-body 
is  destroyed  in  one  case  by  a  certain  temperature,  and  in  another  by  a  different 
temperature,  in  the  manner  already  described. 

It  has  been  previously  remarked  that  the  temperature  at  which  freezing  takes 
place  also  depends  upon  the  stage  of  development  of  the  plants.  It  is  generally 
known  that  woody  trunks  and  branches,  foliage,  and  flower-buds,  and  especially 
seeds  bear  quite  extraordinary  winter  temperatures  when  they  have  been  poor  in 
water  in  the  autumn.  In  Yakutsk  and  Werchojansk  in  Siberia,  where  the  mean 
temperature  in  January  amounts  to  —  42'8°  and  —  49'0°,  and  where  —  62'0°  and 
—  63'2  (the  lowest  temperature  hitherto  generally  observed  on  the  earth)  were 
noticed,  where  for  months  the  temperature  in  the  shade  does  not  rise  above  —  30°, 
numerous  herbs  and  shrubs  are  found  whose  upper  organs  are  exposed  for  weeks 
to  a  degree  of  cold  at  which  mercury  freezes;  even  birches  and  larches  flourish 
there  with  the  most  vigorous  growth,  and  there  can  be  no  doubt  that  the  wood  and 
buds  of  these  trees  are  every  year  cooled  down  for  a  long  time  to  —  30°,  and  yet 
are  not  frozen.  Moreover,  every  winter  the  wood  of  the  juniper  and  of  spruce,  of 
silver  firs  and  arollas  sinks  down  to  — 10°  in  inclement  situations  on  the  Central 
European  mountains,  and  the  evergreen  leaves  of  these  woody  plants  become  cooled 
far  below  the  freezing  point  of  water  without  suffering  the  slightest  damage.  On 
this  account  the  seeds  inclosed  in  the  berries  and  cones  of  the  trees  named  bear  the 
lowest  temperatures  without  injury,  which  is  so  much  the  more  remarkable  since 
these  seeds  require  two  summers  for  ripening,  and  therefore  must  pass  through  the 
severe  winter  of  the  first  year  in  a  still  unripe  condition.  The  seeds  of  other  plants 
also  are  exposed  to  great  cold  through  the  winter.  Thus,  for  example,  those  of  the 
Laburnum  (Cytisus  Laburnum)  do  not  fall  off  as  soon  as  they  are  ripe,  but  remain 
hanging  to  the  sides  of  the  dehisced  pods,  and  as  these  are  not  detached  from  the 
branches  until  the  following  spring,  the  temperature  of  the  seeds  during  the  winter 
falls  far  below  zero.  They  nevertheless  maintain  their  germinating  powers. 
Laburnum  seeds,  which  had  been  during  the  winter  for  weeks  under  the  influence 
of  a  temperature  of  —15°,  germinated  in  the  following  summer,  and  so  had 
evidently  suffered  no  injury  from  the  cold.  Other  seeds,  too,  even  from  tropical 
regions,  which  had  been  experimentally  subjected  to  temperatures  of  —40°,  were 
not  found  to  have  lost  their  germinating  capacity,  and  consequently  their  proto- 
plasm had  not  been  killed  even  by  this  excessive  temperature. 

Since,  on  the  other  hand,  it  is  known  that  the  young  fruits  and  seeds  of  the 
laburnum,  and  still  more  those  of  tropical  plants,  are  already  congealed  by  lowering 
the  temperature  to  -2°,  it  follows  that  portions  of  that  same  plant  in  various 


FREEZING   AND   BURNING.  545 

stages  of  development  are  affected  differently  by  the  lowering  of  their  temperature 
below  the  freezing  point.  For  the  majority  of  plants  the  rule  holds  that  death  in 
consequence  of  frost  occurs  the  sooner,  the  younger  and  richer  the  tissues  in  question 
are  in  water.  The  foliage  of  beeches,  hornbeams,  and  deciduous  oaks,  which  is  not 
killed  in  the  autumn,  even  after  repeated  frosts,  withers,  shrivels,  and  dries  up 
when  young  if  the  temperature  sinks  below  zero  only  for  a  single  night  in  spring. 
Even  many  Alpine  plants  which  bear  very  low  temperatures  without  injury,  when 
completely  developed,  may  suffer  harm  if  they  are  surprised  by  a  frost  at  the  period 
of  most  active  growth.  When  on  one  occasion  at  the  end  of  June  the  temperature 
sank  to  -6°  on  the  mountains  near  Innsbruck,  already  free  from  snow,  at  an  alti- 
tude of  2000  metres,  the  young  foliage-leaves  of  the  Rhododendron  (Rhododendron 
hirsutum),  which  had  just  sprouted,  and  were  not  yet  fully  grown,  were  destroyed 
on  all  the  plants.  They  became  brown  and  dried  up,  while  the  old  fully-formed 
green  leaves,  remaining  on  the  same  plants  from  the  previous  year,  underwent  no 
alteration  from  the  frost. 

Such  phenomena  can  only  be  explained  by  the  assumption  that  in  young,  un- 
developed organs  much  water  is  present,  and  is  not  under  the  control  of  the  living 
protoplasm.  As  such  water  we  may  consider  that  which  is  conducted  from  the 
roots  to  the  green  tissue,  to  be  there  liberated  in  the  form  of  vapour;  that  which 
mounts  through  the  vascular  bundles  of  the  stem,  streams  through  the  veins  of  the 
leaves,  under  certain  circumstances  is  even  forced  into  the  intercellular  spaces,  and 
passes  out  by  the  water-pores  in  the  form  of  drops.  This  water  is  not  retained  by 
molecular  forces,  nor  protected  against  freezing,  but  turns  into  ice  even  at  a  tem- 
perature of  —  1°.  Since  it  is  abundantly  present  in  the  young  tissue  when  freezing 
takes  place,  extensive  ruptures  and  mechanical  injuries  to  the  water-conducting 
tubes  and  rows  of  cells  are  unavoidable.  But  if  the  conduction  of  the  crude  food- 
sap  is  interrupted  in  a  young  plant-organ  during  its  growth,  transpiration  in  it  can 
no  longer  occur  properly,  and  the  transpiring  cells  become  withered  and  dried  up 
even  although  their  protoplasm  should  have  suffered  no  direct  harm  from  the  frost. 

Naturally  connected  with  this  discussion  is  the  question  whether  a  plant  can 
freeze  at  a  degree  of  temperature  above  that  of  the  freezing-point  of  water.  By 
the  majority  of  gardeners  this  question  would  be  answered  in  the  affirmative,  and 
their  reply  would  be  founded  upon  the  fact  that  tropical  Acanthaceae,  variegated- 
leaved  Coleus,  basils,  melons,  tobacco-plants,  &c.,  become  withered,  dry  up  and  die 
if  they  are  exposed  for  only  a  single  night  to  a  temperature  of  +2°.  In  spite 
of  the  great  outward  resemblance  between  this  phenomenon  and  freezing,  it  must 
nevertheless  not  be  called  freezing,  for  the  most  distinctive  processes  of  the  freezing 
of  living  protoplasm,  viz.  the  excretion  of  water  from  the  cell-body,  the  hardening 
of  this  water  into  ice,  and  its  inability  to  return  to  the  cell-body,  do  not  occur  in 
plants  which  are  destroyed  under  the  influence  of  temperatures  above  zero.  It  has 
been  clearly  shown  that  this  so-called  freezing  of  plants  at  temperatures  above  zero 
is  really  a  drying-up  in  consequence  of  the  disproportion  between  the  transpiration 
from  the  leaves  and  the  absorption  of  water  by  the  roots.  In  consequence  of  the 

VOL.  I.  35 


546  FREEZING   AND   BURNING. 

fall  of  temperature  in  the  soil,  the  absorbent  activity  of  the  roots  is  so  restricted 
that  the  loss  of  water  by  the  leaves  through  evaporation  can  no  longer  be  replaced. 
The  leaves  then  become  flaccid,  shrivel  and  dry  up,  are  blackened,  and  look 
exactly  like  leaf-structures  which  have  been  killed  by  frost.  It  can  be  demon- 
strated by  a  very  simple  experiment  that  the  cause  of  death  is  only  the  fall  of 
temperature  in  the  soil.  If  on  autumn  nights,  when  the  temperature  falls  to  + 1° 
or  _|_  2°,  "  very  sensitive  "  Coleus  plants  in  pots  are  brought  from  the  warm  green- 
house into  the  open,  the  pots  not  being  protected  against  cooling,  these  plants  dry 
up  even  the  next  day.  If,  on  the  other  hand,  the  plants  are  sunk  in  warm  sawdust 
over  which  cotton- wool  is  strewn,  and  thus  care  is  taken  that  the  temperature  of 
the  earth  in  the  pots  does  not  sink  below  +  7°,  then  the  Coleus  does  not  dry  up,  and 
generally  suffers  no  harm  even  although  the  temperature  of  the  air  and  of  the  air- 
surrounded  leaves  should  fall  during  the  night  to  +0'5°.  Since  the  conduction  of 
water  to  the  transpiring  leaves  is  sustained  by  the  warmth  of  the  soil,  these  leaves 
may  be  protected  from  the  so-called  "freezing"  even  when  they  cool  down  to  +0*5°. 

Do  means  also  exist  by  which  plants  may  be  protected  from  actual  freezing? 
To  this  question  the  answer  follows  naturally  from  the  above  discussion  about  the 
real  nature  of  freezing.  If  the  plants  in  question  can  be  hindered  from  assuming 
that  temperature  in  which  their  protoplasm  is  killed,  then,  of  course,  a  protection 
against  freezing  may  be  afforded.  Usually  bad  heat-conductors  are  used  as  protec- 
tive agents.  The  plant  organs  to  be  protected  are  clothed  with  dry  straw  and  twigs, 
or  covered  with  dried  foliage.  In  regions  with  continental  climates  vines  are  pro- 
tected against  freezing  by  surrounding  the  lower  portions  of  the  stock  with  earth. 
Often  plants  are  also  protected  by  heaping  up  snow,  and  gardeners  very  generally 
use  snow  as  an  excellent  protection  against  freezing.  As  a  matter  of  experience 
numbers  of  plants  perish  with  us  during  those  winters  in  which  no  snow  falls, 
while  they  survive  without  injury  the  coldest  periods  of  winter  when  the  snow  is 
abundant.  Many  species  of  shrubs  and  low  trees,  of  which  only  the  lower  half  is 
snowed  up,  while  the  upper  half  rises  above  the  surface  are  found  after  severe 
winters  to  be  frozen  from  the  apices  of  the  branches  down  to  the  level  to  which 
the  snow  has  reached.  This  happened,  for  example,  in  the  Vienna  Botanic  Gardens 
(1880)  with  several  young  trees  of  the  deodar  (Cedrus  Deodora),  with  the  bushes  of 
Fontanasia  jasminoides,  and  with  shrubs  of  many  species  of  jasmine  and  indigo. 
But  all  these  protective  agents,  twigs,  straw,  leaves,  earth,  and  snow,  fulfil  their 
function  only  in  neighbourhoods  where  the  cold  period  is  of  comparatively  short 
duration.  In  reality  they  ward  off  only  the  first  onset  of  cold,  and  their  principal 
use  lies  in  the  fact  that  the  radiation  of  heat  from  the  covered  portions  is  retarded. 
In  long  and  continuous  cold  the  temperature  of  the  coverings  not  only  gradually 
sinks,  but  finally  that  of  the  covered  bodies  also,  and  in  Yakutsk  a  plant  whose 
protoplasm  is  killed  at  — 10°  can  no  longer  be  protected  even  by  the  thickest  cover- 
ing of  straw,  leaves,  or  earth. 

Moreover,  in  nature  we  can  only  speak  conditionally  of  a  natural  protection 
against  freezing,  and  only  in  those  regions  where  periods  of  great  cold  alternate 


FREEZING   AND   BURNING.  547 

with  milder  intervals  during  the  winter,  and  where  as  a  rule  a  warmer  day  succeeds 
the  cold  night,  which  is  the  case  wherever  the  sun  does  not  remain  during  the 
winter  below  the  horizon  for  weeks  or  perhaps  months.  All  coverings  which 
protect  from  freezing  in  temperate  zones  are  therefore  entirely  useless  in  Arctic 
regions.  The  snow  which,  as  stated,  is  in  the  north  temperate  zone  one  of  the  best 
protective  measures  against  severe  temperatures,  cannot  in  the  Arctic  regions  at  all 
hinder  the  penetration  of  the  cold.  Kane  found  the  temperature  in  North-west 
Greenland  at  63  cm.  under  the  snow  to  fall  to  —21-3°  and  to  — 16'3°  at  126  cm. 
below.  The  observations  which  were  undertaken  during  the  wintering  of  the 
Swedish  Polar  Expedition  in  Mussel  Bay  on  the  north  coast  of  Spitzbergen,  showed 
that  on  the  14th  February  1873,  when  the  temperature  of  the  air  was  —35°,  the 
snow  had  fallen  to  —  26°  at  26  cm.  below  the  surface,  and  at  a  depth  of  35  cm.  to 
—  20°.  On  the  23rd  February  the  snow  at  a  depth  of  30  cm.  showed  a  temperature 
of  —21°,  while  the  temperature  of  the  air  was  —32°.  On  the  North  Siberian  coast 
the  snow  at  a  depth  of  30  cm.  was  found  by  the  Vega  Expedition  on  the  22d  March 
to  be  cooled  down  to  —161°,  and  the  earth  below  it  to  — 15'1°,  while  the  temperature 
of  the  air  was  — 18'2°.  At  the  middle  of  March  the  sandy  soil  penetrated  by  the 
roots  of  the  Northern  Bent  Grass  (Mymus  mollis)  exhibited  at  a  depth  of  63  cm.  a 
temperature  of  —20°. 

It  is  quite  different  in  the  north  temperate  zone.  When  the  sun  shines  on  the 
snow,  if  only  for  a  few  hours  of  the  day,  it  becomes  warmed  and  usually  melted  at 
the  surface.  In  the  Alps,  during  the  shortest  days  in  December,  when  the  temper- 
ature of  the  air  in  the  shade  is  —10°  to  —15°,  melted  drops  may  be  seen  in  mid-day 
trickling  down  from  the  sun-illumined  roofs  of  the  hay  chalets  situated  high  up  on 
the  mountain  slopes.  Three  Swiss,  who  had  determined  for  the  purpose  of  meteoro- 
logical observations  to  pass  the  winter  of  1865-66  in  the  hut  situated  at  an  altitude 
of  3333  metres  on  the  Matterhorn,  observed  on  the  18th  December,  1865,  and  on 
several  other  days,  that  in  the  sunshine  the  snow  was  melted.  When  the  sun  sets 
behind  the  mountains  the  melted  water,  of  course,  again  freezes,  but  the  next  day 
the  same  process  is  repeated,  while  in  Arctic  regions,  in  the  months  of  uninterrupted 
winter  night,  the  fallen  snow  remains  powdery.  On  mountains  of  the  temperate 
zones,  in  consequence  of  melting  under  the  influence  of  the  sun's  rays  and  the 
succeeding  hardening  during  the  nights,  the  superficial  layer  of  snow  forms  a  crust 
of  ice  which  in  time  becomes  so  thick  that  wide  stretches  of  snow-field  may  be 
traversed  without  breaking  through  it. 

This  alternation  of  thawing  and  freezing  in  the  upper  layers  of  the  winter  coat 
of  snow  has  this  important  use,  that  in  neighbourhoods  where  the  sun  shines  in 
the  winter  the  deeper  layers  of  snow,  and  the  solid  earth  bearing  the  snow,  are 
never  so  much  cooled  as  in  the  far  north,  where  the  cooling  may  continue  for 
months,  and  where,  as  the  above  figures  show,  it  actually  does  so  continue. 
Minimum  thermometers  which  were  placed  in  the  earth  in  the  year  1869  on  various 
mountain  heights  in  the  Tyrol,  and  at  the  end  of  the  winter  were  dug  up,  showed 
the  following  temperatures:  On  the  rocky  summit  of  the  Hafelekar,  at  Innsbruck. 


548  FREEZING  AND   BURNING. 

at  an  altitude  of  2343  metres,  and  40  cm.  below  the  surface,  —5-3°;  on  the  north 
side  of  the  summit  of  the  Blaser,  at  Trins,  at  an  altitude  of  2239  metres,  40  cm. 
below  the  surface,  —  4'0° ;  on  the  northern  slopes  of  the  Patscherkof el,  at  Innsbruck, 
1535  metres  above  the  sea-level,  60  cm.  below  the  surface,  —  2'9°.  The  layer  of  snow 
lying  on  the  ground  at  these  three  points  was  not  a  thick  one,  and  varied  from  30 
to  60  cm.  Where  the  snow-layer  was  at  least  three  times  as  thick,  the  minimum 
thermometer  gave  the  following  results:  On  the  south  side  of  the  summit  of  the 
Blaser,  at  Trins,  at  an  altitude  of  2239  metres,  40  cm.  below  the  surface,  +0'1°; 
rather  lower  on  the  same  mountain,  viz.  at  an  altitude  of  2086  metres,  near  the 
cottage  of  my  experimental  garden,  under  a  snow-drift  3  metres  thick,  +0'2°;  on 
the  Patscherkof  el,  at  Innsbruck,  1921  metres  in  height,  in  the  vicinity  of  Kreuz- 
brunnen,  65  cm.  below  the  surface  of  the  ground,  +0*1°;  near  the  Heiligwasser, 
at  Innsbruck,  at  an  altitude  of  1261  metres,  where  the  winter  coat  of  snow  had 
attained  a  thickness  of  almost  2  metres,  75  cm.  below  the  surface,  +1'35°.  These 
statements  sufficiently  show  what  a  great  significance  is  to  be  assigned  to  snow  as 
a  protective  measure  against  cold  in  those  regions  which  are  not  deprived  of  the 
sun  in  winter-time.  While  the  temperature  of  the  soil  penetrated  by  roots  of  plants 
sank  even  under  deep  snow  to  —  20°  on  the  winter  station  of  the  Vega  in  Siberia, 
the  soil  occupied  by  the  roots  of  plants  on  the  Alpine  heights  of  the  Tyrol  in  places 
covered  with  abundant  snow,  was  never  once  frozen,  and  where  the  snow  layer  was 
very  thin,  was  cooled  down  only  to  —5 '3°.  Accordingly  in  the  Alps,  and  generally 
in  high  mountains  of  the  north  temperate  zone,  a  thick  layer  of  snow  plays  the 
part  of  an  excellent  protective  measure  to  the  soil,  and  consequently  to  the  plants 
rooted  therein. 

There  are  also  plants  in  Alpine  regions  which  are  apparently  dependent  upon 
this  protective  measure,  and  whose  structure  makes  it  possible  for  them  to  survive 
through  the  severe  winter  hidden  under  thick  masses  of  snow.  To  these  belong, 
in  the  first  place,  numerous  bush-like  woody  plants  of  which  Finns  humilis 
represented  opposite  may  serve  as  an  example.  The  stems  of  these  pines  are 
not  erect  like  those  of  other  species,  but  assume  a  horizontal  position  even  when 
they  attain  a  considerable  thickness.  Stems  of  even  20  cm.  diameter,  which  would 
certainly  be  able  to  sustain  the  ample  crown  in  an  erect  position,  grow  almost 
parallel  to  the  ground  without,  however,  directly  resting  upon  it.  In  this  respect 
it  is  very  remarkable  that  on  the  slope  of  the  mountain  the  growing  end  of  the 
stem  is  always  directed  towards  the  valley,  and  it  is  also  noticeable  that  this 
peculiar  habit  of  growth  occurs  not  only  in  the  mountain  pines  growing  wild  in 
the  Alps,  but  also  in  those  raised  from  seed  in  the  botanic  gardens  of  towns,  and 
must  therefore  be  regarded  as  an  inherent  peculiarity.  The  boughs  and  twigs  which 
curve  upwards  from  the  main  stems  are  exceedingly  elastic,  and  when  pressed 
down  stretch  themselves  out  along  the  ground.  Since  all  the  boughs  of  the  crown 
are  turned  upwards,  we  get  here  a  considerable  accumulation,  so  that  in  many  old 
clumps  of  mountain  pines  the  numerous  boughs  are  so  thickly  crowded  and  so  closely 
interwoven  that  progress  through  them  is  almost  impossible.  The  extensive  tracts 


FREEZING   AND   BURNING. 


549 


550  FREEZING   AND   BURNING. 

of  mountain  pines  are  therefore  avoided  and  left  alone,  and  many  of  them  have 
never  been  penetrated  by  the  foot  of  man  during  their  whole  existence.  Woe  to 
him  who  has  the  misfortune  to  lose  his  way  in  such  a  tangled  wood!  The  difficulties 
one  has  to  encounter  in  a  tropical  primeval  forest  beset  with  lianes  are  not  greater 
than  those  with  which  one  must  struggle  in  attempting  to  press  forward  here. 
Frequently  the  mountain  pines  grow  so  high  that  one  is  considerably  overtopped 
even  when  standing  upright  by  the  highest  prickly  branches.  It  is  perhaps  pos- 
sible to  make  a  little  progress  by  climbing  over  the  horizontal,  arm-thick  stems,  but 
it  is  vain  to  endeavour  to  find  one's  way  and  to  gain  an  outlook.  If  we  mount  on 
one  of  the  curved  ascending  boughs  in  order  to  see  above  the  highest  branches, 
the  bough  bends  down  to  the  earth  under  our  weight,  along  with  the  stem  from 
which  it  arises,  and  we  again  sink  despairingly  into  the  sea  of  the  dark-green 
crowns.  Just  such  a  down-bending  occurs,  however,  under  the  burden  of  the 
winter  snow.  If  then  by  chance  the  ordinary  mantle  of  snow  is  added  to  by  that 
from  avalanches,  the  pressure  increases  so  much  that  the  branches  are  pressed 
down  to  the  soil.  This  process  may  go  on  to  such  an  extent  that  even  many 
branches,  which  in  the  summer  stand  a  metre  above  the  ground,  lie  in  the  winter 
directly  on  the  soil  on  account  of  the  snow  pressure.  When  the  snow  melts  in  the 
following  spring,  and  the  branches  are  gradually  lightened,  they  rise  up  again 
in  consequence  of  their  extraordinary  elasticity,  and  resume  that  position  which 
they  occupied  in  the  preceding  summer.  The  process  which  is  here  carried  on 
automatically  strongly  reminds  us  of  the  manipulations  of  gardeners,  who  in  the 
autumn  bend  down  rose-trees  to  the  earth  and  cover  them  with  non-conductors, 
keep  them  in  this  position  throughout  the  winter,  and  not  till  the  next  spring  raise 
them  again  and  fasten  them  to  erect  sticks.  In  the  summer  the  old  leaves  on  the 
ends  of  mountain  pine  branches,  which  wave  above  the  ground  more  than  a  metre 
high,  may  be  frequently  seen  plastered  over  with  earth  and  small  stones,  and 
anyone  knowing  nothing  of  the  processes  above  described  would  not  easily  under- 
stand how  these  small  stones  had  come  to  be  fixed  in  these  situations.  As 
a  matter  of  fact  the  earth  which  lies  on  the  branches  through  the  winter, 
moistened  by  the  snow-water,  forms  the  adhesive  agent,  which  is  so  efficient  that 
stones  more  than  1  cm.  in  diameter  are  attached  by  it  to  the  old  tufts  of  leaves. 
Many  other  Alpine  shrubs  behave  like  the  mountain  pines,  as,  for  example,  the 
Dwarf  Juniper  (Juniperus  nana)  and  the  Alpine  Alder  (Alnus  viridis).  In  like 
manner  the  rhododendron  bushes  are  also  pressed  to  the  ground  by  the  snow, 
although  not  to  such  a  great  extent,  and  are  thus  protected  against  the  great 
cold,  and  particularly  against  extreme  radiation. 

In  forest  regions  the  dry  foliage,  which  falls  from  the  trees  and  overspreads 
the  ground  and  undergrowth  to  a  greater  or  less  thickness,,  appears  also  to  be 
usually  an  excellent  protective  agent.  This  foliage  layer  is  thickest  in  the  beech 
forests  of  Central  Europe,  and  the  sheltered  plants  include  the  Woodruff,  Lungwort, 
Hepatica,  Asarabacca,  Sanicle,  and  Waldsteinia  (Asperula  odorata,  Pulmonaria 
officinalis,  Hepatica  triloba,  Asarum  Europceum,  Sanicula  Europcea,  and  Wald- 


FREEZING   AND   BURNING. 


551 


steinia  geoides)  maintain  themselves   beneath   it,  unfrozen,  even  in  very  severe 
winters. 

Other  plants,  again,  appear  to  be  protected  against  extreme  cold  by  the  fact 
that  they  retire  underground  during  the  winter.  Large  numbers  of  bulbous  and 
tuberous  plants  manufacture  organic  compounds  in  their  green  leaves  in  the  warm 
sunbeams  of  summer,  at  once  transmitting  them  below  to  their  subterranean 
portions.  There,  thick  stems  and  tubers,  fleshy  scale-like  leaves,  and  the  rudi- 
ments of  new  foliage  and  flowers  (which,  however,  do  not  appear  above-ground 


Fig.  136.— Detachment  of  special  shoots  of  Potamogeton  crispus,  for  hibernation  under  water. 

the  same  year)  are  produced  from  the  materials  supplied.  Throughout  the  winter 
these  structures  remain  buried  in  the  earth,  and  are  there  protected  against 
excessive  cold,  just  like  roots.  After  the  winter  is  over,  the  flower-stalks  and 
foliage-leaves,  commenced  in  the  previous  year,  rise  up  in  order  to  bloom  and 
fructify,  and  to  form  anew,  in  the  sunlight,  organic  materials  for  the  subterranean 
bulbs,  tubers,  and  root-stock*.  It  is  very  interesting  to  notice  that  bulbs  and  tubers 
bury  themselves  deeper  in  the  earth  the  more  exposed  their  habitat  to  radiation 
and  cooling,  the  more  they  are  threatened  with  the  danger  that  the  earth  will  be 
covered  by  only  a  thin  mantle  of  snow.  While,  for  example,  the  bulbs  and  tubers  of 
Gagea  lutea  and  Corydalis  cava,  when  growing  in  the  black  humus  of  beech  fores! 
under  withered  foliage,  lie  only  a  few  centimetres  below  the  surface,  in  open 


552  FREEZING    AND   BURNING. 

meadows  they  are  only  to  be  found  at  a  depth  three  or  four  times  as  great.  The 
position  of  the  tuberous  roots  of  many  orchids,  and  of  the  corms  of  the  Meadow 
Saffron  (Colchicum  autumnale)  may  be  actually  used  as  marks  to  indicate  how 
deeply  in  a  given  neighbourhood  the  ground  is  frozen,  for  these  occur  imbedded 
just  at  that  depth  to  which  the  winter  frost  fails  to  penetrate. 

The  same  thing  is  also  observed  in  aquatic  plants.  In  the  still  waters  of  lakes 
and  ponds  the  plants  bodily  withdraw  before  the  advancing  cold  of  winter,  and  an 
actual  retreat  into  the  depths  takes  place.  The  Water  Soldier  (Stratiotes  aloides) 
sinks  down  before  the  commencement  of  winter  to  the  bottom  of  the  lake,  where 
it  scarcely  ever  freezes;  it  passes  the  winter  there,  and  does  not  rise  again  to  the 
surface  till  the  following  spring.  Potamogeton  crispy^,  figured  above,  produces 
late  in  the  autumn,  near  the  surface  of  the  water,  shoots  possessing  short  leaves 
which  are  detached  from  the  old  stem  before  the  uppermost  layer  of  water  is  frozen. 
These  sink  into  the  depths,  and  bore  their  way  into  the  mud  by  their  pointed  lower 
extremities.  There,  in  their  winter  quarters,  where  there  is  never  any  formation 
of  ice,  these  sprouts  are  excellently  protected  against  injury  from  excessive  cold. 

Erect  trees  and  shrubs,  which  rise  up  column-like  above  the  earth,  are  little 
affected  by  the  presence  or  absence  of  a  covering  of  snow  upon  the  ground. 
Generally  the  leaves  have  been  already  shed,  after  they  have  delivered  up  such 
substances  as  they  contained  of  value.  The  leafless  branches  and  the  next  year's 
buds  indeed  remain  above  the  ground,  being  thus  exposed  to  the  winter  cold,  which 
they  must  be  capable  of  bearing  without  injury.  The  branches  are  covered  with  a 
tough  and  compact  investment;  and  it  would  seem  as  if  such  a  covering  would  be 
able  to  protect  the  portion  clothed  by  it  against  cold  better  than  a  mere  epidermis. 
For  a  very  short  period  of  cold  weather  such  may  be  the  case,  but  for  a  longer 
period  even  the  thickest  coat  would  be  unable  to  keep  the  cold  from  the  covered 
portions,  just  as  little  m  fact  as  the  bark  on  old  boughs  and  trunks.  In  long- 
continued  winters,  with  uninterrupted  severe  cold  weather,  the  interior  of  the 
branches  and  trunks  assumes  the  temperature  of  the  environment,  and  it  depends 
entirely  upon  the  resisting  capacity  of  the  protoplasm  whether  the  cooling  is  fatal 
or  not.  From  various  appearances  it  may  be  concluded  that  this  resisting  capacity 
is  greater  the  better  the  opportunity  afforded  to  the  protoplasm  of  suitably  preparing 
itself  in  the  foregoing  summer  and  autumn.  If  the  summer  was  warm,  and  the 
autumn  mild,  if  the  advent  of  the  first  frost  was  much  retarded,  and  the  plant  had 
time  to  become  a  chrysalis  slowly,  in  preparation  for  the  winter,  then  the  branches 
do  not  freeze  dead;  but  if  the  summer  was  cold  and  wet,  and  frosts  appeared  early 
in  autumn,  if  the  water  of  imbibition  was  not  removed  at  the  right  time,  and  the 
wood,  as  gardeners  say,  is  not  "  ripened",  then  a  tolerably  severe  winter  may  result 
in  the  death  of  the  branch,  of  the  same  branch,  indeed,  which  perhaps  in  previous 
years  survived  without  injury  much  greater  cold. 

Accordingly  we  always  come  back  to  this,  that  the  freezing  of  a  plant  to  death, 
or  not,  depends  upon  whether  or  not  the  condition  of  the  protoplasm  is  such  that 
its  molecular  constitution  becomes  permanently  disorganized  in  consequence  of  tho 


FREEZING    AND    BURNING.  553 

cooling,  and  that  the  most  effective  protection  must  be  sought  for  in  the  constitution 
of  the  protoplasm  itself.  Since  we  do  not  know  the  constitution  of  the  protoplasm, 
it  is  idle  to  puzzle  ourselves  in  surmises  about  it;  this  only  being  certain,  that  the 
resisting  capacity  of  protoplasm  differs  much  from  plant  to  plant,  as  well  as  at 
different  times  in  one  and  the  same  plant. 

The  results  which  have  been  obtained  by  the  study  of  the  burning  of  plants  are 
analogous  to  those  afforded  by  researches  into  the  nature  of  freezing. 

When  a  plant  organ  loses  its  capacity  of  absorbing  food,  of  breathing,  and  of 
further  development,  in  consequence  of  the  rise  of  temperature,  we  say  then  that 
it  is  burnt.  The  outwardly  visible  appearances  of  burnt  plants  resemble  exactly 
those  which  have  been  observed  in  plants  killed  by  freezing;  the  green  tissue  is 
discoloured,  exhibits  a  darker  tint,  is  more  transparent,  fades  and  dries  up,  and 
neither  the  supply  of  water  nor  the  reduction  of  the  temperature  can  reproduce 
the  previous  conditions.  The  protoplasm  in  the  interior  of  the  cells  is  massed 
into  balls,  and  is  detached  from  the  cell- wall;  water  is  excreted,  which  had 
stood  hitherto  in  molecular  combination  with  the  protoplasm.  These  observations 
can  be  followed  very  easily  in  aquatic  plants  whose  cell-walls  are  so  transparent 
that  they  allow  us  to  see  into  the  interior  of  the  cell-chambers.  If  the  cells  of 
the  water-plant  Elodea,  illustrated  in  fig.  53  (page  25),  are  examined  under  the 
microscope  while  the  temperature  of  the  surrounding  water  is  30°  C.,  the  proto- 
plasm will  be  seen  to  exhibit  that  active  streaming  movement  described  on  p.  33. 
If  the  temperature  is  raised  to  40°,  the  streaming  becomes  slower,  and  at  41° 
ceases  entirely,  although  the  protoplasm  shows  no  other  particular  alteration. 
Even  if  the  temperature  is  raised  to  45°,  and  gradually  to  50°,  nothing  is  altered 
in  appearance;  not  until  52°  does  any  very  noticeable  alteration  occur.  Then  the 
starch-granules  imbedded  in  the  protoplasm  split  up;  the  protoplasm  shrinks 
together  and  forms  clump-like  masses  around  the  fractured  starch-granules.  The 
protoplasm  now  becomes  rigid,  the  albuminous  materials  in  it  are  curdled  or 
coagulated.  Subsequently,  if  the  temperature  again  sinks  to  30°,  it  does  not 
become  again  living  and  active,  and  we  must  therefore  assume  that  its  molecular 
constitution  has  suffered  at  52°  an  irreparable  alteration,  in  fact,  that  it  has  been 
killed. 

In  the  main,  therefore,  burning  depends  upon  the  coagulation  of  the  albumi- 
nous compounds,  upon  the  destruction  of  the  starch-granules,  and  the  decom- 
position of  the  protoplasm.  If  the  coagulation  of  the  albuminous  compounds  and 
the  alteration  of  the  starch-granules  were  always  brought  about  by  one  and  the 
same  temperature  then  probably  all  plants  would  be  "burnt"  at  this  same  tem- 
perature. But  such  is  not  the  case.  The  various  albumens  not  only  coagulate  at 
different  temperatures  (viz.  60°-80°),  but  the  point  of  coagulation  of  the  same 
albumen  is  materially  affected  by  the  watery  contents,  and  by  the  presence  of  salts 
and  acids.  When,  for  example,  many  salts  are  present,  coagulation  may  occur  at 
50°.  Nor  does  the  destruction  of  the  starch-granules  always  occur  at  the  same  tem- 
perature; large  starch-grains,  swollen  with  water,  at  55°,  smaller  ones  not  till  65°; 


554  FREEZING    AND   BURNING. 

and,  in  order  that  dry  starch -grains  may  be  destroyed,  a  still  higher  temperature 
is  necessary.  Under  such  conditions  it  is  not  to  be  wondered  at  that  plants, 
whose  protoplasm  exhibits  different  coagulation  points,  should  be  "burnt"  at  very 
different  temperatures.  The  processes  which  have  been  observed  in  the  above- 
mentioned  Elodea  at  30°,  41°,  and  52°,  are  seen  to  occur  in  other  water-plants  at 
other  temperatures.  In  the  cells  of  Vallisneria  spiralis,  represented  in  fig.  52, 
the  streaming  of  the  protoplasm  does  not  stop  till  43°  has  been  reached,  and  the 
protoplasm  is  not  formed  into  balls  in  consequence  of  the  coagulation  of  the 
albumen  till  53°-54°.  In  the  lattice-leaved  Aponogeton  fenestrale,  growing  in 
Madagascar,  the  coagulation  and  death  of  the  protoplasm  first  occur  at  55°.  Many 
algae  bear  even  still  higher  temperatures.  In  the  channels  through  which  the  hot 
water  of  the  Carlsbad  spring  flows,  dusky  oscillarias  flourish  even  at  a  tempera- 
ture of  55°  to  56°;  in  the  springs  of  Abano,  which  reach  a  temperature  of  nearly 
60°,  Sphcerotilus  thermalis  is  to  be  found,  and  in  the  Solfatara  at  Naples,  the 
side- walls  of  the  rocky  clefts,  from  which  vapour  issues  at  a  temperature  of  55° 
to  60°,  are  covered  with  a  green  film  of  algae. 

In  plants  which  are  not  submerged  in  water,  the  watery  contents  as  well  as  the 
specific  constitution  of  the  protoplasm  have  a  material  influence  on  the  burning. 
If  the  exposed  tissues  are  poor  in  water  they  can  sustain  much  higher  tem- 
peratures than  when  very  turgid.  The  highest  temperature  which  the  turgid 
cells  of  b'thophytes  and  land  plants  can  endure  without  being  burnt  is  in  most 
cases  55°.  In  the  sun,  succulent  plants  can  endure  for  a  long  time  without  injury 
temperatures  of  50°  to  53°.  The  spores  of  moulds  (RMzopus  nigricans  and  Peni- 
cillium  glaucum)  have  been  seen  to  germinate  and  develop  at  from  54°  to  55°. 
When  dry,  those  cells  and  tissues  which  can  be  dried  up  without  harm  do  not 
perish  even  under  the  influence  of  far  higher  temperatures.  The  crustaceous 
lichens  adhering  to  the  limestone  rocks  of  the  wild  regions  of  the  Karst  of  Istria 
and  Dalmatia  (Aspicilia  calcarea,  Verrucaria  purpurascens,  and  V.  calciseda) 
are  regularly  exposed  on  cloudless  summer  days  to  a  temperature  of  58°  to  60° 
without  injury,  and  the  edible  lichen  (Lecanora  esculenta),  illustrated  opposite,  is 
often  heated  in  the  deserts,  along  with  the  stone  on  which  it  grows,  to  fully  70°, 
and  yet  is  not  destroyed.  Moreover,  seeds  which  are  deposited  on  the  desert 
sands,  and  survive  in  this  position  the  long  periods  of  drought,  do  certainly 
assume  the  temperature  of  their  environment,  and  although  at  noon  this  often 
amounts  to  60°-70°,  it  does  not  injure  these  seeds;  since,  when  the  rainy  season 
returns,  they  are  roused  from  their  summer  sleep,  and  germinate  in  the  cool  and 
moistened  soil.  The  highest  temperature  in  the  superficial  layer  of  soil  has  been 
observed  near  the  equator  at  Chinchoxo  on  the  Loango  coast.  Here,  in  many  cases 
it  exceeds  75°,  often  attains  80°,  and  once  attained  to  even  84'6°.  Nor  is  this  soil 
destitute  of  annuals  during  the  rainy  season,  and  without  doubt  the  dry  seeds  of 
these  plants  have  been  lying  for  months  in  the  sand,  sometimes  heated  to  over 
80°,  without  losing  their  germinating  power.  It  has  been  proved  experimentally 
that  seeds,  which  have  been  deprived  by  calcium  chloride  of  as  much  water  as 


FREEZING   AND   BURNING. 


555 


possible,  are  not  killed  even  at  the  boiling  point  of  water.  Of  various  seeds  from 
which  water  has  been  withdrawn  for  fifty  hours,  and  which  have  then  been 
heated  for  three  hours  up  to  100°,  those  of  duckweeds  (49  per  cent  of  the  seeds 
experimented  upon)  still  germinated;  of  vetches,  50  per  cent;  of  garlic,  60  per  cent; 
of  wheat,  75  per  cent;  of  sweet  marjoram,  78  per  cent,  and  of  melons  96  per  cent.' 
Even  of  seeds  previously  dried,  which  had  been  exposed  for  about  fifteen  months 
to  a  temperature  of  110°  to  125°,  a  small  percentage  always  germinated,  and  it  is 
possible  that  there  are  species  whose  seeds  bear  without  injury  still  higher 
temperatures. 

From  these  experiments  it  is  clearly  shown  that  the  albuminous  substances 
in   the  protoplasm   may  give  up   with    impunity  much  water,  and  that  by  this 


Fig.  137.— Edible  Lichen  (Lecanora  esculenta)  in  the  desert. 

surrender  a  protection  is  obtained  against  coagulation  and  burning,  up  to  a  certain 
point. 

In  nature,  most  contrivances  by  which  plants  are  protected  against  burning 
amount  in  reality  to  a  periodic  surrender  of  water.  Lithophytes,  especially  crus- 
taceous  lichens,  which  are  most  threatened  with  the  danger  of  being  burnt,  are 
so  organized  that  they  can  give  up  a  great  deal  of  water  in  a  very  short  time. 
They  then  become  stiff  and  brittle,  and  can  be  rubbed  into  powder,  and  it  appears 
scarcely  credible  that  these  dried-up  structures  can  ever  live  again.  In  the  rock- 
lichens  the  same  thing  occurs.  Also  several  Volvocinese,  Sphcerella  pluvialis, 
and  various  other  simply  organized  plants,  living  in  shallow  pools  and  ditches, 
dry  up  to  dust  along  with  the  mud,  after  the  evaporation  of  the  water  which  had 
accumulated  in  their  habitat,  and  are  protected  in  this  dried  condition  against 
burning.  If  the  dust,  which  is  warmed  daily  for  several  hours  up  to  60°  during 
the  period  of  drought,  becomes  moistened  later  on,  all  the  tiny  plants  wake  up 
again  from  their  trance,  and,  as  should  not  be  overlooked,  the  rotifers  and  various 
infusoria,  which  are  present  in  the  same  heated  dust,  again  bestir  themselves, 
flourish  their  cilia,  and  give  evidence  that  the  surrender  of  water  at  the  right 
moment  affords  the  best  protective  measure  against  "burning"  for  animal  proto- 


55(5  FREEZING  AND   BURNING. 

plasm  also.  In  deserts  and  steppes,  and  in  all  regions  where  the  earth  is  warmed 
up  to  70°  in  hot  rainless  seasons,  there  are,  it  is  well  known,  a  great  many  annuals. 
When  the  hot  period  commences,  leaves,  stems,  and  roots  are  already  dead,  and 
the  plants  have  scattered  their  seeds.  These  seeds,  however,  possess  very  little 
water,  and  yet  can  give  up  a  portion  of  that  they  contain  without  injury;  they 
are  thus  protected  in  the  best  way  possible  against  being  burnt. 

One  portion  of  the  perennial  plants  of  these  regions  throws  off  its  foliage  at 
the  close  of  the  rainy  period,  and  lives  through  the  hot,  dry  period  with  leafless 
and  apparently  dried-up  branches;  others  expose  the  whole  of  their  organs  above 
the  ground  to  burning,  maintaining  themselves  below  the  soil  only,  where  the 
earth  never  acquires  such  a  high  temperature ;  these  sleep  through  the  hot  period 
as  resting  tubers,  bulbs,  and  root-stocks.  It  should  also  be  remembered  here 
that  in  regions  where  high  temperatures  are  not  combined  with  great  dryness,  the 
excessive  heat  can  be  diminished  by  the  evaporation  from  the  succulent  tissue, 
since,  as  is  well  known,  evaporating  bodies  always  undergo  a  cooling.  Finally, 
the  fact  is  to  be  considered  that  many  plants  choose  places  for  their  settlement 
where  they  are  not  exposed  to  burning,  even  on  the  hottest  days  of  the  year. 
Under  the  protection  of  shade-giving  rocks,  and  wherever  the  sun's  rays  are  not  able 
to  operate  directly  and  untempered,  the  soil,  even  at  the  equator,  may  not  exceed 
those  temperatures  at  which  succulent  plant-organs  cannot  be  burnt,  and  still  less 
could  the  normal  warmth  of  the  air  in  shady  places  bring  about  such  an  effect; 
for  the  highest  temperatures  hitherto  observed  in  the  shade  rise  scarcely  above  40° 
(42°  in  Abu-Arisch,  in  Arabia;  43*1°  on  the  river  Macquarie,  in  Australia),  and  at 
these  temperatures  the  albuminous  substances  are  never  coagulated  in  any  single 
plant. 

The  question  now  is  how  the  results  obtained  from  researches  into  the  phenomena 
of  freezing  and  burning  can  be  brought  into  harmony  with  the  earlier  ascertained 
relations  of  heat  to  living  plants,  and  especially  with  the  theory  of  growth.  We 
have  conceived  growth  as  a  form  of  molecular  work  of  living  protoplasm,  and  we 
imagine  the  molecules  and  groups  of  molecules  to  be  in  a  condition  of  heat- vibration 
of  definite  extent;  or,  in  other  words,  that  for  all  work,  and  especially  for  growth, 
a  definite  degree  of  heat  is  necessary.  If  the  heat-vibrations  exceed  the  fixed 
limit,  the  position  and  the  mutual  relations  of  the  molecules  in  the  protoplasm  are 
completely  altered,  and  disarrangements  result  which  cannot  subsequently  be  made 
good.  The  protoplasm  has  then  lost  the  capacity  of  further  maintaining  itself 
and  increasing — it  is  burnt  and  killed.  The  same  happens  if  the  intensity  of  the 
heat-vibration  sinks  below  a  certain  degree.  Then  again  changes  are  produced 
in  the  substance  of  the  protoplasm  which  are  irreparable,  and  are  followed  by 
death.  Consequently,  a  superfluity  as  well  as  a  want  of  heat  can  retard  the 
molecular  action  of  the  living  protoplasm  which  appears  as  growth,  and  can  even 
completely  stop  it.  And  the  interruption  is  brought  about  in  the  protoplasm  of 
different  species  under  the  influence  of  different  degrees  of  heat;  just  as  water, 
alcohol,  and  mercury  solidify  at  certain  temperatures,  and  become  vaporized  at 


ESTIMATION   OF   THE   HEAT   NECESSARY   TO   GROWTH.  557 

certain  others,  so  there  is  a  temperature  for  the  protoplasm  of  every  species  at 
which  it  freezes,  and  another  at  which  it  is  burnt.  But  this  leads  to  the  conclusion 
that  the  molecules  and  groups  of  molecules  in  all  protoplasm  vibrate  definitely  as 
to  extent  and  intensity  so  long  as  the  protoplasm  is  living,  even  if  it  is  not  exactly 
performing  that  work  which  appears  to  us  as  growth— in  other  words,  that  a 
definite  amount  of  heat  is  necessary  to  the  maintenance  of  life  even  in  protoplasm 
apparently  resting;  and  that  consequently  it  is  not  correct  to  suppose  that  all  the 
heat  supplied  to  the  plants  is  used  up  in  growth. 

ESTIMATION   OF  THE  HEAT  NECESSARY  TO  GROWTH. 

According  to  the  mechanical  theory  of  heat,  which  gives  the  best  explanations 
of  numerous  phenomena  of  life,  all  motion  can  be  converted  into  heat,  and  measured 
as  such.  Should  it  not  be  possible  to  apply  this  principle  to  the  vegetable  kingdom, 
especially  to  the  phenomena  of  growth?  Ought  we  not  to  be  able  to  estimate 
definitely  how  much  heat  is  required  for  plants  for  each  of  their  performances 
within  a  definite  period,  and  therefore  to  determine  their  heat-requirement  as  a 
constant  numerical  quantity  ?  This  question  has  often  been  put,  and  experiments 
have  not  been  wanting  to  supply  the  answer.  It  would  not  be  only  of  theoretical 
but  also  of  great  practical  value,  to  be  able  to  tell  how  much  heat  our  forest  trees, 
our  cereals  and  other  economic  plants,  need  for  the  accomplishment  of  their  yearly 
cycle  of  life,  to  know  how  much  heat  is  necessary  for  the  germination  of  this  or 
that  cultivated  plant,  how  much  in  order  that  the  germinated  plants  may  blossom, 
and  what  degree  of  heat  they  require  to  produce  ripe  seeds  of  full  weight  and 
capable  of  germination.  If  it  were  practicable  to  determine  those  quantities  of 
heat,  which  might  be  called  the  thermal  constants  of  vegetation,  we  should  be 
able  to  estimate  beforehand  from  the  heat-conditions  prevailing  in  any  particular 
place,  whether  this  or  that  plant  species  would  thrive,  whether  it  could  produce 
ripe  fruits,  and  whether  or  not  its  cultivation  would  be  advantageous  and  worthy 
of  encouragement. 

Hitherto  the  results  obtained  in  this  direction  leave  very  much  to  be 
desired,  but  are  nevertheless  so  interesting  that  they  cannot  be  passed  over  in 
silence  here.  First  of  all,  it  has  been  proved  with  regard  to  the  earliest 
phases  of  growth,  the  germination  of  spores  and  seeds,  that  not  a  few  species  are 
able  to  germinate  even  at  very  low  temperatures.  The  seeds  of  the  White 
Mustard,  of  hemp,  of  wheat  and  rye,  of  the  Norway  Maple,  and  of  the  wild 
violet,  germinate  at  a  temperature  very  near  freezing,  between  0°  and  1°C.;  others, 
such  as  the  garden  cress,  flax,  spinach,  onions,  poppy,  beet-root,  and  the  English 
rye-grass,  germinate  at  temperatures  between  1°  and  5°;  French  beans,  sainfoin, 
millet,  maize,  sunflowers,  at  temperatures  between  5°  and  11°;  tomato,  tobacco,  and 
gourds  at  temperatures  between  11°  and  16°;  cucumbers,  melons,  and  cocoa  beans 
not  until  above  16°.  That  is  to  say,  that  melon  seeds,  if  placed  in  damp  soil 
whose  temperature  lies  below  15°,  absorb,  it  is  true,  the  moisture,  and  swell  up, 


558 


ESTIMATION   OF   THE    HEAT   NECESSARY   TO   GROWTH. 


but  that  those  alterations  which  really  constitute  growth  are  not  produced  in 
the  cells  of  the  embryo  at  this  temperature.  Not  until  the  temperature  of  the 
soil  rises  above  15°  does  the  embryo  elongate,  and  the  radicle  bore  its  way  through 
the  seed-coats.  But  all  these  figures  would  give  by  themselves  a  very  incomplete 
idea  of  the  heat-requirements  of  germinating  seeds,  were  it  not  also  ascertained 
how  long  the  seed  must  be  exposed  to  the  given  temperatures  in  order  that  its 
embryo  should  increase  and  develop.  If  a  hen's  egg  is  exposed  for  only  two  or 
three  days  to  a  temperature  of  35°  to  40°,  it  will  not  be  hatched;  hatching  can 
only  take  place  if  the  egg  remains  for  20-21  days  under  the  influence  of  this 
constant  temperature.  With  seeds  the  case  is  the  same.  The  following  is  a 
selection  of  the  results  obtained  in  this  relation: — 


The  seeds  of 

germinated  at  a  con- 
stant temperature  of 

in  No.  of 
days. 

The  seeds  of 

germinated  at  a  con- 
stant temperature  of 

in  No.  of 
days. 

Gold  of  Pleasure  -\ 

4 

Pimpernel            "N 

10 

Pea 

5 

Maize 

11 

Spinach 

9 

Millet 

10-5° 

13 

Poppy 
Beetroot 

4'6°C 

10 
22 

Coriander 
Sunflower 

16 

25 

Guinea  grass        J 
French  Beans       -| 

24 
3 

Tomato                 \ 
Tobacco                J 

15-6° 

6 
9 

Timothy-grass       >• 
Sainfoin                J 

10'5°C 

6 

7 

Cucumber            -> 
Melons                 J 

18-5° 

5 
17 

If  the  number  of  days  is  multiplied  by  the  temperature,  the  product  may  be 
looked  upon  as  an  empirical  formula  for  the  heat  necessary  to  the  process  of 
germination.  It  may  be  considered  that  this  product  is  of  regular  amount,  and 
it  is  regarded  as  a  "thermal  constant".  Thus,  for  purposes  of  comparison,  the 
thermal  constants  for  the  germination  of  the  seeds  of  the  Gold  of  Pleasure  might 
be  expressed  as  18'4,  of  the  Poppy  46*0,  of  Maize  115*5,  and  so  forth. 

In  these  calculations,  of  course,  only  the  constant  temperatures  of  the  soil 
not  directly  illumined  by  the  sun's  rays  come  under  consideration.  The  matter 
becomes  far  more  complicated  when  it  is  a  question  of  determining  the  constants 
for  other  stages  in  the  development  of  plants,  such  as  the  bursting  of  foliage 
from  the  buds,  the  opening  of  the  first  flowers,  and  the  ripening  of  the  first  fruits. 
These  phenomena  of  growth  in  the  majority  of  plants  in  the  open  do  not  occur 
in  the  shade,  but  in  the  sun.  Moreover,  in  the  places  under  observation,  the 
temperature  is  not  constant,  but  changes  from  hour  to  hour,  attaining  its  minimum 
shortly  before  sunrise,  and  its  maximum  in  the  first  hours  of  the  afternoon.  Since 
experience  has  shown  that  the  extent  of  growth  is  regulated  according  to  the 
highest  temperature  in  the  sunshine,  it  follows  that  neither  the  shade  temperature 
nor  the  mean  temperature,  but  the  readings  of  the  maximum  thermometer,  exposed 
to  the  sun,  must  be  used  for  the  estimation  of  the  constants  in  the  above-mentioned 
phenomena  of  growth. 

To  obtain  the  thermal  constants  for  foliage-production,  flower-opening,  and 
seed-ripening,  of  a  plant  growing  in  a  situation  illuminated  by  the  sun,  one  must 


ESTIMATION   OF   THE   HEAT   NECESSARY   TO   GROWTH. 


559 


add  together  the  daily  maxima  of   sun-temperatures   from    the  first  of   January 
until  the  event  in  question  takes  place. 

A  selection  of  constants  obtained  in  this  way  from  observations  extending  over 
many  years  in  Central  Germany  (Giessen)  may  be  suitably  inserted  here. 


CONSTANTS  FOR  THE  ISSUE  OF  THE  FOLIAGE-LEAVES  FROM  THE  BUDS. 


Gooseberry  (Ribes  Grossularia)  478°. 
Hazel  (Corylus  Avellana)  1061°. 
Beech  (Fagus  silvatica)  1439°. 


Plane  (Platanus  acerifolia)  1503°. 
Walnut  (Juglans  regia)  1584°. 


CONSTANTS  FOR  THE  OPENING  OF  THE  FIRST  FLOWERS. 


Hazel  (Corylus  Avellana)  226°. 

Mezereon  (Daphne  Mezereum)  303°. 

Snowdrop  (Oalanthus  nivalis)  311°. 

Sweet  Violet  (  Viola  odorata)  576°. 

Cornel  (Cornus  mas)  576°. 

Apricot  (Prunus  Armeniaca)  843°. 

Corydalis  (Corydalis  cava)  863°. 

Violet  Willow  (Salix  Daphnoides)  968°. 

Cowslip  (Primula  veris)  968°. 

Norway  Maple  ( Acer  platanoides)  1100°. 

Peach  (Persica  vulgaris)  1100°. 

Gooseberry  (Ribes  Grossularia)  1138°. 

Almond  (Amygdalus  communis)  1196°. 

Gean  (Prunus  avium)  1265°. 

Sloe  (Prunus  spinosa)  1265°. 

Pear  (Pirus  communis)  1304°. 

Bird  Cherry  (Prunus  Padus)  1325°. 

Apple  (Pirus  Mains)  1423°. 

Plum  (Prunus  domestica)  1423°. 

Alpine  Woodbine  (Lonicera  alpigena)  1458°. 

Oak  (Quercus  pedunculata)  1556°. 

Lilac  (Syringa  vulgaris)  1556°. 

Walnut  (Juglans  regia)  1584°. 

Barberry  (Berberis  vulgaris)  1615°. 

Poet's  Narcissus  (Narcissus  poeticus)  1615°. 

Hawthorn  (Cratcegus  Oxyacantha)  1649°. 

Lily  of  the  Valley  (Convallaria  majalis)  1649°. 

Horse  Chestnut  (JEsculus  Hippocastanum)  1708° 


Peony  (Pceonia  ojjicinalis)  1818°. 

Laburnum  (Cytisus  Laburnum)  1818°. 

Mountain  Ash  (Sorbus  aucuparia)  1844°. 

Norway  Spruce  (Abies  excelsa)  1904°. 

Plane  (Platanus  acerifolia)  2115°. 

Elder  (Sambucus  nigra)  2313°. 

Deadly  Nightshade  (Atropa  Belladonna)  2346°. 

Acacia  (Robinia  Pseudacacia)  2404°. 

Scotch  Pine  (Pinus  sylvestris)  2404°. 

White  Water  Lily  (Nymphcea  alba)  2506°. 

Arnica  montana  2538°. 

Tulip  Tree  (Liriodendron  tulipifera)  2538°. 

Rosa  centifolia  2538°. 

Fox-glove  (Digitalis  purpurea)  2640°. 

Carthusian  T?mk(J)iantkus  Carthusianorum)  2640°, 

Vine  (  Vitis  vinifera)  2878°. 

Broad-leaved  Lime  (Tilia  grandifolia)  3033°. 

Small-leaved  Lime  (Tilia  parvifolia)  3274°. 

Oat  (Arena  sativa)  3444°. 

White  Lily  (Lilium  candidum)  3378°. 

Chestnut  (Caxtanea  sativa)  3660°. 

Immortelle  (Helichrysum  arenarium)  3918°. 

Ling  (Calluna  vulgaris)  4164°. 

Trumpet-tree  (Catalpa  syringcefolia)  4275°. 

Blue  Aster  (Aster  Amettus)  4874°. 

Syrian  Marsh-Mallow  (Hibiscus  Syriacus)  4986°. 

Meadow  Saffron  (Colchicum  autumnale)  5024°. 

Ivy  (Hed,era  Helix)  5910°. 


CONSTANTS  FOR  THE  RIPENING  OF  FRUIT. 


Wild  Strawberry  (Frag aria  vesca)  2671°. 
Gean  (Prunus  avium)  2778°. 
Mezereon  (Daphne  Mezereum)  2935°. 
Red  Currant  (Ribes  rubrum)  3069°. 
Gooseberry  (Ribes  Orossularia)  3596°. 
Alpine  Woodbine  (Lonicera  alpigena)  4164° 
Mountain  Ash  (Sorbus  aucuparia)  4339°. 
Barley  (Hordeum  vulgare)  4403°. 
Apricot  (Prunus  Armeniaca)  4435°. 
Apple  (Pirus  Mains)  4730°. 


Barberry  (Berberis  vulgaris)  4765°. 

Carthusian  Pink  (Dianthus  Carthusianorum)^!^. 

Elder  (Sambucus  nigra)  4913°. 

Pear  (Pirus  communis)  5024°. 

Cornel  (Cornus  mas)  5416°. 

Plum  (Prunus  domestica)  5780°. 

Vine  (Vitis  vinifera)  5780°. 

Peach  (Persica  vulgaris)  6004°. 

Horse  Chestnut  (^Esculus  Hippocastanum)  6034°. 

Oak  (Quercus pedunculata)  6236°. 


560 


ESTIMATION   OF   THE    HEAT   NECESSARY   TO   GROWTH. 


CONSTANTS  FOR  THE  COMMENCEMENT  OF  LEAF-FALL. 


Bird  Cherry  (Prunus  Padus)  6179°. 

Small-leaved  Lime  (Tilia  parvifolia)  6644°. 

Elder  (Sambucus  nigra)  6644°. 

Alpine  Woodbine  (Lonicera  alpigena)  6759°. 

Pear  (Pirus  communis)  6788°. 

Walnut  (Juglans  regia)  6816°. 

Trumpet-tree  (Catcdpa  syringcefolia)  6816°. 

Violet  Willow  (Saliv  daphnoides)  6838°. 

Horse  Chestnut  (dEsculus  Hippocastanum)  6863°. 


Hazel  (Corylus  Avellana)  6884°. 
Gooseberry  (Ribes  Grossularia)  6884°. 
Beech  (Fagus  silvatica)  6884°. 
Vine  (  Vitis  vinifera)  6913°. 
Oak  (Quercus  pedunculata)  6979°. 
Apple  (Pirus  Mains)  6999°. 
Chestnut  (Castanea  sativa)  7023°. 
Gean  (Prunus  avium)  7023°. 
Plane  (Platanus  acerifolia)  7145°. 


Although  the  computations  which  have  been  made  at  different  places  and  over 
several  years,  by  way  of  trial,  have  given  figures  which  do  not  differ  materially 
from  the  above,  and  it  seems  as  if  these  constants  actually  justified  that  term,  yet 
confidence  in  them  has  been  to  some  extent  diminished  by  the  following  considera- 
tions. 

With  regard  to  the  germination  of  seeds  it  is  concluded  from  various  phenomena 
that  the  heat  liberated  in  respiration  from  the  cells,  as  well  as  the  temperature  of 
the  soil,  has  not  a  little  influence  also  on  the  process  of  growth.  Seeds  in  whose 
cells  the  protoplasm  has  once  been  set  in  action  by  an  external  impulse,  perhaps  by 
a  minimum  of  radiated  or  conducted  heat,  respire  with  a  fair  amount  of  activity. 
In  this  way  the  reserve  materials  stored  up  in  them  are  consumed,  and  so  much 
heat  is  liberated  that  not  only  is  the  embryo  able  to  develop,  but  heat  may  be 
even  given  up  to  the  environment.  Radicles  of  germinating  maple  and  wheat 
seeds,  which  by  chance  were  found  in  an  ice  cellar,  were  observed  to  grow  down 
into  the  blocks  of  ice,  and  this  could  only  have  happened  from  the  melting  of  the 
ice  by  the  radicles,  which  push  their  way  into  the  cavities  formed,  like  the  flower- 
buds  of  the  Soldanellas  already  described.  In  many  cases  of  observed  germina- 
tion it  may  therefore  be  doubted  whether  the  growth  of  the  embryo  alone  is  to 
be  laid  to  the  account  of  the  measured  heat,  supplied  to  the  seeds  from  their  sur- 
roundings. On  the  other  hand,  it  is  doubtful  whether  the  heat  (registered  by  the 
thermometer),  which  reaches  the  plants  from  outside,  is  employed  only  in  growth. 
One  part  may  be  used  in  order  to  maintain  the  plant-organ  in  question  alive; 
another  portion  may  be  useful  in  the  production,  and  in  the  transformation  and 
transmission  of  constructive  materials,  and  only  a  residual  portion  can  then  partici- 
pate in  growth.  But  this  is  not  all.  It  is  also  doubtful  whether  the  positive  heat 
entering  the  plants  from  outside,  can  be  always  completely  disposed  of,  within 
the  given  space  of  time,  in  the  various  chemical  transformations  and  molecular 
arrangements  carried  on  in  the  interior  of  the  plant,  and  whether  an  unused 
surplus  is  not  sometimes  present  which  should  be  really  withdrawn  from  the 
calculation.  It  is  tacitly  implied  in  the  reckonings  that  if  the  plants  are  exposed 
to  a  constant  temperature  of  20°  for  12  hours,  the  total  heat  which  was  able  to 
raise  the  mercury  up  to  20°  in  that  time  would  also  be  turned  to  account  by  the 
plants.  But  that  this  is  not  so,  is  shown  by  the  following  observations: 


ESTIMATION   OF  THE   HEAT   NECESSARY   TO  GROWTH. 


561 


No.  of  Hours  required 

for  germination  of  the  seeds  of 

at  a  temperature  of 

Constants  so  reckoned. 

48  J 
36  ( 

White  Mustard  (Sinapis  alba),  ... 

J               4-6° 
\             10-5° 

220-8 
378-0 

72  ) 

48  f 

Hemp  (Cannabis  sativd),  

4-6° 
10-5° 

331-2 
504-0 

144  ) 
96  j 

Flax  (Linum  usitatissimum), 

4'6° 
10-5° 

662-4 
1008-0 

144  ) 
80  f 

Maize  (Zea  Mais),  

16-1° 
44-0° 

2318-4 
3520-0 

From  these  observations  it  is  easily  gathered  that  in  those  instances  in  which 
the  seed  was  exposed  to  a  higher  temperature,  only  a  portion  of  the  heat  supplied 
was  actually  employed  in  germination,  and  that,  therefore,  the  constants  calculated 
on  the  basis  of  these  observations  are  much  too  high. 

If  the  thermometer  could  tell  us  the  amount  of  heat  actually  needed  within  a 
certain  time  by  plants  growing  together,  then  only  might  the  constants  reckoned 
from  these  readings  be  regarded  as  accurate  and  become  useful  for  comparison.  But 
these  conditions  are  not  fulfilled.  Usually  here  the  conclusions  are  only  post  hoc 
propter  hoc.  Thermometric  readings  are  brought  into  calculation  which  include  the 
surplus  of  heat  not  used  by  the  plants,  and  consequently  the  constants  are  not  the 
correct  expression  of  the  quantity  of  heat  actually  consumed  in  growth. 

The  bases  on  which  the  calculations  are  founded,  for  growing  organs  directly 
under  the  influence  of  the  sun's  rays,  are  much  more  uncertain  than  those  for  seeds 
germinating  in  shaded  ground.  Besides,  doubts  must  arise  from  the  fact  that  the 
sun's  rays  have  a  widely  differing  effect  on  foliage,  flowers,  and  fruits  from  that 
which  they  have  on  the  mercury  of  the  thermometer.  This  defect  may  indeed  be 
removed  by  using  the  same  instrument  in  all  observations  and  employing  suitable 
corrections;  but  it  is  a  more  serious  matter  that  we  have  no  means  of  ascertaining 
how  much  light  is  changed  into  heat  in  growing  organs  exposed  to  the  sun's  rays. 
With  increasing  altitudes  the  intensity  of  the  light  increases,  and  its  significance  for 
growth  increases  in  a  corresponding  manner.  But  it  is  impossible  to  determine 
these  relations  numerically,  more  especially  to  determine  them  in  plants  and 
thermometers  observed  in  the  open. 

Nor  must  it  be  forgotten  that  the  absorption  of  heat  also  depends  upon  the 
individuality  of  the  plant  observed,  and  upon  the  constitution  of  the  protoplasm  of 
the  particular  species.  The  seeds  of  the  White  Mustard  are  incited  to  growth  even 
by  temperatures  little  removed  from  the  freezing-point,  while  the  seeds  of  melons  do 
not  germinate  until  they  have  been  exposed  to  the  influence  of  a  temperature  of 
18'5°  C.  for  at  least  17  days.  This  shows  that  every  species  has  to  a  certain  extent 
its  own  zero  at  which  growth  begins,  and  all  calculations  of  the  heat  required  for 
the  growth  of  the  stem  and  foliage  of  any  particular  species  should  always  be 
reckoned  only  from  this  zero.  Moreover,  it  is  a  matter  of  experience  confirmed  by 
all  gardeners  that  higher  temperatures  are  required  for  the  development  of  flowers 
than  for  foliage,  and  that  for  the  proper  ripening  of  seeds  higher  temperatures  still 


VOL.  I. 


ESTIMATION   OF   THE    HEAT   NECESSARY   TO   GROWTH. 

are  necessary.  Isolated  species  of  course  exhibit  puzzling  deviations  in  this  respect. 
The  Acacia  (Robinia  Pseudacacia)  develops  its  flowers  in  Southern  Italy  before  its 
leaves,  and  when  the  acacia  trees  are  already  in  full  bloom  their  foliage-leaves  are 
still  minute  and  unexpanded.  North  of  the  Alps  the  foliage-leaves  everywhere 
unfold  at  the  same  time  as  the  flowers;  and  yet  we  always  reckon  the  heat 
indicated  by  the  thermometer  as  if  it  were  utilized  in  an  identical  manner  by 
associated  plants  in  all  stages  of  development. 

Finally,  it  must  be  pointed  out  that  certain  alterations  which  are  carried  on  in 
the  interior  of  a  seed  or  plant  during  its  apparent  rest,  and  which  have  a  great 
significance  for  those  later  phenomena  of  growth  visible  to  the  eye,  are  completely 
excluded  from  observation  and  registration.  If  potato-tubers  are  dug  up  in  autumn 
and  put  in  a  cellar,  it  seems  as  if  all  movements  and  chemical  transformations  were 
entirely  stopped  in  their  individual  cells.  The  potato-tuber  lies  tranquilly  resting 
in  the  dark  cellar,  in  which  a  constant  temperature  of  10°  prevails,  throughout  the 
winter.  Spring  arrives;  above-ground  everything  germinates  and  sprouts  from  the 
sun-warmed  soil,  and  we  connect  this  phenomenon  with  the  powerful  heating  caused 
by  the  rays  of  a  more  vertical  sun.  No  heat-giving  sunbeams  reach  the  cellar, 
however.  The  temperature  of  the  air,  of  the  earth,  and  of  the  potatoes  which  have 
been  lying  there  for  months  is  always  the  same,  10° — even  perhaps  now  a  fraction 
lower,  since  according  to  experience  the  lowest  temperature  in  the  cellar  is  not 
reached  until  the  end  of  the  winter.  Nevertheless  the  potato  begins  to  grow  and  to 
send  out  a  slender  shoot  from  one  of  its  buds,  as  if  it  knew  that  spring,  the  proper 
time  for  sprouting  and  growing,  had  arrived.  Why  does  the  growth  not  begin 
until  now  in  March?  Why  did  it  not  commence  in  December,  since  external 
influences,  particularly  the  temperature  of  the  environment,  was  not  in  any  way 
different  within  the  cellar  then  from  what  it  is  now  in  spring?  There  can  be  only 
one  answer  to  the  question,  which  is,  that  in  December  the  potatoes  were  not  yet 
equipped  for  growth.  They  were  only  apparently  in  absolute  rest;  in  reality 
chemical  transformations,  the  preparation  and  production  of  constructive  materials, 
were  being  carried  on  in  the  cells,  and  in  December,  January,  and  February,  these 
were  not  far  enough  advanced  for  the  tubers  to  be  able  to  produce  stems,  leaves, 
and  roots.  Not  until  now  in  March  are  the  preparations  for  development  completed, 
and  not  until  now  can  that  transformation  of  the  constructive  materials  occur 
which  is  outwardly  manifest  as  growth.  The  organic  compounds,  contained  by  the 
cells  of  the  tubers  in  autumn,  were  not  fit  for  the  formation  of  stems,  leaves,  and 
roots,  even  under  the  influence  of  a  temperature  of  20°.  All  these  processes  require, 
therefore,  a  definite  period  of  time,  and  this  can  neither  be  replaced  nor  sensibly 
shortened  by  a  rise  of  temperature. 

In  the  underground  bulb  of  the  Snowdrop  (Galanthus  nivalis)  the  rudiments 
of  the  leaves  and  blossoms  of  the  following  spring  are  already  formed  during 
the  summer,  and  at  the  end  of  September  all  portions  of  the  future  flowers 
can  be  recognized  between  the  enveloping  sheaths  and  bulb-scales.  It  might 
be  thought  an  easy  matter  to  force  this  bulb  by  raising  the  temperature  and 


ESTIMATION   OF   THE   HEAT   NECESSARY   TO   GROWTH.  563 

moistening  the  surrounding  soil,  so  that  we  might  have  the  Snowdrop  blossoming 
even  in  November.  But  very  many  experiments  have  shown  that  bulbs  so 
treated,  although  they  develop  leaves  and  an  inflorescence,  do  not  properly 
develop  their  flowers,  and  always  perish  prematurely;  while  four  months  later 
the  growth  of  the  leaves  and  flowers  takes  place  easily  and  quickly  at  tempera- 
tures which  are  not  much  above  zero.  And  in  many  root-stocks,  in  most  buds  of 
branches  above  the  ground,  and  in  numerous  seeds  and  spores,  the  same  thing 
occurs  as  in  tubers  and  bulbs  of  which  the  Potato  and  Snowdrop  have  been  selected 
as  well-known  examples.  How  many  plants  there  are  which  blossom  early  in  the 
spring,  ripen  their  fruits  in  the  early  summer,  and  whose  seeds,  being  detached  from 
the  parent  plant  in  the  height  of  summer,  lie  scattered  on  the  ground!  Although 
the  soil  in  which  they  are  imbedded  is  damp  and  sufficiently  warm,  and  although  all 
the  external  conditions  of  germination  are  fulfilled,  yet  they  do  not  germinate  in  the 
same  year  in  which  they  have  been  produced.  Not  until  the  following  spring  do 
the  embryos  put  forth  their  rootlets,  and  then  usually  under  conditions  apparently 
much  less  favourable  than  those  of  the  previous  summer  and  autumn.  These  seeds 
are  not  yet  ripe,  or  rather,  perhaps,  they  are  not  yet  capable  of  germination  when 
they  fall  from  the  parent  plants.  The  materials  contained  in  their  cells  must  first 
pass  through  a  process  of  transformation  before  they  can  promote  the  development 
of  the  embryo,  and  this  transforming  process  can  in  no  wise  be  hastened  by  in- 
creased supplies  of  heat  and  moisture.  In  many  large  seeds,  as,  for  example,  those 
of  hazel,  beech,  and  almond,  this  difference  between  seeds  just  fallen  from  the  tree 
not  yet  capable  of  germination,  and  the  seeds  which  have  been  mellowed  and  can 
germinate,  may  be  easily  perceived  in  their  consistency,  taste,  and  smell.  The 
phenomenon  here  described  is  found  in  a  specially  remarkable  manner  in  the  fruits 
of  the  Water  Chestnut  (Trapa  natans).  If  in  autumn  water-chestnuts  just  ripe  be 
placed  in  water,  and  the  temperature  of  the  water  be  kept  through  the  winter  at 
15°  C.,  the  rootlets  of  the  embryos  will  not  emerge  until  the  coming  spring,  and  not 
then  on  account  of  a  higher  temperature,  but  at  the  same  temperature  to  which  they 
have  been  continuously  exposed  for  six  months.  If  the  temperature  of  the  water  be 
raised  to  20°  the  growth  of  the  rootlets  is  not  accelerated,  and  the  increased  tempera- 
ture cannot  become  effective  as  an  incitement  to  growth  until  after  the  seeds 
have  been  suitably  prepared  for  a  period  of  six  months.  Gardeners  say  that  such 
seeds  must  "mellow"  and  "ripen  after  gathering",  and  they  have  indeed  hit  the 
mark  with  this  latter  expression.  Some  spores  also  must  mellow  and  ripen  for 
a  much  longer  time.  Many,  of  course,  germinate  immediately  after  their  detach- 
ment from  the  parent  plant.  The  so-called  resting  spores,  however,  always  pass 
through  a  quiescent  period,  whose  duration  usually  can  be  determined  with  great 
accuracy  and  may  be  shortened  a  little  by  altered  external  influences.  Very 
remarkable  is  the  fact  that  in  the  seas  of  tropical  regions,  whose  waters  possess 
the  same  chemical  composition,  temperature,  and  illumination  from  one  year's  end 
to  the  other,  certain  species  of  red  seaweeds  develop  in  March,  others  in  June,  and 
others  again  in  October.  In  these  instances  all  grounds  for  an  explanation  are 


5(34  ESTIMATION   OF   THE   HEAT   NECESSARY   TO   GROWTH. 

wanting.  It  can  only  be  stated  with  certainty  that  the  increase  or  diminution 
of  heat  does  not  take  any  part  in  this  remarkable  periodicity.  It  would,  however, 
be  going  too  far  to  assert  of  all  species  that  the  resting  period,  normally  observed 
by  them,  could  not  be  shortened  by  external  influences,  especially  by  rise  of 
temperature.  Many  seeds,  such  as  those  of  cress,  mustard,  barley,  and  numerous 
so-called  weeds,  which  appear  as  unwelcome  guests  on  cultivated  land,  have  no 
resting  stage,  germinate  at  any  season  if  they  are  supplied  with  the  necessary 
moisture;  and  the  warmer  the  soil  the  more  rapid  is  their  development.  It  is 
also  well  enough  known  that  there  are  plants  which,  to  use  the  language  of  gar- 
deners, may  be  "forced".  Tulips,  Lilies  of  the  Valley,  and  Lilac,  whose  resting 
period  lasts  in  Central  Europe  from  the  ripening  time  of  the  seed  in  summer  until 
the  spring  of  the  next  year,  may  be  forced  even  late  in  autumn  if  planted  in  a 
greenhouse  in  warm,  damp  soil,  soon  after  they  have  ripened  their  seeds  and 
have  begun  to  rest  Under  these  circumstances  they  produce  their  flowers  even 
in  January,  and  in  these  plants,  consequently,  the  materials  manufactured  in  the 
previous  summer  may  be  used  as  constructive  materials  for  growth  almost  at 
once.  I  remember  once  drawing  the  shoot  of  a  Clematis  plant  rooted  in  the  open, 
after  it  had  lost  its  foliage  in  the  autumn,  through  a  narrow  crevice  3  metres  above 
the  soil  into  the  interior  of  a  neighbouring  hothouse.  Leafy  shoots  were  developed 
from  the  buds  of  the  upper  portion  thus  warmed  even  in  December;  while  the 
lower  portion  of  the  same  plant,  situated  outside  the  hothouse  and  surrounded  by 
cold  air,  was  still  frozen.  In  this  plant  also,  the  materials  manufactured  in  the 
summer  could  be  used  as  constructive  materials  as  soon  as  ever  they  had  been 
deposited  in  the  reserve  storehouses. 

The  same  must  indeed  be  the  case  in  those  plants  which  bloom  normally  in  the 
spring,  but  yet  often  in  years  characterized  by  particularly  mild  autumns,  burst 
open  in  October;  the  buds  destined  for  the  next  spring  thus  sending  out  fresh  leafy 
shoots  and  blossoms  twice  in  the  same  year — for  example,  many  apples  and  horse- 
chestnuts,  violets  and  strawberries,  many  primulas,  gentians,  and  anemones. 

Although  it  may  be  doubted  whether  the  constants  hitherto  computed  can  be 
taken  as  an  accurate  expression  of  the  heat  consumed  by  plants  for  growth  in  their 
various  stages  of  development,  nevertheless  their  value  must  not  be  under-estimated. 
Comparisons  of  results  obtained  in  various  places  by  the  same  methods,  with  the 
same  instruments,  and  on  the  same  species,  will  without  doubt  yet  lead  to  many 
interesting  conclusions.  The  determination  of  the  commencement  of  the  various 
phenomena  of  development,  the  determination  of  the  unfolding  of  the  foliage  and 
flowers,  of  the  ripening  of  the  fruit,  and  of  the  autumnal  leaf -fall — at  as  many 
stations  of  observations  as  possible — is  not  only  a  highly  attractive  problem  in  it- 
self, but  is  also  of  great  scientific  value;  and  this  no  less  in  its  bearing  upon  the  life 
of  plants  generally,  than  upon  the  geography  of  plants,  since  the  barriers  which  con- 
fine plants  in  their  distribution  can  be  in  great  part  explained  by  the  fact  that  the 
species  in  question  are  unable  to  complete  their  annual  cycle  of  development  on  the 
further  side  of  the  boundary.  Finally,  also  for  climatology,  since  the  yearly  process 


ESTIMATION   OF   THE   HEAT   NECESSARY   TO   GROWTH. 


565 


of  development,  in  many  cases,  much  more  clearly  exhibits  the  climate  of  a  district 
than  the  readings  of  instruments  erected  in  the  places  in  question.  The  so-called 
phonological  observations,  that  is,  the  determination  of  the  awakening  of  nature  at 
the  close  of  the  winter,  or  at  the  end  of  the  summer  drought,  the  ascertaining  of  the 
times  at  which  growth  and  blossoming  reach  their  maximum,  and  the  fixing  of  the 
period  at  which  the  organism,  on  account  of  the  unfavourable  external  conditions, 
falls  into  a  winter  or  summer  sleep,  are  consequently  of  interest  even  if  we  are 
unable  to  reckon  the  heat  constants  for  the  commencement  of  these  phenomena. 
The  results  of  such  phonological  observations  have  been  already  made  use  of 
repeatedly  on  pp.  519  and  525,  and  it  has  been  there  shown  how  valuable  these 
may  be  in  questions  concerning  the  relations  of  heat  to  growth. 

We  cannot  close  this  chapter  without  touching  upon  two  valuable  results  of 
phonological  observations,  although  only  in  passing.  The  following  table  gives 
first  of  all  a  view  of  the  retardation  of  vegetative  development  with  increasing 
latitudes  in  Europe  in  the  spring. 

Comparisons  with  Lesina  in  the  Adriatic  Sea,  4^  H'  Nor.  Lat.,  16°  1ft  East  Long. 


Places  between 

Retarda- 

Places between 

Retarda- 

Places between 

Retarda- 

North 

0  and  10 

tion  in 

10  and  30 

tion  in 

30  and  45 

tion  in 

Lat. 

Meridian. 

days. 

Meridian. 

days. 

Meridian. 

days. 

48-49° 

Paris 

43 

Pressburg 

58 

Sarepta 

66 

50-51° 

Brussels 

50 

Prague 

59 

Kiew 

68 

52-53° 

Osnabriick 

63 

Warsaw 

65 

Orel 

79 

59-60° 

Christiania 

86 



— 

Pulkowa 

100 

As  the  starting-point  in  the  comparison  we  choose  the  Island  of  Lesina  off  the 
Dalmatian  coast,  because  there  the  climatic  conditions  lie  midway  between  those  of 
places  situated  in  the  same  latitude  in  Western  Oceanic  and  in  Eastern  Continental 
Europe.  The  stations  of  observation,  situated  not  more  than  300  metres  above  the 
sea-level,  which  are  here  compared  with  Lesina,  have  been  arranged  in  three  columns 
—a  western  between  0  and  10  meridian,  a  central  between  10  and  30,  and  an  eastern 
between  30  and  45.  Reviewing  the  retardation  due  to  the  increasing  latitude  with 
regard  to  Lesina,  we  have  the  interesting  result  that  the  retardation  in  the  column 
of  the  eastern  continental  stations  is  from  two  to  three  weeks  more  than  in  that  of 
the  western  column.  Thus,  when  in  Paris  many  spring  plants  are  in  full  bloom, 
vegetation  on  the  Russian  steppes  (Sarepta),  at  the  same  latitude,  is  still  deep  in 
winter  slumber,  and  does  not  reach  the  same  stage  until  23  days  later. 

In  a  second  small  table  inserted  on  next  page,  very  remarkable  results  are  given 
with  respect  to  the  blossoming  of  the  same  species  in  Western  Europe  and  in 
Eastern  North  America. 

Here  those  American  and  European  stations  are  placed  side  by  side  in  which 
the  blossoming  of  the  same  species  occurs  simultaneously,  and  hence  the  comparison 
shows  that  the  geographical  position  of  these  places  differs  by  about  8-10  degrees 
of  latitude;  so  that,  for  example,  in  New  York  (which  has  the  same  latitude  as 


566      FORM   AND   SIZE   OF   PARTICLES   EMPLOYED    IN    CONSTRUCTION    OF   PLANTS. 


Naples)  plants   blossom  at  the  same  time  as   in  Marburg,  situated  10°  further 

north. 

Stations  at  which  Spring  Plants  blossom  simultaneously. 


North  Americ 

i, 

Geographical 
Latitude. 

Europe. 

Geographical 
Latitude. 

Difference  of 
Latitude. 

New  Albany 
Sykesville 
Belle  Centre 

38°   17' 
39°  23' 
40°  28' 

Dijon 
Kremsmiinster  . 
Heidelberg 

47°   19' 
48°  30' 
49°  28' 

9°  20' 
9°  07' 
9°  00' 

New  York 

40°  42" 

Marburg  (Hesse) 

50°  47' 

10°  05' 

Germanstown 
Baldwinville 

42°  80' 
43°  40' 

Antwerp 
Utrecht  ... 

51°  13' 
52°  03' 

8°  33' 
8°  90' 

3.   ULTIMATE  STEUCTUEE  OF  PLANTS. 

Hypotheses  as  to  the  Form  and  Size  of  the  Smallest  Particles  employed  in  the  Construction  of 
Plants.— Visible  Structural  Activity  of  Protoplasm. 


HYPOTHESES    AS    TO    THE    FORM    AND    SIZE   OF   THE   SMALLEST  PARTICLES 
EMPLOYED  IN  THE  CONSTRUCTION  OF  PLANTS. 

When  anywhere  within  the  limits  of  a  flourishing  town  buildings  are  raised 
in  great  number  and  quick  succession  by  the  skilful  hands  of  men,  it  is  said  that 
the  houses  grow  up  with  astonishing  rapidity  from  the  ground.  In  the  same  way 
the  growth  of  plants  is  appropriately  compared  by  botanists  with  the  erection  of 
human  habitations.  In  this  book  the  latter  comparison  has  already  been  made  as 
occasion  offered,  and,  even  at  the  risk  of  repetition,  I  must  again  make  use  of  the 
simile  here  in  discussing  the  building  up  of  plants. 

As  in  the  erection  of  human  habitations,  so  in  the  production  of  the  plant 
structure,  it  is  a  question  of  providing  a  home  for  living  beings,  of  securing  this 
home  against  injury  by  weather,  and  other  dangers  which  might  terminate  the 
existence  of  the  inhabitants.  At  the  same  time  the  inhabitants  in  these  settle- 
ments must  be  able  to  take  in  food  from  the  exterior,  breathe,  work  up  the  food- 
stuffs, and  extend  themselves  further.  Where  very  numerous  portions  of  proto- 
plasm live,  associated  together  in  a  plant  in  social  union,  and  where  corresponding 
to  this  a  division  of  labour  has  occurred,  the  whole  structure  becomes  naturally 
divided  up  into  open  spaces  where  there  is  no  lack  of  air  and  light,  into  con- 
trivances for  ventilation,  into  mechanisms  for  conveying  gas  and  water,  and  into 
chambers  for  storing  up  nourishment;  in  short,  it  is  a  question  of  varied  mechan- 
isms within  and  defences  without,  of  the  ensuring  of  strength  throughout  the 
whole  structure,  and  of  the  necessary  supporting  framework  for  the  individual 
parts.  Each  part  occupies  a  position  corresponding  to  the  demand  upon  it;  the 
light-requiring  parts  are  exposed  to  the  sun's  rays;  the  mechanisms  for  conveying 


FORM   AND   SIZE   OF   PARTICLES   EMPLOYED    IN    CONSTRUCTION   OF   PLANTS.      567 

gas  and  water  begin  and  end  in  the  manner  best  adapted  to  the  given  conditions; 
while  the  pillars  and  beams  are  placed  where  something  has  to  be  protected,  borne, 
or  prevented  from  breaking  down. 

Such  structures,  just  like  the  buildings  produced  by  the  hand  of  man,  convey 
the  idea  of  fitness  of  means  to  ends.  Indeed,  they  often  surpass  mere  human 
creations  in  the  suitability  of  their  arrangement.  It  can  hardly  be  invariably 
said  of  man's  designs  that  they  are  carried  out  in  a  way  completely  suited  to  the 
requirements  of  the  case;  while  no  plant  lives  and  maintains  itself  which  is  not 
adapted  to  the  given  conditions  of  life  in  the  most  advantageous  manner.  The 
most  remarkable  thing  about  it  is  that  this  adaptation  in  plants  is  not  produced 
directly  by  external  influences,  but  that  rather  the  individual  portions  assume  the 
most  suitable  form  and  position,  even  in  their  first  rudiments  and  very  early 
stages  of  development;  that  is,  at  a  time  in  which  the  forces  acting  outside  the 
plant  can  have  no  considerable  influence  in  directly  moulding  its  form.  Such  an 
adaptation  presupposes,  however,  a  law  of  form;  in  other  words,  a  plan  of  con- 
struction, a  plan  concerning  the  division  of  space  best  suited  to  the  future  division 
of  labour,  a  plan  of  the  most  advantageous  construction  of  the  whole  framework, 
the  most  suitable  position  of  the  conducting  and  ventilating  mechanisms,  and  much 
besides,  which  will  benefit  the  plant  in  the  future. 

This  supposition  being  forced  upon  us,  the  question  arises  as  to  whether  it  is 
correct  to  speak  of  a  constructive  plan  in  plants.  In  the  sense  in  which  we  speak 
of  the  constructive  plan  of  a  house,  certainly  not.  Plants  are  not  built  according 
to  a  plan  devised  by  themselves,  but  their  organs  receive  their  definite  form,  as  if 
according  to  a  prescribed  law,  from  inward  necessity,  like  the  crystal  whose  shape 
is  dependent  upon  and  founded  in  the  chemical  composition  of  the  fluid  from 
which  it  is  formed.  But  just  as  we  can  speak  of  the  plan  and  elevation,  of  the 
symmetrical  arrangement,  even  of  the  plan  of  construction,  of  a  crystal,  equally 
well  can  we  speak  of  the  plan  of  construction,  or,  if  we  prefer  it,  the  law  of  form, 
of  growing  plants.  The  plan  of  construction  is  given  and  traced  out  for  every 
plant  by  its  specific  constitution,  and  so  far  every  species  has  its  own  plan  quite 
independently  of  the  external  influences  which  it  follows,  indeed,  is  compelled  to 
follow,  as  long  as  the  constitution  is  not  altered. 

But  by  specific  constitution  we  do  not  merely  understand  the  chemical  com- 
position, the  definite  number  of  atoms,  and  their  characteristic  grouping  into 
molecules.  We  understand  further  the  union  of  molecules  in  definite  groups  of  a 
higher  order,  which  must  be  regulated  in  the  plant  body  just  as  in  the  body  of  a 
crystal.  This  arrangement  of  the  molecules  is  characteristic,  we  must  suppose,  for 
every  species  of  plant;  further,  we  must  believe  that  the  substance  which  is 
associated  with  the  growth  of  the  molecular  groups,  already  present,  is  always 
subordinated  to  the  laws  of  symmetry  prevailing  there,  and  that  this  grouping  is 
not  only  specific,  but  also  constant  and  invariable. 

When  we  speak  of  crystals,  we  do  not  mean  to  say  that  the  processes  in  ques- 
tion in  them  and  in  plants  are  identical.  It  is  much  more  probable  that  there  is 


568      FORM   AND   SIZE   OF   PARTICLES   EMPLOYED   IN   CONSTRUCTION    OF   PLANTS. 

a  fundamental  difference  between  the  construction  of  crystalline  bodies  and  plant 
bodies;  that  this  very  difference  is  bound  up  with  the  distinction  between  inani- 
mate and  living  structures,  and  that  especially  are  the  organized  parts  of  plants 
fitted  by  their  characteristic  structure  to  those  movements  which  appear  to  us  as 
life. 

Molecules,  united  in  the  growth  of  crystals,  admit  of  no  further  insertion  of 
plastic  substance,  of  no  rearrangement  and  transformation,  of  no  addition  of  new 
molecules  between  those  already  present,  as  is  the  case  with  the  molecules  of  living 
organized  bodies.  When  the  molecules  of  water  penetrate  into  a  salt  crystal,  the 
molecules  of  salt  separate  from  one  another,  and  break  away,  so  that  we  have  a 
disintegration  and  solution  of  the  crystal,  and  not  its  further  development.  The 
crystal,  moreover,  never  shows  at  any  time  those  displacements  and  movements 
of  the  smallest  constructive  particles  which  characterize  the  living  organized 
parts  of  plants,  which  in  the  aggregate  we  call  life.  Crystals,  therefore,  cannot 
be  considered  as  organized  bodies;  they  are  not  directly  concerned  in  the 
phenomena  of  life,  and  form  no  object  susceptible  to  the  influence  of  that  specific 
natural  force  which  we  call  vital  force.  They  are  not,  and  will  never  be  living, 
just  as  they  cannot  die. 

The  analogy  between  the  structure  of  crystals  and  of  plants  consists  only  in 
the  fact  that  in  both  cases  the  grouping  of  the  molecules  cannot  proceed  irre- 
gularly, but  must  always  follow  definite  laws  of  symmetry.  In  both  cases  the 
external  visible  form  of  the  finished  structure  is  the  expression  of  a  particular 
and  specific  grouping  of  the  molecules,  and  of  molecular  aggregates  known  as 
micellce. 

Many  attempts  have  been  made  to  glean  some  idea  of  the  actual  shape  of  these 
groups  of  molecules  or  micellae,  the  bricks — so  to  speak — of  which  the  plant  is 
constructed.  That  the  hypotheses  brought  forward  are  very  divergent  is  not  sur- 
prising when  we  remember  how  few  are  the  data  of  actual  fact  that  have  been 
observed,  and  how  readily  these  data  admit  of  varying  interpretation,  and  how 
full  a  scope  they  offer  to  the  imagination  of  the  investigator. 

Not  long  ago  the  idea  found  almost  general  acceptance  that  micellae  were 
crystalline  in  form.  In  many  cell- walls,  and  especially  in  certain  Desmidieae,  very 
regular  systems  of  striae  were  observed,  which  ran  off  into  three  dimensions  of 
space,  and  strongly  resembled  the  striae  connected  with  the  cleavage  planes  of 
certain  crystals  (e.g.  of  calc-spar).  Since  these,  and  generally  all  cell- walls,  light  up 
the  dark  field  in  the  polarizing  microscope,  that  is  to  say,  appear  doubly  refractive, 
the  assumption  was  supposed  warranted  that  the  cell- walls  and  other  organized 
substances  consist  of  crystalline  doubly  refractive  micellae,  which  lie  loosely  but 
in  regular  arrangement  next  one  another.  It  was  imagined  that  every  micella  was 
surrounded,  when  moist,  by  an  envelope  of  water,  and  that  on  drying,  the  micellae 
came  into  mutual  contact.  But  later  researches  have  shown  that  the  double 
refraction  can  be  produced  by  pressure  and  strain  in  substances  which  do  not 
normally  exhibit  this  property,  and  that  the  refraction  in  the  polarizing  micro- 


FORM    AND   SIZE   OF   PARTICLES    EMPLOYED    IN    CONSTRUCTION   OF   PLANTS.      569 

scope  is  not  always  indicative  of  the  crystalline  nature  of  micellae.  The  striation 
is  brought  about  by  dissimilar  chemical  constitution  and  unequal  quantities  of 
water  in  the  successive  strata  of  molecular  groups,  and  may  be  present,  equally 
well,  where  the  groups  of  molecules  do  not  possess  crystalline  form.  Moreover, 
the  results  which  have  been  obtained  by  the  so-called  carbonization  or  pulveriza- 
tion of  the  cell-walls  goes  against  the  assumption  of  crystal-like  micellae.  By 
treating  with  sulphuric  acid,  heating  up  to  60°-70°C.,  and  then  operating  with 
hydrochloric  acid,  the  cell- wall  is  broken  up  into  extraordinarily  small  fragments, 
exhibiting  parallel  strise  and  frequent  clefts;  and  these  often  subdivided  into  short, 
very  fine  filaments,  which  filaments  break  up  by  pressure  into  granules  imbedded 
in  a  homogeneous  gelatinous  matrix.  A  definite  geometrical  crystalline  form 
cannot  be  demonstrated  in  this  ground-substance.  Moreover,  the  granules  are  not 
bounded  by  plane  surfaces  and  rectilineal  edges,  and  have  no  resemblance  to  the 
smallest  visible  portions  of  crystals.  All  the  observations  obtained  by  this  class 
of  experiment  tend  rather  to  show  that  the  granules  are  grouped  into  filaments, 
or  lamellae,  or  both,  that  they  are  joined  together  by  extremely  delicate  proto- 
plasmic threads,  and  that  the  cell-wall  possesses  a  reticular  structure.  If  these 
granules  and  filaments  are  not  themselves  the  micellae,  but  groupings,  rather,  of  a 
higher  order,  still  their  outlines  in  no  case  suggest  the  forms  of  crystalline  micellae. 
The  idea  that  the  micellae  possess  a  reticular  form  was  corrected  much  earlier.  If 
the  same  rule  which  prevails  in  the  grouping  of  the  molecules  into  micellae  would 
also  hold  in  the  association  of  micellae  into  groups  of  high  order,  and  ultimately 
into  bodies  which  are  in  their  outline  recognizable  by  our  senses,  then  one  might 
hope  to  derive  the  form  of  the  micellae,  and  even  the  form  of  the  molecules 
themselves,  from  the  form  of  the  smallest  visible  portions  of  the  plant.  This 
supposition  would  lead  to  the  conception  of  reticular  micellae  and  reticular  mole- 
cules in  the  organized  parts  of  plants.  It  is,  however,  very  noticeable  that  all 
researches  concerning  the  form  of  the  smallest  visible  elements  of  protoplasm  point 
to  a  reticular  structure.  In  the  dry  coating  of  the  so-called  plasmodia  of  myxo- 
mycetes,  which  contains  no  cellulose,  but  consists  of  protoplasm  (in  which  are 
deposited  crystals  of  calcium  oxalate),  for  example,  in  the  plasmodium  of  Leocarpus 
fragilis,  it  is  seen  that  the  entire  papery  skin  consists  of  twisted  threads  extend- 
ing in  all  directions,  which  anastomose  in  a  reticular  manner,  and  that  the  meshes 
of  this  net- work  are  filled  with  a  highly  refringent  substance. 

In  the  hyaline  ectoplasm  of   the  living  protoplasm  which  inhabit   the  cell 
chamber,  very  fine  threads  have  been  observed  lying  side  by  side,  and  if  this 
protoplasm  is  displaced  and  killed  by  alcohol,  it  can  be  ascertained  by  the  aid  of 
colouring  matters  that  the  whole  cell-body  is  built  up  of  very  minute  threa, 
connected  into  a  net-work,  and  that  the  meshes  of  this  fine  net-work  are  filled  wit 
a  fluid  substance.     Within  the  threads  are  to  be  seen,  however,  corpuscles  arrangec 
in  rows,  which  have  received  the  name  of  microsomata. 

The  whole  protoplasmic  cell-body,  including  the  cell-nucleus,  appears  gen 
to  possess  this  same  structure,  for  in  the  processes  which  lead  up  to  the  divis 


570      FORM   AND   SIZE   OF   PARTICLES   EMPLOYED   IN   CONSTRUCTION   OF   PLANTS. 

of  the  cell  we  always  see  therein  granules,  rods,  and  shorter  or  longer,  straight 
and  curved  tortuous  threads,  twisted  in  balls,  and  anastomosing  into  net-works, 
which  undergo  the  most  wonderful  displacements,  as  will  be  described  in  the 
following  pages. 

All  these  observations  at  any  rate  do  not  contradict  the  supposition  of  reticular 
micellse;  and  since  the  conception  of  molecules  built  up  from  atoms  grouped  in 
this  manner  has  not  been  contradicted  by  chemists,  the  hypothesis  should  find 
support  from  this  fact.  Of  course  the  hypothesis  of  the  net-like  form  of  the 
micellse  is  based  upon  an  assumption,  the  accuracy  of  which  is  subject  to  many 
doubts.  It  is  questionable  whether  the  same  rule  always  holds  in  all  these 
groupings  and  connections.  Just  as  pointed  crystals  often  join  up  into  spherical 
groups,  whose  construction  follows  other  laws  of  symmetry  than  are  observed 
by  the  molecules  of  which  the  individual  crystals  are  composed,  so  it  is  always 
possible  that  the  combination  of  the  micellse  into  visible  bodies  follows  other 
rules  than  the  union  of  the  molecules  into  the  micellas. 

This  change  in  the  relations  of  symmetry,  occuring  in  minerals,  gives  rise 
to  the  idea  of  the  possibility  that  micellse  may  possess  a  spherical  shape,  that 
is  to  say,  the  highest  degree  of  symmetry  which  can  be  imagined  in  a  body. 
Some  form  of  symmetry  must  exist  under  all  conditions,  and  if  the  crystalline 
form  of  micellse  is  excluded,  then  there  remains  the  possibility  of  reticular  and 
spherical  micellse. 

Although  our  thirst  for  knowledge  finds  but  little  satisfaction  in  hypotheses 
of  this  kind,  still  they  are  not  on  this  account  to  be  held  in  contempt.  The 
minutest  structure  of  every  substance,  whose  movements  appear  to  the  perception 
of  our  senses  as  life,  is  far  too  complicated  for  us  to  be  able  to  bring  it  into  the 
scope  of  our  observations  on  the  life  of  plants;  and  in  order  that  we  may  be 
able  to  form  a  clear  picture  of  all  these  matters,  it  is  better  at  any  rate  to 
imagine  the  groups  of  molecules  as  net- works  and  spheres  than  to  imagine  nothing 
at  all. 

Though  we  may  deny  to  the  micellae  a  crystalline  nature,  actual  crystals  can 
be  produced  by  many  organized  portions  of  plants.  Groups  of  crystals  of  calcium 
oxalate  (see  fig.  123 4)  are  found  very  regularly  deposited  in  the  net- work  which 
forms  the  pellicle  of  myxomycetes.  Such  groups  of  crystals  are  also  to  be  found 
in  the  cell-membranes  of  many  flowering  plants  (Cactacese,  Nyctaginese,  Com- 
melynacese,  &c.).  The  carbonate  of  lime  excreted  in  the  cell-walls  of  Litho- 
thamniese,  is  likewise  crystalline.  In  other  cases  these  excretions  and  depositions 
of  lime  and  of  silica  are  not  crystalline,  but  amorphous,  which  literally  means 
without  form.  But  we  must  be  careful  not  to  be  misled  by  this  expression. 
These  substances  cannot  be  conceived  of  without  a  definite  shape  governed  by  con- 
ditions of  symmetry,  only  they  are  not  composed  according  to  the  laws  of  symmetry 
governing  crystals,  and  the  word  amorphous  should  therefore  be  interpreted  here 
as  non-crystalline.  It  does  not  lie  within  the  scope  of  these  remarks  to  enter 
into  details  about  the  hypotheses  as  to  the  shape  of  the  molecules  and  groups  of 


FORM   AND   SIZE   OF   PARTICLES   EMPLOYED   IN   CONSTRUCTION   OF   PLANTS.     571 

molecules  of  amorphous  lime  and  amorphous  silica;  but  this  much  must  be  said 
with  regard  to  these  depositions,  that  they  cannot  be  looked  upon  as  organized 
substances. 

Here  is  the  proper  place  to  consider  investigations  as  to  the  size  of  molecules. 
In  these  researches,  especially  for  the  ascertainment  of  the  size  of  molecules  of 
gas,  very  various  physical  facts  offer  themselves  as  data,  such  as  the  coefficients 
of  condensation,  the  deviations  from  Boyle's  law,  the  variability  of  the  coefficients 
of  expansion,  the  heat  of  evaporation,  and,  finally,  the  constants  of  dielectrics. 
The  results  differ  considerably.  For  example,  the  estimates  of  sizes  given  for  a 
certain  gas  by  different  methods  differ  from  one  another  far  more  than  those 
which  have  been  obtained  from  different  gases  by  one  and  the  same  method.  But 
all  calculations  agree  that  the  diameter  of  the  hypothetically  spherical  molecules 
of  gas  must  lie  between  the  hundred-thousandth  and  the  millionth  part  of  a 
millimetre,  and  that  these  limits  cannot  be  overstepped,  either  above  or  below,  to 
any  great  extent  even  in  the  extremest  cases.  A  cubic  millimetre  of  gas  would 
therefore  contain  about  866  billions  of  molecules,  and  if  the  gas  were  condensed 
into  a  fluid,  the  number  in  a  millimetre  would  increase  to  a  trillion. 

The  length  of  light-waves  is  of  the  smallest  of  measurable  dimensions.  If 
the  diameter  of  a  molecule  is  taken  in  round  numbers  at  the  millionth  part  of  a 
millimetre,  this  is  700  times  smaller  than  the  wave-length  of  red  light,  and  the 
diameter  of  a  molecule  bears  about  the  same  proportion  to  a  millimetre,  as  a 
millimetre  to  a  stretch  of  2  kilometres.  Particles  of  these  dimensions  are 
beyond  the  conception  of  our  senses;  even  the  highest  powers  of  the  microscope 
are  unable  to  disclose  them  to  us,  as  is  shown  by  the  following  considerations. 
Sheets  of  gold-leaf  are  produced,  whose  thickness  amounts  to  only  a  hundredth 
part  of  the  wave-length  of  light,  and  which  accordingly  contain  only  3-5  molecules 
of  gold  above  one  another.  These  gold-leaves  are  transparent  to  white  light, 
and  this  may  be  regarded  as  a  proof  that  rays  of  light  penetrate  through  the 
chinks  between  the  molecules.  Nevertheless  this  leaf  appears  as  a  continuous 
mass  under  the  best  microscopes,  and  it  is  not  possible  to  recognize  the  individual 
molecules  composing  it.  Under  the  most  favourable  circumstances,  our  microscopes 
are  able  to  render  visible  only  particles  which  comprise  perhaps  two  million 
molecules.  Since  there  are  no  certain  data  to  enable  us  to  measure  how  great 
is  the  number  of  molecules  from  which  micellae  are  built  up,  and  in  what  manner 
the  molecules  are  grouped  in  them,  it  would  be  rash  to  attempt  any  conjectures 
as  to  the  size  of  micellse.  The  possibility  of  perceiving  micellae  with  the  micro- 
scope in  their  outline  and  shape,  especially  those  of  albuminous  bodies,  whose 
molecules  are  composed  of  such  a  large  number  of  atoms  (see  p.  456)  is  not  to  be 
wholly  excluded,  particularly  since  our  microscopes  are  still  capable  of  much 
improvement.  Still,  the  probability  is  but  a  remote  one,  and  as  matters  stand  at 
present,  all  conclusions  on  this  subject  would  be  of  the  nature  of  theory,  in 
which  one  uncertain  hypothesis  has  to  furnish  the  foundation  for  a  second, 
still  more  doubtful. 


572  VISIBLE   CONSTRUCTIVE   ACTIVITY    IN   PROTOPLASM. 


VISIBLE  CONSTRUCTIVE  ACTIVITY  IN  PROTOPLASM. 

Though  it  is  improbable  that  we  shall  ever  succeed  in  seeing  the  micellae  of 
which  the  organized  living  portions  of  plants  are  built  up,  and  though  all  attempts 
to  form  a  picture  of  these  tiny  units  are  only  founded  upon  conjecture  and  hypo- 
thesis, still  we  can  follow  with  our  eyes  the  general  operations,  the  constructive 
and  shaping  activity  of  the  protoplasm. 

This  formative  activity  can  be  most  easily  observed  in  the  comparatively  large 
protoplasmic  bodies  of  myxomycetes,  in  their  so-called  plasmodia;  therefore  some  of 
the  most  striking  of  these  processes  will  now  be  briefly  described. 

The  myxomycete  Leocarpus  fragilis,  which  commonly  occurs  on  the  bark  of 
dry,  fallen  branches  of  the  Pine,  forms  a  viscous  yellow  mass,  looking  deceptively 
like  the  spilt  yolk  of  an  egg.  The  dead  branch  is  covered  by  a  thin  layer  of  this 
substance,  in  which  no  particular  projections  can  be  recognized.  Quite  late  in  the 
evening  Leocarpus  can  be  seen  in  this  plasmodial  stage.  During  the  night,  how- 
ever, it  rises  up  in  certain  places  into  knobs  and  warts,  and  the  whole  mass  then 
has  a  coarsely  granular  appearance.  Towards  morning,  pear-shaped  bodies,  sup- 
ported on  thin  stalks,  are  produced  from  these  protuberances,  which  are  now  no 
longer  viscous,  but  exhibit  a  thin  dry  pellicle.  Within,  they  have  become  trans- 
formed into  numerous  hair-like  threads,  with  black  powdery  spores  lying  between 
the  threads.  Leocarpus  needs  about  12  hours  for  this  manifestation,  and  if  one  has 
the  patience  to  observe  the  mass  shaping  itself  throughout  the  night,  one  may 
actually  see  how  it  rises  from  the  substratum,  rounds  itself  off,  forms  a  skin,  and 
assumes  the  pear-shape  form.  Dictydium  umbilicatum  develops  its  plasmodia  in 
the  same  way  as  Leocarpus.  The  light  brown,  irregular,  flowing  mass  of  proto- 
plasm gathers  itself  up  into  a  round  cord,  which  becomes  thickened  in  a  club- 
shaped  manner  at  its  upper  end,  and  then  spreads  out  into  a  delicate  net -work 
with  spherical  outline.  Between  the  meshes  of  this  net-work  the  protoplasm 
separates  out  into  black  powdery  spores,  which  are  at  the  mercy  of  the  slightest 
breath  of  wind.  The  slimy  protoplasm  of  Stemonitis  fusca,  on  the  other  hand, 
rises  up  in  the  shape  of  numerous  closely-compacted  strands  about  1J  cm.  long. 
Each  individual  strand  is  divided  into  a  lower,  stalk-like  portion,  and  an  upper, 
thick,  cylindrical  body.  This  is  at  first  of  slimy  consistency,  but  soon  becomes  dry 
and  divides  into  a  central  axis,  from  which  proceed  all  round  an  endless  number  of 
very  fine  reticulating  threads  which  break  up  into  thousands  of  powdery  spores, 
and  at  the  periphery  into  a  very  delicate  skin,  which  later  on  ruptures  and  allows 
the  spores  to  fall  out.  This  entire  shaping  of  the  protoplasm,  with  which  is  con- 
nected a  change  of  colour  from  white  to  purple,  is  accomplished  under  the  eye  of 
the  observer  in  about  ten  hours.  The  protoplasm  of  Chondrioderma  difforme  can 
scarcely  be  distinguished  from  that  of  Stemonitis  fusca,  and  yet  how  very  different 
is  the  form  which  it  assumes  as  a  plasmodium.  First,  it  is  massed  into  a  round 
ball,  and  in  this  is  separated  out  an  enveloping  skin  of  innumerable  single  slender 


VISIBLE   CONSTRUCTIVE   ACTIVITY    IN    PROTOPLASM.  573 

threads,  and  a  large  quantity  of  dark  spores  which  fill  up  the  space  inclosed  by  the 
skin.  Soon  after,  the  skin  breaks  up  into  stellate  projecting  lobes  at  the  free  apex 
of  the  spherical  body,  and  now  the  dark  spores  can  pour  out  of  the  open  vesicle. 

The  protoplasm  of  Didymium  shapes  itself  quite  differently,  and  that  of 
Clatroptychium  differently  again.  If  we  were  to  exhaust  the  multiplicity  of  form 
which  the  protoplasm  of  this  group  of  plants  assumes,  we  should  be  obliged  here 
to  actually  describe  the  shapes  of  all  myxomycetes.  The  above  examples  will 
suffice  for  the  establishment  of  the  fact  that  apparently  quite  similar  protoplasm 
becomes,  in  each  species,  speedily  transformed  into  a  definite  structure.  It  only 
remains  to  be  noticed  that  the  shape  assumed  by  the  specifically  different  proto- 
plasm is  quite  independent  of  external  conditions ;  and  that  in  the  same  light,  with 
the  same  degree  of  humidity,  and  at  the  same  temperature,  under  the  same  glass 
shade,  the  pear-shaped  Leocarpus,  and  the  cylindrical  strands  of  Stemonitis  develop 
side  by  side  (for  illustrations  of  Myxomycetes  cf.  vol.  II.,  fig.  355). 

The  pellicle  which  bounds  the  plasmodia  of  myxomycetes  contains  no  deposited 
cellulose,  and  there  is  consequently  in  these  plants  generally  no  distinction 
between  the  pellicle  and  the  body  of  the  cell.  The  protoplasm  of  other  plants, 
however,  always  provides  itself,  sooner  or  later,  with  an  envelope  in  which  cellulose 
can  be  demonstrated.  Of  course,  cellulose  is  often  present  in  the  cell-wall  only  in 
small  amount ;  thus,  in  yeast,  as  well  as  in  the  majority  of  fungi,  the  main  part  of 
the  membrane  is  formed  of  nitrogenous  compounds.  Various  phenomena  lead  to 
the  conclusion  that  by  the  development  of  cellulose  in  the  skin,  advantages  are 
obtained  which  are  not  enjoyed  by  myxomycetes,  with  their  brittle  pellicle  built  up 
of  firm  nitrogenous  compounds.  The  soft  protoplasm  is  better  protected  against 
injurious  external  influences  by  the  cellulose  wall,  and  the  whole  structure  obtains 
that  firmness  and  strength  which  are  absolutely  necessary,  especially  to  plants 
composed  of  numerous  cells. 

Moreover,  the  cell- wall  must  not  be  conceived  as  always  a  rigid  covering, 
as  a  chamber  with  immovable  walls.  In  many  instances  it  is  rather  to  be 
compared  to  the  skin  of  an  animal,  which  adapts  itself  to  each  alteration  in 
the  shape  of  the  body.  In  no  case  is  the  elasticity  of  the  protoplasm  hindered 
by  the  surrounding  cell-wall.  Frequently  the  cell-wall  takes  no  share  in  the 
visible  plastic  processes  of  the  protoplasm  which  it  incloses,  and  it  usually  perishes 
when  the  transformations  have  been  completed  in  the  space  it  surrounds  and 
protects.  In  many  instances,  on  the  other  hand,  the  outline  and  shape  of  the  cell- 
wall  alter  in  correspondence  with  the  alteration  of  the  protoplasm  inclosed  by  it. 

These  remarks  had  first  to  be  made  in  order  to  rightly  understand  the  plastic 
processes  to  be  described  successively  as  Segregation,  Gemmation,  and  Cell  Division. 

In  the  case  of  the  Segregation  associated  with  most  of  the  previously  described 
plasmodia,  it  is  to  be  pointed  out  as  characteristic  that  the  protoplasm  divides 
within  a  rigid,  enveloping  cell- wall  into  completely  separate  portions  of  identical 
shape,  and  develops  no  partitions  continuous  with  the  surrounding  cell -wall. 
The  inclosing  cell-wall  stands  in  no  direct  contact  with  the  formed  protoplasmic 


574  VISIBLE   CONSTRUCTIVE   ACTIVITY   IN   PROTOPLASM. 

masses.  Even  when  the  wall  remains,  and  is  not  ruptured  nor  disintegrated,  it  is 
separated  from  the  protoplasmic  masses  by  the  new  cell-walls,  with  which  these 
have  meanwhile  surrounded  themselves.  For  every  species  of  plant  the  number, 
size,  and  shape  of  the  bodies  arising  in  the  interior  of  a  cell  by  division  are 
quite  definite,  though  they  vary  from  species  to  species.  In  the  cell-chambers 
of  some  species  several  thousand  minute  protoplasmic  bodies  arise.  In  others, 
again,  the  number  is  very  limited.  Frequently,  indeed,  the  protoplasm  only  splits 
up  into  two  similar  halves.  If  the  number  is  large,  the  individual  masses  are 
exceedingly  small,  and  can  only  be  recognized  when  very  greatly  magnified.  If 
the  number  is  limited,  the  divided  portions  are  comparatively  large.  The  shape 
of  the  structures  is  exceedingly  various.  Some  are  spherical,  elliptical,  or  pear- 
shaped;  others  elongated,  fusiform,  filamentous,  or  spatulate;  some  are  straight, 
others  are  spirally  twisted,  and  many  are  drawn  out  into  a  thread;  others  are 
provided  over  the  whole  surface  with  short  cilia,  others  again  with  a  crown  of 
cilia  at  a  particular  spot,  or  with  only  a  single  pair  of  long  cilia.  The  illustration 
on  p.  29  represents  the  most  widely  differing  forms,  without,  however,  exhausting 
the  wealth  of  configuration.  In  the  majority  of  cases  the  small  bodies  exhibit 
active  movements,  and  that  even  within  the  cell-covering  which  surrounds  the 
dividing  protoplasm ;  but  sooner  or  later  they  come  to  rest,  and  then  assume  another 
shape,  or  fuse  with  another  protoplasmic  body. 

With  regard  to  the  further  changes  experienced  by  the  bodies  formed  by 
division,  many  events  may  be  distinguished.  In  one,  the  cell  in  which  the  division 
of  the  protoplasm  has  taken  place  opens,  the  bodies  formed  glide  out  separately 
and  swarm  in  the  surrounding  fluid.  Often  they  are  concerned  in  fertilization,  and 
fuse  with  other  protoplasmic  bodies  in  a  manner  to  be  described  later  in  detail.  If 
not,  they  surround  themselves  with  a  cell-wall,  but  do  not  adhere  together,  or 
develop  into  a  cell-colony. 

In  the  Water-net  (Hydrodictyori),  described  on  p.  36  (cf.  fig.  197,  vol.  II.),  the 
parietal  protoplasm  of  a  cell  divides  up  into  7000-20,000  minute  clumps  which 
exhibit  the  so-called  swarming  movement.  At  first  a  definite  aim  cannot  be 
assigned  to  these  movements,  but  after  a  short  time  the  particles  appear  arranged 
very  regularly  in  a  net  with  hexagonal  meshes.  They  assume  the  form  of  short 
rods,  each  of  which  joins  at  its  poles  with  two  others,  being  cemented  to  them 
by  excreted  cellulose.  Instead  of  a  protoplasmic  parietal  layer  in  the  cell  in 
question  a  miniature  water-net  is  now  seen  to  have  arisen.  This  becomes  free 
with  the  disintegration  of  the  parent-cell;  its  cells  grow  and  increase  in  all 
directions  without,  however,  altering  the  shape  once  assumed.  The  process  which 
is  observed  in  Pediastrum  (fig.  197,  vol.  II.),  a  very  small  water  plant  allied  to  the 
water-net,  is  very  much  the  same.  Here  also  the  protoplasm  of  a  cell  which  has 
isolated  itself  from  the  others  divides  up  into  small  clumps  which  round  themselves 
off,  and  swarm  about  for  a  short  time.  Gradually  they  come  to  rest,  assume  an 
angular  form,  and  arrange  themselves  so  as  to  form  two  concentric  rings  in  one 
plane.  Where  they  come  into  contact  with  each  other,  they  excrete  cellulose  and 


VISIBLE   CONSTRUCTIVE   ACTIVITY    IN    PROTOPLASM.  575 

thus  become  connected  into  a  tiny  disc.  This  disc  consists  of  as  many  cells  as  there 
are  connected  clumps  of  protoplasm,  and  presents  almost  the  appearance  of  a  honey- 
comb. Out  of  this  combination  each  cell  can  again  separate  itself  from  its  com- 
panions, its  protoplasm  can  divide  up  afresh,  and  generally  the  whole  process 
described  above  may  be  repeated. 

The  Water-net  and  the  discs  of  Pediastrum  develop  young  nets  and  discs 
accordingly,  from  the  divided  protoplasm  in  the  individual  cells.  These  escape  as 
small  colonies  of  cells  from  the  space  in  which  they  were  formed,  and  here  a 
definite  isolation  of  the  young  cell-colony  occurs.  In  Glceocapsa,  on  the  contrary, 
of  which  a  species  (Glceocapsa  sanguinea)  is  represented  in  figure  25A,  n  and  o, 
the  young  cell-groups  remain  joined  together.  Each  cell  always  divides  up,  two 
and  two,  into  protoplasmic  clumps,  which  surround  themselves  immediately  with  a 
thick  cell-wall.  The  old  cell-wall,  however,  does  not  disintegrate  nor  rupture ;  it 
does  not  allow  the  young  cell-colony  to  escape,  but  it  stretches,  and  the  young  and 
old  cell-walls  are  now  seen  layered  one  above  another.  If  this  process  is  repeated 
many  times,  protoplasmic  balls  arranged  in  pairs  are  to  be  seen  inserted  within  a 
whole  system  of  concentrically  stratified  cell-walls.  A  process  similar  to  that  just 
described  is  observed  in  the  ovules  of  seed-plants,  and  has  been  called,  though  not 
very  happily,  "free  cell-formation". 

Gemmation  is  essentially  different  from  the  process  just  described.  It  is 
observed  in  plants  both  with  and  without  chlorophyll,  but  is  not  really  frequent 
in  the  vegetable  kingdom.  Its  characteristic  feature  is  that  the  protoplasm  at  a 
certain  point  of  the  circumference  of  a  cell  pushes  outwards,  and  in  this  way  a  wart 
or  bud-like  elevation  of  the  cell-wall,  an  actual  protuberance,  arises  which,  though 
at  first  not  very  prominent,  soon  increases  in  area,  and  in  the  end  assumes  the  size 
and  shape  of  the  body  from  which  it  was  produced.  We  may  distinguish  two 
kinds  of  gemmation.  Either  an  open  communication  is  maintained  between  the 
outgrowth  and  the  structure  from  which  it  was  produced,  and  no  separation  occurs 
at  the  place  of  origin ;  or,  the  parent  cell  is  shut  off  from  the  outgrowth  by  a  cell- 
wall  which  subsequently  splits,  and  the  outgrowth  is  detached  from  the  cell-body 
from  which  it  arose.  Very  pretty  examples  of  the  first  kind  are  exhibited  by  the 
Siphoneae,  especially  in  Vawcheria,  illustrated  in  figure  25A,  a.  The  tubular  cells 
appear  branched,  each  branch  consisting  of  a  tube  ending  blindly,  and  all  these 
branched  tubes  are  in  free  communication  with  one  another.  The  entire  Vaucheria 
is  really  only  a  single,  much-branched  cell — of  course  a  cell  which  must  be  called 
gigantic  in  comparison  with  ordinary  plant-cells.  Species  of  the  genus  Bryopsis 
shape  themselves  similarly,  but  in  these  the  outgrowths  are  much  more  regular 
than  in  Vaucheria,  the  whole  cell,  branched  and  thus  pouched,  almost  resem- 
bling a  moss  with  axes,  leaves,  and  rhizoids.  In  the  genus  Caulerpa  the  cell 
also  produces  outgrowths,  some  of  which  resemble  roots,  whilst  others  imitate  the 
shapes  of  leaves,  reminding  one,  in  some  species,  of  small  fern-fronds. 

Of  the  second  kind  of  gemmation  yeast  may  be  taken  as  a  type.  The  shape 
of  individual  yeast-cells  is  ellipsoidal.  When  the  yeast-cell  grows,  the  elliptical 


576  VISIBLE   CONSTRUCTIVE   ACTIVITY    IN   PROTOPLASM. 

form  of  the  body  is  retained  for  a  time,  and  the  ellipsoid  increases  equally  on  all 
sides.  When  it  has  once  attained  a  certain  size,  the  protoplasm  bulges  out  at  a 
particular  place,  and  a  wart-like  protuberance  arises  at  the  periphery,  at  first 
exceedingly  small,  but  gradually  increasing  in  extent,  and  at  length  reaching  the 
size  of  the  ellipsoid  from  which  it  was  produced  (cf.  vol.  II.  figs.  371 3  and  371  6). 
To  say  that  the  cell- wall  of  the  yeast-cell  protrudes  or  grows  out,  and  that  proto- 
plasm immediately  enters  into  the  protuberance,  is  not  a  correct  account  of  this 
process.  The  cell-wall  here  is  only  passive :  it  projects  beyond  the  periphery  of  the 
ellipsoidal  parent-cell  only  because  it  is  itself  the  skin  of  the  protoplasm  pushing 
its  way  out  at  that  point.  From  one  yeast-cell  two  outgrowths  may  arise  at 
different  places,  and  each  of  them,  when  it  has  once  reached  a  certain  size,  may 
again  form  protuberances.  In  this  way  the  yeast  shapes  itself  into  a  structure 
which  strongly  recalls  to  our  mind  the  Prickly  Pear  in  outline  (cf.  vol.  II.  fig. 
371  3).  When  the  protuberance  has  grown  to  an  ellipsoid,  equal  in  size  to  that  from 
which  it  originated,  the  slightest  pressure  is  sufficient  to  disconnect  the  two,  and 
to  separate  the  individual  members  of  the  irregular  opuntia-like  chain.  Even 
without  any  external  stimulus  the  cells  separate,  as  may  be  well  observed  in 
brewers'  yeast  (Saccharomyces  cerevisice),  which  of  all  the  species  of  yeast  has 
been  most  investigated. 

The  formation  of  yeast  by  the  development  of  a  cell- wall  as  a  partition  between 
two  adjoining  cells  reminds  one  of  the  division  of  cells  which  has  now  to  be 
described  as  the  third  formative  process  connected  with  growth.  The  division  of 
the  cells  is  always  accomplished  in  the  following  manner: — The  protoplasm, 
inclosed  in  its  cell-wall,  develops  a  partition  in  its  interior  by  which  it  becomes 
divided  into  two  halves,  and  the  cell-space  into  two  compartments  or  chambers. 
In  some  plants  the  sister-cells  produced  in  division  separate  from  one  another,  the 
partition- wall  becoming  completely  split,  but  in  most  cases  the  neighbouring  cells 
remain  connected,  and  then  in  each  of  these  the  same  process  is  repeated;  in  this 
way  arise  multicellular  structures,  that  is,  aggregates  of  cells. 

A  separation  of  the  two  cells  arising  from  a  division,  due  to  the  splitting  of 
the  intervening  wall,  is  observed  in  the  Desmidiese,  those  small  green  aquatic 
plants,  of  which  two  species  are  represented  in  figure  2  5 A,  i,  k.  Although  the 
Desmidiea8  consist  only  of  a  single  cell,  their  multiplicity  of  form  is  considerable. 
We  have  cylindrical,  semilunar,  tetrahedral,  stellate,  and  disc-shaped  forms  in  inex- 
haustible variety,  often  occurring  in  a  restricted  area,  and  forming  a  gay  assemblage 
like  the  various  herbs  growing  in  a  meadow.  The  cell  of  each  species,  however, 
adheres  with  wonderful  tenacity  to  its  plan  of  construction,  and  always  develops 
only  to  a  definite  size.  When  once  this  size  is  attained,  and  after  the  cell  has  re- 
mained unaltered  in  form  for  a  time,  a  noticeable  change  begins  to  take  place.  The 
central  portion  of  the  cell  (which  is  constricted  in  all  species)  quickly  elongates  and 
expands.  The  protoplasm  then  develops  a  dividing-wall,  and  two  cells  are  now 
produced  from  the  one.  These  remain  connected  for  only  a  little  while;  the  inter- 
calated cellulose  wall  splits;  the  two  cells  separate,  and  each  forthwith  assumes 


VISIBLE   CONSTRUCTIVE   ACTIVITY   IN   PROTOPLASM.  577 

exactly  the  shape  which  the  parent  had  possessed.  These  elegant  desmids  claim 
our  interest  because  although  their  wall  is  composed  principally  of  cellulose,  and 
is  comparatively  thick,  it  has  a  determinate  outline,  and  in  this,  and  in  its  pro- 
tuberances, and,  generally,  in  its  entire  shape,  it  is  governed  by  the  living  cell-body 
which  has  formed  it.  If  such  a  desmid-cell  extends  in  length  or  breadth,  if  it 
bulges  out  in  one  place  and  remains  constricted  in  another,  this  is  caused  only  by 
the  activity  of  the  protoplasm,  which  shapes  and  transforms  the  body  in  accordance 
with  the  constructive  plan  of  the  species. 

The  continued  connection  of  the  cell-couples  produced  by  division,  and  the 
origination  of  extensive  cell-aggregates  by  the  repeated  formation  of  partition 
walls,  is  much  more  usual  than  their  separation.  No  less  than  five  different 
modifications  may  be  distinguished  of  this  process,  which  is  connected  with  the 
construction  of  so  many  plants. 

In  the  green  aquatic  filaments,  of  which  two  species  (Zygnema  pectinatum 
and  Spirogyra  arcta)  are  illustrated  in  figure  25A,  m  and  I,  a  wall  may  be  de- 
veloped by  the  protoplasm  of  each  cell,  which  is  first  formed  as  a  ring-like  band 
on  the  already  existing  cell-wall,  and  resembles  the  diaphragm  in  the  tube  of  a 
microscope.  Gradually  from  this  circular  band  a  completely  closed  partition-wall 
is  produced,  and  the  single  cell  becomes  divided  into  two.  In  each  of  these  cells 
this  process  may  be  repeated,  and  thus  in  a  very  short  time  may  arise  a  row  of 
four,  eight,  sixteen,  &c.,  cells.  These  remain  connected  with  one  another,  and  the 
whole  row  constitutes  a  cylindrical  tube  divided  up  by  numerous  transverse  walls. 
If  the  single  cells  are  much  swollen  at  the  sides,  the  row  of  cells  has  the  appearance 
of  a  string  of  pearls.  The  intercalated  partition-walls  in  these  plants  are  all 
developed  parallel  to  one  another,  and  are  placed  at  right  angles  to  the  axis  of  the 
cell-filament. 

The  fact  of  these  intercalated  partition-walls  being  parallel  distinguishes  this 
process  from  another,  which  is  characterized  by  the  fact  that  the  insertion  of 
partition- walls  occurs  in  two  dimensions  of  space.  In  this  case  neither  partitioned 
tubes  nor  strings  of  pearls  arise,  but  groups  of  cells  arranged  in  one  plane,  which 
are  plate-like  in  appearance,  and,  to  the  naked  eye,  look  like  membranes  or  leafy 
structures.  This  kind  of  structure  is  often  shown  by  marine  algae  which  grow  on 
stones.  If  all  the  cells  adhere  to  the  substratum,  as  in  Hildenbrandtia,  the  out- 
line of  the  plate  is  more  or  less  circular,  and  green  or  red  patches  are  to  be  seen  on 
the  stone,  which  continually  increase  in  size  without  altering  their  general  form. 
In  this  case  there  is  no  obstacle  which  could  restrict  the  circular  shape  of  the  cell- 
plate.  If,  on  the  other  hand,  only  some  of  the  cells  adhere  to  the  substratum, 
while  the  others  rise  up  from  the  stone,  so  that  the  whole  floats  in  the  water  as 
a  thin  film  (only  attached  to  the  substratum  at  one  point),  then  the  further 
development  is  unsymmetrical.  It  is  suppressed  towards  the  substratum,  and  the 
whole  layer  usually  has  a  fan-like  appearance. 

If  the  arrangement  of  partition- walls  in  a  cell  occurs  in  three  dimensions  of 
space,  a  tissue  is  then  formed.  The  tissue-body  developing  most  regularly  in  this 

VOL.  I.  37 


578  VISIBLE   CONSTRUCTIVE   ACTIVITY   IN   PROTOPLASM. 

manner  is  such  as  is  exhibited  by  Sarcina  ventriculi,  a  vegetable  structure  which 
will  be  presently  treated  of  in  detail.  Here  the  eight  daughter-cells  produced  from 
one  cell  appear  so  connected  with  one  another,  that  they  present,  taken  together, 
almost  the  form  of  a  cube  (cf.  vol.  II.  fig.  372 10).  One  cell  always  comes  to  lie 
in  each  of  the  eight  corners.  Structures  of  such  regularity  are  of  course  rare. 
Usually  manifold  variations  take  place.  In  the  so-called  pollinia  of  orchids 
hundreds  of  daughter-cells  are  developed  by  repeated  division,  grouped  into  small 
balls  which  again  form  a  large,  irregular,  clumpy  mass.  It  frequently  happens 
that  a  group  of  cells,  which  increases  at  the  periphery  in  three  dimensions  of  space, 
in  consequence  of  the  intercalation  of  division-walls,  does  not  exhibit,  as  would 
have  been  expected,  a  symmetrical  growth  on  all  sides,  but  increases  chiefly  in  one 
of  the  three  dimensions.  This  form,  which  is  specially  observed  in  stem  structures, 
depends  upon  the  development  of  a  so-called  apical-cell.  By  this  is  meant  a  cell 
which  forms  to  some  extent  the  apex  of  a  cellular  body  constructed  on  a  horizontal 
base.  By  the  insertion  of  a  partition-wall  a  chamber,  a  so-called  segment,  is 
formed  from  the  lower  half  of  the  apical-cell.  While  fresh  divisions  are  being 
accomplished  in  this  segment  the  upper  half  of  the  apical-cell  again  grows  up  to 
the  original  size;  and  if  one  did  not  know  that  a  segment  had  been  cut  off  from 
it  a  short  time  before,  it  might  be  thought  unaltered  with  regard  to  size,  position, 
and  shape.  After  a  little  time  the  segmentation  just  described  is  repeated  and 
forthwith  it  again  recovers  from  the  loss,  and  attains  to  its  original  size.  Thus  the 
apical-cell  cuts  off  one  segment  after  another  at  the  base,  and  builds  a  pedestal  on 
whose  highest  point  it  enthrones  itself.  The  apical-cell  comes  in  this  way  to  be 
raised  always  higher  and  higher,  as  it  were  pushing  its  way  through  the  surround- 
ing air  or  water  at  the  head  of  a  group  of  cells;  and  to  a  certain  extent  the 
direction  of  growth,  as  well  as  the  internal  tissue  of  the  groups  of  cells  cut  off  from 
the  apex,  are  ruled  and  ordered  by  the  processes  of  division  carried  on  within  it. 

This  results  from  the  fact  that  the  position  of  the  segments  separated  from 
the  apical-cell  (i.e.  of  the  intercalated  separation-walls),  is  always  arranged  in  a 
definite  manner.  If  the  division-wall,  which  arises  in  the  lower  part  of  the 
apical-cell,  parallel  to  the  base  and  at  the  same  time  at  right  angles  to  the 
direction  of  growth  of  the  cell,  and  if  the  further  divisions  arising  in  the  re- 
peatedly-divided segments  occur  in  three  dimensions  of  space,  as  is  the  case,  for 
example,  in  the  Characese,  then  the  whole  plant  is  built  up  in  stories.  The 
chambers  of  the  lower  story  are  produced  from  the  first  segment  cut  off  from  the 
apical-cell,  those  of  the  next  higher  story  from  the  second,  and  so  forth.  The 
whole  structure,  however,  is  terminated  above  by  the  indefatigable  apical-cell, 
which  continues  to  divide  in  the  same  way  as  at  the  commencement  of  the  edifice. 

In  other  cases  the  separation- walls,  which  have  been  intercalated  successively  in 
the  lower  part  of  the  apical  cell,  take  up  an  essentially  different  position  from  that 
in  the  Characeae.  They  are  frequently  placed  obliquely  to  the  direction  of  growth 
of  the  shoot-axis,  and  the  base  of  the  cell  is  either  wedge-shaped  or  three-sided.  It 
is  wedge-shaped,  for  example,  in  some  liverworts  (Aneura  and  Metzgeria)  as  well 


VISIBLE   CONSTRUCTIVE   ACTIVITY    IN    PROTOPLASM.  579 

as  in  Selaginella  (belonging  to  the  family  of  the  Lycopodinese).  Here  we  have 
inclined  walls  formed  alternately  on  the  right  and  left,  and  thus  arise  two  rows  of 
segment-cells,  arranged  with  regard  to  the  axis  of  growth  like  the  barbs  of  a 
feather  on  their  axis.  The  base  of  the  apical-cell  is  three-sided  in  the  stems  of 
horse-tails,  most  ferns,  and  mosses.  Such  an  apical-cell  may  be  best  compared  to 
a  three-sided  pyramid,  whose  sides  are  not  flat  but  somewhat  convex.  The  side  of 
this  cell,  which  corresponds  to  the  base  of  the  pyramid,  forms  the  free  end  which  is 
not  bordered  by  other  cells,  but  by  the  air,  or  earth,  or  water;  the  three  other  sides, 
directed  towards  the  base  of  the  growing  plant-organ,  converge  at  a  point  which 
lies  in  the  axis  of  growth  of  the  organ.  The  insertion  of  division-walls  occurs 
parallel  to  these  three  slightly  arched  sides,  and  in  a  regular  succession,  so  that  the 
segments  cut  off  appear  arranged  like  the  steps  of  a  spiral  staircase.  The  walls 
which  are  afterwards  inserted  in  the  segment-cells  are  partly  parallel,  partly 
at  right  angles  to  the  first-formed  walls.  On  the  whole  it  cannot  be  questioned 
that  in  this  building,  as  in  the  buildings  of  men,  the  walls  are  intercalated  at  right 
angles  to  one  another  in  three  dimensions  of  space. 

In  the  root-tips  of  ferns  and  horse-tails,  we  also  have  a  three-sided,  pyramidal 
apical-cell,  as  just  described,  but  the  construction  is  to  some  extent  complicated 
by  the  fact  that  division- walls  also  arise  parallel  to  that  side  which  corresponds  to 
the  base  of  the  three -sided  pyramid.  The  segments  so  cut  off,  which  divide 
up  again  into  many  cells  by  radial  walls,  cover  the  apical-cell  like  a  cap.  This 
structure,  which  has  been  called  the  root-cap,  serves  to  protect  the  apical-cell  at 
the  root-tip  as  it  pushes  its  way  into  the  earth,  and  would  otherwise  be  exposed  to 
many  dangers. 

In  some  ferns,  and  in  most  flowering  plants,  two,  or  even  a  group  of  cells  are 
to  be  found  at  the  tips  of  the  growing  stems.  Some  trouble  has  been  taken  to 
reduce  the  arrangement  of  these  to  three  types,  but  it  does  not  lie  within  the  scope 
of  this  work  to  describe  these  in  detail.  That  the  construction  in  these  cases  is 
extremely  complicated,  that  in  many  cases  it  is  very  difficult,  frequently  even 
impossible,  to  follow  and  to  establish  with  certainty  the  plastic  processes,  does  not 
in  the  least  alter  our  conviction  that  the  construction  of  the  growing  organs  in 
these  plants  is  accomplished  according  to  rule,  and  that  a  definite  plan  underlies 
the  form  of  every  species,  which  is  indicated  beforehand  by  the  specific  constitution 
of  the  protoplasm. 

It  must  also  be  here  remarked,  to  prevent  misunderstanding,  that  in  plants  in 
which  numerous  organs  are  developed  with  various  functions,  all  the  growing 
parts  are  not  formed  in  the  same  manner.  This,  however,  is  not  opposed  to  the 
fact  that  in  each  species  the  same  constructive  plan  is  invariably  adhered  to.  The 
directions  of  the  septa  inserted  in  the  growing  rhizoids,  leaflets,  and  capsules  of 
a  species  of  moss  may  differ  much  among  themselves,  but  in  each  species  they 
are  always  the  same  in  the  various  organs.  In  flowering  plants,  too,  the  processes 
in  the  formation  of  the  root-cap,  the  stomata,  the  pollen-grains,  and  so  forth,  vary 
very  much  among  themselves,  but  these  processes  are  retained  in  each  species  of 


580  VISIBLE    CONSTRUCTIVE    ACTIVITY    IN    PROTOPLASM. 

plant  with  great  constancy.  In  the  same  species  the  root-cap,  stomata,  and  pollen- 
grains  are  always  found  to  be  constructed  on  the  same  plan.  In  poppy  flowers 
which  had  developed  two  thousand  years  ago  in  Egyptian  soil,  and  which  were 
then  placed  in  tombs  as  ornaments  of  the  dead,  the  cells  of  the  anthers  and  pollen 
are  formed  precisely  as  in  poppy  flowers  which  grow  in  our  fields  to-day.  It  is 
important  to  hold  firmly  to  the  fact  of  this  constancy.  On  it  is  founded  not  only 
the  possibility  of  distinguishing  between  species  of  plants,  but,  generally,  the 
conception  of  kind  or  species,  to  which  we  shall  repeatedly  return. 

The  alteration  of  shape  in  the  protoplasm  and  its  walls,  just  described,  refers  in 
each  case  really  only  to  the  external  contour.  Obviously,  definite  displacements 
and  arrangements  in  the  interior  of  the  living  protoplasm  lie  at  the  foundation 
of  these  alterations,  and  it  is  reserved  for  further  investigation  to  establish  these 
latter  as  far  as  they  are  visible  and  recognizable.  Hitherto  the  alterations 
occurring  in  cell-division  in  the  substance  of  the  protoplasm,  especially  in  the 
so-called  cell-nucleus,  have  alone  been  accurately  observed,  and  what  has  been  seen 
there  has  already  been  briefly  stated  on  a  previous  occasion.  This  is  the  place  to 
return  to  these  remarkable  phenomena,  and  to  collect  together  the  most  important 
results  in  a  brief  review. 

Let  us  look  at  a  cell  in  which  the  protoplasm  fills  the  whole  interior.  A  large 
cell-nucleus  is  visible  in  the  centre  of  the  cell-body.  The  protoplasm  exhibits 
when  very  highly  magnified,  granules,  and  fibrils,  the  latter  long  and  short,  curved 
and  straight,  knotted  and  twisted  or  rolled  into  balls,  and  anastomosing  into 
a  net- work.  This  structure  appears  most  plainly,  especially  the  filamentous 
formation,  in  large  nuclei.  The  twisted  threads  there  visible  have  been  termed 
nuclear  fibrils.  In  many  instances  there  seems  to  be  only  a  single  much-twisted 
thread  present  in  the  nucleus.  In  other  instances  more  are  to  be  seen,  and  they 
appear  to  be  distributed  with  some  uniformity  in  the  nucleus,  as  shown  in  fig.  138 1. 
The  change  begins  first  of  all  with  the  division  of  the  nuclear  threads;  from  them 
are  formed  numerous  short,  twisted  rod-like,  or  granular  portions,  which  journey 
towards  the  centre  of  the  nucleus,  take  up  a  position  there  corresponding  to  the 
equator  of  the  cell-nucleus,  which  may  be  compared  to  a  geographical  globe  (see  fig. 
1382),  and  arrange  themselves  into  a  plate  which  has  been  called  the  nuclear  plate. 
Soon,  however,  a  detachment  again  occurs  of  the  constituents  of  this  plate.  They 
separate  from  each  other,  each  fibril  splitting  into  two,  and  seek  the  poles  of  the 
spindle  (fig.  1383).  As  they  do  so  the  fibrils  turn  and  bend  themselves,  usually  so 
that  those  going  one  way  have  the  form  of  a  U,  and  in  the  other  of  n.  Arrived  at 
the  region  of  the  pole,  the  filamentous  portions  fuse,  contract  on  every  side  into  a 
dense  skein  (fig.  1384),  and  thus  from  one  cell-nucleus  two  nuclei  result. 

A  system  of  very  delicate  filaments  also  plays  a  part  in  these  movements  of  the 
elements  of  the  nuclear  plate.  These  filaments,  as  may  be  seen  in  fig.  138  2>  3>  4-  form 
a  spindle.  This  spindle  arises,  not  from  the  nucleus,  but  from  the  surrounding 
protoplasm.  The  spindle  appears  to  serve  the  nuclear  fibrils  for  support  and 
guidance  in  their  movements,  leading  the  fibrils  to  the  poles,  where  they  join 


VISIBLE    CONSTRUCTIVE   ACTIVITY   IN   PROTOPLASM. 


581 


together  again  into  two  fresh  nuclei.  After  they  have  performed  this  function, 
these  spindle  filaments  have  a  further  and  no  less  important  part  to  play.  Almost 
at  the  identical  place  where  the  nuclear  plate  was  previously  to  be  seen,  an 
accumulation  of  exceedingly  small  granules  arises,  the  repeatedly  mentioned  micro- 
somata;  and  these  are  arranged  to  form  a  plate,  the  so-called  cell-plate,  which 
ultimately  divides  the  whole  cell  into  two  compartments.  Apparently  these  spindle- 
threads  serve  also  as  conductors  to  these  microsomata,  and  many  of  the  small 
granules  are  conveyed  along  them  to  the  equator.  But  occasionally  they  arise 
there  directly,  and  help  to  produce  the  cell-plate.  The  development  of  the  cell- 
plate  does  not  seem  to  be  always  quite  the  same  in  different  species,  but  it  is 
established  with  certainty  that  in  it  cellulose  micellae  are  always  formed,  and  that 
the  partition-wall  produced  from  them  possesses  the  characteristics  of  a  cellulose 
wall,  that  is  to  say,  of  a  cell-wall.  Already  it  has  been  mentioned  (p.  44)  that  in 


Fig.  138.— Changes  in  the  Protoplasm  of  the  Cell-nucleus  during  its  Division. 


i  The  nuclear  fibrils  distributed  through  the  whole  nucleus.    2  The  broken-up  nuclear  fibrils  arranged  as  the  nuclear  plate. 
»  The  elements  of  the  plate  separating  from  one  another.    «  The  same  elements  forming  two  skeins  at  the  pol< 
spindle.    (After  Guignard.) 

this  cell- wall,  at  least  at  first,  albuminous  portions  of  protoplasm  are  retained  by 
means  of  which  the  intercalated  membrane  can  undergo  manifold  metamorphoses, 
and  that  by  them,  if  required,  the  communication  is  maintained  between  neighbour- 
ing masses  of  protoplasm. 

In  the  cells  of  those  green  water-threads  known  as  Spirogyra,  Zygnema,  and 
Cladophora,  as  well  as  in  those  of   desmids  and  many  other  simple  plants,  the 
plants  never  seem  to  come  to  an  end  of  this  dividing.     Each  cell  continues  to  grow 
until  it  has  attained  certain  dimensions;  it  then  divides  into  daughter-cells  in  the 
manner  peculiar  to  it,  and  in  these  the  process  which  has  been  performed  in  the 
parent  cell  is  repeated  afresh.     This  process  continues  perpetually  under  favourabl 
external  conditions,  and  an  interruption  occurs  only  when  there  is  lack  of  necessary 
food,  or  when  the  living  protoplasm  is  killed  by  unfavourable  circumstances, 
these  plants,  of  which  we  can  enumerate  more  than  a  thousand  different  spec 
there  is  thus  no  distinction  into  portions  in  course  of  formation  and  those  whi 
have  been  completed,  and  are  no  longer  capable  of  development.     It  i 
in  large  plants  in  which  a  division  of  labour  and  a  corresponding  organic 
have   taken   place,   in   those   plants   whose  different   members   perform    dil 
functions.     In  these,  stability  of  some  members  is  of  the  greatest  advantage,  and 


582  VISIBLE   CONSTRUCTIVE   ACTIVITY    IN   PROTOPLASM. 

accordingly  besides  the  cells  which  are  still  formative  and  promote  growth,  many- 
others  are  present  which  undergo  no  further  changes,  whose  size  and  shape  is 
permanently  retained,  and  which  have  therefore  been  termed  permanent  cells. 

Organically-connected  groups  of  permanent  cells  are  called  permanent  tissue  in 
opposition  to  the  groups  of  constructive,  dividing,  and  changing  cells,  the  so-called 
meristematic  tissue.  All  permanent  tissue  is  obviously  produced  from  meriste- 
matic  tissue,  and  the  meristem  is  ultimately  derivable  from  a  single  cell  capable  of 
division. 

The  cells  of  meristems  exhibit  only  very  slight  variations  in  form.  It  is 
impossible  to  recognize  what  forms  the  permanent  tissues  produced  from  them  will 
in  time  assume.  Of  four  exactly  similar  meristematic  cells,  the  first  may  become 
the  starting-point  of  several  flattened  epidermal  cells  devoid  of  chlorophyll;  the 
second  for  the  formation  of  a  group  of  green  palisade-cells;  the  third  for  a  bundle 
of  elongated,  thick- walled  bast-cells;  the  fourth  for  several  delicate- walled,  large 
parenchymatous  cells.  It  is  difficult  to  explain  how  this  comes  about,  and  we 
relinquish  the  attempt  to  give  a  full  explanation  here. .  Only  this  much  may  be 
remarked,  that  whilst  the  stimulus  to  these  metamorphoses  comes  from  outside,  and 
external  conditions  have  a  determinative  influence  on  the  size  of  the  developing 
permanent  tissue,  the  shape,  outline,  and  definite  configuration  which  the  indi- 
vidual cells  in  the  permanent  tissue  assume,  as  well  as  the  arrangement  of  the 
various  cells  in  space,  are  independent  of  external  influences.  Just  as  in  a  plant  the 
first  division-walls  assume  a  position  defined  beforehand  in  the  dividing  apical-cell, 
the  further  metamorphoses  of  the  daughter-cells  proceed  within  the  limits  settled 
by  the  specific  constitution  of  the  protoplasm,  so  the  transformation  of  the  cells  of 
the  meristem  into  cells  of  permanent  tissue  is  governed  according  to  a  plan  of 
construction  peculiar  to,  and  hereditary  in,  each  species. 

This  law,  derived  from  numerous  facts,  of  the  independence  to  external 
influences  of  the  constructive  plan  and  character  of  the  cells,  seems  to  be  con- 
tradicted by  the  fact  that  alteration  in  the  outline  of  individual  cells  can  be 
produced  by  strain  and  pressure.  Spherical  cells  with  elastic,  flexible  walls  may  be 
changed  by  strain  into  ellipsoids;  in  consequence  of  all-round  pressure  a  spherical 
cell  may  assume  the  form  of  a  rhombic  dodecahedron,  or  by  lateral  pressure,  the 
form  of  a  six-sided  prism.  In  explanation  of  these  conditions  it  has  been  pointed 
out  that  peas  which  are  made  to  swell  up  in  a  cubical,  thick-walled  vessel,  by 
pouring  water  over  them,  assume  the  shape  of  rhombic  dodecahedra,  because  each 
individual  pea  is  in  this  way  allowed  the  greatest  possible  room  together  with  the 
utmost  economy  of  space.  We  are  again  reminded  of  the  fact  that  the  structure  of 
slate-like  stones  is  dependent  upon  the  pressure  acting  upon  the  mass  so  far  at  least 
that  the  planes  of  cleavage  and  stratification  are  always  at  right  angles  to  the 
direction  of  the  pressure.  But  however  valuable  these  facts  are  in  the  explanation 
of  the  condition  of  the  form  of  inorganic  bodies,  they  are  of  little  significance  to  the 
question  in  hand.  No  one  will  deny  that  spherical  cells  on  which  an  equal  pressure 
operates  from  all  sides  may  assume  the  shape  of  dodecahedra,  but  this  form  is  not 


VISIBLE   CONSTRUCTIVE   ACTIVITY   IN    PROTOPLASM.  583 

transmitted  to  the  descendants.  In  the  next  generation  of  these  same  plants  it  is 
a  group  of  spherical  and  not  of  dodecahedral  cells,  which  arises  at  the  particular 
place.  The  latter  will,  indeed,  only  reappear  if  the  aforesaid  pressures  be  again 
exerted. 

How  little,  however,  external  influences  define  the  form  and  grouping  of 
permanent  cells,  is  shown  by  the  fact  that  from  one  and  the  same  meristem,  under 
the  same  pressure,  the  same  temperature,  and  equal  illumination,  arise  in  the  closest 
proximity  the  most  different  permanent  cells;  and  that,  on  the  other  hand,  the 
formation  and  grouping  of  these  cells  is  not  essentially  different  when  the  develop- 
ment of  the  meristem  takes  place  under  wholly  different  external  pressure,  or 
different  temperature.  We  always  come  back  to  this  important  thesis: — the  forces 
operating  on  plants  from  outside  are  only  stimuli  to  the  formative  processes; 
these  latter  are  accomplished  independently  of  external  influences  in  a  manner 
established  for  each  species,  and  founded  in  the  specific  construction  of  its  living 
protoplasm. 


PLANT-FORMS  AS  COMPLETED  STRUCTURES. 


1.— PROGRESSIVE  STAGES  IN   COMPLEXITY  OF 
STRUCTURE  FROM  UNICELLULAR  PLANTS   TO   PLANT-BODIES. 

Though  all  plants  are  mortal,  they  have  the  capacity  of  renewing  themselves 
and  rejuvenating,  so  that,  in  spite  of  their  perishable  nature  and  limited 
duration,  the  species  now  existing  on  the  earth  are  in  no  danger  of  extinction. 
The  rejuvenescence  is  always  effected  by  means  of  the  protoplasm  of  a  single 
cell;  i.e.  by  a  small  mass  of  slimy  substance  which  can  only  be  perceived  by 
the  naked  eye  in  the  rarest  instances  on  account  of  its  minute  dimensions. 
The  largest  palm  in  its  rejuvenescence  must  pass  through  this  unicellular  stage 
exactly  in  the  same  way  as  the  smallest  of  mould-fungi.  The  only  difference  is 
that  in  large  and  usually  long-lived  plants  a  longer  time  elapses  before  this 
stage  is  reached,  while  in  small  plant-forms  many  generations  may  pass  away 
and  be  replaced  in  the  course  of  a  single  year.  The  protoplasm  in  the  minute 
rejuvenating  cell  always  grows  at  the  expense  of  its  surroundings,  moulds  itself 
in  the  manner  peculiar  to  its  species,  and  divides  when  it  has  attained  to  a 
certain  size  into  two  or  more  masses,  which  have  inherited  the  capacity  of 
dividing  afresh. 

Each  one  of  these  protoplasmic  masses  is  to  be  regarded  as  an  individual. 
When  the  adjacent  masses  of  protoplasm,  the  result  of  continued  division,  remain 
in  connection  with  one  another,  as  indeed  seems  to  be  usually  the  case,  each 
retains  a  certain  degree  of  independence;  nor,  should  a  severance  take  place, 
are  they  necessarily  abandoned  to  destruction.  Under  favourable  conditions 
they  may,  although  separated  from  their  companions,  enlarge,  divide,  and 
continue  to  grow.  In  not  a  few  unicellular  plants  it  is  even  customary  for 
each  mass  of  protoplasm  immediately  after  its  formation  to  separate  itself 
entirely,  and,  for  the  future,  to  live  independently.  It  is  remarkable  that 
in  most  of  these  unicellular  plants  a  time  arrives,  i.e.  the  time  of  pairing, 
when  they  again  seek  each  other  with  the  view  of  uniting;  but  this  period  is 
of  short  duration  compared  with  the  length  of  the  isolated  life.  Moreover, 
a  definite  bond  of  union  has  been  recognized  between  the  separate  individuals 
produced  from  one  cell.  Just  as  caterpillars  which  creep  out  of  the  eggs 
laid  by  a  butterfly  are  seen  not  to  disperse,  but  to  follow  common  paths  and 
.ways,  so  the  swarm-spores  of  Sphcerella  pluvialis  swim  together  in  groups 
from  one  place  to  another,  and  select  a  suitable  spot  for  settling  down.  The 


584 


PROGRESSIVE   STAGES   IN   COMPLEXITY   OF   STRUCTURE.  585 

single  cells  of  diatoms  and  desmids  form  similar  social  groups  living  within 
restricted  areas.  We  must  suppose  that  here — exactly  as  in  the  young  brood 
produced  from  the  spawn  of  a  fish,  which  swim  in  company  through  the 
water,  or  in  midges  hatched  simultaneously,  which  dance  up  and  down  in  the 
evening  sun — here  must  be  some  kind  of  family  feeling  which  binds  the  different 
individuals  together,  although  we  cannot  fully  comprehend  these  relations  between 
the  several  organisms. 

When  the  single  genetically-connected  masses  of  protoplasm,  each  retaining 
its  own  individuality,  can  transfer  themselves  in  common  from  one  place  to 
another,  like  caterpillars,  midges,  grasshoppers,  fishes  and  the  like,  the  com- 
munity is  called  a  swarm;  if,  on  the  other  hand,  the  isolated  individuals  settle 
quite  close  to  one  another  on  a  substratum,  and  there  take  up  a  definite  position 
for  their  lifetime,  then  we  speak  of  a  colony.  The  amoeboid  bodies  of  some 
myxomycetes,  several  unicellular  Palmellaceae,  Desmidiese,  and  Diatomaceae,  live 
in  swarms;  the  numerous  Siphoneae,  on  the  other  hand,  as  well  as  the  species 
of  the  genera  Synedra  and  Gomphonema  belonging  to  the  family  of  the  Dia- 
tomacese,  dwell  in  colonies  (cf.  vol.  II.,  fig.  369 T  and  369 14).  These  colonies  often 
attain  to  considerable  dimensions.  The  Acetabularias  attached  to  stones  and 
mussel-shells  at  the  bottom  of  the  sea,  the  swollen  bladder-like  Valonias,  the 
moss-like  forms  of  Bryopsis,  and  the  dusky  species  of  Codium,  form,  arranged 
in  thousands  side  by  side,  very  extensive  colonies.  The  Vaucherias,  dwelling  on 
damp  earth  and  in  cold  springs,  appear  as  extensive  cushions  which  cover  the 
ground  over  a  wide  distance  with  a  green  felt.  Besides  the  swarms  and  colonies, 
we  have  a  third  form  of  assemblage,  the  cell-union,  in  which  the  genetically-con- 
nected masses  of  protoplasm  grow  together  /n  a  body.  This  union,  again,  differs 
essentially  according  as  to  whether  the  individual  masses  of  protoplasm  forming 
it  are  devoid  of,  or  are  surrounded  by,  a  cell-wall.  In  the  former  case  they  are 
fused  into  a  mass  in  which  the  limits  of  the  single  individuals  can  no  longer 
be  recognized,  as  is  the  case,  for  example,  in  many  myxomycetes.  The  expres- 
sion "fusion"  can  here  be  employed  figuratively  with  the  utmost  propriety,  for 
indeed  the  process  strongly  resembles  the  fusing  of  fluid  metallic  globules 
into  a  larger  mass,  or  the  fusing  of  numerous  drops  of  oil  floating  on  the 
surface  of  water  into  a  larger  drop,  in  which  the  contours  of  the  single  fused 
portions  are  obliterated.  It  is  indeed  doubtful  whether  the  fused  masses  of 
protoplasm  do  actually  surrender  their  individuality.  Certain  phenomena  tell 
rather  against  than  for  this  view.  Thus  many  myxomycetes  form  so-called 
sclerotia,  i.e.  they  lose  their  mobility  and  pass  into  a  temporary  state  of  rest. 
The  whole  mass  becomes  rigid,  assumes  a  wax-like  consistency,  and  dries  up, 
and  the  shapeless  protoplasm  divides  into  innumerable,  clearly-defined,  rounded  or 
angular  particles.  When  at  the  end  of  the  resting  period  the  stiffened  mass  is 
to  be  again  transformed  into  the  mobile  condition,  the  individualized  particles 
become  fluid  and  a  fresh  fusion  takes  place.  The  phenomenon  observed  in  the 
whole  series  of  myxomycetes  suggests  the  idea  that  the  isolated  corpuscles  in 


586  PKOGRESSIVE   STAGES   IN   COMPLEXITY   OF   STRUCTURE. 

the  sclerotia  correspond  to  the  single  masses  of  protoplasm  from  which  the  whole 
mass  had  previously  been  formed,  and  that  these  have  not  surrendered  their 
individuality,  although  their  boundaries  cannot  be  recognized  in  the  mass.  The 
unions  of  fused  masses  of  protoplasm  devoid  of  cell-wall  are  inconsiderable  in 
number  in  comparison  with  the  enormous  quantity  of  those  combinations  in 
which  each  portion  of  protoplasm  is  surrounded  by  a  cell-wall,  by  means  of 
which  the  cohesion  of  the  whole  is  brought  about.  The  latter  are  classed  as 
cell-complexes  or  tissues  and  are  for  the  sake  of  clearness  divisible  into  four 
groups,  which  may  be  distinguished  as  rows,  nets,  plates,  and  masses. 

Its  name  tells  us  what  a  filamentous  cell-complex  looks  like.  As  regards 
its  production,  it  is  to  be  noticed  that  the  partition- walls,  which  are  formed  by 
the  segmentation  of  its  cells,  always  stand  at  right  angles  to  the  long  axis  of 
the  cell-filament,  and  are  therefore  parallel  to  one  another.  The  general  appear- 
ance of  this  tissue  is  regulated  according  to  the  particular  shape  of  the  single 
cells.  If  the  individual  members  of  the  row  are  spherical,  chain-like  strings 
of  pearls  are  produced,  such  as  are  found  in  the  Nostocacese;  if  the  individual 
cells  are  cylindrical,  long  or  short,  then  thread-like  structures  arise  from  their 
end-to-end  arrangement,  as  may  be  frequently  observed  in  the  Zygnemese  and  the 
CEdogonieae.  If  the  cylindrical  cells  decrease  in  thickness  as  the  filament 
increases  in  length  in  one  direction,  whip-like  forms  arise,  as,  for  example,  in 
the  species  of  the  genus  Mastichonema.  Occasionally  the  single  members  of 
the  row  are  tabular,  and  the  tablets  are  joined  to  one  another  by  their  narrow 
edges,  in  which  case  ribbon-like  rows  are  produced,  as  in  Odontidium]  or  the 
neighbouring  tabular  cells  are  only  connected  by  their  corners,  in  which  case 
the  row  has  a  zigzag  appearance,  as  in  the  genus  Diatoma  (c.f.  vol.  II.  fig.  369  15). 

In  the  reticular  cell-complexes  the  numerous  cells  are  seen  to  be  so  arranged 
that  they  adjoin  one  another  by  comparatively  small  contiguous  surfaces, 
joining  together  at  two  or  three,  more  rarely  four,  angles  of  corresponding  size. 
The  partition -walls  intercalated  during  the  division  are  not  all  parallel  with 
one  another,  but  are  arranged  in  more  than  one  dimension  of  space.  Nets 
may  be  distinguished  as  open  and  closed.  In  the  former,  which  may  be  best 
compared  with  the  net-work  of  rivers  on  a  map,  the  cells  only  seldom  form 
closed  meshes,  but  start  out  from  one  another  like  the  prongs  of  a  fork.  Open 
nets  occur  very  often,  especially  in  the  mycelia  of  fungi,  in  species  of  the  green 
Confervoidese  living  in  water  (Cladophora  and  Chaetophora)  and  in  numerous 
red  Floridese.  Much  rarer  are  the  closed  nets  with  hexagonal  meshes,  as,  for 
example,  those  of  the  Water-net  (Hydrodictyori)  described  on  p.  36,  and  the 
remarkable  nets  of  Volvox  globator,  comparable  to  hollow  spheres,  which  were 
considered  on  p.  37.  Open  reticular  cell -complexes  permeate  the  decayed 
trunks  of  trees,  the  mould  of  the  forest-soil,  and  the  humus  of  the  meadow- 
ground.  Here  they  exist  as  saprophytes,  or  on  living  plants  and  animals  as 
parasites ;  or  they  are  only  attached  by  a  few  cells  to  the  substratum,  and  the 
forked  ramifications  stretch  out  from  these  starting-points  like  fans  and  radiate 


PROGRESSIVE   STAGES   IN   COMPLEXITY   OF   STRUCTURE.  587 

forth  into  the  surrounding  water,  as  in  most  water-plants  belonging  to  this  cate- 
gory. The  closed  nets,  on  the  other  hand,  are  never  joined  to  a  substratum,  but 
remain  floating  in  the  water  from  which  they  derive  their  nourishment. 

The  plate-like  cell-complexes  are  composed  of  cells  arranged  in  one  plane, 
and  adjoining  one  another  so  as  to  leave  no  intercellular  spaces.  The  partition- 
walls  inserted  in  the  separate  chambers  during  the  development  of  this  form 
are  arranged  in  two  dimensions  of  space,  and  frequently  intersect  at  right 
angles.  These  cell-complexes  either  form  a  thin  coating  on  stones  or  other 
solid  bodies,  and  then  adapt  themselves  closely  to  all  inequalities  of  the  sub- 
stratum, as  e.g.  in  Protoderma  viride,  which  covers  the  stones  and  old  tree- 
trunks  in  mountain  -  brooks ;  or  they  appear  as  membranes,  ribbons,  and 
delicate  leaf-like  structures,  which  are  attached  to  the  substratum  only  at  one 
point,  and  for  the  rest  float  freely  in  the  water.  This  is  what  occurs  in  the 
Sea  Lettuce  ( Ulva),  and  in  many  Floridese,  as,  for  example,  Porphyra.  Sometimes 
the  plate-shaped  complexes  are  developed  as  quite  independent,  unattached 
tablets  and  discs,  as  in  the  genus  Pediastrum  (c.f.  vol.  II.  fig.  197 6).  The 
leaf  and  ribbon-like  forms  which  float  in  water  are  but  seldom  quite  flat; 
usually  they  appear  much  bent,  undulated,  and  pitted;  the  margin,  also,  is 
generally  crinkled  or  slit,  and  divided  into  lobes  and  fringes,  and  these  forms 
thus  furnish  transitional  stages,  half  cell-plates,  and  half  cell-nets.  In  the  matter 
of  size,  all  possible  gradations  are  to  be  found,  from  the  minute  discs  of  Pedi- 
astrum, and  the  small  membranes  of  Prasiola  flourishing  in  glacier-streams,  up 
to  the  Ulvas,  living  in  the  sea,  many  of  which  grow  up  into  membranes  a  square 
metre  in  area. 

Mass-like  cell -complexes  are  those  whose  constituents  adjoin  one  another 
in  three  dimensions  of  space.  Both  in  transverse  and  longitudinal  sections 
of  their  tissues  we  have  at  least  two,  but  as  a  rule  several  stratified  cell-layers. 
Usually  the  whole  body  is  elongated  much  more  in  one  direction  than  in  the 
others;  frequently  it  has  the  shape  of  a  solid  cylinder  or  prism,  or  it  forms 
thick  fibres,  cords,  and  ropes.  Many  remind  one  of  earth-worms,  or  they 
resemble  the  tentacles  of  polyps  and  sea -anemones.  In  many  Florid  eae,  and 
especially  in  the  brown  leathery  sea -wracks,  these  cell -complexes  are  strap- 
shaped,  or  they  are  contracted  into  a  stalk  below,  where  they  are  attached  to 
the  substratum,  and  above  widen  out  into  leaf-like  structures,  as,  for  example, 
in  the  Laminarias  of  the  North  Sea  (see  fig.  139),  and  in  many  other  cases. 
These  strap-like,  ribbon-like,  and  leaf-like  structures  occasionally  remind  one 
of  the  similar  plate-shaped  cell-tissues  of  Ulvacese,  previously  mentioned,  but 
are  distinguished  from  them  by  the  fact  that  they  are  always  built  up  of  two 
or  more  stratified  layers  of  cells,  so  that  a  section  taken  at  right  angles  to  the 
leaf-like  structure  always  exhibits  two  cell-layers  at  least.  Cake-like  and  ball- 
shaped  tissues  are  rarer.  As  examples  of  the  latter  various  species  of 
Glceocapsa  may  be  instanced,  one  of  which  is  illustrated  in  fig.  25A,  n. 

In  most  of  these  simple  cell-complexes  the  bulk  of  the  cells  are  shaped  similarly. 


588 


PROGRESSIVE   STAGES   IN   COMPLEXITY   OF   STRUCTURE. 


Usually  only  the  portions  serving  for  reproduction  exhibit  differences  of  shape,  and 
these  are  so  subordinate  in  number  and  extent  that  the  appearance  of  the  whole  is 
very  little  altered  whether  they  are  present  or  not.  It  is  of  more  importance  with 
regard  to  the  general  appearance,  that  most  of  the  simple  tissues  enumerated, 


Fig.  139.—  Laminarias  in  the  North  Sea. 

multiply  and  divide  without  the  portions  thus  produced  becoming  separate  or 
detached.  The  nets  of  Hydrodictyon  indeed  multiply  by  the  formation  of 
daughter-nets  within  single  cells,  which  then  become  detached  from  the  parent 
plant.  The  disc-shaped  plates  of  Pediastrum  also  multiply  in  a  similar  manner,  and 
in  this  sort  of  plant  whole  swarms  of  cell-complexes  are  always  developed,  so  that 


PROGRESSIVE    STAGES   IN    COMPLEXITY   OF   STRUCTURE.  589 

in  the  pools  where  these  species  grow  hundreds  and  thousands  of  separate  nets  are 
to  be  found  living  together  within  a  limited  area.  But  the  number  of  instances 
of  swarm-forming  cell-complexes  is,  however,  utterly  insignificant  in  comparison 
with  the  enormous  number  of  those  forms  in  which  the  tissues  arising  by  re- 
juvenescence remain  connected.  We  call  these  permanently-connected  cell-tissues 
systems;  and  distinguish  between  systems  of  cell-rows,  cell-nets,  cell-plates,  and 
cell-masses.  The  arrangement  of  the  individual  parts,  and  the  fitting  together 
of  the  systems  is  quite  irregular,  but  is  defined  for  each  plant-species  in  the 
established  manner,  inherited  from  generation  to  generation.  The  simple  cell- 
tissues  which  build  up  an  extensive  system  can  be  distinguished  as  separate  parts, 
and  may  be  compared  to  the  members  of  a  body,  and  even  called  members  of  that 
system.  There  are,  of  course,  systems  which  consist  of  very  many  simple  cell- 
tissues,  and  therefore  have  a  much-membered  appearance;  and  others  which  exhibit 
only  a  slight  organization,  i.e.  are  built  up  of  only  a  few  simple  tissues.  Setting 
aside  the  question  of  greater  or  smaller,  the  kind  and  manner  of  connection  must 
be  taken  into  consideration  in  a  general  review  of  the  forms  of  plants,  and  these 
systems  can  be  comprehended  under  two  divisions. 

The  first  division  comprises  those  whose  members  (i.e.  cell-complexes)  are  all 
of  similar  shape,  so  that  the  whole  plant-body  consists  only  either  of  cell-filaments, 
or  of  cell-nets,  or  of  cell-plates,  or,  finally,  of  cell-masses.  These  uniform  systems 
are  found  more  especially  in  plants  growing  under  water,  which  reproduce  them- 
selves by  spores,  as  well  as  in  fungi,  and  the  commonest  forms  to  be  noticed  are  as 
follows: — first,  the  clusters  of  tortuously  twisted  and  intertwined  rows  of  cells,  like 
strings  of  pearls,  such  as  occur  in  the  Nostocaceae,  the  bundles  of  elongated, 
straight  filamentous  rows  of  the  Oscillatorieae,  the  flock-like  Scytonema  and 
other  aquatic  plants,  and  the  dark  cushions  of  whip-like  rows  grouped  in  bundles, 
as  shown  in  the  genera  Euactis  and  Dasyactis.  Among  the  series  of  complex 
systems  a  particular  interest  is  claimed  by  those  which  are  produced  from  the 
frequently-mentioned  hyphse.  When  the  branched  hyphae,  often  knitted  into 
meshes,  and  united  into  net-works,  are  crowded  together  in  great  numbers,  plexuses 
and  strands  arise  which  have  exactly  the  appearance  of  a  cell-mass,  but  which 
may  be  distinguished  therefrom  by  the  fact  that  neighbouring  cells,  whose  sides 
adjoin  one  another,  are  not  produced  by  the  intercalation  of  partition- walls.  The 
fungal  hyphse  have  a  common  development  and  manner  of  growth;  hundreds  of 
hyphal  threads  which  are  joined  together  into  a  strand  or  plexus  continue  to  grow 
at  the  apices  with  equal  rapidity  and  in  the  same  direction;  they  carry  out  in  com- 
mon the  same  curves  and  twistings,  often  divide  into  single  threads,  then  reunite, 
and  thus  form  the  most  peculiar  shapes.  The  so-called  Hercules-club  (Coryne 
pistillaris),  the  strange  forms  of  Clavaria,  resembling  pieces  of  coral,  the  Cap-fungi, 
divided  into  cap  and  stalk,  the  Helvellas  and  Morels,  the  very  peculiar  puff-balls 
and  earth-stars,  and  many  other  forms  are  built  up  of  hyphal  strands  and  plexuses, 
which,  as  already  stated,  are  nothing  else  but  conglomerated  cell-nets.  Systems  of 
cell-plates  are  more  rarely  to  be  met  with.  This  construction  is  found  most 


PROGRESSIVE   STAGES    IN    COMPLEXITY   OF   STRUCTURE. 

noticeably  in  the  marine  Padina  Pavonia,  older  species  of  which  are  composed  of 
superimposed,  thin,  leaf-like  cell-plates.  Systems  of  cell-masses  are  found  in 
many  Floridese  and  especially  in  the  large  brown  sea- wracks,  Cystosira,  Sargas- 
sum,  and  Fwcus.  The  separate  cell-complexes,  which  form  a  system  in  these 
plants,  frequently  assume  the  form  of  leaves,  and  these  sea- wracks  are  occasionally 
ranked  with  leafy  plants,  which  will  be  described  later.  Hydrurus  and  the 
Stonewort  (Chara)  are  systems  of  cell-complexes;  but  while  the  individual  com- 
plexes in  Hydrurus  are  connected  very  irregularly,  they  exhibit  in  Chara  an 
extremely  regular,  geometrical,  whorled  arrangement  (cf.  vol.  II.,  fig.  206  1). 

Following  this  first  division  of  systems,  which  have  a  uniform  construction,  is 
the  second,  in  which  the  body  is  built  up  of  different  kinds  of  cell-complexes. 
These  are  called  mixed  systems.  Each  member  of  such  a  mixed  system,  regarded 
by  itself,  exhibits  a  simple  homogeneous  cell-tissue;  but  the  simple  complexes  are 
so  combined  that  in  one  case  cell-rows  are  carried  by  a  cell-plate,  while  in  another 
case  a  cell-mass  forms  the  starting-point  for  several  open  cell-nets,  and  so  forth. 
All  possible  combinations  are  realized  in  nature,  but  none  more  frequently  than 
that  in  which  a  cylindrical  cell-mass  forms  the  centre  or  axis  of  the  whole  plant- 
body,  whilst  cell-plates  or  nets  are  laterally  articulated.  In  many  sorts  of  Bat- 
rachospermum  open  nets  are  seen  which  are  borne  on  a  central  pillar  of  cell-masses; 
and  in  a  liverwort  (Jungermannia  trichophylla)  the  same  thing  occurs,  except 
that  here  there  are  cell-rows  which  proceed  from  the  lower  parts  of  the  central 
mass  (fig.  1406).  Many  mosses  and  liverworts  (e.g.  Hooker ia  splendens  and 
Jungermannia  polyanthos)  exhibit  a  stem-like  central  tissue  which  does  not  carry 
cell-nets,  but  single-layered  cell-plates.  As  shown  in  the  illustration  opposite, 
all  possible  stages  are  to  be  seen  in  moss  vegetation  between  central  supports 
provided  with  cell-nets,  and  those  with  cell-plates;  this  must  be  particularly  noted 
here,  in  order  to  establish  the  fact  that  all  classifications  and  distinctions  based 
upon  external  forms  are  really  only  artificial,  and  that  sharp  limits  between 
the  various  forms  do  not  exist.  Still  it  conduces  to  clearness,  none  the  less,  if 
we  collect  together  and  classify  the  various  forms  as  well  as  we  can.  The  mixed 
systems  which  are  represented  by  the  liverworts,  illustrated  here,  claim  an 
especial  interest,  inasmuch  as  they  are  to  a  certain  extent  the  prototypes  of  plant- 
bodies,  i.e.  of  those  complex  forms  which  botanists  in  earlier  times  alone  recognized 
when  speaking  of  the  configuration  of  plants;  these  alone  were  considered,  for 
example,  in  Goethe's  Theory  of  Metamorphosis. 

The  Plant-body  is  always  membered,  and  each  of  its  members  is  composed  of 
cell-complexes  of  the  most  varied  kinds.  In  this  lies  the  distinction  between 
plant-bodies  and  the  previously-described  forms.  The  members  of  a  simple  and 
of  a  mixed  system  are  simple  cell-complexes: — cell-rows,  cell-plates,  and  the  like. 
The  members  of  a  plant-body  are,  on  the  other  hand,  combinations  of  cell-rows, 
cell-nets,  cell-plates,  and  cell-masses.  The  cell-complexes  combined  in  a  member 
of  a  plant-body  are  connected  from  their  first  origin.  A  single  cell  is  always  the 
starting-point  for  the  particular  member  of  the  body;  this  divides;  the  compart- 


PROGRESSIVE  STAGES   IN   COMPLEXITY  OF  STRUCTURE.  591 

ments  are  again  divided,  and  from  the  single  compartments  (i.e.  cells)  originate 
here  plate-like,  there  mass-like  complexes,  in  this  place  cell-rows,  in  that  cell-nets; 
these,  however,  are  not  isolated,  but  remain  joined  and  produce  small,  wonderfully- 
arranged  structures.  The  result  of  this  modelling  process  is  therefore  a  plant- 
member  composed  of  varied  cell-complexes,  with  a  definite  internal  structure,  with 
definite  external  contour,  and  also  with  entirely  definite  functions  in  the  economy 
of  the  plant.  In  spite  of  the  variety  of  shape  which  plant-members,  formed 
from  various  constituent  cell-tissues,  exhibit  in  the  many  thousands  of  plant- 
species  which  develop  into  bodies,  they  can  yet  be  referred  to  a  few  fundamental 
forms,  viz.,  to  the  leaf,  stem,  and  root.  These  members  of  the  plant-body  are  in 


4   Hi  5 

Fig.  140.— Liverworts  with  Cell-nets,  Cell-plates  and  Cell-rows  in  various  transitional  forms. 

»  Jungermannia  pumila.     2  Jungermannia  quinquedentata.     »  Polyotus  magellanicus.     *  Ptilidium  ciliare.     «  Trichocolea 
tomentella.    «  Jungermannia  trichophylla.    (All  the  figures  magnified.) 

most  cases  so  arranged  that  a  stem  represents  the  starting-point  and  support  of 
many  leaves  and  roots.  In  the  simplest  form  the  plant-body  appears  as  embryo 
and  as  bud.  The  latter  consists  of  a  very  short  stem,  beset  with  leaves  lying 
•closely  above  one  another,  and  grows  later  into  a  shoot  which  agrees  in  structure 
with  the  parent  plant  producing  the  bud,  of  which  it  actually  forms  a  replica. 
If  th,e  young  body  remain  connected  with  the  old,  it  is  called  a  branch;  the 
branches  may  again  form  buds,  and  these,  again,  twigs;  and  in  this  way  originate 
much-branched  plant-structures  which  often  attain  to  considerable  dimensions, 
and  must  be  regarded  as  compound.  In  rare  instances  the  laterally-inserted  buds 
are  detached  from  the  body  producing  them,  before  they  develop  further;  and 
these  buds,  which  are  known  as  bulbils,  give  rise  to  an  independent  plant-body. 
This  process  reminds  one  of  the  swarm  formation  of  cell-complexes  which  has 
been  spoken  of  above. 

This  is  also  the  place  to  notice  the  analogy  between  vegetable  and  animal 


592  PROGRESSIVE   STAGES   IN   COMPLEXITY   OF   STRUCTURE. 

bodies.  In  polyp-colonies,  the  individual  polyps  formed  by  budding  remain  in 
connection  with  the  parent-animal,  and  behave  accordingly  like  the  branches  of  a 
compound  plant-body.  Yet  between  the  parts  there  exists  this  remarkable 
reciprocal  relation,  that  the  digestive  cavities  of  individual  polyps  communicate 
with  one  another,  and  that  the  liquids  acquired  by  the  individuals  are  at  the 
common  service  of  the  colony.  This  connection  of  the  individual  parts  by  com- 
municating, sap-conducting  channels  also  exists  in  plant-bodies.  We  call  these 
conducting  channels  vascular  bundles,  and  have  already  had  repeated  occasion 
to  speak  of  them.  They  are  a  peculiarity  of  plant-bodies,  and  are  absent  in  all 
other  forms  of  cell-unions,  even  in  mixed  systems,  many  of  which  have  a  great 
resemblance  to  true  plant-bodies,  as,  for  example,  the  mosses.  The  difference 
existing  in  this  respect  was  the  reason  for  placing  the  plants  in  two  great  divisions 
in  respect  of  their  construction — into  (1)  the  group  of  those  in  which  vascular 
bundles  are  present  as  architectural  elements  in  their  bodies,  and  (2)  that  in  which 
this  form  of  cell-system  is  absent.  The  former,  which  are  called  vascular  plants, 
form  a  natural  group;  the  latter,  which  are  called  Thallophytes,  are,  on  the  other 
hand,  classed  quite  unsuitably.  By  "thallus"  we  understand  the  most  different 
vegetable  structures  which  are  devoid  of  vascular  bundles,  that  is  to  say,  not  only 
all  possible  tissues  and  systems,  but  also  the  masses  of  myxomycetes,  even  the 
colonies  and  swarms  of  unicellular  plants,  structures  which  could  not  differ  more 
widely  in  constitution. 

It  is  a  remarkable  phenomenon  that  the  majority  of  aquatic  plants  are  devoid 
of  vascular  bundles,  and  therefore,  according  to  the  older  signification,  belong  to  the 
Thallophytes;  and,  on  the  other  hand,  that  those  plants  which  have  assumed  the 
shape  of  plant-bodies  with  vascular  bundles,  belong  almost  entirely  to  land-plants. 
This  difference  can  be  more  accurately  formulated  as  follows: — plants  which 
throughout  their  life,  or  at  the  time  when  they  absorb  nourishment,  are  surrounded 
by  water,  saprophytes  which  are  wholly  imbedded  in  humus,  and  parasites 
which  are  situated  entirely  within  their  hosts,  absorb  nourishment  by  the  whole  of 
their  superficial  cells.  Such  structures  do  not  require  common  sap-conducting 
mechanisms,  penetrating  and  connecting  the  several  members.  Those  plants,  on  the 
other  hand,  whose  shoots  are  surrounded  by  air;  which  derive  their  fluid  food  from 
the  soil,  and  have  to  conduct  it  to  the  aerial  organs,  especially  the  leaves;  which 
finally  conduct  to  the  growing  parts  in  a  fluid  form  the  organic  compounds  manu- 
factured in  the  green  tissues  in  sunlight;  such  plants  require  special  transmitting 
mechanisms,  and  as  such,  vascular  bundles  are  developed  in  all  land-plants.  It  is 
necessary  for  the  stability  of  the  conducting  mechanisms  that  the  cells  and  vessels 
in  question  should  be  lignified,  or  that  so-called  mechanical  cells,  i.e.  hard  bast, 
should  be  placed  near  or  in  contact  with  them.  Thus  it  is  again  made  evident 
that  there  is  a  difference  between  water-plants  and  land-plants  in  the  matter 
of  rigidity.  The  numerous  submerged  plants  do  not  possess  woody  and  bast 
cells,  while  these  are  always  abundantly  developed  in  land-plants,  and  to  a 
greater  extent  the  more  the  plant  in  question  requires  in  its  natural  habitat  to 


DEFINITION    AND   CLASSIFICATION    OF   LEAVES.  593 

resist  strains  and  bending  pressures.  Just  as  we  distinguish  soft,  pulpy  animals 
from  such  as  are  provided  with  skeletons,  so  also  we  distinguish  soft  plants,  without 
wood  and  hard  bast,  from  hard  plants  possessing  these  tissues.  I  would  only  point 
out  this  analogy  in  passing,  and  avoid  entering  into  any  further  discussion  upon  it 
lest  thereby  misconceptions  might  arise.  In  discussing  the  hypotheses  relating  to 
the  history  of  development  of  the  whole  vegetable  kingdom  in  the  second  volume, 
I  shall  take  the  opportunity  to  return  to  these  analogies,  as  well  as  to  the  relation 
of  the  habitat  to  the  structure  and  form  of  plants.  There  the  speculations  about 
the  evolution  of  plants  on  the  ground  of  the  comparison  here  only  indicated  will 
receive  an  impartial  consideration.  In  this  place,  however,  such  discussions  would 
be  premature,  and  our  remarks  might  share  the  same  fate  as  the  speculations  of 
the  Nature-Philosophers  of  which  examples  were  quoted  on  p.  13  of  the  Intro- 
duction. 


2.    FOEM   OF   LEAF-STRUCTURES. 

Definition  and  classification  of  Leaves.— Cotyledons.— Scale-leaves,  Foliage-leaves,  Floral-leaves. 

DEFINITION   AND    CLASSIFICATION    OF   LEAVES. 

— T  is  written — "  IN  THE  BEGINNING  WAS  THE  WORD." — 

Already  at  a  stand— and  how  proceed ! 

Who  helps  me?     Is  the  WORD  to  have  such  value, 

Impossible — if  by  the  spirit  guided. 

Once  more— "!N  THE  BEGINNING  WAS  THE  THOUGHT." — 

Consider  the  line  first  attentively, 

Lest  hurrying  on  the  pen  outrun  the  meaning. 

Is  it  Thought  that  works  in  all,  and  that  makes  all? 

—It  should  stand  rather  thus :—"  IN  THE  BKO  INNING 

WAS  THE  POWER."— yet  even  as  I  am  writing  this 

A  something  warns  me  we  cannot  rest  there. 

Of  this  speech— which  Goethe  puts  into  the  mouth  of  Faust— the  naturalist  is 
involuntarily  reminded  when  he  attempts  to  explain  terms  which  popular  language 
from  time  immemorial  has  associated  with  certain  ideas.     These  terms  have  later 
gained  admission  into  scientific  terminology,  and  here,  once  adopted,  have  gradually 
been   employed   to   indicate   things  which  no  longer  correspond  to  the   original 
current  notions.     Whosoever  introduced  into  common  language  the  words  "leaf", 
"stem",  and  "root",  little  suspected   how  difficult  it  would  come  to  be,  to  say, 
shortly  and  exclusively,  what  botanists  mean  by  these  designations-to  write  down 
what  the  man  of  science  understands  by  a  leaf,  a  stem,  a  root;  nor  did  he  surmis, 
that  over  the  question  as  to  whether  or  not  certain  plant-structures  should 
regarded  as  leaves,  and  should  be  so  named,  continuous  eager  strife  womd 
amongst   the  learned,  and  that  the  polemic  writings  on  this  matter,  if  care* 
collected,  would  fill  a  book  much  more  extensive  than  the  present  one,  in  wto 
am  attempting  to  describe  the  life  of  the  whole  Vegetable  World.  ^ 

VOL.  I. 


M.riMTloN    AND    CLASSIFICATION    OK    LK.AVKS. 

When  a,  botanist  of  tlio  KJtli  or  J7l.li  e.-nl.ury  used  the  word  "leaf"  in  describing 
plants,  it  \\as  exclusively  in  the  sense  of  the  popular  acceptation  of  that  term.  He 
understood  by  "leaf",  a  flattened  outspread  structure,  such  as  appears  on  the 
branches  of  trees  as  a  foliage-leaf,  green  in  colour,  or,  as  a  floral-leaf,  adorn »•<!  with 
red,  blue,  and  other  colours.  Not  until  the  18th  century,  and  in  great  part  through 
the  influence  of  Goethe's  Essay  on  Metamorphosis  (</.  p.  10),  did  botanists  apply  the 
term  "  leaf  "  also  to  the  thick  fleshy  scales  of  bulbs,  to  the  scales  of  winter-buds,  to 
many  spines  and  tendrils,  to  stamens,  and  to  parts  of  the  fruit-capsule.  The  causes 
of  the  movement  in  this  direction  were  threefold.  First,  the  wish  to  collect  the 
extremely  manifold  phenoim-na.  sy  i  ioj  >(  .\t".\  I  ly  ;  tin-,  stru^ide  t,o  find  a  simple  gem-nil 
law  of  nature  to  which  the  shapes  of  innumerable  single  living  organisms  would 
conform;  further,  the  similarity  of  origin — the  agreement  actually  observed  over 
and  over  again  in  the  earliest  stages  of  development  of  structures  which  afterwards 
become  so  did'erent;  and,  tin;i  My,  <he  eireumstance  that  occasionally  under  abnormal 
external  influences,  viz.,  under  the  influence  of  mites,  plant-lice,  and  other  animals, 
given  leaves  are  actually  formed  from  the  sj>ines,  tendrils,  stamens,  and  fruit- 
capsules.  Now,  an  original  or  fundamental  type  of  leaf  was  imagined,  of  which 
naturally  the  shape  of  the  ordinary  green  foliage-leaf  became  a  standard  of  com- 
parison. It  was  represented  that  the  other  structures  enumerated,  which  do  not 
agree  in  their  shape,  although  they  agree  in  their  origin  with  the  green  leaves, 
had  been  produced  from  these  by  modification,  and  that  they  also  must  be  regarded 
as  leaves,  of  course  as  changed  or  metamorphosed  leaves.  According  to  this  view, 
the  bulb-scales,  the  stamens,  and  the  parts  of  the  fruit-capsule  are  metamorphosed 
leaves,  although  they  do  not  correspond  in  their  adult  form  to  the  idea  of  a  leaf 
conceived  by  people  who  are  not  botanists. 

The  struggle  after  perfection,  the  gradual  refinement  of  the  sap  conveyed  to  the 
leaves  in  their  first  stages,  and  many  other  things  were  at  first  supposed  to  be  (he 
causes  of  the  transformations.  In  modern  times  this  metamorphosis  is  associated 
with  the  division  of  labour,  and  with  the  change  of  function  in  the  members  of  the 
plant-body.  The  green  foliage-leaves  effect  the  formation  of  organic  materials 
from  inorganic  food  in  sunlight,  but  they  are  not  suited  at  the  same  time  for 
the  protection  of  seeds  or  for  the  manufacture  of  pollen;  nor  would  they  be  well 
adapted  as  underground  storehouses  for  reserve  materials.  Consequently  certain 
other  leaves  of  the  plant  assume  shapes  better  suited  to  these  functions,  or,  in 
other  words,  they  are  metamorphosed  to  suit  the  particular  function  required  of 
them.  We,  therefore,  do  not  see  green  leaves,  but  stamens  developed  for  the 
manufacture  of  the  pollen;  we  do  not  have  green,  flattened,  outspread  foliage  as  a 
storehouse  for  reserve  materials  in  the  dark  bosom  of  the  earth,  but  thick,  white, 
fleshy  scales.  The  stamens  manufacturing  the  pollen,  the  green  leaf-blades 
preparing  organic  materials  in  sunlight,  and  organs,  of  one  and  the  same  plant, 
fitted  to  various  other  definite  tasks,  are  so  entirely  similar  in  their  origin  and  first 
stages,  of  development,  that  they  are  included  under  a  common  abstraction,  and  the 
word  "  leaf  "  has  been  employed  to  express  it.  As  in  a  beehive  the  adult  workers, 


DEFINITION    AND    <:f.ASSIFICATION   OF    U.A 

the  drones,  and  the  queen  are  of  different  forms  in  accordance  wiUi  their  dill'm-nt 
tasks,  as  demanded  by  the  division  of  labour— HO  the  leaves,  which  agree  in  th«-ir 
. es  of  development,  exhibit,  in  their  fully  formed  condition,  another  con- 
struction in  accordance  with  the  function  assigned  to  them.  Hence  we  come  to 
this  conclusion: — the  variety  of  the  tasks  accomplished  for  the  prosperity  and 
maintenance  of  the  whole  plant,  and  the  con.1-  division  of  labour,  necessitate 

the  metamorphosis  of  the  leaves  in  each  plant-body. 

From  what  has  been  said  it  follows  that  a  definition  of  the  botanical  leaf  must 
be  connected  with  the  first  stages  of  development.  At  the  earliest  stage  each  leaf 
app--.  lateral  swelling  or  protuberance  below  the  growing  point  of  the  stem. 

It,  arises  from  the  peripheral  portions  of  this  region,  which  are  still  in  a  state  of 
active  ^rowUi.  The  growth  of  the  leaf  is  limited,  so  that  in  respect  of  their 
development,  we  may  say  that  leaves  are  laterally  developed  members  of  limited 
growth,  which  H-priny  in  <j '-//metrical  succession  from  the  outer  layers  of  tissue 
below  the  yrovjiny  point  of  tlw  stem. 

In  many  foliage-leaves  we  can  plainly  distinguish  a  plate-like,  outspread,  green 
portion,  traversed  by  lighter  veins,  the  blade  (lamina),  also  a  firm  and  stalk-like 
support  for-  this  blade,  the  leaf-stalk  (petiolus),  and,  finally,  that  portion  which 
connects  the  leaf-stalk  with  the  part  of  the  stem  in  question.  In  many  plants  this 
letter  portion  is  widened,  grooved,  and  occasionally  provided  with  a  membraneous 
border,  so  that  the  stern  is  then  surrounded  by  this  portion  as  the  blade  of  a  dagger 
is  by  the  sheath.  This  part  of  the  leaf  has  in  fact  been  termed  the  sheath  (vagina). 
Where  the  leaf  projects  from  the  stem  there  are  frequently  two  outgrowths,  one  on 
the  i -irrht,  the  other  on  the  left  of  this  sheathing  portion.  These  have  generally  the 
form  of  membraneous  scales  (see  fig.  92  6).  They  are  often  dilated,  as,  for  example, 
in  the  Tulip-tree  (fig.  91),  and  usually  fall  off  when  the  leaf  at  whose  base  they  are 
inserted  is  fully  developed.  In  other  plants  they  have  the  form  of  small  lobes  or 
auricles,  are  coloured  green,  and  are  retained  as  long  as  the  leaf  remains  connected 
with  the  stem.  These  structures  are  called  stipules  (stvpulce). 

Leaves  in  which  the  blade,  leaf -stalk,  sheath,  and  stipules  are  plainly  developed, 

on  the  whole  less  frequently  met  with  than  those  from  which  one  or  other  of 

e  portions  is  absent.     Often  no  trace  is  to  be  seen  of  the  stipules.     Sometimes 

only  the  leaf -sheath  is  present  in  the  form  of  a  concave,  husk-like  scale.     In  other 

instances  the  leaf-stalk  is  absent,  and  the  blade  is  then  situated  directly  on  the 

stern  (fig.  14);  or  it  may  happen  that  the  green  tissue  of  the  blade  surrounds  the 

whole  stem  like  a  collar,  so  that  it  might  be  thought  that  the  stem  had  been  stuck 

through,  or  had  grown  through  this  leaf.     If  two  or  more  of  these  leaves  with 

sessile  blades  arise  together,  they  may  be  fused  into  a  bowl  or  cup,  being  partially 

.vholly  united,  and  then  again  it  looks  as  if  the  stem  from  which  these  leaves 

arise  has  been  thrust  through  the  middle  of  the  united  leaf-group  (see  fig.  56). 

Occasionally  the  green  tissue  of  sessile  leaf-blades  is  seen  to  be  continued  down  the 

stem  in  the  shape  of  two  green  bands  or  wings.     For  the  forms  here  only  very 

briefly  described,  the  botanical  terms  are  sessile  leaves  (folia  sessiUa),  perfoliate 


596  DEFINITION   AND   CLASSIFICATION   OF   LEAVES. 

leaves  (folia  perfoliata),  connate  leaves  (folia  connata),  and  decurrent  leaves  (folia 
decuwentia),  of  which  terminology  this  explanation  must  be  given,  that  in  earlier, 
and  indeed  even  in  modern  times,  the  leaf -blade — as  the  most  noticeable  part  of  the 
lea£ has  been  in  describing  plants,  shortly  called  "  the  leaf  "  (folium). 

The  classification  of  leaves  with  regard  to  their  point  of  origin  from  the  stem  is 
of  particular  importance,  and  in  this  connection  we  must  first  of  all  distinguish 
between  seed-leaves  and  shoot-leaves.  The  former  only  occurs  in  the  embryo,  the 
latter  in  all  those  structures  comprised  under  the  term  "shoot".  The  embryo 
which  has  developed  from  the  fertilized  egg-cell  in  the  embryo-sac — in  a  manner 
which  has  yet  "to  be  described  in  .detail — 'presents  in  many  instances  a  tissue-body 
in  which  as  yet  no  trace  can  be  recognized  of  a  differentiation  into  stem  and  leaf, 
or  rather,  the  embryo,  when  it  leaves  the  fruit-capsule,  is  like  a  stem  in  which 
all  indication  of  leaves  is  absent,  e.g.  in  several  thousand  orchids,  the  numerous 
Balanophoreae  and  Rafflesiaceae,  species  of  Broom-rape  (Orobanche),  Winter-green 
(Pyrola),  Bladderwort  (Utricularia),  Bird's-nest  (Monotropa),  Dodder  (Cuscuta), 
and  many  other  epiphytes,  saprophytes,  insectivorous  plants  and  parasites,  as  well 
as  many  plants  living  together  symbiotically.  In  the  majority  of  instances 
however,  a  distinct  differentiation  can  be  recognized  in  the  embryo  hidden  in  the 
seed,  and  one  or  two  leaves  may  be  seen  issuing  from  the  tissue-mass  which  forms 
the  axis  of  the  embryo.  These  are  the  seed-leaves  or  cotyledons.  The  short  axis  or 
stem-portion  from  which  the  seed-leaves  originate,  and  which  looks  like  the 
pedestal  of  the  cotyledons,  is  called  the  hypocotyl.  At  one  end  of  the  hypocotyl  a 
tissue-mass  is  developed,  termed  the  radicle  (radicula)\  at  the  opposite  end  a  tissue- 
mass  named  the  plumule  (plumula).  (See  figs.  141 1  and  141 2.)  The  plumule  is 
situated  above  the  place  where  the  cotyledon,  or  pair  of  cotyledons,  issue  from  the 
hypocotyl.  It  is  the  rudiment  of  a  new  portion  of  the  stem,  which  is  situated  above 
the  cotyledons,  and  is  called  the  epicotyl.  The  epicotyl  thus  originates  from  the 
apex  of  the  hypocotyl,  and  the  boundary  between  these  two  portions  of  the  stem 
is  the  place  of  origin  of  the  cotyledon,  or  pair  of  cotyledons. 

The  epicotyl  in  the  resting  seed  is  frequently  only  a  tiny  knob  or  cone,  on 
which  no  indications  of  leaves  are  yet  to  be  seen.  In  the  majority  of  instances, 
however;  distinct,  although  as  yet  very  small,  leaflets  may  be  found  on  it,  and 
where  this  is  not  the  case  swellings  sooner  or  later  arise  which  are  the  leaf- 
rudiments.  Each  short  stem-structure,  with  closely-crowded  and  overlapping  leaves 
or  leaf -rudiments,  is  called  a  bud  (gemma);  consequently  the  plumule  is  a  bud,  in 
fact  it  is  the  bud  of  the  embryo,  which  arises  from  the  apex  of  the  hypocotyl. 
This  bud,  at  the  germination  of  the  seed,  elongates;  its  axis,  hitherto  very  short, 
stretches;  the  overlapping  leaflets  are  separated,  new  leaves  arise  under  the 
growing-point,  and  so  the-  bud  develops  into  a  structure  termed  a  "shoot" 
(innovatio).  The  bud  is  accordingly  the  primary  groundwork  of  a  shoot,  and  when 
considering  the  form  of  a  compound  plant-body,  special  regard  should  always  be 
paid  to  the  places  where  the  buds  originate.  The  first  bud,  which  is  established  in 
every  plant-body  at  the  commencement,  is  situated  at  the  apex  of  the  hypocotyl, 


DEFINITION   AND   CLASSIFICATION   OF   LEAVES.  597 

close  above  the  cotyledons.  But  later  on  buds  are  also  developed  on  this  primary 
shoot,  and  most  usually  close  above  the  place  where  leaves  arise  from  the  axis  of 
this  shoot.  Many  of  the  buds  elongate  and  themselves  become  shoots,  and  we  then 
say  the  shoot  has  formed  branches.  Some  of  the  buds,  however,  only  undergo  a 
slight  extension,  and  we  distinguish  between  long  branches  and  short  branches,  to 
which  we  shall  return  subsequently. 

Of  special  interest  to  us  here  are  the  leaves  of  these  shoots,  the  whole  of 
which  are  comprehended  under  the  general  term  shoot-leaves.  They  exhibit 
much  greater  diversity  in  form  than  do  cotyledons,  and  this  is  quite  intelligible, 
since  the  tasks  required  from  a  shoot  "are  much  more  numerous,  and  the  allot- 
ment of  various  functions  to  the  leaves  inserted  on  the  shoot  at  different  heights 
necessitates  a  greater  wealth  of  form.  But  the  extraordinary  abundance  of 
shapes  makes  it  necessary  to  group  the  shoot-leaves  according  to  their  origin, 
their  mutual  position,  and  their  succession  in  time.  We  have  long  ago  complied 
with  this  requirement,  since  we  distinguish  them  as  scale-leaves,  foliage-leaves, 
and  floral-leaves.  Lowest  on  the  shoot  we  see  the  scale  -  leaves.  They  are 
developed  earliest,  and  their  rudiments  are  frequently  to  be  seen  even  in  the 
bud  from  which  the  shoot  is  produced.  They  generally  appear  only  as  the 
sheathing  portions  of  leaves — as  scales  devoid  of  chlorophyll,  and  exhibit  relatively 
small  dimensions.  Following  these  scale-leaves  further  up  the  shoot  are  the 
foliage-leaves;  these  arise  later,  are  larger  in  size,  and  generally  developed  with 
green  laminae  directed  towards  the  sun's  rays  as  foliage.  Finally,  above  these, 
the  floral-leaves,  which  form  the  termination  of  the  series  of  leaves  growing  on 
a  shoot,  and  take  part  either  directly  or  indirectly  in  the  production  and  union 
of  the  sexual  cells.  One  and  the  same  shoot  does  not  always  bear  the  three 
kinds  of  leaf -structures  one  above  the  other  at  the  same  time.  There  are  some 
plants  whose  shoots  never  bear  foliage-leaves,  and  it  is  a  very  common  occur- 
rence for  a  compound  plant -body  to  develop  no  floral-leaves  on  one  shoot,  and 
no  foliage -leaves  on  another,  while  in  the  Lathrophytum  Peckoltii,  one  of  the 
Balanophorese  described  on  p.  196,  only  floral-leaves  are  formed,  and  neither  a 
foliage-leaf  nor  a  scale-leaf  has  ever  been  seen  on  this  plant. 

The  leaves,  hitherto  distinguished  only  with  regard  to  their  succession  in  age, 
relative  position,  and  insertion  on  the  stem,  must  now  also  be  described  in 
connection  with  the  shapes  which  they  assume  and  the  functions  which  they 
fulfil.  Touching  this  I  cherish  the  conviction  that  the  special  form  is  always 
correlated  with  a  special  function,  and  that  the  recognition  of  the  relation  of 
shape  to  the  performance  of  work  is  the  highest  problem  of  the  science  of  plants. 


598  COTYLEDONS. 


COTYLEDONS. 

The  cotyledons  or  seed-leaves  are  borne  by  the  embryonic  stem.  Their 
function  in  the  first  instance  is  to  provide  this  organ,  as  well  as  the  rudiments 
of  the  radicle  at  the  one  end  and  the  small  bud  at  the  other,  with  food.  These 
portions  of  the  embryo  as  long  as  they  are  still  surrounded  by  the  skin-like 
envelope  of  the  seed — the  so-called  "seed-coat" — and  even  still  later,  when  they 
have  burst  through  these  envelopes,  cannot  at  once  absorb  inorganic  food  from 
their  environment,  and  still  less  can  they  transform  this  into  organic  materials. 
And  yet  they  require  these  substances  for  growth,  that  is  to  say,  they  require 
materials  for  the  building  of  the  foundation  of  the  plant-body  which  is  to  issue 
from  the  seed.  Only  when  the  radicle  has  penetrated  into  the  soil  and 
produced  its  root-hairs,  and  green  leaves  have  forced  their  way  to  the  sunlight 
from  the  little  bud  which  formed  the  rudiment  of  the  epicotyl,  is  the  young, 
newly-settled  plant  placed  on  its  own  feet;  henceforth  it  can  nourish  itself 
independently.  But  up  to  the  moment  of  this  independence  it  draws  its  food 
from  a  store  which  is  deposited  in  the  seed ;  it  lives  on  materials  derived  from  the 
mother-plant,  i.e.  on  a  supply  of  starch,  fat,  and  proteid  formed  by  the  parent 
and  deposited  in  special  cells  for  the  benefit  of  the  embryo.  A  fully-equipped 
embryo  is  provided  with  food  reservoirs  in  either  of  two  ways.  Sometimes 
the  cotyledons  themselves  form  the  storehouse  for  the  food  to  be  consumed 
later  on.  In  this  case  the  reserve  materials  are  deposited  by  the  parent  plant 
in  the  cell-chambers  of  the  cotyledon,  and  when  the  suitable  time  arrives,  and 
when  the  need  for  them  has  arisen,  these  materials  are  employed  in  the 
further  construction  of  the  hypocotyl,  and  of  the  radicle,  springing  from  one 
end  of  it,  and  of  the  bud  at  the  other  end.  In  the  second  case  a  special  store- 
house is  formed  within  the  enveloping  seed-coat  beside  the  embryo.  The  cells 
of  this  storehouse  are  quite  filled  with  fat  and  starch  and  proteid  granules. 
The  tissue  of  this  particular  store-chamber  of  the  embryo  is  in  most  instances 
composed  of  cells  which  have  arisen,  together  with  the  germ-cell,  in  the  so-called 
embryo-sac  (the  large  cell  in  which  the  egg  is  produced),  and  it  is  then  termed 
endosperm.  Less  frequently  this  tissue  is  formed  outside  the  embryo -sac,  in 
the  nucellus,  and  is  then  called  perisperm.  This  distinction  is  without  signifi- 
cance in  the  processes  here  to  be  discussed,  and  therefore  in  the  following 
description,  endosperm  and  perisperm  are  included  under  the  term  reserve- 
tissue. 

When  the  cotyledons  themselves  form  the  reserve -tissue,  the  maintenance 
of  the  young  plant  is  relatively  simple.  The  transformation  and  transportation 
of  the  reserve-materials  are  carried  on  in  the  manner  already  described.  In 
proportion  as  the  radicle  of  the  embryo  develops  into  the  root,  and  a  leafy 
shoot  is  produced  from  the  embryonic  bud  at  the  cost  of  the  building  materials 
conveyed  to  them,  the  cells  of  the  cotyledons  lose  their  store  of  food  materials, 


COTYLEDONS. 


599 


and  their  role  of  nurse  is  at  an  end.  Often,  indeed,  the  cotyledons  take  on 
later  another  function,  but  they  have  ceased  to  be  of  importance  as  reserve- 
tissue  for  the  use  of  the  developing  embryo.  In  those  instances  where  the 
supply  of  food  given  by  the  parent  plant  is  not  stored  up  in  the  cotyledons, 


IS 


Fig.  141.— Cotyledons. 

i  Longitudinal  section  of  seed  of  Ricinus,  parellel  to  the  plane  of  the  embryo.  2  Longitudinal  section  of  the  same  seed,  taken 
at  right  angles  to  the  two  parallel  cotyledons.  »  Longitudinal  section  through  a  grain  of  Wheat  (Triticum  vulgare);  x  4. 
*  Longitudinal  section  through  a  grain  of  Wheat  after  germination  has  taken  place ;  x  4.  *  The  embryo  with  its  scutellum 
in  a  grain  of  Wheat ;  x  80.  «  Absorbent  cells  on  the  surface  of  the  scutellum  of  a  grain  of  Wheat ;  x  210.  r  Germinating 
seed  of  the  Corn-cockle  (Agrostemma  Githago) ;  slightly  magnified.  8  Longitudinal  section  of  the  same.  »  Seedling  of  the 
Corn-cockle  in  a  later  stage  of  development.  10  The  same  in  longitudinal  section.  n  Absorbent  cells  on  the  surface  of 
the  cotyledon  adjoining  the  reserve-tissue  in  the  seed  of  the  Corn-cockle ;  x  210.  ia  Germinating  seed  of  Tradescantia 
Virginica ;  slightly  magnified.  18  The  same  in  a  later  stage  of  development.  1*  Transverse  section  through  the  knob- 
like  end  of  the  cotyledon  of  Tradescantia  Virginica  embedded  in  the  reserve-tissue ;  x  10.  »  Absorbent  cells  on  the 
surface  of  this  knob-like  end;  x  180.  16  Germinating  seed  of  the  Onion  (Allium  Cepa);  natural  size.  »7  The  snme  cut  in 
half ;  slightly  magnified.  *»  Seedling  of  the  Onion  in  a  later  stage  of  development ;  natural  size.  «  The  same  cut  through 
longitudinally;  slightly  magnified. 

but  is  deposited  as  a  special  reserve  near  the  embryo,  its  nourishment  is  more 
complicated. 

In  this  state  of  affairs  the  cotyledons  have  an  essentially  different  function. 
They  serve  as  agents,  and  their  first  task  is  to  take  up  the  building-materials. 
These  have  been  liquefied  in  the  reserve -tissue,  either  by  changes  in  that  tissue 
itself,  or  by  the  direct  solvent  action  of  the  cotyledons.  They  are  then  conducted 


(500  COTYLEDONS. 

to  the  growing  parts  of  the  embryo.  In  order  to  do  this,  it  is  necessary  that  those 
cells  of  the  cotyledon  which  adjoin  the  special  reserve-tissue  should  have  the  power 
of  absorbing  organic  compounds  from  it,  and  of  leading  them  away.  The  cotyle- 
dons in  this  respect  resemble  the  suckers  of  parasites,  and,  like  these,  are  provided 
with  absorbent  cells.  In  many  species,  e.g.  in  the  Corn-cockle  (see  fig.  141  ") 
they  remain  short,  form  a  continuous  cell-layer  which  borders  on  the  special 
reserve-tissue,  and  remind  one  of  the  absorbent  cells  of  the  Bird's  Nest  (fig.  162); 
in  others,  as,  for  example,  in  Tradescantia  (see  fig.  141 15),  they  appear  as 
papillae,  slightly  separated  from  one  another  at  the  sides,  and  resembling  the 
absorbent  cells  of  a  Gentian  root  (cf.  fig.  16 x);  again,  in  other  instances,  as, 
for  example,  in  the  Wheat  (fig.  141 6),  they  increase,  at  the  time  of  absorp- 
tion, to  ten  or  twelve  times  their  previous  length,  and  then  their  side-walls 
separate  from  one  another  so  that  they  are  comparable  to  the  absorbent  cells 
of  Cuscuta  (fig.  35 2).  If  the  embryo  is  entirely  embedded  in  the  special 
reserve-tissue,  it  may  happen  that  all  its  superficial  cells  in  contact  with  the 
food-containing  tissue,  and  not  only  those  on  the  exterior  of  the  cotyledons, 
act  as  absorbent  ceUs.  If,  on  the  other  hand,  the  embryo  only  adjoins  the 
reserve-tissue  on  one  side,  the  absorbent  cells  also  are  only  developed  on  this 
side.  The  embryo  of  the  Corn-cockle,  which  is  bent  like  a  horse-shoe  around  the 
special  reserve -tissue  (fig.  141s),  exhibits,  for  example,  absorbent  cells  only  on  the 
lower  side  of  one  of  its  two  cotyledons,  which  is  directed  towards  the  middle 
of  the  seed.  Frequently  only  a  very  small  part  of  the  cotyledon  possesses 
absorbent  cells  adjoining  the  reserve-tissue,  as,  for  example,  in  the  Onion,  where 
only  the  end  of  the  cotyledon  bears  absorbent  cells  (figs.  141 17>  18>  19);  or  in 
Tradescantia,  where  the  end  of  the  cotyledon  presents  a  knob-like  absorbent 
tubercle  (fig.  141  u).  It  is  worthy  of  notice  that  in  many  instances  where  the 
reserve-tissue  is  ample  and  the  embryo  very  small,  the  extent  of  the  absorbent 
surface  of  the  cotyledon  becomes  enlarged  during  germination.  As  the  reserve 
materials  are  absorbed,  and  the  exhausted  reserve-tissue  shrinks,  the  absorbing 
portion  of  the  cotyledon  advances.  The  knob-like  termination  of  the  cotyledon 
of  Tradescantia,  originally  of  small  dimensions,  becomes  larger  in  proportion  as 
the  reserve-tissue  diminishes.  The  absorbent,  hollow,  conical  or  inflated  end  of 
the  cotyledons  of  many  palms,  e.g.  of  the  Date  and  Cocoa-nut  Palms,  increases 
in  proportion  as  the  reserve-tissue  diminishes,  presses  forwards  just  as  far  as  the 
tissue  to  be  absorbed  shrinks  back,  and  occupies  the  space  vacated  by  it  (figs. 
1449  and  144 10).  A  similar  relation  is  to  be  seen  in  rushes  and  sedges.  In 
the  embryo  of  Coffee  and  Ivy  seeds,  the  cotyledons  are  at  first  very  small, 
but  grow  further  and  further  into  the  reserve -tissue  during  the  process  of 
germination,  till  they  gradually  fill  up  the  whole  space  in  the  seed.  The 
cotyledons  of  umbelliferous  plants  also  behave  in  a  very  characteristic  manner. 
The  small  embryo  lies  in  the  seed  at  the  base  of  the  reserve-tissue,  and  its 
minute  cotyledons  project  into  a  space  occupied  by  empty  cells,  which  are 
however,  surrounded  by  the  well-filled  cells  of  the  reserve-tissue.  Now  when 


COTYLEDONS.  601 

germination  commences  the  two  cotyledons  grow  in  length,  penetrate  through 
this  loose  central  cell-layer  and  attach  themselves  to  the  reserve-tissue  which 
has  to  be  absorbed. 

On  the  whole  it  may  be  taken  as  correct  that  the  surface  of  contact  between 
the  absorbent  part  and  that  which  has  to  be  absorbed  is  greater  the  quicker 
the  absorption  has  to  be  accomplished,  on  account  of  the  local  climatic  conditions. 
Starch  is  best  suited  for  rapid  liquefaction  and  absorption;  fat  takes  much 
longer  to  become  changed  into  a  form  adapted  for  absorption;  and  the  trans- 
formation of  layers  of  cellulose  requires  a  still  longer  time.  In  accordance  with 
this  the  embryo  comes  into  contact  with  the  reserve -tissue  whose  cells  are 
crowded  with  starch  (as,  for  example,  in  the  seeds  of  pinks,  oraches,  poly- 
gonums,  and  grasses),  presenting  a  broad  surface,  or  else  wrapped  with  its  long 
cotyledons  around  the  tissue  either  horse-shoe-wise  or  spirally.  On  the  other 
hand,  in  plants  whose  special  reserve-tissue  is  principally  filled  with  fat,  the 
surface  in  contact  is  much  smaller,  and  the  seeds  of  those  plants  whose  reserve- 
food  consists  chiefly  of  cellulose,  e.g.  those  of  the  Date,  usually  exhibit  only 
a  very  limited  area  of  contact  between  the  cotyledon  and  the  reserve- tissue. 
But  in  these  latter  the  liquefaction  and  absorption  continue  for  months,  while 
the  same  processes  in  the  starchy  seeds  of  grasses  and  oraches  are  completed 
in  a  few  days. 

In  addition  to  this  first  task  of  the  cotyledons,  which  we  have  just 
described,  in  many  instances  we  have  a  second  function,  viz.  the  extrusion  of 
the  hypocotyl  and  its  crowning  bud  from  the  interior  of  the  seed-coats.  After 
the  formation  of  the  embryo  from  the  parent  plant,  it  remains  quiescent  for 
a  time,  and  during  this  period  appears  to  be  protected  in  the  most  diverse 
ways  by  coverings  against  the  external  dangers  that  might  threaten  its  exist- 
ence. When  a  special  reserve-tissue  is  present,  the  embryo  is  frequently  found 
embedded  in  the  centre  of  it,  or  hidden  in  its  folds.  The  reserve -tissue  is 
often  horn-like,  or  as  hard  as  bone,  as,  for  example,  in  the  seeds  of  Date  and 
Coffee,  and  therefore  an  excellent  protection  is  provided  by  this  tissue  for  the 
dormant  embryo.  In  any  case  the  embryo  is  surrounded  by  the  seed-coat,  which 
may  consist  of  a  single  or  a  double  layer.  In  very  many  plants  the  seed  is 
also  walled  in  by  a  non-dehiscent  pericarp  (or  fruit -covering)  and  occasionally 
by  other  structures.  The  seed-coat  forms  an  envelope  which  allows  of  the 
-entrance  of  moisture  into  the  interior  only  by  a  very  restricted  opening.  It  is 
not  flexible  or  extensible  to  a  great  degree,  and  consequently  if  the  contents 
swell  up  and  the  growth  of  the  embryo  begins,  then  the  portion  of  the  embryo 
designed  for  further  development  must  either  find  an  exit  through  the  above- 
mentioned  aperture  or  else  it  must  burst  through  the  husk;  or  both  kinds  of 
escape  may  occur  together. 

This  process,  in  which  the  cotyledons  take  a  very  prominent  part,  is  carrie 
on  in  a  manner  defined  for  every  species,  but  in  different  species  by  an  incalculable 
variety   of   methods.      Occasionally   larger   alliances   of    the   vegetable    kingdom 


(502  COTYLEDONS. 

exhibit  a  remarkable  agreement,  but  it  also  happens  that  even  closely  related 
species  of  one  and  the  same  genus  differ  considerably  with  respect  to  the  liberation 
of  the  embryonic  plant  from  the  bondage  of  the  seed-coat.  That  some  idea  may 
be  conveyed  of  the  methods  obtaining  at  germination,  eight  different  cases  will 
be  described. 

Let  us  begin  with  one  of  the  most  remarkable  cases,  viz.  with  the  germination 
of  mangroves,  which  grow  in  extensive  forests  on  the  tidal  swamps  of  tropical 
coasts. 

The  species  which  I  select  as  example,  and  whose  whole  process  of  development 
is  clearly  shown  in  the  figures  opposite,  is  called  Rhizophora  conjugata.  A  longi- 
tudinal section  through  the  pendent  flower  of  this  species  (figs.  142 1>  2>  8> 4-)  exhibits 
two  compartments  of  equal  size  in  the  ovary,  and  in  each  compartment  is  dis- 
covered the  commencement  of  a  seed.  After  fertilization  the  corolla  and  stamens 
fall  off;  the  calyx  remains,  and  the  much-enlarged  ovary  assumes  the  form  of  a 
stunted  cone,  whose  apex  bears  two  stigmas,  now  transformed  into  shrivelled 
points.  If  the  ovary  is  cut  through  longitudinally  at  this  stage  of  development, 
it  may  be  seen  that  one  compartment  (fig.  142  5)  with  its  young  seed  is  atrophied, 
and  the  other  with  its  seed  has  widened  and  enlarged  very  much.  Within  the 
young  seed  (which  is  attached  to  one  side  of  the  originally  central  wall  of 
the  ovary)  an  embryo  can  now  be  plainly  distinguished,  surrounded  by  its  reserve 
tissue.  Together  they  fill  the  egg-shaped  cavity,  open  below,  formed  by  the  thick 
seed-coat.  The  embryo  consists  of  the  hypocotyl,  whose  free  end  is  directed  down- 
wards, that  is,  towards  the  point  of  the  pendent  ovary,  and  the  cotyledon  which 
forms  the  upper  termination,  tubular  below,  and  above  not  unlike  a  Phrygian  cap. 
The  cotyledon  covers  like  an  inverted  bell  the  embryonic  bud,  which  is  inserted 
upon  the  apex  of  the  hypocotyl.  In  the  lower  tubular  portion  of  the  cotyledon 
are  numerous  vascular  bundles  which  pass  down  into  the  hypocotyl  and  supply  it 
with  food.  A  true  radicle  is  not  developed  at  the  lower  end  of  the  hypocotyl,  and 
that  which  was  formerly  regarded  as  a  root  may  be  more  accurately  interpreted 
as  the  hypocotyl  itself.  Strangely  enough,  the  fruits  of  mangroves  do  not  become 
detached  from  the  branches  after  the  formation  of  the  embryo;  nor  do  they  dehisce 
to  allow  the  seeds  to  fall  out.  On  the  contrary,  these  germinate  while  still  inclosed 
in  the  fruit  hanging  on  the  tree.  The  embryo  develops  within  the  seed-coat  at 
the  cost  of  the  reserve-food  in  which  it  is  embedded,  absorbing  this  nourishment  by 
means  of  the  cotyledon.  The  whole  of  the  exterior  of  the  upper  portion  of  the 
cotyledon  is  covered  with  absorbent  cells,  and  the  materials  drawn  by  these  cells 
from  the  surrounding  slimy,  gelatinous  mass  are  conducted  by  the  aforesaid 
vascular  bundles  to  the  hypocotyl.  Since,  in  spite  of  this,  the  amount  of  the  food 
stored  up  does  not  diminish,  and  since  it  is  not  proportioned  to  the  size  of  the 
growing  embryo,  it  may  be  safely  concluded  that  whatever  food  is  absorbed  by 
the  cotyledon,  and  employed  for  the  growth  of  the  hypocotyl,  is  continuously 
replaced  by  the  parent  plant. 

When  the  hypocotyl  has  attained  a  length  of  2  centimetres,  the  tubular  portion 


COTYLEDONS. 


603 


of  the  cotyledon  also  extends  and  pushes  the  hypocotyl  in  front  of  it  until  the 
apex  has  bored  its  way  through  the  wall  of  the  fruit  and  come  out  into  the  day- 
light (see  figs.  142  •.«•».  *).  The  hypocotyl  now  elongates  in  a  month  to  about 
4  cm.,  and  in  from  7  to  9  months  attains  a  length  of  30-50  cm.,  and  from  1-5  cm 


Fig.  142. — Rhizophora  conjugata. 

i  Flower,  cut  in  half  longitudinally,  a  Fruit.  »  Twig  with  two  fruits,  whose  conical  ends  have  been  broken  through  by  the 
pressure  of  the  elongating  hypocotyls.  *  Longitudinal  section  through  the  ovary;  about  twice  the  natural  size.  •  Longi- 
tudinal section  through  a  fruit;  the  cap-like  cotyledon  surrounded  by  reserve-tissue;  the  lower  end  of  the  hypocotyl 
having  grown  through  the  seed-coat  has  reached  the  lower  hollow  conical  apex  of  the  pericarp.  •  Longitudinal  section 
through  a  fruit  two  months  later;  the  tubular  sheath  of  the  cotyledon  has  elongated  and  pushed  the  hypocotyl  quite  out 
of  the  pericarp.  1  Longitudinal  section  through  a  fruit  eight  months  later.  The  hypocotyl  is  separating  from  the  tubular 
portion  of  the  cotyledon,  a  part  of  the  same ;  slightly  magnified.  9  Upper  end  of  the  hypocotyl  with  the  embryonic  bud. 
The  two  lowest-leaves  of  the  bud  are  expanding,  the  two  upper  are  still  folded  together. 

in  thickness.  It  is  thickest  in  its  lower  third,  and  is  there  slightly  curved.  Its 
weight  now  amounts  to  almost  80  grammes.  These  long  heavy  hypocotyls  pro- 
jecting from  the  fruits  sway  to  and  fro  with  every  breath  of  wind.  At  length  the 
vascular  bundles,  by  which  the  connection  with  the  tubular  portion  of  the  cotyledon 
was  retained,  are  ruptured  (see  figs.  142 7  and  142 8).  The  embryo  falls  away,  and 


604 


COTYLEDONS. 


its  lower  end  bores  deeply  into  the  mud.  Even  a  layer  of  water  half  a  metre  deep 
is  pierced  by  it  with  such  force  that  it  remains  standing  upright  in  the  mud 
beneath.  After  a  few  days  the  pericarp,  with  the  cotyledon  inside,  is  also  de- 
tached. At  the  upper  end  of  the  fallen  hypocotyl  the  bud  which  was  formerly 
covered  over  by  the  tubular  cotyledon  is  now  to  be  seen.  The  four  small  green 
scale-leaves  of  this  bud  only  increase  slightly  in  length;  but  immediately,  from 
the  shoot  arising  from  it,  large  elliptical  shiny  green  leaves  are  developed  which 
become  active  as  foliage;  whilst  from  the  lower  end  of  the  hypocotyl  which  has 
bored  into  the  mud,  as  well  as  from  the  epicotyl  itself,  roots  arise  which  are  at 
once  the  means  of  fixing  the  plant  in  the  muddy  shore,  and  of  conducting  food- 
salts  to  it.  In  the  neighbourhood  of  old  mangrove  trees,  dozens  of  these  young 
plants  may  be  seen,  which  have  fallen  and  bored  their  way  into  the  mud;  and  on 
the  shoots  produced  from  their  upper  ends  sometimes  only  scale-leaves,  and  some- 
times foliage-leaves  are  developed.  The  illustration  opposite,  taken  from  a  sketch 
near  Goa,  on  the  coast  of  Bombay,  drawn  from  nature  by  Ransonnet,  shows  all 
this  very  clearly. 

The  second  form  of  cotyledon  to  be  brought  forward  is  that  which  occurs  in 
grasses,  and  is  called  by  botanists  the  scutellum.  Although  variously  modified, 
it  is  in  the  main  developed  similarly* in  the  many  thousands  of  different  species. 
The  small  embryo  of  the  grass  is  in  lateral  contact  with  one  end  of  the  large  starchy 
reserve-tissue,  by  means  of  its  cotyledon,  as  shown  in  the  grain  of  wheat  chosen 
as  type  (see  figs.  141 4  and  141 5).  The  free  edges  of  the  cotyledon  arch  over  the 
embryo  bud,  sometimes  actually  curling  round  it,  forming  a  sheath-like  envelope. 
Below,  the  cotyledon  is  continued  into  a  sac  which  incloses  the  radicle  of  the 
embryo.  When  the  materials  are  conveyed  from  the  reserve-tissue  to  the  hypo- 
cotyl, radicle,  and  embryo-bud,  by  means  of  the  absorbent  cells  of  the  cotyledon 
described  on  p.  600,  these  portions  quickly  increase  in  length.  The  radicle  pierces 
the  sac-like  envelope,  penetrates  into  the  ground,  and  unites  by  abundant  root- 
hairs  with  the  particles  of  the  soil.  The  bud  also  elongates  and  the  leaves  grow  up 
into  the  light  from  the  sheath-like  envelope  of  the  cotyledon.  The  lower  leaves 
are  usually  scale-leaves  without  green  blades,  but  the  leaves  following  these  all 
exhibit  large  green  laminae  which  function  as  foliage.  The  starch  of  the  reservoir 
is  soon  completely  consumed  in  the  rapid  growth  of  the  embryo.  As  soon  as  this 
has  happened  the  cotyledon  has  no  further  task  to  fulfil,  it  shrivels  and  perishes, 
but  the  young  grass-plant  with  its  roots  and  its  green  foliage-leaves  is  now  in 
a  position  to  manufacture  for  itself  the  substances  necessary  for  its  further 
construction. 

The  third  form  of  cotyledon  is  shown  in  the  embryos  of  sedges  and  rushes, 
of  irises,  snowdrops,  narcissus,  aloes,  and  butcher's-broom,  of  flowering  rushes, 
bananas,  and  palms,  and  numerous  other  plants  belonging  to  the  class  of  monocoty- 
ledons. In  all  these  plants  the  embryo  is  embedded  in  the  reserve-tissue  of  the 
seed,  and  the  cotyledon  proceeding  from  the  hypocotyl  forms  a  sheath  surrounding 
the  bud  situated  upon  it.  The  cotyledon  is  provided  with  absorbent  cells  only  at 


COTYLEDONS. 


605 


ite  apex  and  1S  connected  with  the  cells  of  the  reserve-tissue  at  that  point.  In 
germination  the  cotyledon  increases  in  length  and  pushes  the  hypocotyl  with  the 
embryonic  bud  and  radacle  out  of  the  seed.  The  food  absorbed  from  the  reserve- 
fassue  by  the  remaining  portion  of  the  cotyledon  is  conducted  from  the  interior 
the  seed  to  the  extruded  embryo  by  the  lengthened  part  of  the  cotyledon  With 


Fig.  143.— Mangroves  near  Goa  on  the  West  Coast  of  India  at  ebb-tide. 

the  help  of  food  thus  conveyed  to  it,  the  embryo  is  enabled  to  develop  its  radicle 
into  an  absorbent  root  penetrating  into  the  ground,  and  also  to  develop  its  leaf- 
rudiments  into  green  leaves.  Numerous  modifications  of  the  process  here  only 
sketched  quite  generally  may  be  distinguished,  and  these  consist  chiefly  in  the 
varying  direction  and  length  of  the  portions  of  the  cotyledon  thrust  out  from 
the  seed.  In  sedges,  rushes,  and  cyperuses  germinating  in  marshy  ground,  or  even 


COTYLEDONS. 

in  the  mud  under  water,  the  extruded  portion  of  the  cotyledon  surrounding  the 
embryonic  stem,  with  its  bud  and  first  shoot-leaves,  becomes  bent  upwards  after  it 
has  issued  from  the  interior  of  the  seed  (see  figs.  144  u  and  144 15),  while  in  species 
of  Yucca  and  Tradescantia  it  grows  downward  in  an  arch  (see  fig.  1419);  and  in 
cycads  and  palms,  growing  in  soil  exposed  superficially  to  drought,  it  bends  round 
immediately  after  its  exit  from  the  seed,  and  penetrates  vertically  into  the  deeper 
layers  of  earth  which  are  always  somewhat  moist  (see  figs.  1447'9'10).  In  the 
Areca-palm  and  the  slender  Chamcedorea  the  sheath-like  extruded  portion  of  the 
cotyledon  is  very  short,  while  in  the  Commelynaceae  it  is  much  elongated,  so  much, 
indeed,  that  it  looks  as  if  the  sheath-like  portion  surrounding  the  hypocotyl  and 
the  bud  were  connected  by  a  long  thread  with  the  absorbent  portion  which 
remains  behind  in  the  seed.  This  central  portion  of  the  cotyledon  is  also 
much  elongated  in  the  Date  palm  and  in  the  Cocoa-nut  palm,  as  well  as  in  the 
cycads  Zamia,  Ceratozamia,  Encephalartos.  The  figs.  7,  8,  9,  10  of  the  illustra- 
tion opposite  show  all  the  stages  of  development  in  the  Date  seedling.  As  long  as 
the  cotyledon  has  not  pushed  out  from  the  interior  of  the  seed,  it  forms  a  mantle- 
like  envelope  for  the  bud  of  the  hypocotyl,  and  is  continued  into  a  sac-like  covering 
for  the  radicle.  At  germination  the  cotyledon  increases  much  in  length;  the 
free  end  is  sheath-like,  the  middle  portion  forms  a  stalk -like,  rolled-up  structure, 
and  the  part  remaining  behind  in  the  seed  forms  a  hollow  cone  which  becomes 
dilated  like  a  vesicle  where  absorption  of  the  reserve  materials  occurs  (figs.  1449 
and  144 10).  In  a  still  later  stage  the  radicle  develops  into  a  root,  and  breaks 
through  its  sac-like  covering,  while  the  scale-leaves  of  the  epicotyl  stretch,  and 
push  their  way  out  of  the  cotyledonary  sheath  (fig.  1448).  Gardeners  employ 
what  they  call  a  "  dibble  ",  a  tool  by  the  help  of  which  the  seeds  and  seedlings  are 
planted  in  a  suitable  depth  of  earth.  One  is  involuntarily  reminded  of  these 
dibbles  in  observing  how  the  tubular,  rolled,  stalk-like  cotyledon-sheath — which 
grows  out  of  the  seed — not  only  pushes  the  embryo  out  of  the  interior,  but  presses 
it  deeper  and  deeper  into  a  layer  of  earth  which  by  its  depth  is  protected  from 
drying  up;  there  it  is  planted  in  a  suitable  place — actually  in  the  most  favourable 
position.  In  many  palms  the  cotyledonary  sheath  is  half  a  metre  long,  and  many 
months  pass  before  all  the  reserve-materials  of  the  gigantic  seed,  often  weighing 
as  much  as  8  kilograms,  are  conducted  by  this  sheath  to  the  embryo  planted  below. 
Numerous  species  of  Onion  (Allium),  and  of  Reed-mace  (Typha)  exhibit 
our  fourth  form  of  cotyledon.  The  extrusion  of  the  embryo  by  the  cotyledon 
is  conducted  in  the  same  way  as  in  the  type  just  described,  but  there  is  this 
essential  difference,  that  here  the  cotyledon,  after  it  has  absorbed  the  reserve- 
materials  of  the  seed  by  its  apex,  entirely  vacates  the  cavity  of  the  seed-coat, 
becomes  green,  and  then  acts  like  a  foliage -leaf.  In  the  seed  of  the  Garlic 
(Allium  sativum)  the  embryo  is  embedded  in  the  centre  of  the  reserve  (cf.  fig. 
141 17).  As  soon  as  germination  begins,  the  cotyledon  pushes  its  way  out  of  the 
seed-coat,  and  grows  first  upwards,  then  bending  round  at  an  angle,  so  that 
the  extruded  end  surrounding  the  hypocotyl  and  the  bud,  comes  to  lie  below  the 


COTYLEDONS. 


607 


level  of  the  seed  (figs.  141  »  and  141  «).  Here  long  root-fibres  develop  from  the 
radicle  and  from  the  base  of  the  hypocotyl;  these  burst  through  the  cotyledon 
grow  down  into  deeper  layers  of  earth,  and  fix  the  young  plants  in  the  spot 
where  the  cotyledon  has  placed  it.  The  apex  of  the  cotyledon  remains  in  the 


Fig.  144.— Germinating  Seeds  and  Seedlings. 


1  Seedling  of  the  Nasturtium  (Tropceolum  majus).  *  The  same  at  an  earlier  stage  of  development.  •  Water  Chestnut  (Trapo 
natans),  from  which  the  embryo  is  emerging.  *  Later  stage  of  development.  «  Young  seedling  of  the  Austrian  Oak 
(Quercus  Austriacd).  e  The  same,  further  developed.  »  Seed  of  the  Date  (Phoenix  dactylifera)  from  which  the  embryo  is 
emerging.  8  The  same  eight  weeks  later,  after  the  seedling  has  already  developed  root  and  scale-leaves.  •  Young  Date 
in  longitudinal  section.  ">  Older  Date  in  longitudinal  section.  "  Seed  of  the  Reed-mace  Typha  ShutOeworthii. 
12  The  same  with  protruding  embryo,  "  The  same  at  a  later  stage  of  development.  ",  »  Seedling  of  the  Sedge  Carex 
vulgaris.  Fig.  1-8,  natural  size;  9,  10,  x  8;  11-13,  x  4;  14,  15,  x  6. 

seed,  and  here  absorbs  the  last  remnants  of  the  reserve-materials.  When  these 
are  at  last  exhausted,  one  limb  of  the  bent  cotyledon  grows  upwards,  and  its 
apex  is  drawn  out  from  the  emptied  seed-coat.  All  this  occurs  underground. 
Now  the  cotyledon  also  has  to  reach  the  sunlight  and  become  green.  This  is 


608 


COTYLEDONS. 


brought  about  by  the  knee  of  the  upwardly-growing  cotyledon  acting  like  a 
wedge,  and  thus  making  a  path  upwards  through  the  ground.  This  penetration 
of  the  ground  is  materially  assisted  by  the  presence  of  cells  on  the  convex 
side  of  the  knee  which,  unlike  the  other  superficial  cells  of  the  cotyledon,  are 
somewhat  curved  outwardly,  and  highly  turgescent — a  contrivance  which  will 
be  described  more  in  detail  later  on.  When  finally  the  free  end  of  the  cotyledon 
has  been  drawn  out  of  the  ground,  the  knee-shaped  bend  is  obliterated  as  the 
green  cotyledon  straightens  out. 

The  germination  of  the  Reed-mace  (Typha)  is  quite  peculiar.  The  small 
fruits  which  are  blown  off  the  spike,  fall  on  to  the  surface  of  the  water  and 
remain  floating  for  some  days.  Then  the  pericarp  opens  and  the  seed  sinks 
slowly  down  into  the  water.  The  husk  of  the  seed  is  pointed  at  one  end,  and 
at  the  other  is  closed  by  an  extremely  pretty  trap-door  (cf.  fig.  14411).  While 
sinking  through  the  water  the  pointed  end  is  turned  downwards,  and  the 
covered  end  upwards.  At  the  bottom  the  seed  lies  in  the  position  indicated 
and  germination  commences.  The  cotyledon  grows  in  length,  pushes  open  the 
trap-door,  and  makes  its  appearance  at  the  mouth  of  the  seed-coat  (fig.  144 12). 
It  now  describes  an  arch  and  the  end  in  which  are  concealed  the  hypocotyl 
and  the  bud  reaches  the  mud.  Scarcely  has  it  done  so,  however,  when  its 
epidermal  cells  elongate  and  form  long  tubular  structures  which  penetrate 
into  the  slime,  and  the  free  end  of  the  cotyledon  is  thus  firmly  fixed  (fig.  144  13). 
Later  on  rootlets  make  their  appearance,  which,  proceeding  from  the  hypocotyl, 
break  through  the  unresisting  cotyledon.  Meanwhile  the  reserve  food  has  been 
sucked  up  by  the  apex  of  the  cotyledon  which  remained  in  the  seed;  this  apex 
is  now  drawn  out  of  the  seed-coat,  the  cotyledon  straightens  itself,  turns  green, 
and  functions  as  a  foliage-leaf. 

In  the  four  cases  just  described  the  embryo  only  possesses  one  cotyledon, 
and  each  seed  contains  a  reserve  -  tissue  beside  the  embryo.  In  the  fifth  case 
now  to  be  described,  however,  the  embryo  is  equipped  with  two  cotyledons, 
and  the  building  materials  which  are  at  its  disposal  for  the  first  period  of 
growth  are  stored  up  in  the  embryo  itself,  almost  entirely  indeed  in  the 
cotyledons.  To  this  class  belong  plants  with  stone-fruits  as  well  as  most 
species  with  seeds  and  fruits  of  nut-like  appearance,  and  many  the  seeds  of  which 
have  a  softer,  leathery  covering.  As  examples  may  be  named  the  Walnut  and 
Hazel,  the  Oak,  Chestnut  and  Horse-chestnut,  the  Almond,  Cherry,  Apricot,  and 
Peach,  the  Laurel  and  Pistachio-nut ;  the  Nasturtium  (Tropoeolum),  Broad-bean,  the 
Scammony  (Cynanchum),  and  the  Bastard-Balm  (Melittis).  The  two  leaves  pro- 
ceeding from  the  hypocotyl  almost  completely  fill  the  space  inclosed  by  the  seed- 
coats  in  all  these  plants ;  and  the  small  embryonic  bud  as  well  as  the  radicle  are 
situated  between  the  two  cotyledons,  just  like  a  dried  plant  between  the  sheets 
of  paper  in  a  herbarium.  The  cotyledons  are  thick,  swollen,  and  tense,  of  fleshy 
appearance  in  section,  and  always  comparatively  heavy.  Many  of  them  are  wavy, 
and  they  rarely  look  leaf -like.  Occasionally  the  two  cotyledons  are  fused  together 


COTYLEDONS.  (J09 

by  their  adjacent  surfaces,  as,  for  example,  in  the  Chestnut,  Horse-chestnut, 
and  Nasturtiums.  Everything  which  one  is  generally  accustomed  to  consider 
an  attribute  of  a  leaf  is  entirely  wanting.  When  these  seeds  take  up  water 
from  the  environment  and  begin  to  germinate  and  grow,  first  of  all  the  seed- 
coat  bursts  at  one  end  of  the  seed,  and  the  radicle  together  with  the  axis  and 
also  the  thick  stalks  of  the  two  cotyledons  are  extruded  through  the  rupture. 
The  cotyledons  themselves,  however,  remain  inside,  lose  weight  in  proportion  as 
they  give  up  materials  to  the  growing  parts,  dwindle,  and  finally  appear  quite 
shrivelled  and  emptied.  The  extruded  radicle  has,  on  the  contrary,  visibly 
increased,  it  bends  downwards,  penetrates  vertically  into  the  ground,  and  produces 
lateral  roots  with  root-hairs,  which  now  absorb  nourishment  from  the  soil.  The 
bud  which  was  hemmed  in  between  the  short  thick  stalks  of  the  two  cotyledons 
has,  on  the  other  hand,  curved  upwards,  elongated  pretty  quickly,  and  become 
a  shoot  which  in  the  Nasturtium  immediately  develops  green,  lobed  foliage-leaves. 
In  other  plants,  e.g.  in  the  Oak,  first  scale-leaves  appear  and  then  green  foliage- 
leaves  above  them.  In  fig.  1441'2-  5«  6  these  conditions  are  depicted  both  in  the 
Nasturtium  and  the  Oak.  The  cotyledons  have  here  a  threefold  part  to  play; 
first  of  all  they  function  as  storehouses  for  reserve  materials,  and  at  the  same 
time  as  protecting  envelopes  for  the  small  squeezed  rudiment  of  the  future  plant; 
in  addition  they  also  have  the  task  of  thrusting  the  embryo  out  of  the  cavity 
of  the  seed  so  far  that  its  members  can  elongate  as  required — some  towards  the 
light,  and  some  into  the  dark  ground.  When  they  have  performed  these  duties 
they  die  off  and  disintegrate  so  completely  that  at  the  place  where  they  were 
connected  with  the  hypocotyl,  scarcely  a  trace  of  their  insertion  is  to  be  recognized. 
A  peculiar  condition  of  the  cotyledons,  the  sixth  in  the  series  here  described, 
is  observed  in  the  Water  Chestnut  (Trapa).  One  of  the  cotyledons  is  small 
and  scale-like,  containing  no  reserve  materials;  the  other  is  very  large,  and  so 
completely  fills  up  the  nut  that  it  looks  as  if  someone  had  poured  wax  into  the 
interior  of  the  fruit,  and  that  it  had  there  become  hardened  into  a  solid  mass. 
The  Water  Chestnut  germinates  on  the  mud  under  water.  As  soon  as  germina- 
tion commences,  a  white  worm-like  body  is  extruded  from  the  aperture  of  the 
nut,  and  though  many  consider  this  to  be  the  hypocotyl,  it  should,  strictly 
speaking,  be  regarded  as  a  root  (of.  fig.  144 3).  This  structure  elongates  under 
the  water  and  grows  directly  upwards.  Of  the  two  cotyledons  only  that  one 
which  was  inserted  as  a  tiny  scale  on  the  short  hypocotyl,  leaves  the  cavity 
of  the  nut  and  is  connected  by  a  long  stalk  with  the  hypocotyl.  This  long 
stalk,  together  with  the  very  small  hypocotyl  and  the  root,  pass  so  gradually 
into  one  another  that  they  resemble  a  single  unjointed  white  cord  (cf.  fig.  144 4). 
The  reserve  materials  deposited  in  the  large,  fleshy  cotyledon  are  conducted  by 
the  stalk-like  connection  to  the  growing  parts  of  the  embryo  in  the  water; 
a  process  which  occupies  a  considerable  time.  By  the  time  that  this  cotyledon 
has  yielded  up  the  reserve  food,  the  root  has  grown  so  strong  that  it  is  able 
to  take  up  materials  from  its  surroundings;  it  bends  down  towards  the  mud 

IT^T      T  39 


VOL.  I. 


(510  COTYLEDONS. 

in  which  it  fixes  itself  by  numerous  lateral  fibres.  The  bud  has  meanwhile 
grown  out  and  formed  a  shoot  which  develops  scale-leaves  below,  with  green 
foliage-leaves  above  these,  and  so  grows  up  to  the  surface  of  the  water.  The 
exhausted  cotyledon  never  leaves  the  interior  of  the  nut,  but  gradually  decays 
with  it  Thus  we  have  here  the  rare  instance  of  one  cotyledon  being  extruded 
from  the  cavity  of  the  seed  (that  is  to  say,  of  the  fruit)  while  the  other  remains 
behind. 

In  the  seventh  case  the  embryo  exhibits  two  (only  exceptionally  more  than 
two)  cotyledons,  which  are  drawn  out  of  the  seed -coat  during  germination, 
and  spreading  out  in  the  sunlight,  turn  green  and  serve  as  foliage -leaves. 
These  foliage -leaves  first  function  as  absorbent  organs;  that  is  to  say,  they 
adjoin  a  special  reserve-tissue  in  the  seed  from  which  they  derive  the  materials 
required  for  their  first  growth,  and  do  not  issue  from  the  cavity  of  the  seed 
until  the  storehouse  is  exhausted  and  emptied  of  food.  This  is  the  case,  for 
example,  in  the  repeatedly-mentioned  Corn-cockle  (Agrostemma  Giihago),  whose 
two  cotyledons,  folded  together,  are  bent  like  a  horse-shoe  round  the  reserve- 
tissue,  but  are  withdrawn  from  the  seed -coat  after  the  consumption  of  this 
food,  when  they  separate  and  become  green  (cf.  141 7>  8>  9>  10).  Much  more  rarely 
the  seed-coat  bursts  at  the  beginning  of  germination,  the  large  folded  cotyledons 
together  with  the  surrounding  reserve-tissue  are  drawn  out  so  that  the  absorp- 
tion of  the  reserve -food  does  not  take  place  till  after  vacating  the  seed -coat, 
after  which  follows  the  unfolding  and  colouring  of  the  two  cotyledons  in  the 
sunlight.  The  seeds  of  Ricinus  (cf.  figs.  141 l  and  141 2)  show  this  process  of 
development,  which  on  the  whole  is  very  uncommon.  On  the  other  hand,  it 
frequently  happens  that  no  special  reserve -tissue  (endosperm)  is  present,  that 
the  small  amount  of  reserve-food  is  deposited  in  the  cotyledons  themselves,  and 
that  immediately  after  germination  has  commenced  the  two  cotyledons  leave 
the  cavity  of  the  seed-coat  and  become  green  foliage-leaves.  As  an  example  of 
this  the  germination  of  the  Gourd  (Cucurbita  Pepo)  is  given  in  fig.  145  l. 

The  way  in  which  cotyledons  are  withdrawn  from  the  cavity  of  the  seed-coat 
is  very  characteristic,  and  it  is  worth  while  to  inspect  the  most  remarkable  con- 
trivances of  this  kind  somewhat  more  carefully.  One  of  the  most  peculiar  is 
observed  in  the  seed  and  embryo  of  the  Gourd,  which  is  figured  opposite  in  natural 
size.  The  seed  of  the  Gourd  is  pretty  large,  flattened,  oval  in  outline,  rounded  at 
one  end,  and  somewhat  tapering  at  the  other,  and  here  cut  off  short,  and  provided 
with  a  small  aperture.  If  these  seeds  are  disseminated  they  lie  flat  on  the  ground, 
and  easily  glue  themselves  to  the  soil,  especially  if  their  surface  is  covered  with  the 
adhesive  juice  of  the  fleshy  fruit,  as  is  always  the  case  when  the  seeds  are  naturally 
distributed.  Since  the  embryo  inclosed  by  the  seed-coat  is  straight,  it  has  a  position 
parallel  to  the  surface  of  the  ground.  When  germination  begins  the  radicle  is  first 
of  all  pressed  out  through  the  small  opening  mentioned.  It  immediately  arches  and 
grows  quickly  downwards  into  the  earth  by  the  help  of  the  food  conveyed  to  it  by 
the  two  cotyledons.  It  there  develops  lateral  rootlets,  and  unites  firmly  with  the 


COTYLEDONS. 


611 


particles  of  soil  by  means  of  abundant  root-hairs.  The  hypocotyl  also,  into  which 
the  root  merges,  grows  at  first  downwards  into  the  earth,  but  of  course  only  for  a 
short  time,  for  this  is  very  soon  altered;  and  growth  then  takes  place  in  an  opposite 
direction  towards  the  light,  and  immediately  after  this  alteration  of  direction  the 
withdrawal  of  the  cotyledons  commences.  It  follows  from  what  has  been  said  that 
the  hypocotyl  is  fixed  both  above  and  below— below  by  the  root  which  has  grown 


Fig.  145.— Liberation  of  the  Cotyledons  from  the  cavity  of  the  seed  or  fruit  husk. 

i  Gourd  (Cucurbita  Pepo).  2  Asafoetida  (Scorodosma  Asa  fcetidd).  «  Immortelle  (Helichrygum  annuum).  *  Cross-section 
through  the  cotyledons,  showing  them  curled  up  in  the  pericarp  of  the  Immortelle.  «  Cardopatium  corymbosum  (after 
Klebs).  Fig.  1-3,  natural  size ;  fig.  4-6,  somewhat  enlarged. 

firmly  into  the  ground,  above  by  the  firmly  glued  seed-covering  in  which  che 
cotyledons  lie.  As  soon  as  it  increases  in  length  it  forms  a  well-marked  arch, 
frequently  even  a  loop,  with  the  convex  side  turned  upwards  (cf.  fig.  145  *). 
Naturally  it  thus  exercises  a  severe  strain  on  both  ends.  The  root,  well  planted  in 
the  earth,  can  no  longer  be  disturbed  from  its  position,  but  the  effects  of  the  tension 
make  themselves  felt  on  the  cotyledons,  which  still  lie  in  the  seed.  The  coat  of  the 
Gourd  seed  bursts,  the  cotyledons  are  drawn  out  from  the  yawning  cleft,  the 
hypocotyl  straightens  itself,  and  the  two  cotyledons  separating  from  each  other 
turn  their  upper  sides  towards  the  light  (fig.  145  \  on  the  left). 


(512  COTYLEDONS. 

The  splitting  of  the  seed-coat  and  the  withdrawal  of  the  cotyledons  in  the 
Gourd  are  materially  assisted  by  the  development  of  a  projecting  lip  at  the  union  of 
the  radicle  and  hypocotyl.  This  lip  presses  against  the  lower  edge  of  the  hard  seed- 
coat,  and  holds  it  to  the  ground,  so  that  after  the  bursting  takes  place  the  upper 
portion  of  the  seed-coat  is  raised  up  lid- wise  from  the  lower.  A  smaller  projection 
is  also  developed  on  the  hypocotyl  in  the  embryo  of  the  Sensitive  Plant  (Mimosa 
pudica),  and  in  that  of  Cuphea,  and  here  again  it  presses  against  the  lower  part  of 
the  seed-husk,  and  so  assists  both  the  bursting  and  the  withdrawal.  When  the 
seeds  are  surrounded  by  a  pericarp,  sometimes  bands  and  projecting  corners 
are  developed  on  it,  sometimes  projecting  edges  of  the  dried  calyx  and  the  like, 
which  serve  as  a  fulcrum  to  the  lip  of  the  hypocotyl.  The  presence  of  numerous 
structures  formerly  considered  to  be  stunted  organs  useless  to  the  plant  thus  receives 
its  natural  explanation. 

Many  plants,  e.g.  certain  Umbelliferse,  develop  a  very  short  hypocotyl.  This 
does  not  bend,  and  exercises  only  an  insignificant  strain,  or  perhaps  none  at  all,  on 
the  cotyledons,  and  so  would  not  be  able  to  release  them  from  the  integument  of 
the  seed  or  fruit-husk.  In  all  these  plants  the  cotyledons  have  long  stalks,  and 
these  assume  the  function  of  the  hypocotyl,  at  least  in  so  far  as  the  withdrawal  of 
the  blades  of  the  cotyledons  is  caused  by  them  in  the  same  way.  This  phenomenon 
is  very  noticeable  in  the  germination  of  the  Asafoetida  (Scorodosma  Asa  foetida),  as 
is  clearly  shown  in  fig.  145  2.  The  stalks  of  the  cotyledons,  arising  from  the  very 
short  hypocotyl,  rapidly  elongate,  and  exhibit  the  same  s-shaped  bend  as  that 
formed  in  the  hypocotyl  of  the  Gourd  seedling.  These  stalks  also  produce  a  similar 
effect  on  the  blades  of  the  cotyledons  still  lying  within  the  fruit-husk,  and  actually 
draw  them  out.  As  soon  as  this  has  happened,  the  stalks  straighten,  and  the 
blades  borne  by  them  turn  their  upper  surfaces  towards  the  light. 

Almost  a  third  of  all  seed-bearing  plants  have  cotyledons  whose  liberation  from 
the  bondage  of  the  seed-coat  or  pericarp  is  effected  in  this  manner,  and  this 
consequently  is  the  form  of  cotyledons  which  has  been  most  frequently  observed 
and  described.  Much  less  frequently  the  two  cotyledons  make  their  appearance  at 
one  end  of  the  pericarp  or  seed-coat,  while  the  radicle  grows  out  at  the  opposite 
end.  In  this  case,  which  must  be  regarded  as  the  eighth  of  the  series  here  given, 
the  embryo  is  straight,  and  the  hypocotyl  is  short  and  bears  two  thick  cotyledons 
whose  apices,  lying  close  together,  form  a  truncated  cone.  When  the  radicle  has 
been  once  pushed  out,  and  has  planted  itself  firmly  in  the  ground,  the  hypocotyl 
at  once  elongates  in  the  opposite  direction  without  bending,  pushes  the  folded 
cotyledons  in  front  of  it,  and  presses  them  out  of  the  fruit-husk.  The  tissue  of  the 
fruit-husk  lying  above  the  cotyledonary  cone  must  be  pierced,  and  this  is  not 
difficult  to  do,  since  this  tissue  consists  of  thin- walled  cells.  When  the  radicle  has 
grown  out  from  one  pole,  and  the  pair  of  cotyledons  from  the  other,  the  seedling 
is  surrounded  half-way  up  by  the  vacated  fruit -husk,  as  though  by  a  girdle 
(cf.  fig.  145 5).  The  apices  of  the  cotyledons  still  folded  cone-like  together 
usually  bore  through  the  soil  above  the  husk  after  they  have  left  the  cavity,  and 


COTYLEDONS.  613 

not  until  this  has  occurred  do  they  unfold  and  become  green.  In  this  penetration 
of  the  earth  the  cotyledons  are  exposed  to  so  many  dangers  that  special  arrange- 
ments are  frequently  to  be  found  with  a  view  to  protecting  their  advancing  points. 

As  the  cotyledons  push  through  the  ground,  a  pressure  is  exercised  upon  the 
layers  of  soil  above  by  the  straightening  of  their  stalks.  The  cotyledons  raise 
portions  of  soil  on  their  backs,  so  to  speak,  without  actually  bursting  or  boring 
through  them.  In  this  way  the  danger  of  injury  is  at  any  rate  much  diminished, 
and  the  supposition  is  fully  justified  that  cotyledons  which  develop  after  the  type 
of  the  Gourd  or  Asafcetida  are  those  which  occur  most  frequently.  Plants  whose 
straight  embryo  has  to  pierce  through  the  fruit-husk  and  the  layer  of  earth  above 
it  by  means  of  their  conically-folded  cotyledon  apices  are,  as  already  stated,  less 
common.  Fig.  145 5  shows  this  rare  form  in  Cardopatium  corymbosum.  It  has 
also  been  observed  in  many  other  species  allied  to  this  composite,  and  in  the 
Mediterranean  Atractylis  cancellata. 

In  all  those  cases  where  the  cotyledons  are  withdrawn  through  a  cleft  or  hole 
in  the  seed -coat  it  seems  quite  obvious  that  the  aperture  should  have  a  diameter 
at  least  equal  to  that  of  the  organs  to  be  withdrawn.  As  a  rule  this  is  so;  but 
occasionally  the  cotyledons  are  actually  broader  than  the  cleft,  and  one  asks  in 
astonishment  how  the  withdrawal  could  have  been  accomplished  without  injury  to 
their  fabric.  The  feat  is  performed  in  the  following  way.  Before  the  strain  on 
the  cotyledons  comes,  they  are  folded  together,  and  are  then  drawn  out  as  a  long 
roll  through  the  narrow  opening  of  the  integument.  Scarcely  have  they  been 
liberated  ere  they  begin  to  unroll  and  spread  themselves  out  flat.  This  is  the  case, 
for  example,  in  the  Immortelle  (Helichrysum  annuum)  (see  figs.  1453  and  1454),  also 
in  the  umbellifer,  Smyrnium  Olusatrwm,  and  in  many  others.  In  some  plants,  e.g. 
in  the  Beech  (Fagus  sylvatica),  the  cotyledons,  as  long  as  they  remain  within  the 
husk,  are  folded  together  lengthwise  like  a  fan,  and  in  this  position  occupy  but  a 
limited  space.  They  are  also  withdrawn  from  the  nut  through  a  comparatively 
small  aperture,  and  then  expand  in  a  very  short  time  after  they  are  free  (see  figs. 
1481'2-3).  The  two  cotyledons  of  Pterocarya,  caucasica  are  each  divided  into 
four  lobes,  and  each  pair  of  lobes  lying  close  to  one  another  are  imbedded  in  a  special 
excavation  in  the  seed.  Altogether  the  fruit  presents  four  compartments,  in  each 
one  of  which  lies  such  a  pair  of  narrow,  closely-packed  lobes.  The  aperture  of  the 
nut-like  pericarp  now  affords  sufficient  space  for  each  pair  of  folded  lobes  to  be 
drawn  out,  nor  does  their  withdrawal  occur  simultaneously,  but  rather  so  that 
the  pairs  of  lobes  emerge  one  after  the  other.  The  cotyledons  of  Schizopetalon 
Walkeri  behave  in  a  similar  manner,  each  of  them  being  divided  into  two  long 
narrow  lobes  which  are  drawn  out  successively  through  the  small  aperture  of  the 
spherical  seed.  Moreover,  in  the  embryos  of  Pinus  there  are  five  or  more  whorled 
linear  cotyledons  (see  fig.  148 6).  These  also  leave  the  cavity  of  the  seed  one  after 
the  other.  Speaking  generally,  it  would  seem  that  the  dimensions  and  form  of  the 
cotyledons  are  correlated  with  the  shape  of  the  seed-coat  or  other  investment,  and 
with  its  manner  of  opening. 


614  COTYLEDONS. 

The  external  form  of  the  seed  and  the  position  which  it  consequently  assumes 
on  falling  to  the  ground  is  by  no  means  an  unimportant  item  in  this  connection. 
If  the  seed  comes  to  lie  so  that  the  axis  of  the  hypocotyl  is  perpendicular  to  the 
surface  and  the  tip  of  the  radicle  is  directed  downwards,  we  seem  at  first  sight  to 
have  a  very  favourable  position  for  germination;  but  it  is  not  so  in  reality.  In 
this  position  the  hypocotyl  would  have  to  perform  the  most  complicated  curves  in 
order  to  be  able  to  withdraw  the  cotyledons  from  the  seed.  On  the  other  hand, 
the  most  favourable  condition  is  obtained  when  the  axis  of  the  hypocotyl  and 
radicle  lie  parallel  to  the  surface  of  the  ground,  as,  for  example,  in  the  gourd 
seeds  illustrated  in  fig.  145  \  Here  the  radicle  immediately  after  leaving  the  seed- 
coat  can  bend  down  at  a  right  angle  and  grow  into  the  earth,  and  the  hypocotyl 
is  able  to  withdraw  the  cotyledons  very  rapidly.  When  seeds  of  this  sort  are 
sown,  they  usually  assume  the  last -mentioned  position.  Flat  or  compressed 
seeds  lie  with  their  broad  surface  on  the  ground;  oval  and  elongated  cylindrical 
seeds  fall  so  that  their  longer  axis  is  parallel  to  the  substratum;  whilst  in 
spherical  seeds  the  centre  of  gravity  is  so  situated  that  the  most  favourable  posi- 
tion possible  for  germination  is  obtained. 

The  importance  of  numerous  developments  on  the  exterior  of  the  seed- 
coat  or  pericarp  will  at  once  become  evident  to  anyone  who  observes  attentively 
the  process  of  withdrawal  of  cotyledons.  It  is  manifest  that  the  withdrawal 
only  occurs  without  delay  when  the  seed  is  in  some  way  or  other  firmly  fixed 
and  when  arrangements  are  present  which  prevent  a  favourable  position  being 
lost  when  once  assumed.  This  would  not  be  so  were  the  seed  the  plaything  of 
every  gust  of  wind  or  current  of  water.  Equipments  for  retaining  fruits  and 
seeds  in  the  position  of  germination  occur  in  great  number  and  variety.  Even 
the  wing-like  and  hairy  appendages,  the  curved,  pointed,  and  barbed  processes, 
and  the  various  adhesive  arrangements  of  fruits  and  seeds,  which  in  the  first 
instance  have  the  function  of  agents  for  distributing  the  fruits,  often  afford 
this  advantage,  viz.  that  by  their  means  the  seeds  are  fixed  where  germination 
can  successfully  take  place.  If  we  look  at  the  damp  mud  by  a  river's  bank, 
towards  the  end  of  May,  when  the  fluffy  seeds  of  willows  and  poplars  are 
escaping  from  the  dehisced  fruit-capsules  and  are  carried  along  by  the  wind, 
we  there  see  countless  numbers  of  these  seeds  sticking  by  their  hairs  to  the 
mud  so  tightly  that  they  cannot  readily  be  displaced.  All  such  seeds  (differing 
from  the  generality  of  seeds)  germinate  in  a  few  days,  while  seed  lying  on  the 
ground  in  loose  flakes  close  by  do  not  germinate.  The  hairy  coat  which  first 
served  as  an  agent  for  distributing  the  seed,  now  functions  as  an  agent  for 
fixing  it  in  the  germinating  bed.  This  also  applies  to  the  tufts  of  silk 
adorning  the  email  seeds  of  tropical  tillandsias  (Tillandsia  usneoides  and 
T.  recurva)  which  grow  as  epiphytes  on  the  bark  of  trees.  These  first  serve 
as  wings,  and  the  tiny  seeds  are  carried  by  the  wind  far  away  from  the 
burst  fruit-capsules.  If  these  seeds  are  stranded  on  the  bark  of  a  tree- 
trunk  which  is  swept  by  the  wind,  the  hairs  cling  firmly  and  bring  the  seeds 


COTYLEDONS.  015 

into  contact  with  the  substratum.  Accordingly  the  weather  side  of  the  trunk 
is  seen  to  be  beset  with  large  numbers  of  such  seeds,  and  many  of  them, 
pressed  into  the  crannies  of  the  substratum,  begin  to  germinate.  A  similar 
process  is  observed  in  the  settling  of  the  seeds  of  Anemone  sylvestris  and  many 
composites.  To  cite  yet  another  example,  we  may  name  the  hooked  fruits  of 
Xanthium  spinosum  and  Lappago  racemosa.  When  detached  from  their  place 
of  origin  by  wandering  animals,  these  seeds  remain  fixed  by  their  barbed  processes 
to  the  hairy  coats  of  the  animals,  and  thus  are  often  removed  considerable 
distances.  Naturally  the  animals  try  to  free  themselves  from  these  irritating 
appendages  by  rubbing  themselves  against  the  ground  until  they  detach  the 
fruits  from  their  coats.  In  this  way  a  part  of  the  fruits  are  pressed  into  the 
soil,  and  are  there  anchored  by  their  barbed  spines.  Only  the  embryos  of  the 
firmly-anchored  fruits  develop  into  vigorous  plants;  those  seeds  which  lie  more 
loosely  on  the  ground,  on  the  contrary,  either  do  not  germinate  at  all,  or  the 
seedlings  whose  cotyledons  are  imperfectly  withdrawn  from  the  pericarp  soon 
perish. 

Besides  these  outgrowths,  which,  as  we  see,  possess  a  double  function,  there 
are  also  those  which  have  no  connection  whatever  with  distribution,  and  have 
apparently  no  other  use  than  to  fasten  the  seeds  to  the  germinating  bed.  In 
this  connection  we  have  first  to  notice  adhesive  materials  which  exude  from 
the  surface  of  the  seed-husk,  whereby  the  seeds  are  cemented  to  the  soil.  These 
make  their  appearance  when  the  surface  of  the  seed  is  moistened,  as  when 
water  is  sucked  up  by  the  seed  from  the  soil  of  the  germinating  bed.  Usually 
the  slimy  cement  arises  from  the  superficial  cells,  as,  for  example,  in  the  many 
species  of  flax  and  plantain  (Linum  and  Plantago),  in  the  Cress  and  the 
Gold-of -pleasure  (Lepidium  sativum  and  Camelina  sativa),  in  Teesdalia,  Gilea, 
and  Collomia,  and  in  many  other  species  of  the  most  diverse  genera.  All, 
however,  agree  in  this  particular,  that  the  seed-coat  possesses  a  smooth,  polished 
surface.  In  the  Basil  (Ocymum  Basilicum)  and  in  the  numerous  species  of 
Salvia  and  Dracocephalum,  the  mucilaginous  substance  arises  from  the  smooth 
surface  of  the  pericarp.  Frequently  the  adhesive  mucilage  is  only  developed 
in  certain  cells  arranged  in  rows  on  the  surface  of  the  fruit  or  seed-husk,  as 
in  the  New  Zealand  Selliera  and  in  numerous  Composite,  of  which  the  Wild 
Chamomile  (Matricaria  chamomilla)  may  be  cited  as  the  best-known  example. 
In  Oxybaphus  there  are  five  longitudinal  ridges  on  the  integument  of  the  seed 
covered  with  small  mucilage -organs.  When  the  integument  is  moistened,  five 
white  slimy  lines  appear  on  it,  and  these  bring  about  its  adhesion  to  the 
germinating  bed.  In  many  Composite,  e.g.  in  the  common  Groundsel  (Senecio 
vulgaris)  and  in  Euriops,  Doria,  Trichocline,  and  in  many  other .  genera,  special 
hairs  are  developed  on  the  fruit-husk  which  excrete  adhesive  mucilage.  In 
other  instances,  again,  as  in  many  aroids,  the  cement  is  not  developed  by  super- 
ficial cells,  but  a  part  of  the  fleshy  pericarp,  in  which  the  seeds  were  inclosed, 
remains  as  a  dried-up  crust.  If  these  seeds  be  subsequently  moistened,  the 


COTYLEDONS. 

crust  again  becomes  changed  into  a  mucilaginous  adhesive  mass  which  glues 
them  to  the  substratum.  The  whole  succulent  decaying  pericarp  often  becomes 
the  fixing  agent  of  the  seeds,  as,  for  example,  in  gourd-like  plants,  and  in  many 
plants  with  berries  and  stone-fruits. 

In  numerous  plants,  e.g.  in  the  Corn-cockle  (cf.  figs.  141 7>8> 9' 10),  and  in 
Neslia  paniculata  which  grow  abundantly  in  loamy  fields,  the  fixing  of  the 
fruits  or  seeds  to  the  soil  is  not  effected  by  mucilaginous  cement-materials,  but 
by  inequalities  on  the  surface  of  the  integument.  Here  are  developed  extremely 
diversified  warts,  pegs,  ridges,  or  net-works,  and  between  them  pit-like  depres- 
sions into  which  the  earth  -  particles  penetrate,  and  when  moistened  become 
closely  connected  with  the  superficial  cells.  The  adhesion  is  therefore  very 
close,  and  if  we  try  to  cleanse  these  seeds  or  fruits,  and  to  remove  the  clinging 
soil  from  all  the  small  hollows,  we  shall  not  completely  succeed  even  after  a 
great  deal  of  trouble.  And  here  we  must  point  out  the  interesting  distinction 
between  rugged  seeds  like  these,  and  such  as  become  slimy.  Seeds  with  rough, 
wrinkled,  and  pitted  surfaces  never  develop  adhesive  agents,  since  they  are  fixed 
to  the  soil  by  these  inequalities  of  the  seed-coat;  on  the  other  hand,  seeds  with 
smooth  surface,  which  would  otherwise  be  easily  displaced,  adhere  by  means  of 
mucilage  developed  by  their  epidermal  cells. 

The  Water-chestnut  (Trapa),  whose  germination  was  described  on  p.  609, 
behaves  in  a  very  peculiar  manner.  Each  of  its  large  fruits  exhibits  two  pairs 
of  projecting  spines  arranged  cross- wise,  which  have  been  formed  from  the 
sepals,  and  which  protect  it  during  ripening  from  the  attacks  of  aquatic 
animals.  These  spines,  as  well  as  the  whole  fruit,  are  as  hard  as  stone,  but 
only  in  the  interior.  The  outer  cell-layers  are  soft,  decompose  quickly  under 
water,  and  separate  from  the  deeper  tissue  in  irregular  tatters  and  shreds. 
At  the  apex  of  the  spines,  after  the  detachment  of  the  soft  portion  there 
remain  not  only  the  strong  hard  midrib,  but  also  the  commencements  of  some 
recurrent  bundles  of  very  firm  elongated  cells  which  spring  from  the  midrib 
immediately  behind  the  apex.  These  spines  therefore  have  the  appearance  of 
anchors  (see  fig.  146),  and  indeed  they  function  as  anchors,  adhering  at  the 
bottom  of  the  lake  by  means  of  their  barbed  points  to  various  vegetable 
remains  which  cover  the  mud,  and  are  actually  anchored  there.  The  seedling 
arising  from  the  nut  does  not  consequently  lift  the  pericarp  with  it,  but  this 
remains  fixed  in  the  place  where  it  fell. 

Peculiar  contrivances  for  anchoring  fruits  to  spots  suited  for  germination  are 
observed  in  many  steppe-grasses,  especially  in  the  Feather-grass  (Stipa)  and  in 
the  Stork's -bill  genus  (Erodium).  The  feather-grasses  are  a  striking  feature 
of  steppes;  indeed,  together  with  various  Papilionacese — especially  with  tragacanth 
shrubs  (Astragalus) — and  with  numerous  Composite,  pinks,  and  low  irises,  they 
compose  the  chief  constituent  of  the  vegetation.  The  appearance  of  the  Feather- 
grass is  appropriate  to  its  name,  consisting  as  it  does  of  tufts  of  white  feathery 
streamers  swinging  in  the  wind.  This  characteristic  feature  makes  its  appearance 


COTYLEDON'S. 


617 


after  the  flowers  have  been  pollinated,  and  is  due  to  the  awns  which  belong  to 
the  flowering  glumes,  of  which  one  surrounds  each  ovary,  undergoing  a  very 
remarkable  elongation  and  hair-like  branching,  recalling  a  like  behaviour  in  the 
styles  of  Clematis  and  some  species  of  Anemone. 

The  glume,  which  is  crowned  with  the  feather-like  awn,  together  with  a  second 
short  glume,  destitute  of  awn,  incloses  the  small  fruit.  As  soon  as  it  is  ripe,  the 
fruit,  wrapped  in  its  glumes,  becomes  detached;  the  first  breeze  carries  it  off  and 
blows  it  like  down  over  the  steppe.  The  long  feathery  awn  arising  from  the 
glume  has  thus,  in  the  first  instance,  the  significance  of  a  flying  apparatus,  like 
so  many  of  the  feathery  or  wing -shaped  structures  which  beset  or  envelop  seeds 
and  fruits.  It  effects  the  distribution  of  the  feather-grass  in  question  over  wide 


Fig.  146.— Anchoring  of  the  Water-chestnut  (Trapa). 

tracts  of  country.  But  after  the  awn  has  become  stranded  somewhere  on  the 
soil  of  the  steppes,  it  has  yet  another  function  to  perform. 

Let  us  suppose  that  a  feather-grass  fruit  has  fallen  on  to  the  bare  earth,  as 
in  the  illustration  on  p.  619;  that  part  containing  the  fruit  inclosed  in  the  glume, 
as  it  is  the  heaviest,  will  obviously  come  first  into  contact  with  the  ground;  and 
since  the  tip  of  this  portion  is  hard  and  sharply  pointed,  the  stranded  fruit  often 
sticks  in  the  ground  immediately  upon  alighting  (fig.  147  l).  Should  it  fall  obliquely, 
the  tip  will  penetrate  into  the  ground  by  the  continued  twisting  of  the  long 
feather  waving  in  the  air.  This  first  penetration  is  materially  favoured  by  the 
fact  that  the  point  is  bent  a  little  obliquely  towards  one  side. 

When  once  the  point  has  penetrated  into  the  soil,  the  other  portions  of  the 
glume  surrounding  the  fruit  soon  follow,  owing  to  the  action  of  the  following 
contrivance.  Close  above  the  point  of  the  enveloping  glume  are  inserted  up- 
wardly-directed hairs  which  are  at  once  elastic,  flexible,  and  yet  stiff.  As  long 
as  these  stiff  hairs  lie  close  they  offer  no  resistance  to  the  penetration  of  the 
glume  into  the  soil,  and  some  of  them  are  actually  embedded  in  the  soil  even  at 
the  first  penetration  of  the  point.  Now  if  the  fruit  as  it  pierces  the  ground  be 
inclined  to  one  side,  by  some  pressure  operating  ever  so  lightly  from  above,  then 
the  hairs  on  that  side  are  pressed  still  more  closely  against  the  glume,  while  those 


(518  COTYLEDONS. 

on  the  other  are  made  to  stand  off  somewhat;  these  latter  press  against  the 
particles  of  earth  above  them,  and  act  as  levers,  by  means  of  which  the  whole 
fruit  at  the  moment  of  bending  in  the  given  direction  becomes  pressed  down 
deeper  into  the  ground.  This  action  is  continued  every  time  the  fruit  wobbles 
from  side  to  side,  so  that  bit  by  bit  it  is  buried.  The  only  question  is,  how  these 
sudden  alterations  in  position  of  the  fruit  fixed  in  the  ground  can  be  brought 
about.  A  glance  at  fig.  147  shows  that  every  wind,  even  though  slight,  which 
strikes  the  long  feathery  portion  of  the  awn,  must  immediately  be  followed  by 
an  alteration  in  the  position  of  the  fruit.  Just  as  a  weather-cock  on  the  top  of 
a  roof  in  a  strong  east  wind  does  not  point  invariably  towards  the  east,  but 
generally  makes  short  veerings  to  the  north  and  south,  so  the  plumed  awns 
fluttering  to  and  fro  in  the  east  wind  swerve  momentarily  towards  the  north  and 
south,  and  this  change  of  direction  causes  the  fruit  sticking  in  the  soil  to  incline 
to  various  sides.  When  the  wind  veers  round  of  course  an  alteration  occurs  in  the 
direction  of  the  feathery  awn,  and  consequently  a  more  strongly-marked  inclination 
of  the  fruit  occurs,  so  that  a  see-sawing  motion  of  the  latter  will  be  unavoidable. 
The  wind,  therefore,  is  an  important  factor  in  driving  the  fruit  into  the  ground. 
The  awns  of  the  Feather-grass,  however,  have  two  other  peculiar  contrivances, 
viz.,  below  the  feathery  portion  they  are  bent  twice  like  a  knee,  and  they  are 
also  spirally  twisted  like  a  corkscrew.  This  bent  and  twisted  part  of  the  awn  is 
exceedingly  hygroscopic;  in  rainy  weather  the  knee-shaped  bend  almost  entirely 
disappears;  the  awn  bristles  and  straightens  itself,  the  spiral  unwinds  in  damp 
weather,  and  twists  up  again  in  dry  air.  These  movements  are  evidently  conveyed 
to  the  glume,  and  produce  alterations  in  its  inclination,  which  again  cause  an 
advancement  of  the  tip  into  deeper  layers  of  earth. 

These  movements  of  the  lower  portion  of  the  awn  produced  by  the  varying 
humidity  of  the  air  make  themselves  specially  felt  when  the  upper  portion  has  in 
some  way  become  entangled  with  the  stems  and  leaves  of  the  other  steppe-plants,  as 
frequently  happens.  When  in  dry  weather  the  fruits  of  the  Feather-grass  become 
detached  and  are  blown  by  the  wind  over  the  steppes,  it  is  almost  unavoidable 
that  they  should  remain  fastened  by  their  knee-like  bent  portion  to  haulms,  stems, 
and  the  like — that  the  feathery  part  should  be  hemmed  in  between  two  neighbour- 
ing stems  of  other  plants,  or  occasionally  even  entangled  with  them  (cf.  fig.  147  2). 

But  as  soon  as  the  upper  portion  of  the  awn  is  fixed,  and  later  on  in  damp 
weather  the  lower  knee-shaped  portion  of  the  same  awn  has  become  straightened 
and  the  spiral  twists  uncoiled,  the  fruit  is  necessarily  forced  into  the  ground  with 
a  twisting  movement,  and  is  also  pressed  now  to  this  side  and  now  to  that  by  the 
unequal  straightening  of  the  knee-shaped  bend.  Any  backward  movement  of  the 
fruit  from  a  subsequent  drying  up  of  the  awn  is  prevented  by  the  above-named 
stiff  hairs,  in  the  manner  already  described.  It  is  much  more  likely  that  one  of 
the  stems  to  which  the  awn  has  attached  itself  should  be  somewhat  bent  by  this 
contraction  of  the  awn,  than  that  the  glume  already  driven  a  certain  depth  into 
the  ground  and  there  anchored  should  be  drawn  out. 


COTYLEDONS. 


619 


of  tl  VrT  °m)  get  Pknted  in  the  sa™  ™y  -  those 

of  the  Feather-grass.     The  five  mericarps  (or  fruit  segments)  in  this  plant  detach 

hrteriStiC  manaer  fr°m  ^  SUPP°rt'  -  ^  be  s^n 
the  lower  thick  end  inclosing  the  seed  splits  off/and  later 


, 

Fig.  147.— Showing  the  boring  of  fruits  into  the  ground, 
i,  2  Fruits  of  the  Feather-grass  (Stipa  pennata).    *,  *  Fruits  of  the  Stork's-bill  (Erodium  deuterium). 

also  the  long  drawn-out  point  of  the  carpel.  A  part  of  the  latter  twists  up 
spirally,  and  only  its  free  end  stretches  out  in  a  slight  curve,  like  the  hand  of  a 
watch.  It  is  well  known  that  this  fallen  fruit-segment  is  used  as  a  hygrometer. 
It  is  placed  with  its  lower  thick  end  which,  like  the  fruit-end  of  the  Feather-grass, 
possesses  a  sharp  point,  on  a  board  covered  with  paper,  in  the  centre  of  a  circle. 
Marks  are  made  on  the  circumference  of  the  circle  corresponding  to  the  position 


620 


COTYLEDONS. 


of  the  pointer- like  end  of  the  Stork's -bill  fruit  in  very  damp  and  in  very  dry 
weather  respectively,  and  we  can  then  draw  conclusions  from  the  position  of  the 
pointer  as  to  the  relative  dampness  of  the  air.  In  this  application  of  the  fruit  we 
have  an  exhibition  of  the  torsions  which  take  place  in  the  course  of  its  penetration 
into  the  ground,  and  which  are  produced  by  the  alterations  in  the  humidity  of  the 
air.  When  such  a  fruit  falls  on  the  ground,  however,  it  is  not  the  lower  thick  end 
inclosing  the  seed  which  is  fixed,  as  in  the  hygrometer,  but  the  pointer-like  process, 
and  consequently  in  nature  it  is  the  seed-end  and  not  the  pointer  which  is  set  in 
motion  by  an  alteration  of  humidity  in  the  air.  The  fixing  of  the  fruit  in  the 
ground  is  naturally  effected  thus:  the  point  of  the  arm  lies  on  the  soil,  and  in 
consequence  of  the  untwisting  of  the  spirals  in  damp  weather,  the  thicker  seed- 
containing  end  (which  is  provided  with  a  sharp  point)  bores  deeply  into  the 
ground.  More  frequently  the  fallen  fruits  hang  between  the  entangled  stems,  &c., 
of  other  plants  lying  on  the  ground,  as  shown  in  fig.  147 4.  Here  again  the  arm  is 
fixed,  and  the  thicker,  lower  end  is  set  in  motion.  The  movement  may  be  compared 
to  that  of  an  augur,  although  in  consequence  of  the  swaying  and  alteration  in 
position  of  the  beak,  unavoidable  in  windy  weather,  see-sawing  movements  occur 
in  the  boring  part,  and  these  are  apparently  advantageous.  Like  the  fruits  of  the 
Feather-grass  those  of  the  Stork's-bill  are  beset  above  the  sharp  point  with  erect, 
stiff  hairs.  These  hairs  also  play  the  same  part  as  in  the  Feather-grass.  Accord- 
ing to  the  species  various  divergencies  are  found  in  the  fruits  of  Feather-grasses 
and  Stork's-bills.  The  twisting  of  the  lowest  portion  of  the  awn  generally  differs 
from  that  of  the  knee-like  bent  part;  the  bristles  on  the  glumes  are  sometimes 
arranged  in  two  longitudinal  rows  and  sometimes  they  form  a  ring  below  and 
are  continued  upwards  as  a  one-sided  longitudinal  stripe,  and  so  forth.  Many 
species  of  Stipa  have  no  plume  to  the  awn,  and  approximate  closely  in  form 
to  the  fruit  of  the  Stork's-bill.  The  same  remark  applies  to  fruits  of  the  genera 
Aristida  and  Heteropogon,  which  are  allied  to  Stipa.  But  in  the  main  all  these 
developments  agree  with  one  another.  The  aim  and  object  of  the  wonderful 
mechanism  just  described  is  not  so  much  the  penetration  of  the  pericarp  or  seed- 
husk  to  a  great  depth  into  the  soil,  as  the  fixing  of  it  firmly  in  the  germinating  bed. 
It  still  remains  to  be  pointed  out  that  the  cotyledons  are  only  withdrawn  with- 
out delay  from  their  investments  when  the  latter  are  firmly  cemented,  anchored,  or 
fixed  in  some  way  to  the  substratum.  When  this  is  not  the  case,  it  often  happens 
that  the  fruit,  or  seed-coat,  is  raised  up  like  a  cap  by  the  enlarging  cotyledons. 
The  pull,  otherwise  exerted  by  the  elongating  hypocotyl,  cannot  under  these 
circumstances  assist  the  cotyledons  in  their  liberation.  Often,  indeed,  the  cotyle- 
dons throw  off  the  seed-coat  unaided,  but  this  is  not  always  the  case.  In  many 
instances  their  apices  remain  squeezed  up  in  the  cavity  of  the  husk,  stunted  and 
yellow,  and  this  reacts  injuriously  on  the  seedling,  often  causing  weakness  and 
even  death.  It  is  therefore  a  mistake  for  gardeners  to  plant  seeds  in  loose  earth 
where  no  good  hold  is  afforded,  since  then,  on  germination,  the  seed-coats  are  raised 
up  by  the  only  half -liberated  cotyledons,  whose  apices  are  still  imprisoned. 


COTYLEDONS. 


621 


With  regard  to  the  forms  assumed  by  the  cotyledons  now  withdrawn  from  the 
seed  under  favourable  conditions,  and  which  have  become  green  in  the  sunlight,  it  is 
to  be  noticed  that  they  present  far  fewer  variations  than  those  of  ordinary  foliage- 
leaves.  Usually  their  margins  are  entire,  their  form  elliptical  or  linear,  more 
rarely  circular  and  obovate.  Sometimes  the  cotyledons  are  indented  in  front, 
resembling  a  heart  in  outline;  this  is  especially  the  case  where  the  embryo  is  folded 


Fig.  148.— Cotyledons. 

*,  2,  8  Fagus  sylvatica.  *  Fumaria  offlcinalis.  *  Galeopsis  pubescent.  «  Abie*  orientalis.  *  Convolvulus  arvensig.  •  Borago 
officinalis.  *  Senecio  eruccefolius.  u>  Rosa  canina.  "  Erodium  Cicutarium.  «  Quamoclit  coccinea.  «  TUia  grandifolia. 
i*  Lepidium  satimtrn.  «  Eucalyptus  orientalis.  16  Eucalyptus  coriaceus.  "-2°  Streptocarput  Rexii. 

in  the  seed,  so  that  the  radicle  lies  close  to  the  outer  margin  of  the  cotyledons,  and 
may  be  explained  as  an  economy  of  the  scanty  space  within  the  seed.  Most  rarely 
of  all  the  cotyledons  are  two-lobed  (Raphanus  sativus),  and  bisected  (Eucalyptus 
orientalis,  Eschscholtzia  Califomica),  three-lobed  (Erodium  Cicutarium),  and  tri- 
sected (Lepidium  sativum),  four-lobed  (Pterocarya  Caucasica),  and  five-lobed 
(Tilia).  It  is  still  to  be  mentioned  that  in  all  seedlings  whose  hypocotyl  is  short, 
the  blade  of  the  cotyledons  has  a  long  stalk,  while  in  seedlings  with  elongated 
hypocotyls  the  blade  is  sessile.  This  is  connected  with  the  processes  already 
mentioned,  and  also  partly  with  the  fact  that  it  is  of  importance  to  seedlings  that 


(J22  COTYLEDONS. 

their  green  blades  should  be  exposed  as  much  as  possible  to  the  sun,  and  that  so 
they  should  rise  above  other  objects  which  might  place  them  in  the  shade.  The 
accompanying  figure  148  shows  the  most  noticeable  forms  of  cotyledons  after  they 
have  unfolded  and  spread  out  in  the  sunlight. 

When  two  green  cotyledons  are  present  they  are  usually  similar  in  shape  and 
size,  only  that  which  has  served  in  the  seed  as  an  absorbent  organ  is  generally 
somewhat  smaller  in  the  adult  condition,  as,  for  example,  in  the  Corn-cockle, 
Mustard,  and  Hemp.  Frequently  the  limited  character  of  the  space  within  the  seed 
makes  it  necessary  that  one  of  the  cotyledons  should  give  place  to  the  radicle,  or 
that  it  should  only  attain  to  an  inconsiderable  development,  as,  for  example,  in 
Petiveria  and  Abronia.  In  species  of  Streptocarpus  belonging  to  the  Gesneracese 
(see  figs.  148 17> 18' 19>  2(X),  the  two  cotyledons  have  the  same  shape  and  size  in 
the  seed,  after  they  have  left  the  seed-coat  they  are  still  entirely  similar,  but  later 
on  the  growth  of  one  is  retarded,  and  it  dies,  while  the  other  increases  to  an  extra- 
ordinary degree,  and  develops  into  a  green  foliage-leaf  lying  on  the  ground,  22  cm. 
long,  and  12  cm.  broad.  Strangely  enough,  many  species  of  this  genus,  e.g. 
Streptocarpus  polyanthus  develop  no  other  green  leaves,  but  content  themselves 
with  the  development  of  the  one  cotyledon  into  a  gigantic  foliage-leaf  prostrate  on 
the  soil,  with  which  later  on  the  epicotyl  appears  to  be  united,  and  from  whose 
thick  midrib  it  rises  up  as  a  flowering  axis. 

It  is  without  question  that  cotyledons  which  become  green  possess,  in  common 
with  other  green  tissues,  the  property  of  manufacturing  organic  materials  in  the 
sunlight  from  the  absorbed  food-gases,  salts,  and  water.  As  a  rule  chlorophyll  does 
not  appear  until  the  cotyledons  have  issued  from  the  seed-coat,  and  have  spread  out 
in  the  sunlight.  It  is,  however,  sometimes  formed  even  while  the  cotyledons  are 
still  in  the  seed  and  shrouded  in  darkness,  as,  for  example,  in  firs  and  pines,  in 
maples,  and  some  Cruciferse,  in  Loranthus,  Mistletoe,  and  the  Japanese  Sophora. 
Green  cotyledons  exhibit  all  the  characteristics  of  foliage;  their  epidermis  is  pro- 
vided with  stomata,  whilst  palisade-cells  and  spongy  parenchyma  can  usually  be  dis- 
tinguished in  the  green  tissue.  Many  plants,  especially  those  which  subsequently 
develop  subterranean  tubers,  or  tuberous  roots,  e.g.  many  species  of  Ranunculus, 
Monkshood,  Corydalis,  Eranthis,  Leontice,  Bunium,  Smyrnium  perfoliatum,  and 
Chcerophyllum  bulbosum,  do  not  in  the  first  year  after  germination  go  beyond  the 
formation  of  green  cotyledons;  green  shoot-leaves  are  not  developed  from  the  bud 
or  plumule  until  the  next  year.  Many  plants,  on  the  other  hand,  unfold  green 
shoot-leaves  almost  simultaneously  with  the  cotyledons,  but  the  cotyledons  function 
with  them  as  foliage,  and  sometimes  remain  fresh  and  green  until  the  time  of 
flowering,  or  even  until  the  ripening  of  the  fruit.  Examples  of  these  are  afforded 
by  numerous  quick-growing  annual  weeds  in  our  fields  and  kitchen-gardens 
(e.g.  Fumaria  officinalis,  Scandix  Pecten-Veneris,  Arnoseris  pusilla,  Urtica  urens, 
Adonis  wstivalis).  The  cotyledons,  in  rapidly-developing  annuals,  sometimes  attain 
dimensions  scarcely  inferior  to  those  of  the  green  shoot-leaves.  For  example,  the 
cotyledons  of  the  Gourd  are  more  than  a  decimeter  long,  and  4-5  cm.  broad.  It  is 


SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES. 

to  be  expected  that  these  green  cotyledons,  whose  function  is  precisely  the  same  as 
that  of  the  green  leaves  of  the  shoot,  should  also  be  protected  in  exactly  the  same 
way  against  external  injurious  influences,  and  as  a  matter  of  fact  many  of  the  protec- 
tive contrivances  are  found  on  them  which  have  been  described  in  detail  previously. 
The  cotyledons  of  many  Boragineae  are  beset  with  stiff  bristles  (e.g.  Borago, 
Caccinia,  Anchusa,  Myosotis  (see  fig.  148 8);  those  of  roses  are  fringed  with  glandular 
hairs  (see  fig.  148 10);  and  those  of  many  nettles  bear  stinging  hairs  on  their  upper 
surface.  It  has  been  already  pointed  out  on  p.  350  that  cotyledons  protect 
themselves,  and  the  young  shoot-leaves  hidden  between  them,  against  the  injury 
which  might  happen  from  loss  of  heat  on  clear  nights  by  folding  together  and 
assuming  a  vertical  position. 

SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES. 

When  the  leaves  borne  on  the  shoot  were  distinguished  as  scale-leaves,  foliage- 
leaves,  and  floral-leaves,  it  was  not  implied  that  these  three  kinds  of  leaf -structures 
were  actually  developed  on  all  shoots.  Scale-leaves  are  only  found  developed  on 
perennial  plants.  In  annual  plants  they  are  entirely  absent.  Even  the  bud  which 
arises  at  the  apex  of  the  hypocotyl  of  an  annual  plant  begins  at  once  with  green 
foliage-leaves,  nor  are  traces  of  scale-leaves  to  be  seen -in  the  buds  which  are  subse- 
quently developed  on  the  epicotyl.  Now,  what  can  be  the  cause  of  this  difference 
between  annual  and  perennial  plants?  Obviously  annual  plants  require  no  scale- 
leaves.  It  is  of  great  importance  for  them  that  they  should  develop  fruits  and  seeds 
in  the  short  period  of  a  single  summer;  for  this  they  must  manufacture  the  building 
materials  necessary  by  the  help  of  their  green  foliage-leaves.  A  portion  of  the 
building  materials  is  employed  in  the  formation  of  the  embryo  in  the  seed;  another 
part  in  the  production  of  well-stocked  food-reserves  associated  with  the  embryo. 
The  seeds  become  detached  and  scattered,  whilst  the  parent  plant  which  has 
produced  them  shrivels  up  and  dies.  It  leaves  no  buds  behind  to  persist  through 
the  winter  and  sprout  next  year;  consequently  any  provision  for  the  maintenance 
of  such  buds  would  be  superfluous.  It  is  different  in  perennial  plants,  as  the  buds 
formed  by  them  must  be  provided  with  the  necessary  reserve-food,  and  protected 
during  the  period  of  inactivity,  throughout  the  winter  sleep  and  summer  rest, 
against  extremities  of  cold  and  heat,  from  freezing,  burning,  and  drying  up.  They 
must  also  be  protected  as  well  as  possible  against  the  attacks  of  animals,  and  these 
tasks  are  assigned  to  the  scale-leaves,  which  serve  on  the  one  hand  as  storehouses 
for  reserve  food-materials,  and  on  the  other  as  protective  envelopes  covering  the 
still  short  axis  with  its  rudiments  of  foliage  or  floral-leaves.  Of  course  no  green 
leaf -blades,  and,  generally  speaking,  no  green  tissue  is  required  for  the  fulfilment  of 
these  functions.  The  brown  or  colourless  leaf -sheath  is  sufficient  for  the  purpose; 
which  explains  why  the  scale-leaves  appear  on  all  shoots  as  pale,  husky  or  scale-like 
structures  without  green  blades.  Even  the  first  bud  of  the  plant  arising  at  the 
apex  of  the  hypocotyl  is  provided  in  most  perennial  plants  with  pale  scaly  leaves, 


(524  SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES. 

and  this  is  the  case  not  only  in  woody  plants,  e.g.  in  the  Oak,  as  illustrated  in 
fig.  144  5  and  144  6,  but  also  in  quite  small  herbaceous  plants,  as  in  the  Moschatel 
(Adoxa  Moschatellina),  in  which  small  scale-leaves  without  chlorophyll,  followed  by 
green  foliage-leaves,  are  developed  above  the  cotyledons  on  the  epicotyl,  and  above 
these  floral-leaves.  All  the  shoots  (that  is  to  say,  buds)  developed  later  on  in 
perennial  plants  start  below  with  scale-leaves  from  which  the  green  blade  is  absent, 
perhaps  because  it  would  be  superfluous. 

The  scale-leaves  which  are  developed  on  subterranean  shoots,  especially  on 
bulbs,  rhizomes,  and  turions,  differ  considerably  from  each  other  according  to  the 
various  conditions  of  growth  of  these  three  kinds  of  shoot-structures.  By  bulb 
(bulbus)  we  understand  an  erect  subterranean  shoot,  whose  very  short,  thick  axis  is 
covered  with  relatively  long,  closely-packed,  scale-leaves  lying  one  above  another. 
The  resting  bulb  is  really  a  bud,  and  its  form  is  occasioned  almost  entirely  by  the 
shape  of  its  scale-leaves.  These  are  in  most  instances  broad  and  concave,  and  they 
are  arranged  so  that  the  inner  ones  are  completely  invested  by  the  outer,  as,  for 
example,  in  tulips  and  species  of  onion;  and  they  are  elongated,  ovate,  or  lanceolate, 
and  lie  on  each  other  like  the  tiles  of  a  roof,  as  in  the  lilies  (Lilium  Martagon, 
album,  &c.).  The  adjacent  scale-leaves  are  sometimes  united,  as,  for  example,  in  the 
Crown  Imperial  (Fritillaria  imperialis).  Those  of  bulbs  function  chiefly  as 
storage-organs.  The  shoot,  whose  base  they  cover,  when  it  begins  to  develop, 
withdraws  the  necessary  building  materials  from  this  storehouse  until  its  foliage- 
leaves  become  green  and  emerge  above  the  ground;  then  the  leaves  are  able  to 
manufacture  new  organic  materials  in  the  sunlight.  Bulbs  are  protected  against 
the  risk  of  drying  up  by  the  earth  surrounding  them,  but  it  is  very  important  for 
them  that  they  should  also  be  protected  against  the  attacks  of  animals  which  live 
underground,  and  particularly  from  their  nibbling.  In  addition  to  the  poisonous 
materials  for  warding  off  these  attacks,  further  protection  is  afforded  chiefly  by  the 
fact  that  the  exhausted  and  dead  older  scale-leaves  do  not  entirely  decay  and 
disintegrate,  but  remain  as  a  sheath.  Sometimes  they  form  a  tough  parchment-like 
investment,  or  their  thick  reticular  and  latticed  strands  remain  as  a  sort  of  cage, 
within  which  the  young  and  succulent  bulbs  are  inclosed  and  protected,  as  may  be 
particularly  well  seen  in  crocuses,  gladioluses,  and  tulips. 

The  scale-leaves  also  perform  the  part  of  storage-tissues  in  subterranean, 
horizontally  elongating  shoots,  called  rhizomes  or  root-stocks  (rhizoma).  They 
also  often  serve  as  protecting  envelopes,  especially  when  they  cover  the  apex 
of  the  stem  as  it  pushes  its  way  through  the  ground.  In  the  latter  case  theii 
cells  are  strongly  turgescent,  or  more  frequently  very  hard,  almost  horny,  an<? 
are  folded  closely  over  the  apex  of  the  shoot,  forming  a  stiff,  pointed  cone  which 
is  able  to  penetrate  even  clayey  soil  like  a  borer. 

By  turion  (turio)  is  meant  a  bud  originating  laterally  on  underground  stem- 
structures  and  developing  in  the  summer  into  a  shoot  which  rises  above  the 
ground.  In  the  autumn  its  upper  part  dies  off,  whilst  its  lowermost,  subten 
ranean  portion  persists  through  the  winter  and  originates  new  buds.  Here 


SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES.  625 

the  scale-leaves  principally  function  as  protecting  envelopes  for  the  foliage- 
leaves.  The  young  and  still  very  delicate  foliage-leaves,  folded  together  within 
the  bud,  are  entirely  surrounded  and  over-arched  by  them.  The  sheath-like 
scales  close  together  like  a  dome  over  the  bud,  and  form  an  actual  shield  for 
it.  Either  hard,  much-thickened  cells,  or  more  usually,  strongly  turgescent 
cells  are  present  at  the  apex  of  each  of  these  scale-leaves,  and  often  these 
coverings  are  injured  in  penetrating  the  soil;  but  this  is  not  of  great  import- 
ance because  the  scale -leaves  become  superfluous  and  perish  when  once  the 
foliage -leaves  have  emerged  and  expanded  above  the  ground.  If  earth  is 
thrown  up  over  the  underground  stock  of  such  a  plant  as  the  Rhubarb, 
the  scale-leaves  of  the  turions  increase  in  length  in  proportion  to  the  thickness 
of  the  heaped -up  stratum.  The  growth  of  the  leaves  keeps  pace  with  the 
growth  of  the  enveloped  shoot;  scarcely  has  the  earth  been  penetrated  when 
the  scale-leaves  stop  growing,  and  the  shoot — no  longer  requiring  a  protection 
against  the  ruggedness  of  the  soil — rises  up  from  its  sheathing  envelope  and 
unfolds  its  young,  green  foliage -leaves  in  the  sunlight.  If  the  layer  of  earth 
which  has  been  piled  up  above  the  subterranean  stock  is  too  thick,  and  if  the 
store  of  building -materials  for  the  lengthening  of  the  sheathing  scale -leaves 
is  inadequate,  then  the  young,  green  foliage -leaves  are  forced  to  leave  their 
protecting  envelopes  even  below  the  ground,  and  make  their  appearance  above 
usually  damaged,  torn,  and  mutilated.  Many  fumitories  (e.g.  Corydalis  fabacea) 
have  only  a  single  sheathing  scale-leaf  which  surrounds  that  part  of  the  shoot 
possessing  green  foliage -leaves.  Here  also  it  can  be  plainly  seen  that  the 
scale -leaf  affords  protection  only  as  long  as  it  is  necessary,  i.e.  the  scale 
stretches  up  from  the  lowest  portion  of  the  shoot-axis  until  it  has  reached  the 
surface  of  the  ground,  where  the  delicate,  green  foliage-leaves  no  longer  require 
protection,  and  can  unfold  in  the  air.  If  the  Corydalis  is  rooted  only  super- 
ficially in  the  earth,  the  scale-leaf  is  raised  a  very  little,  often  scarcely  a  single 
centimeter,  but  if  it  is  very  deeply  rooted,  or  if  earth  is  heaped  up  over  it 
either  purposely  or  accidentally,  then  this  lengthening  of  the  lower  portion 
of  the  stem  amounts  sometimes  to  more  than  20  centimeters.  In  either  case 
that  portion  of  the  stem  by  which  the  sheathing  scale-leaf  is  raised  stops 
growing  as  soon  as  the  apex  of  the  sheathing  envelope  has  reached  the  surface 
of  the  soil,  and  it  looks  as  if  the  Corydalis  had  deliberately  adapted  itself  to 
the  existing  conditions. 

Many  plants  have  two  kinds  of  underground  scale -leaves.  Firstly,  those 
whose  cells  are  filled  with  starch  and  other  food -reserves.  These  are  always 
thick  and  fleshy,  and  they  do  not  continue  to  grow,  but  are  absorbed  by  the 
growing  shoots.  Secondly,  sheath-like  ones,  which  elongate,  inclose,  and  protect 
the  green  foliage-  or  floral-leaves,  in  their  passage  through  the  layers  of  earth 
as  they  grow  up  towards  the  light;  these  do  not  cease  growing  nor  lose  their 
turgescence  until  the  delicate  structures  within  reach  the  surface,  when  they 
are  in  no  more  danger,  and  require  protection  no  longer. 

VOL.  L 


(J26  SCALE-LEAVES,   FOLIAGE-LEAVES,    FLORAL-LEAVES. 

Scale-leaves  situated  above-ground  are  found  on  the  buds  of  all  woody 
plants,  both  on  foliage  and  floral  buds,  i.e.  both  on  the  lowest  portions 
of  those  rudimentary  shoots  which  are  destined  to  become  leafy  shoots,  and  in 
those  which  develop  floral-leaves  immediately  above  the  scale-leaves.  As  a  rule 
they  present  a  hard,  tough  epidermis,  are  frequently  covered  externally  with 
adhesive  substances,  hairs,  and  the  like,  and  chiefly  serve  as  a  protection  against 
the  drying  up  of  the  little  shoot  within.  When  in  springtime  this  axis  begins 
to  elongate,  they  are  either  immediately  detached  and  thrown  off,  as  in  willows, 
or  they  may  separate  just  sufficiently  to  permit  the  shoot  to  grow  through,  as  in 
Koelreuteria  paniculata.  In  many  species  they  remain  undisturbed  and  unaltered 
in  their  position;  in  others  they  separate  and  remain  for  some  time  at  the 
base  of  the  new  shoot,  as  in  the  Walnut  and  Ash;  whilst  in  others,  again,  they 
are  turned  back  and  soon  fall  off,  as  in  the  Mountain-ash  (Sorbus  Aucuparia), 
and  in  most  species  of  the  Horse-chestnut  (AZsculus).  ^Esculus  neglecta  is 
especially  noticeable  in  this  respect,  since  its  bud -scales,  which  are  detached 
almost  simultaneously,  are  large  and  red  in  colour,  and  when  they  fall  off  they 
cover  the  ground  under  the  tree  quite  thickly  as  if  with  autumnal  foliage.  In 
most  instances  the  scale-leaves  on  the  buds  of  woody  plants  are  brown  and 
devoid  of  chlorophyll,  and  increase  in  size  only  slightly  during  their  separation 
from  one  another;  those  of  Gymnocladus,  however,  have  a  green  colour,  and 
increase  in  the  spring  to  twice  or  even  three  times  their  former  size. 

On  the  buds  of  willows  only  a  single  scale-leaf  is  to  be  seen;  limes  have 
two,  alders  three,  manna -ashes  four,  while  in  the  beech,  hornbeam,  elm,  and 
Celtis  occidentalis  there  are  very  many  bud -scales.  If  only  a  single  scale 
exists,  as  in  willows,  it  is  deeply  hollowed,  and  surrounds  the  part  of  the  bud 
to  be  protected  like  a  husk.  If  only  a  few  scale-leaves  are  developed,  as  in 
Gymnodadus,  they  arch  like  a  dome  over  the  young  green  foliage-leaves ;  but  if 
many,  then  they  lie  close  above  one  another  like  the  slates  of  a  roof.  It  remains 
yet  to  be  noticed  that  in  all  cases  where  the  bud  is  protected  by  a  single 
or  only  a  few  scale-leaves,  their  texture  is  always  very  tough  and  hard ;  but 
where  many  are  present  they  are  thin  and  membraneous.  It  has  been 
previously  mentioned  that  the  stipules  of  many  plants,  as,  for  example,  of  fig- 
trees,  magnolias,  and  the  tulip-tree  (fig.  91),  take  the  place  of  scale-leaves  as 
protective  coverings. 

Foliage-leaves,  unlike  scale-leaves,  exhibit  an  almost  inexhaustible  variety 
in  their  internal  structure  and  external  form,  a  fact  partly  due,  no  doubt,  to 
the  multifarious  duties  they  have  to  discharge.  The  most  important  of  all 
these  functions  is  the  manufacture  of  organic  materials  from  inorganic  food— 
on  the  efficient  discharge  of  which  the  existence,  not  only  of  individual  plants 
but  of  the  whole  organic  world  depends.  This  almost  entirely  devolves  upon 
the  foliage -leaves.  Of  course,  in  numerous  instances  cotyledons  and  floral- 
leaves,  the  cortex  of  branches,  and  in  some  plants  even  aerial  roots  discharge 
this  function;  but  all  these  are  so  subordinate  that  we  may  say  that  more 


SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES.  627 

than  90  per  cent  of  the  organic  matter  manufactured  throughout  the  whole 
world  every  year  should  be  reckoned*  to  the  account  of  the  green  foliage- 
leaves. 

That  those  members  of  the  plant  to  which  is  allotted  the  manufacture  of 
organic  matter  should  exhibit  such  a  marvellous  diversity  can  hardly  astonish 
us.  For  how  infinitely  varied  are  the  conditions  under  which  this  function 
is  performed  in  the  different  zones  and  regions  of  the  globe!  Even  within  the 
narrow  confines  of  a  restricted  area,  one  may  find  habitats  damp  and  dry, 
sunlit  and  shady,  tranquil  and  tempest -tossed.  Nor  should  we  be  surprised  to 
find  leaves  of  diverse  shape  at  different  heights  on  one  and  the  same  shoot, 
and  that  the  foliage  borne  by  any  plant  may  exhibit  variations  in  form  in 
successive  seasons  of  the  year.  And  then  we  must  remember  that  besides  the 
most  important  function  mentioned,  foliage-leaves  have  often  to  provide  for  the 
irrigation  of  rain-water  to  the  absorbent  roots,  to  play  the  part  of  climbing 
organs,  or  to  serve  as  weapons ;  more  than  this,  they  even  act  as  organs  for 
digesting  imprisoned  animals,  with  which  last  function  is  associated  very 
curious  metamorphoses  of  foliage-leaves.  By  the  segmentation  of  the  leaf  into 
those  parts,  into  the  blade,  leaf -stalk,  and  sheath  with  stipules,  an  allotment  of 
these  various  functions  becomes  possible;  but  evidently,  in  consequence  of  this 
division  of  labour  in  one  and  the  same  leaf,  the  structure  becomes  much  more 
complex  and  manifold. 

A  distinctive  name  has  been  given  to  each  shape  by  botanists,  who  have 
endeavoured  to  define  the  different  forms  by  descriptions.  For.  foliage-leaves 
alone  perhaps  a  hundred  different  expressions  have  been  used  to  shortly  desig- 
nate the  most  remarkable  varieties.  Since  these  terms  of  botanical  nomen- 
clature can  be  combined  and  varied  according  to  the  actual  facts,  we  are  able 
to  describe  the  many  thousands  of  differently-shaped  foliage-leaves,  briefly  and 
tersely,  and — what  is  of  especial  value,  and  really  the  most  important  aim  of 
these  descriptions  —  another  person  is  able  from  them  to  picture  the  object  to 
himself. 

First  of  all,  let  us  describe  the  leaf -blade,  the  outline  of  which  may  exhibit 
every  imaginable  geometrical  form:  obovate,  circular,  elliptical,  rhombic, 
rhomboidal,  triangular,  pentagonal,  &c.  Very  often,  too,  the  leaf -blade  is  much 
elongated,  and  the  margins  are  parallel  to  one  another;  this  is  known  as  linear. 
The  free  end  of  the  blade  is  sometimes  pointed,  sometimes  blunt,  and  some- 
times drawn  out  into  a  long  point;  occasionally,  again,  it  is  truncate,  pressed 
in,  or  cut  out  in  the  form  of  a  heart.  The  base  of  the  leaf-blade  may  be 
narrowed  and  attenuate  towards  the  stem;  or  its  outline  may  be  kidney- 
shaped,  arrow-shaped,  lanceolate,  ovate,  spathulate,  crescent -shaped,  &c.  The 
blade  is  either  undivided,  when  it  is  termed  entire,  or  the  margin  is  to  a 
greater  or  less  extent  indented;  if  the  indentations  are  but  slight,  the  leaf- 
blade  is  said  to  be  crenate,  serrate,  or  dentate,  but  if  they  are  considerable, 
the  margin  is  said  to  be  sinuous  or  incised;  if,  again,  the  indentations  go  more 


628  SCALE-LEAVES,   FOLIAGE-LEAVES,    FLOKAL-LEAVES. 

deeply  into  the  green  surface  of  the  blade,  the  expressions  lobed,  cut,  divided,  or 
partite  may  be  used.  A  partite  leaf  appears  as  if  composed  of  several  leaflets, 
and  such  leaves  have  also  been  termed  compound  leaves,  especially  when  the 
already-described  pulvini  are  present  at  the  base  of  the  individual  leaflets. 

The  distribution  of  the  strands  traversing  the  green  tissue  is  connected  in 
the  closest  manner  with  the  structure  and  shape  of  the  leaf -blade.  Expressions 
have  been  borrowed  from  the  anatomy  of  the  animal  body  to  designate  these 
strands,  and  they  are  called  indifferently  veins,  ribs,  and  nerves.  The 
term  "vein"  has  some  justification,  since  most  of  these  strands  contain 
cells  and  vessels  which  serve  to  conduct  fluid  materials  to  and  fro ;  but  since 
there  are  also  strands  which  have  nothing  to  do  with  this  conduction,  which  are 
developed  exclusively  for  the  support  of  the  whole  blade,  the  name  is  unsuit- 
able, and  can  only  be  used  figuratively.  The  same  may  be  said  of  the  term 
"ribs".  In  many  instances  the  strands  in  question,  of  course,  do  present  the 
appearance  of  ribs,  and  the  whole  arrangement  of  them  in  a  blade  may  be 
compared  with  a  skeleton  upon  which  the  soft  portions  are  attached.  We  even 
speak  of  "  leaf -skeletons ",  an  expression  which  seems  justifiable,  since  by 
removing  the  soft  portions  a  white  framework  is  obtained  exhibiting  a  great 
analogy  to  the  bony  skeleton  of  an  animal  body.  Thus  if  the  blades  of  green 
foliage-leaves  are  allowed  to  macerate  for  some  time  in  water,  the  epidermis 
and  thin- walled  green  tissues  decay,  while  the  tougher  strands  remain  intact; 
if  these  leaves  are  now  dried  and  brushed,  all  the  soft  disintegrated  parts  are 
removed,  and  only  the  skeleton  of  the  leaf  remains,  in  which,  as  in  the 
skeleton  of  an  animal,  larger  and  smaller  structures  may  be  recognized,  con- 
nected together  in  the  most  varied  manner.  But  from  the  fact  that  most  of 
the  strands,  together  with  those  cells  which  serve  to  strengthen  the  whole 
blade,  also  contain  conducting  tubes;  that  many  of  them  indeed  consist  only  of 
conducting  vessels,  it  is  hardly  permissible  to  speak  of  skeletons,  or  to  apply 
the  term  "ribs"  to  the  strands  so  beautifully  interlaced.  Finally,  the  name 
"nerves"  is  still  more  unfortunately  chosen,  for  the  strands  of  leaf -blades  have 
no  resemblance  to  animal  nerves,  either  in  structure  or  function.  Consequently 
this  designation  must  be  also  condemned,  although  it  is  the  one  most  often 
employed  by  descriptive  botanists. 

It  is  simplest  and  most  correct  to  call  the  structures  in  question  what  they 
really  are,  viz.  strands,  strands  consisting  of  elongated  and  fibrous  cells,  which 
are  combined  in  the  most  diverse  ways  with  tubular  and  pipe-like  vessels,  and 
whose  elements  serve  partly  for  the  conduction  of  fluid  materials  to  and  from 
the  green  tissues,  and  partly  to  afford  the  necessary  protection  to  the  whole 
blade — protection  against  strain,  pressure,  and  bending,  according  to  the  need 
of  the  moment. 

In  looking  for  the  origin  of  the  strands  on  a  leaf-blade,  we  are  always 
directed  to  the  stem  from  which  the  leaf  in  question  springs.  In  other  words, 
the  first  trace  of  those  strands,  which  traverse  the  leaf-blade  as  a  richly-articu- 


SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES.  629 

lating  system,  is  already  found  in  the  stem.  From  this  they  extend  through 
the  leaf-sheath  and  petiole  to  the  base  of  the  blade.  This  last  is  therefore  in 
a  manner  the  entrance-gate  for  the  strands,  and  as  soon  as  they  have  passed  it 
a  division  takes  place  not  unlike  that  of  a  stream  which  flows  from  a  narrow 
valley  into  a  plain,  and  there  breaks  up  into  numerous  larger  and  smaller 
branches;  or  they  may  perhaps  be  still  better  compared  to  an  aqueduct  whose 
main  stream  is  inclosed  and  strengthened  by  masonry  and  embankments,  but 
which  branch  out,  at  the  confines  of  the  town  which  has  to  be  supplied  with 
water,  into  several  subordinate  conduits  which  penetrate  the  different  districts, 
and  then  again  break  up  into  numerous  smaller  water-pipes  leading  to  the 
buildings  and  other  places  of  consumption. 

We  may  distinguish  two  kinds  of  distribution  in  respect  of  the  course  of 
these  strands  as  they  enter  the  leaf.  In  the  one  case  there  is  only  a  single 
thick  strand,  the  primary  strand,  which  distributes  itself  and  breaks  up  inside 
the  narrow  gate.  In  the  other,  three  or  more  distinct  main  strands  pass  over,  side 
by  side,  into  the  blade,  each  following  a  separate  course.  As  a  rule,  these  are 
connected  by  bridges  and  inter -networks.  Thus  we  distinguish  between  leaf- 
blades  with  a  single  main  strand  and  those  with  several. 

Leaf -blades  with  one  main  strand  may  be  sub -divided  into  two  groups 
according  to  the  form  and  course  of  the  lateral  strands  which  arise  from  the 
primary  one.  Either  these  lateral  strands  are  all  weaker  than  the  main  one,  and 
originate  from  it  successively,  at  intervals,  like  the  ribs  of  a  spinal  column,  or 
like  the  barbs  on  the  axis  of  a  feather,  when  we  speak  of  a  feather-like  (or 
pinnate)  arrangement  of  the  lateral  strands  (see  figs.  1491,2,8,4,5,6,7,10,13).  or  tne 
lateral  strands  are  almost  as  strong  as  the  main  one,  arise  from  it  directly  at 
the  base  of  the  blade,  and  run  out  from  this  point  like  rays  towards  the  margin 
of  the  lamina.  This  arrangement  of  the  lateral  strand  is  called  radiating  (see 
figs.  149  8-9-n'12). 

When  the  lateral  strands  are  arranged  like  a  feather,  it  generally  happens 
that  they  are  alike  in  the  matter  of  strength,  that  they  are  distributed 
symmetrically  over  the  whole  blade,  and  originating  at  fairly  equal  intervals 
from  the  main  strand,  take,  at  least  at  first,  a  parallel  course.  More  rarely 
it  happens  that  stronger  and  weaker  lateral  strands  alternate,  and  that  they 
diverge  from  the  primary  one  at  unequal  angles.  In  the  Camphor  Tree 
•(Laurus  Camphora,  fig.  149 4),  the  Cinnamon  Tree  (Cinnamomum),  and  many 
other  plants  related  to  the  Bay  Laurel,  this  peculiarity  is  found,  viz.  that  two 
lateral  strands  which  proceed  from  the  lower  third  of  the  main  one  are 
stronger  than  the  others,  looking  as  though  a  three-pronged  fork  had  been 
inserted  in  the  leaf.  In  the  Wall-Pellitory  (Parietaria),  whose  leaves  show 
a  similar  character,  stronger  and  weaker  lateral  strands  alternate,  and,  strangely 
-enough,  the  stronger  spring  from  the  main  strand  at  an  acute  and  the  weaker 
at  a  right  angle.  For  the  rest,  the  lateral  strands  with  feather-like  arrangement 
lay  be  distinguished  as  reticulate,  looped,  arched,  and  undivided. 


ITU 


630  SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES. 

Those  lateral  strands  are  termed  reticulate  (dictyodromous),  which  break  up 
into  a  delicate  net-work  soon  after  their  origin  from  the  primary  strand,  or 
at  least  before  they  have  reached  the  margin  of  the  blade.  The  meshes  of  the 
net-work  are  of  almost  equal  size,  so  that  it  is  impossible  to  distinguish  in  the 
confusion  of  small  strands  near  the  margin  of  the  blade  any  particular  one 
more  vigorous  than  the  others.  The  leaf  of  the  Wild  Pear  (Pyrus  communis) 
is  given  as  an  example  of  this  form  in  fig.  149 1.  The  same  distribution  of 
strands,  however,  is  found  in  very  many  other  plants  allied  to  pear-trees,  as 
also  in  willows,  rhododendrons,  and  species  of  barberry  and  sage. 

The  lateral  strands  called  looped  (brachydodromous)  run  fairly  straight  and 
distinct  towards  the  margin,  but  before  reaching  it  they  bend  round  in  a 
graceful  sweeping  curve,  towards  the  apex,  uniting  with  the  next  lateral  strand 
above,  and  with  it  form  a  loop.  Such  loops  can  always  be  seen  standing 
plainly  out  from  the  delicate  net-work  of  small  strands,  and  the  arrangement 
may  be  recognized  at  the  first  glance.  It  is  observed  in  the  leaves  of  the 
Mahaleb  and  common  Cherry,  in  the  Buckthorn  (Rhamnus  Frangula  and 
Wulfenii,  see  fig.  149 2),  in  myrtaceous  plants  (Myrtus,  Metrosiderus,  Eugenia, 
see  fig.  14913),  in  many  species  of  dock  and  nightshade,  and  especially  in 
rough -leaved  plants  (Boraginaceae).  The  net- work  of  fine  strands  inserted 
between  the  laterals  is  often  so  delicate  that  it  is  scarcely  visible  to  the  naked 
eye,  and  then  only  a  series  of  bold  loops,  like  arcades,  is  to  be  seen  in  each 
half  of  the  leaf.  In  the  Comfrey  and  Lungwort  (Symphytum  and  Pulmonaria) 
these  loops  are  developed  at  some  little  distance  from  the  margin  of  the  leaf- 
blade.  In  the  cherry  and  buckthorn,  on  the  other  hand,  the  loops  are  quite 
close  to  the  margin.  The  lateral  strands  are  frequently  very  delicate,  and  extend 
in  a  straight  line  from  the  primary  strand  right  up  to  the  margin,  when  they 
bend  suddenly  round,  like  a  knee,  almost  forming  a  right  angle.  The  outer 
limb  of  this  right  angle  then  runs  parallel  to  the  margin,  and  unites  with 
the  knee  of  the  next  upper  lateral  strand.  In  this  way  we  have  a  strand 
running  parallel  with  the  leaf -margin  connected  with  the  central  primary 
strand  by  cross-ties.  This  looped  form  occurs  very  regularly  in  the  Myrtaceae, 
but  many  tropical  Moreae  are  also  distinguished  by  it,  and  the  leaves  of  the 
Forget-me-not  (Myosotis)  also  exhibit  this  peculiar  arrangement  of  lateral 
strands  (see  fig.  149 10). 

Arched  strands  (kamptodromous)  are  those  which  run  out  from  their  place 
of  origin  on  the  main  strand  towards  the  margin  of  the  leaf,  which,  however, 
they  never  reach,  but  turn  in  an  arch  towards  the  leaf -apex,  and  there  lose 
themselves  without  forming  definite  loops.  As  a  rule,  the  places  of  origin  are 
crowded  together  in  the  lower  half  of  the  main  strand,  and  the  two  uppermost 
arched  lateral  strands  then  inclose  an  oval  central  area.  The  Cornel  (Cornus 
mas),  illustrated  in  fig.  149 3,  is  chosen  as  an  example  of  this  form. 

Those  lateral  strands  are  called  undivided  (craspedromous)  which  run  in  a 
straight  line  from  the  main  strand  to  the  margin  and  there  terminate.  They  end 


SCALE-LEAVES,   FOLIAGE-LEAVES,  FLORAL-LEAVES. 


631 


either  in  the  apices  of  the  lobes  or  serrated  teeth  of  the  margin,  as  in  hazels,  oaks, 
chestnuts,  hornbeams,  and  hop-hornbeams  (see  fig.  149'),  or  in  the  indentations 


Fig.  149.—  Arrangement  of  Strands  in  the  blades  of  Foliage-leaves.    Forms  with  one  main  strand. 

Reticulate  (Pyrus  communis).  a  Looped  (Rhamnus  Wulfenii).  »  Arched  (Cornus  mas).  «  Arched  ;  the  two  lowest  lateral 
strands  much  stronger  than  the  others  (Laurus  Camphora).  *  Reticulate-pinnate  (Populut  pyramidalit).  «  Undivided 
strands,  ending  in  the  incisions  of  the  crenate  leaf  margin  (Rhinanthus).  1  Undivided  strands,  terminating  in  the 
projecting  teeth  of  the  margin  (Ostrya).  »  Reticulate  (Hydrocotyle  asiatica).  •  Reticulate  strands  in  the  blade  of  a 
peltate  leaf  (Hydrocotyle  vulgaris).  10  looped  (Myosotis  paluttris).  "  Arched  (Phyllagathis  rotundtfolia).  "  Rad;ate 
and  undivided  (Acer  platanoides).  i»  Looped  (Eugenia). 


of  the  margin,  as  in  Bartsia,  Eyebright,  and  Yellow-rattle  (Bartsia, 

and  Rhinanthus),  and,  generally  speaking,  in  all  Rhinanthacese  (see  fig.  1498). 


(532  SCALE-LEAVES,   FOLIAGE-LEAVES,    FLORAL-LEAVES. 

Lateral  strands  with  radiate  arrangement  exhibit  conditions  quite  similar 
to  those  with  a  feather -like  course.  Frequently  they  are  reticulate  as  in 
geraniums  and  mallows,  the  Judas  Tree  (Cercis  Siliquastrum),  and  many 
Umbelliferse,  as,  for  example,  in  the  leaves  of  Hydrocotyle  asiatica,  illustrated  in 
fig.  1498.  In  some  water-lilies  looped  lateral  strands  are  also  observed,  whilst 
arched  lateral  strands  are  very  characteristic  of  MelastomaceaB.  In  these  Mela- 
stomacese  (see  fig.  149 n)  the  lateral  strands  originate  from  the  main  strand  at 
the  base  of  the  leaf-blade,  and  travel  towards  the  apex  of  the  leaf  in  elegant, 
sweeping  arches  parallel  to  the  margin.  Numerous  cross  strands,  like  ties, 
connect  the  lateral  strands  with  one  another  and  with  the  primary  one,  giving 
an  extremely  ornamental  appearance  to  this  class  of  leaf.  The  leaves  of  maples 
exhibit  lateral  strands  radiating  towards  the  margin;  this  is  particularly  well 
shown  in  the  Norway  Maple  (Acer  platanoides),  the  leaf  of  which  is  illus- 
trated in  fig.  149 12.  Planes  (Platanus)  also  have  lateral  strands  running 
towards  the  margin  and  terminating  in  the  points  of  the  leaf,  but  it  is  worthy 
of  remark  that  in  some  species  the  branching  of  the  lateral  strands  from  the 
primary  one  does  not  take  place  immediately  at  the  base  of  the  blade,  but 
somewhat  above  it.  A  peculiar  modification  of  lateral  strands  with  a  radiating 
arrangement  is  observed  in  many  so-called  peltate  leaves  (see  fig.  149 9).  In 
these  leaves  the  blade  is  more  or  less  circular,  and  is  connected  with  the  central 
stalk  as  the  cover  of  an  umbrella  with  its  stick.  The  strands  radiate  out  in 
all  directions  from  the  point  of  attachment  of  the  stalk,  and  without  close 
investigation  of  the  relations  between  such  a  leaf  and  its  petiole,  it  is  often 
quite  impossible  to  say  which  of  the  radiating  strands  is  to  be  regarded  as  the 
main  one.  This  arrangement  is  found  in  most  species  of  Pennywort  (Hydro- 
cotyle, cf.  fig.  149 9),  in  Nasturtiums,  Ricinus,  and  Nelumbium;  the  last-men- 
tioned plant  has  also  this  peculiarity,  that  its  peltate  leaves  are  somewhat 
depressed  in  the  centre  like  a  bowl. 

Leaf -blades  with  several  main  strands  offer  far  less  variety  than  those  with 
only  one.  The  margin  is  almost  always  entire,  and  they  are  generally  elon- 
gated. The  most  noticeable  variations  consist  in  the  number  of  the  main 
strands  which  enter  the  base,  in  their  varying  thickness,  and  in  the  direction 
which  they  take  in  the  blade.  We  have  also  to  consider  whether  they  divide 
like  a  fork,  and  whether  the  lateral  nerves  which  they  give  off  are  developed 
as  cross-connections,  or  as  a  fine-meshed  net-work. 

When  the  latter  is  the  case,  that  is  to  say,  when  the  main  strands  entering 
separately  into  the  blade,  and  travelling  towards  the  apex  of  the  leaf  are 
linked  together  by  a  net-work  of  lateral  strands  with  angular  meshes,  they 
are  then  said  to  be  apical  (acrodromous).  The  numerous  broad-leaved  species  of 
Plantain  (Plantago),  species  of  Hare's-ear  (Bupleurum)  belonging  to  the  Umbel- 
liferse,  the  leaf  of  one  species  of  which  (Bupleurum  falcatum)  is  represented  in 
fig.  1501,  show  apical  main  strands.  In  the  leaf  of  the  Hare's-ear  the  main 
strands  are  crowded  together  in  the  narrow  base  of  the  blade,  and  the  meshes 


SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES.  633 

of  the  net-work  between  these  strands  are  principally  formed  from  trans- 
versely-running  lateral  strands.  In  the  Australian  Lcucopogon  Cunninghami, 
one  of  the  Epacride*  (see  fig.  160  %  the  very  narrow  meshes  of  the  net-work 
are,  on  the  other  hand,  formed  by  the  longitudinally-running  lateral  strands 
A  very  peculiar  form  of  the  apical  arrangement  of  strands  is  that  which  the 
older  botanists  called  pedate.  Three  distinct  strands  enter  the  base  of  the 
blade  from  the  leaf  -  stalk ;  the  central  strand  is  relatively  thin  and  passes 


Fig.  150.— Distribution  of  Strands  in  the  blades  of  Foliage-leaves :  Forms  with  several  main  strands. 

1  Apical  or  acrodromous  (Bupleurum  falcatum).  2  Curved  or  campylodromous  (Hydrocharis  Morsut-rantc).  '  Curved 
(Haianthemum  bifolium).  *  Curved  (Funkia).  *  Fan-like  or  diadromous  (Oinkgo  biloba).  «  Apical  or  acrodromous 
(Leucopogon  Cunninghami).  'Apical,  "pedate"  (Parnassia  palustris).  «  Parallel  (Bambusa).  •  Parallel  (Oryza 
clandestine^). 

direct  towards  the  leaf -apex;  the  two  lateral  are  thick,  bend  round  like  an 
arch  to  the  right  and  left  as  soon  as  they  have  entered  the  blade,  and  then 
send  arched  lateral  strands  toward  the  upper  margin,  which  are  almost  equi- 
valent to  the  central  main  strand,  and  may  at  first  sight  even  be  taken  for 
main  strands.  This  arrangement  is  found  in  the  Birthwort  and  Asarabacca 
(Aristolochia  Clematitis  and  Asarum  Europceum),  in  numerous  violets  and 
Ranunculaceae,  and  in  the  Grass  of  Parnassus  (Parnassia  palustris),  the  leaf  of 
which  is  shown  in  fig.  1507. 

Main  strands  which  enter  the  blade  in  large  numbers,  but  always  separately, 


634  SCALE-LEAVES,   FOLIAGE-LEAVES,   FLORAL-LEAVES. 

and  of  which  the  external  ones  travel  towards  the  apex  of  the  leaf  in  an 
arch  parallel  to  the  margin,  are  termed  curved  (campylodromous).  The  lateral 
strands  which  are  usually  so  delicate  that  they  cannot  be  recognized  by  the 
naked  eye  always  form  spans,  connecting  the  adjacent  main  strands  trans- 
versely. In  the  leaf  of  the  May  Lily  (Maianthemum  bifolium)  depicted  in 
fig.  1503,  the  number  of  main  strands  is  very  large  and  the  span-like  laterals 
are  short.  In  the  leaf  of  the  Frogbit  (Hydrocharis  Morsus-rance,  see  fig.  1502) 
only  five  main  strands  traverse  the  leaf -blade,  the  connecting  ties  being 
remarkably  long  and  distinct.  In  Bananas  and  Scitaminese  (Musa,  Maranta, 
Zingiber,  Canna)  the  curved  main  strands  look  like  arched  laterals  branching 
off  from  a  single  central  strand,  but  on  looking  closer  it  becomes  evident  that 
the  thick  rib  traversing  the  centre  of  the  leaf,  like  a  keel,  is  not  a  single 
main  strand,  but  consists  of  many  separate  strands  which  are  embedded  in  a 
large-celled  mass  of  tissue.  These  main  strands  are  inclined  one  above  the 
other  laterally  away  from  the  keel,  travel  towards  the  leaf-margin  and  there 
curve  up  towards  the  apex.  In  bananas  this  bundle  of  separate  strands,  surrounded 
by  parenchyma,  extends  from  the  base  to  the  apex;  in  species  of  the  genus 
Funkia  (see  fig.  1504)  only  part  way  to  the  middle  of  the  blade. 

When  several  distinct  main  strands  enter  the  blade  from  the  leaf-sheath  or 
leaf-stalk,  running  parallel  to  one  another  in  a  relatively  wide  area  without 
dividing  and  not  converging  until  the  actual  leaf -apex  is  reached,  they 
are  termed  parallel  (parallelodromous).  Such  an  arrangement  of  strands  is 
found  in  many  liliaceous  plants,  in  orchids,  rushes,  sedges,  and  especially  in  the 
thousands  of  different  grasses.  The  strands  enter  the  blade  either  from  a 
broad  sheath,  as,  for  example,  in  Oryza  clandestina  (see  fig.  1509),  when  their 
separate  nature  can  be  easily  recognized  even  at  the  base  of  the  blade;  or 
they  enter  by  a  sort  of  stalk  on  which  the  blade  is  inserted,  as  in  bamboo 
leaves  (see  fig.  1508),  where  the  strands  entering  the  base  of  the  blade  appear 
bent  like  a  knee.  Parallel  strands  are  usually  of  unequal  thickness,  the  central 
being  almost  always  stronger  and  more  vigorous  than  the  lateral.  But  even 
among  the  lateral,  thicker  and  thinner  often  alternate  in  a  manner  definite  for 
each  species.  In  the  slender  False  Brome  Grass  (Brachypodium  sylvaticum), 
for  example,  from  two  to  five  weaker  strands  always  appear  between  every 
pair  of  stronger  ones;  the  weaker  are  often  so  exceedingly  delicate  that  they 
cannot  be  recognized  by  the  naked  eye.  The  unaided  vision  recognizes  eleven 
almost  equally  thick  strands  in  the  leaf  of  Oryza  clandestina,  represented  in 
natural  size  in  fig.  1509;  under  a  lens  five  more  delicate  strands  are  to  be  seen 
between  every  pair  of  these.  When  lateral  strands  are  present,  connecting  the 
adjacent  parallel  main  strands,  they  always  take  the  form  of  transverse  ties. 

Finally,  we  have  here  to  consider  that  remarkable  arrangement  of  strands 
which  is  called  fan-like  (diadromous).  A  few  separate  main  strands  enter  the 
leaf-blade,  divide  up  repeatedly  into  forked,  straight-running  branches,  and  the 
ultimate  twigs  terminate  at  the  upper  margin  of  the  leaf.  This  course  of  the 


SCALE-LEAVES,  FOLIAGE-LEAVES,  FLORAL-LEAVES.  635 

strands  goes  with  a  very  peculiar  form  of  leaf,  which  may  best  be  compared  to 
an  open  fan.  The  Maiden-hair  Tree  (Ginkgo  biloba,  see  fig.  150 6)  may  serve 
as  an  example  of  this  distribution  of  the  strands,  which,  on  the  whole,  is  not 
common.  It  is  also  observed  in  several  true  ferns  (e.g.  Adiantum  arcuatum, 
Acrostichum  sphenophyllum,  and  Livingstonei).  With  regard  to  the  Ginkgo, 
it  should  be  mentioned  that  as  a  rule  only  four  distinct  strands  enter  the 
blade  from  the  leaf -stalk ;  two  central  which  are  very  delicate,  and  two  lateral 
which  are  very  strong,  and  from  which  arise  a  large  number  of  fine,  forking 
strands  running  upwards. 

Besides  the  arrangements  of  strands  in  leaf-blades  here  described,  there  are 
many  which  cannot  readily  be  brought  under  the  limits  defined;  in  the  same 
way  there  are  intermediate  forms  which  may  be  placed  just  as  well  in  one  as 
in  another  of  our  artificial  divisions,  and  which  we  try  to  describe  clearly  by 
connecting  the  technical  terms  together.  For  example,  we  find  intermediate  forms 
between  arched  and  reticular  strands  which  are  described  as  arched-reticulate,  and 
so  forth. 

The  fact  should  be  emphasized  that  the  distribution  and  arrangement  of  the 
strands  in  any  given  species  is  remarkably  constant.  This,  however,  is  by  no  means 
the  case  in  genera  and  families.  Of  course  there  are  plant-families  the  whole  of 
whose  members  exhibit  marked  agreement  in  this  respect,  as,  for  example,  the 
Rhinanthacese,  Melastomacese,  and  Myrtacese;  but,  on  the  other  hand,  instances  are 
not  lacking  where  the  reverse  is  the  case.  Thus  the  various  genera  of  Primulacess 
present  the  widest  varieties,  and  even  the  individual  species  of  the  genus  Primula 
differ  more  from  each  other  in  the  arrangement  and  course  of  the  strands  than 
perhaps  the  Myrtacese  from  the  Boragineae.  Nevertheless  the  accurate  determina- 
tion and  description  of  the  distribution  of  these  strands  in  the  leaves  is  very  im- 
portant for  that  branch  of  botanical  study,  the  object  of  which  is  to  provide  criteria 
for  the  discrimination  of  species.  The  careful  investigation  of  the  distribution  of 
strands  in  leaves  is,  perhaps,  of  the  greatest  interest  to  the  palaeo-botanist,  the 
investigator  of  pre-existing  vegetation.  Those  parts  of  plants  which  have  come 
down  to  us  from  earlier  periods,  embedded  in  geological  formations,  consist  prin- 
cipally of  single  leaves  or  fragments  of  leaves,  often  of  very  insignificant  appear- 
ance. In  these  fragments  often  we  cannot  even  recognize  plainly  the  edge,  much 
less  the  whole  contour  of  the  blade;  but  the  strands  themselves,  and  the  net- work 
which  intervenes  between  the  coarser  strands,  may  be  distinguished  on  the  smallest 
fragments.  Often  enough  the  palseo-botanist  has  only  such  scanty  remains  to  refer 
to  when  he  seeks  information  about  the  species  of  plants  with  which  our  globe  was 
covered  in  long-past  ages.  Consequently  even  the  most  insignificant-looking  bit  of 
leaf-network  becomes  of  the  highest  importance.  Just  as  an  investigator,  busy 
with  the  history  of  the  human  race,  can  draw  certain  inferences  from  the  characters 
of  a  hardly  decipherable  papyrus  roll  about  the  state  of  the  household,  about  the 
political  institutions,  the  customs,  manners,  and  civilization  of  the  population  settled 
more  than  two  thousand  years  ago  in  the  valley  of  the  Nile,  so  can  the  botanist, 


636  SCALE-LEAVES,   FOLIAGE-LEAVES,    FLORAL-LEAVES. 

investigating  the  history  of  plants  and  attempting  to  clear  up  the  connection 
between  past  and  present,  recognize  from  fossil  leaves  the  species  living  in  periods 
long  past,  and  read  the  condition  of  vegetation  as  it  existed  many  thousands  of 
years  ago.  Although  the  results  of  investigation  hitherto  obtained  in  this  field 
are  still  imperfect,  and  although  these  results  may  receive  manifold  additions  and 
corrections  as  more  abundant  materials  come  to  hand,  still  the  history  of  vegetation 
is  already  exposed  in  its  principal  features,  and  that  which  has  been  obtained  in 
this  respect  during  the  comparatively  short  period  of  half  a  century  is  noteworthy 
in  a  period  of  remarkable  additions  to  natural  knowledge.  In  imagination  we  see 
replaced  the  woods  and  meadows  which  long  ages  ago  adorned  the  continents  of 
the  Coal  period;  colonies  of  slender  calamites,  the  rigid  fronds  of  the  cycads,  and 
thickets  of  countless  ferns  rise  up  before  us;  we  are  able  to  sketch  landscape 
pictures  of  the  Jurassic  and  Cretaceous  periods,  and  to  see  the  banks  of  the  rivers 
fringed  with  species  of  Cinnamomum,  evergreen  oaks,  walnut-  and  tulip -trees. 
And  all  these  pictures  of  the  vegetation  of  the  most  remote  periods  would  hardly 
have  been  possible  except  on  the  basis  of  the  determination  of  species  with  the 
help  of  the  minutest  investigations  into  the  arrangement  and  distribution  of  the 
strands  in  the  fossil  leaves. 

When  the  leaves  of  fossil  and  living  plants  are  compared,  we  notice  that  the 
strands  in  the  former  appear  more  distinct  than  in  the  fresh  succulent  green  blades. 
This  is  in  consequence  of  the  fact  that  in  living  plants  the  strands  are  often  em- 
bedded in  parenchymatous  tissue  so  that  they  cannot  be  seen  on  the  surface,  while 
in  fossil  plant-remains  the  parenchyma  has  been  wholly  destroyed  and  only  the 
strands  have  been  preserved.  When  the  strands  run  in  the  interior  of  the  sub- 
stance of  a  leaf,  and  are  not  visible  at  the  surface,  they  are  hidden,  or,  to  use  the 
technical  term,  hyphodromous.  Succulent  leaves  almost  always  have  such  hidden 
strands,  which  may  be  contrasted  with  those  which  project  above  the  general  level 
on  either  side  of  the  leaf.  On  the  whole  this  latter  condition  is  rare,  most  usually 
the  strands  project  on  one  side  only,  and  that  the  lower  surface.  Often  we  find  a 
plexus  of  ridges  on  the  under  side,  and  one  of  grooves  on  the  upper  side  correspond- 
ing to  the  course  of  the  strands.  The  enormous  circular  leaves  of  the  Victoria 
regia,  which  float  on  the  surface  of  the  water,  have  very  strong  projecting  ribs 
on  the  lower  side.  In  leaves  of  submerged  water-plants,  however,  the  strands  are 
insignificant;  many  are  even  destitute  of  vessels,  and  present  only  strands  of  elon- 
gated cells,  as,  for  instance,  the  leaves  of  the  celebrated  Vallisneria.  This  is  easily 
understood,  as  the  need  of  resisting  pressure  and  bending  in  submerged  leaves  is 
very  slight.  Nor  do  submerged  plants  require  special  conducting  tubes  for  their 
food-salts.  Numerous  other  striking  relations  existing  between  the  inner  structure 
of  the  leaf -blade  and  the  peculiar  conditions  of  the  habitat  of  plants  have  already 
been  discussed,  and  we  need  here  merely  refer  to  the  description  of  flattened, 
rolled,  succulent,  spiral,  arched,  hinged,  and  tubular  leaves  occurring  in  the  section 
which  begins  on  p.  209  of  this  volume. 

The  form  of  the  leaf -stalks,  stipules  and  leaf -sheaths,  in  their  dependence  on 


SCALE-LEAVES,    FOLIAGE-LEAVES,   FLORAL-LEAVES.  637 

the  peculiar  conditions  of  the  environment,  has  also  been  repeatedly  discussed,  and 
it  is  enough  to  remember  here  that  the  principal  duties  of  the  leaf-stalk,  as  the 
support  of  the  light-needing  green  blade,  are  to  turn  and  twist  it,  to  raise  and 
lower  it,  to  bring  it  always  into  a  position  where  it  will  be  properly  illuminated; 
to  keep  it  in  that  position  in  spite  of  storm  and  tempest.  The  chief  function  of 
stipules  consists  in  screening  young  and  tender  leaves— not  yet  emerged  from  the 
bud— from  excessive  illumination,  and  protecting  them  from  too  much  loss  of  heat 
on  clear  nights.  The  stipules  in  many  cases  actually  serve  as  bud-scales,  as  may 
be  seen  in  the  fig-tree,  where  the  tiny  leaf-blades  are  rolled  up  together  and 
inclosed  in  the  spathe-like  stipules.  When  this  is  the  sole  function  of  the  stipules, 
they  become  detached  after  the  unfolding  of  the  leaves  wrapped  round  by  them. 
Consequently,  shortly  after  the  unfolding  of  the  foliage  of  oaks,  beeches  and  other 
trees,  the  floor  of  the  forests  formed  by  these  trees  is  strewn  with  enormous 
quantities  of  fallen  stipules.  When  the  stipules  persist  at  the  sides  of  the  leaf- 
stalk and  become  green,  there  can  be  no  doubt  but  that  they  supplement  the  green 
leaf -blades  in  their  function,  and  like  them  manufacture  organic  substances  from 
inorganic  food.  In  the  Woodruff,  Bed-straw,  Madder  (Asperula,  Galium,  Rubia) 
the  stipules  actually  possess  the  same  size,  shape  and  colouring  as  the  blades  of  the 
real  foliage -leaves,  and  thus  a  star  of  green  leaf -structures  is  formed,  to  which 
these  plants  owe  their  name  of  Stellatae.  In  the  Pansy  ( Viola  tricolor)  and 
numerous  species  of  violet  allied  to  it,  the  stipules  are  green  and  sometimes  larger 
than  the  leaf -blade,  at  the  base  of  which  they  occupy  a  subordinate  position. 

A  peculiar  formation  is  observed  in  the  Yellow  Vetch  (Lathyrus  Aphaca),  a 
common  weed  in  the  fields  of  Southern  Europe,  though  not  so  frequent  in  England. 
In  this  plant  the  leaves  are  completely  transformed  into  tendrils  which  serve  as 
climbing  organs;  the  two  stipules  which  stand  at  the  base  of  the  metamorphosed 
leaf  have,  on  the  other  hand,  assumed  the  function  of  leaf -blades;  they  are  very 
large,  provided  with  green  tissue,  of  arrow-like  or  lanceolate  contour,  and  at  a 
cursory  glance  may  be  easily  taken  for  leaf -blades.  It  has  already  been  mentioned 
on  p.  335  that  a  like  modification  of  function  occurs  in  many  Australian  acacias, 
the  foliage-leaves  of  which  are  devoid  of  green  blades  whilst  the  leaf-stalks  are 
developed  as  green,  flattened,  outspread  organs,  the  so-called  phyllodes. 

In  all  these  cases  we  have  only  treated  of  the  most  important  function  of 
foliage-leaves,  that  is,  the  formation  of  organic  materials  from  inorganic  food  in 
sunlight.  But  as  mentioned  previously,  the  foliage -leaves  of  many  plants  are 
assigned  other  functions,  which  again  require  certain  peculiar  adaptations,  and 
contribute  not  a  little  to  the  great  variety  in  the  form  of  this  organ.  One  series 
of  these  metamorphoses,  viz.  the  transformation  of  the  leaf -blades  and  leaf -stalks 
into  traps  and  digestive  organs  in  insectivorous  plants;  the  metamorphosis  of 
blades,  leaf -stalks,  and  stipules  into  weapons;  and  the  development  of  furrows  and 
channels  on  different  parts  of  the  foliage-leaves  for  the  irrigation  of  rain-water; 
and  finally  the  transformation  of  foliage-leaves  into  mere  scales,  as  in  the  switch 
plants,  &c.— all  these  have  already  been  fully  treated  of  in  earlier  chapters.  But 


638  SCALE-LEAVES,   FOLIAGE-LEAVES,    FLORAL-LEAVES. 

a  further  series  of  such  transformations,  especially  the  metamorphosis  of  parts  of 
foliage-leaves  into  tendrils,  hooks,  and  claws,  with  the  help  of  which  the  stem  is 
able  to  climb  up  firm  supports  towards  the  light,  and  the  transformation  of  the 
leaf -sheaths  into  mechanisms  for  protecting  flowers  against  unbidden  guests;  the 
consideration  of  these  must  be  deferred  till  we  deal  with  climbing  contrivances  and 
protections  for  flowers  in  general;  here,  there  only  remains  to  be  considered  the 
production  of  floating  contrivances  in  certain  marsh  and  water  plants,  and  the 
development  of  special  cells  to  assist  those  foliage-leaves  which  are  unprovided 
with  scale-leaves  in  breaking  through  the  soil. 

Floating  arrangements  occur  in  only  a  few  species  of  plants,  most  noticeably  in 
the  Brazilian  Pontederia  crassipes,  and  in  the  few  species  of  the  water-chestnut 
(Trapa).  In  both  instances  the  leaf-stalks  are  swollen  up  into  floats,  and  remind 
one  to  some  extent  of  the  swollen  utricular  leaf-stalk  of  Cephalotus,  Sarracenia, 
and  of  pitcher-plants.  They  are  distinguished  from  these  by  the  fact  that  the 
buoy-like  swelling  is  quite  closed,  and  that  the  partitioned  interior  neither  contains 
digestive  organs,  nor  is  beset  with  spines,  &c.,  to  hinder  the  exit  of  imprisoned 
animals.  Pontederia  crassipes  is  not  fixed  in  the  mud  beneath  the  water  by  roots, 
but  the  plants  float  freely  on  the  surface  of  the  pond.  It  is  of  great  importance 
to  these  plants  that  they  should  have  a  small  specific  gravity,  and  that  their  leaves, 
grouped  in  rosettes,  which  have  been  unfolded  above  the  water,  should  offer  a  large 
surface  to  the  air,  while  at  the  same  time  the  illumination  of  the  green  portions 
should  not  be  encroached  upon.  Both  these  requirements  are  met  by  the  bladder- 
like  leaf-stalk,  and  these  strange  floating  plants  are  driven  by  the  wind  like  ships 
hither  and  thither  over  the  surface  of  the  water. 

The  plants/of  water-chestnut  are  held  fast  to  the  muddy  bottom  under  water  by 
roots,  and  are  not  adapted  to  floating  freely.  The  submerged  leaves  are  finely 
divided  like  a  comb,  and  have  such  a  small  specific  gravity  that  when  detached 
from  the  stem  they  immediately  rise  to  the  surface  of  the  water.  The  uppermost 
leaves  lying  on  the  surface  of  the  water,  and  grouped  into  rosettes,  have  rhornboidal, 
tough,  almost  leathery  blades,  and  these  also  do  not  sink  when  they  are  isolated, 
and  therefore  it  is  difficult  to  see  what  advantage  is  afforded  in  this  instance  by 
the  swollen  leaf-stalk.  But  when  in  the  height  of  summer  large  heavy  fruits  are 
seen  to  be  produced  from  the  flowers  developed  amongst  the  leaves  of  these  floating 
rosettes,  it  then  becomes  evident  that  the  floating  capacity  of  the  rosette-leaves 
must  be  maintained,  lest  they  be  drawn  underneath  by  the  weight  of  the  nuts  and 
placed  in  a  position  as  unfavourable  as  could  be  imagined  for  the  proper  discharge 
of  their  functions. 

In  the  subterranean  buds  of  perennial  plants  the  rudimentary  foliage-leaves  are 
usually  surrounded  by  scales,  which  function  as  shields  and  screens,  and  in  par- 
ticular play  the  part  of  protective  organs  in  the  work  of  breaking  through  the 
ground.  Most  of  these  sheath-like  scales,  as  already  stated,  grow  up  with  the  elon- 
gating buds  until  the  soil  has  been  pierced,  and  their  points  strengthened  by  turgid 
cells  serve  as  actual  earth-breakers.  But  in  some  plants  which  survive  through 


SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES.  639 

the  winter,  with  underground  buds  or  bulbs,  the  young  sprouting  foliage-leaves  do 
not  have  this  assistance;  they  must  carve  their  way  through  the  soil  unaided,  and 
press  above  the  surface  without  a  sheathing  envelope.  Accordingly  they  have  to 
bore  through  a  more  or  less  thick  layer  of  earth,  often  a  stiff  clay;  or  one  perhaps 
containing  pointed  stones  and  angular  grains  of  sand.  Now  in  order  that  the 
foliage-leaves  traversing  this  rugged  and  uneven  path  may  suffer  no  damage,  they 
are  variously  folded  and  twisted  together  so  as  to  form  a  cone;  and  most  important 
of  all,  the  apex  of  this  cone,  which  operates  like  a  ground-auger,  and  therefore 
exercises  a  strong  pressure  on  the  soil,  is  armed  with  special  cells.  These  cells 
have  a  great  resemblance  to  those  at  the  apex  of  the  sheath-like  scale-leaves,  and  to 
those  of  the  knee-shaped  bent  cotyledon  of  the  onion  (see  p.  606).  In  many  plants 
possessing  lobed  or  deeply-divided  leaf -blades,  the  boring  apex  of  this  cone  is  formed 
by  a  bend  of  the  leaf-stalk,  which  is  doubled  over  like  a  hook.  Thus  in  the  foliage- 
leaves  of  the  Yellow  Monkshood  (Aconitum  Vulparia,  Lycoctonum,  &c.)  it  is  not 
the  apex  of  the  leaf  which  emerges  first  from  the  ground  but  the  convex  part  of 
its  bent  and  knee-like  stalk.  As  long  as  the  leaf  is  still  occupied  in  boring,  the 
delicate  free  apices  of  its  lobes  are  directed  inwards  and  downwards,  and  not  until 
the  hooked  leaf-stalk  has  emerged  above  the  surface  of  the  soil  does  it  straighten 
and  draw  the  leaf -blade  out  of  the  ground.  The  free  points  of  the  leaf -blade,  which 
were  hitherto  directed  downwards,  are  inclined  in  the  opposite  direction  when 
they  arrive  above  the  earth,  and  the  whole  leaf  then  unfolds  into  an  expansion 
parallel  to  the  surface  of  the  ground.  An  exactly  similar  process  is  observed  in 
large  ferns  with  underground  winter  buds,  e.g.  in  the  common  Male-fern  (Aspidium 
Filix-mas).  The  fronds  at  the  end  of  the  root-stock  are  spirally  rolled,  their 
delicate  segments  are  packed  closely  together,  one  above  the  other,  and  covered  by 
the  strong  rachis  of  the  leaf  as  by  a  thick  hoop.  Only  the  back  of  this  rachis 
comes  into  contact  with  the  forest  soil  as  it  is  broken  through;  the  rachis  prises  up 
the  top  layer  of  the  soil  in  its  gradual  unrolling,  and  the  delicate  segments  are  only 
unfolded  when  the  part  of  the  axis  in  question  has  emerged  and  straightened  itself. 
The  earth  is  broken  through  in  a  very  peculiar  manner  by  the  peltate  leaf- 
blades  of  Podophyllum  peltatum.  As  long  as  the  leaves  of  this  plant  are  still 
small  and  below  the  ground,  they  resemble  a  closed  umbrella;  the  folded  blade  is 
directed  downwards,  and  nestles  close  to  the  thick  stalk,  which  grows  straight  up. 
At  the  free  end  of  the  stalk,  which  would  correspond  in  position  to  the  ferule  of  an 
umbrella  held  upright,  is  found  a  group  of  thin-walled,  turgid  cells,  without  chloro- 
phyll, situated  like  a  white  knob  at  the  converging-point  of  the  leaf-strands.  When 
the  leaf -stalk  grows  in  height,  it  is  this  cell-group  which  presses  on  the  layers  of  earth 
above  it,  and  it  is  the  first  to  appear  at  the  surface.  The  leaf -blade,  still  furled  to 
the  stalk,  is  then  raised  up  through  the  hole  thus  bored.  Once  above  the  surface,  the 
blade  expands  just  like  an  opening  umbrella.  The  above-mentioned  group  of  cells, 
having  served  as  a  buffer,  now  loses  its  turgescence,  but  remains  visible  as  a  white 
spot  at  the  centre  of  the  brownish-green  expanded  leaf -blade.  In  the  species  of  the 
genera  Acanthus  and  Hydrophyllum,  which  are  characterized  by  divided  leaves, 


640  SCALE-LEAVES,   FOLIAGE-LEAVES,    FLORAL-LEAVES. 

the  lobes  of  the  blades  whilst  still  under  the  ground  are  depressed  as  in  Podo- 
phyllum,  but  here  the  penetration  is  accomplished  by  means  of  peculiar  bumps  and 
bladder-like  protuberances  on  the  uppermost  lobes,  which  again  consist  of  strongly 
turgid  cells.  In  the  Asarabacca  (Asarum)  it  is  the  apex  of  the  lower  leaf  folded 
together  lengthwise  which  is  composed  of  turgescent  cells,  and  which  growing 
upwards,  presses  the  earth  apart  like  a  wedge.  In  the  Broad-leaved  Allium,  Dog's- 
tooth  Violet  (Allium  ursinum  and  Erythronium  Dens  Canis),  in  the  Star  of 
Bethlehem  and  Hyacinth,  and  many  other  bulbous  plants,  also  in  numerous  orchids 
of  our  meadows  and  woods  whose  buds  pass  the  winter  embedded  in  deep  soil,  the 
apex  of  the  lowest  leaf -blade  is  transformed  into  an  actual  ground-auger,  usually 
shaped  like  a  hood  or  folded  cap-like  over  the  apices  of  the  other  leaf -blades  of  the 
plant.  A  group  of  cells  without  chlorophyll  is  always  found  on  the  apex  of  that 
leaf  which  envelops  the  others,  and  this  apex  may  be  plainly  distinguished  by  its 
white  colour.  In  most  of  the  plants  examined  the  cells  are  thin- walled  but  very 
turgid;  only  a  few  present  thickened  walls,  as,  for  example,  the  Broad-leaved  Allium 
(Allium  ursinum),  where  the  whole  leaf-apex  is  almost  horn-like.  This  group  of 
turgescent  cells  always  forms  the  apex  of  the  leaf-cone  growing  out  from  the 
subterranean  bud;  afterwards  when  this  cone  has  grown  up,  and  the  leaves  are 
spread  out  over  the  soil,  the  formerly  tense  cells  of  the  leaf -apex  collapse,  dry  up 
and  present  a  withered  appearance.  In  the  Asarabacca  and  in  many  orchids  the 
apices  of  the  mature  and  lower  leaves  are  regularly  browned,  and  look  as  if  burnt, 
even  when  they  have  not  been  actually  injured  in  penetrating  the  ground. 

The  term  floral-leaves  comprehends  all  those  which  are  directly  or  indirectly 
concerned  in  the  processes  of  fertilization,  and  in  the  production  of  the  embryo. 
First  of  all  we  have  the  leaf-structures  within  which  the  germ-cell  is  formed,  that 
cell  from  which  the  embryo  proceeds  after  fertilization.  Then  there  are  those  in 
which  arise  the  fertilizing  cells  known  by  the  name  of  pollen-grains.  Finally  all 
those  which  are  concerned  in  bringing  about  the  union  of  the  pollen-cells  with  the 
germ-cells,  or  whose  task  is  to  protect  these  two  kinds  of  sexual  cells  during  their 
development  from  injurious  external  influences.  Since  the  processes  only  shortly 
indicated  here  will  be  fully  described  in  the  second  volume  of  The  Natural  History 
of  Plants,  and  since  the  forms  of  the  floral  leaves  will  be  considered  in  these 
descriptions,  we  need  not  here  give  a  detailed  representation  of  these  structures. 
In  the  pages  which  follow  they  will  only  be  treated  of  so  far  as  is  necessary  for 
the  comprehension  of  the  architecture  of  the  whole  plant,  and  of  a  series  of 
botanical  terms. 

With  regard  to  the  succession  and  arrangement  of  floral-leaves,  it  has  to  be 
noticed  as  one  of  their  most  characteristic  features  that  the  last  and  uppermost 
floral  leaves  are  always  very  close  together,  and  are  usually  developed  as  closely 
appressed  whorls.  These  assemblages  of  floral-leaves  together  form  the  flower.  The 
axis  which  bears  the  flower  at  its  free  end  is  termed  the  flower-stalk  (pedunculus). 

The  axis  which  is  terminated  by  the  flower  is  only  in  rare  instances,  viz.  in  a 
few  annual  herbs,  the  direct  continuation  of  the  shoot  which  is  produced  from  the 


SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES.  641 

first  bud  (plumule)  situated  above  the  hypocotyl  (cf.  fig.  2).  In  this  case  the  floral 
leaves,  collected  together  to  form  the  flower,  follow  directly  above  the  foliage- 
leaves  on  the  same  shoot.  Such  a  flower  is  called  terminal.  Much  more  frequently 
the  flowering  axis  or  peduncle  is  inserted  laterally  on  an  older  shoot,  and  origin- 
ates close  above  a  leaf,  called  a  subtending  leaf;  here  we  speak  of  lateral  flowers. 
Usually  several  flowers  are  grouped  in  a  definite  way,  and  the  term  inflorescence 
(inflorescentia)  has  been  introduced  to  distinguish  these  groupings.  The  subtending 
leaf  (folium  fulcrans)  either  agrees  in  general  character  with  the  lower  foliage- 
leaves,  and  is  then  said  to  be  "  leaf-like  ",  or  it  differs  in  shape  and  size  as  well  as  in 
colouring,  and  is  then  spoken  as  a  bract  (bractea). 

These  leaves,  differing  from  foliage-leaves,  always  have  a  special  relation  to  the 
processes  of  fertilization;  and  are  therefore  to  be  reckoned  with  the  floral-leaves. 
Frequently  a  whole  inflorescence  is  surrounded  and  supported  by  a  single  enormous 
bract,  and  in  such  inflorescences,  which  are  very  characteristic  of  palms  and  aroids, 
the  bracts  at  the  base  of  the  individual  flower-stalks  are  usually  undeveloped.  This 
large  common  bract  is  called  a  spathe  (spatha).  The  Climbing  Palm  (Desmoncus) 
illustrated  in  fig.  157  3,  has  such  a  spathe  beset  with  prickles.  It  sometimes  hap- 
pens that  some  of  the  flowers  of  the  inflorescence  do  not  develop,  and  that  then 
bracts  are  to  be  seen  without  flowers.  If  such  "  empty  bracts  "  are  found  crowded 
together  at  the  base  of  the  inflorescence  arranged  at  one  level,  or  are  there  grouped 
in  very  close  spiral  revolutions,  we  speak  of  an  involucre  (involucrum).  Some- 
times they  are  to  be  seen  at  the  apex  of  the  whole  inflorescence,  the  group  forming 
what  we  may  call  a  crest.  Minute,  stiff,  dry  bracts,  without  chlorophyll,  in  the 
centre  of  thickly  crowded  inflorescences  are  called  palece  (palece). 

In  flowers  we  distinguish  perianth-leaves,  stamens,  and  carpels.  The  perianth- 
leaves,  are  arranged  either  spirally  or  in  whorls.  The  former  arrangement  is 
observed  most  noticeably  in  the  cacti,  of  which  several  species,  including  Cereus, 
Mamillaria,  and  the  remarkable  hedgehog-like  JEchinocactus  capped  by  its  flower, 
are  illustrated  at  vol.  II.,  p.  787.  In  the  many-membered  perianth  inclosing  the 
flowers  of  this  plant  more  than  a  hundred  perianth -leaves  are  so  arranged  at 
small  vertical  intervals  along  a  spiral  line  that  the  smallest  stand  lowest,  the 
largest  uppermost,  not  unlike  the  leaves  of  the  involucral  cup  around  the  capitulum 
of  a  composite.  This  spiral  arrangement,  however,  is  rare,  at  least  in  such  a 
striking  form.  Much  more  frequently  the  perianth -leaves  form  two  successive 
whorls.  If  the  lower  whorl  consists  of  green  leaves,  which  agree  in  texture  and  in 
general  appearance  with  foliage-leaves,  while  the  upper  is  composed  of  more  delicate 
leaf -structures  displaying  all  possible  colours  except  green,  the  lower  is  called 
calyx,  and  the  upper  the  corolla.  If  all  these  perianth -leaves  are  shaped  and 
coloured  very  much  alike,  so  that  there  is  no  marked  contrast  between  the  whorls, 
we  then  speak  of  a  perigone  (perigonium).  This  may  be  either  green  like  a  calyx, 

or  coloured  like  a  corolla. 

The  stamens  (stamina),  the  "attire"  of  the  older  botanists,  are,  like  the  pei 
anth-leaves,  usually  whorled,  or,  more  rarely,  arranged  in  spirals.     Each  stamen 


VOL.  I. 


042  SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES. 

consists  of  the  anther  (anthera),  i.e.  that  part  in  which  the  pollen  is  developed, 
and  of  the  support  to  this  anther,  which  is  usually  threadlike,  and  bears  the  name 
of  filament  (filamentum).  Filaments  and  anthers,  in  many  instances,  correspond 
to  the  sheathing-part  and  stalk  of  the  leaf,  and  in  these  stamens  the  blade  is 
wholly  suppressed;  in  other  instances  the  anther  is  to  be  regarded  as  the  lower 
part  of  the  blade,  and  then  the  apex  of  the  blade  appears  as  a  scale-like  appendage. 
The  blade  of  the  stamen  sometimes  resembles  a  perianth-leaf,  and  this  is  a  case 
to  which  there  will  be  frequent  allusion. 

The  carpels  (carpophylla)  are  arranged,  like  the  perianth-leaves  and  stamens, 
sometimes  in  whorls  and  sometimes  spirally.  In  one  section  of  flowering  plants 
they  are  scale-like,  and  present  free  margins  not  joined  together.  In  another 
section  they  are  rolled  together  and  their  margins  fused,  so  that  a  capsule  called 
the  pistil  (pistillum,  ovarium)  is  formed.  If  many  carpels  are  present  in  one 
flower,  each  of  them  may  form  a  separate  ovary,  and  then  the  more  or  less 
numerous  one-leaved  ovaries  appear  arranged  either  spirally  or  in  a  stellate 
manner  as  the  termination  of  the  shoot  in  the  centre  of  the  flower,  e.g.  in  the 
Ranunculaceae  and  Dryadeae.  In  the  Papilionacese  and  several  others  allied  to 
these  groups  of  plants  there  is  only  a  single  one-leaved  pistil  at  the  end  of  the 
flower-shoot;  but  usually  several  whorled  carpels  are  united  together  to  form  a 
single  ovary  in  the  centre  of  the  flower.  A  great  number  of  different  constructive 
plans  of  many-leaved  pistils  are  distinguished  according  to  the  manner  and  extent 
of  union,  and  these  in  particular  afford  excellent  marks  for  characterizing  the 
families  and  genera.  The  most  striking  differences  are  produced  by  the  whorled 
carpels  being  at  one  time  fused  with  one  another  along  their  whole  length,  while 
at  another,  the  fusion  is  restricted  only  to  the  lower  part;  by  the  fact  that  fre- 
quently the  rolled  united  margins  of  the  adjoining  carpels  become  partition- walls 
in  the  interior  of  the  pistil,  resulting  in  the  formation  of  compartments,  while  in 
other  cases  this  formation  of  septa  does  not  occur,  the  carpels  adjoining  one 
another  like  the  staves  of  a  cask,  and  forming  an  unchambered  capsule. 

The  pistil  may  be  divided  into  the  ovary  (germeri),  style  (stylus)  and  stigma 
(stigma).  The  ovary  corresponds  to  the  sheathing  portion,  the  style  to  the  stalk, 
and  the  stigma  perhaps  to  the  blade  of  the  leaf.  The  ovary  forms  in  most  cases 
an  expanded  structure;  its  contour  and  surface  offer  little  variety,  especially 
when  compared  with  the  inexhaustible  diversity  of  other  parts  of  the  flower. 
Usually  its  shape  is  ovate,  ellipsoidal,  spherical,  or  disc-like,  more  rarely  elongated, 
cylindrical,  or  barrel-shaped;  sometimes  it  is  flattened  from  side  to  side,  and 
has  the  form  of  a  sword  or  sabre.  Projecting  knobs,  cushions,  angles,  ridges, 
and  bands  are  often  found  on  its  circumference  in  accordance  with  the  number 
of  the  carpels  of  which  it  is  composed,  and  three-  or  five-sided  forms  are  met 
with  very  frequently.  The  hairs,  bristles,  spines,  and  wings  appearing  so  noticeably 
on  the  ovary  when  it  has  been  transformed  into  the  fruit-capsule  are  usually  so 
undeveloped  at  the  time  of  flowering  that  perhaps  not  even  the  rudiments  of 
these  outgrowths  can  be  recognized. 


SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES.  643 

The  ovary  contains  structures  which,  from  analogy  with  the  eggs  of  animals, 
have  been  termed  ovules  (ovula).  They  are  also  called  "seed-buds",  as  the  seeds 
are  produced  from  them  after  fertilization.  Formerly  the  name  "germ-buds"  was 
frequently  employed  for  these  structures.  Those  botanists  who  endeavour  to  refer 
the  infinitely  manifold  members  of  plants  to  a  few  fundamental  forms,  and 
•especially  to  settle  whether  a  certain  structure  is  to  be  considered  as  a  stem  or  a 
leaf,  have  fought  very  much  over  the  ovules.  First  of  all,  ovules  were  regarded 
without  exception  as  stem-structures,  as  parts,  that  is,  of  the  axis,  and  the  upper- 
most portion  of  the  stem  which  bears  the  ovules— or  from  which  the  supports  of 
the  ovules  branch  off  —  were  designated  as  fruit-axes.  It  was  thought  that 
these  fruit-axes  divided  up  in  the  most  varied  manner,  and  that  they  sometimes 
also  became  leaf -like,  resembling  flattened  shoots,  in  which  case  the  ovules  would 
arise  from  the  margins  of  the  flattened  expansion.  It  was  also  supposed  that 
such  fruit-axes  might  be  united  with  the  carpels,  and  the  impression  would  then 
be  given  that  the  ovules  were  produced  from  the  carpels.  Later,  the  ovules  of 
all  plants  were  interpreted  as  leaf-structures,  i.e.  as  parts  of  the  carpels,  and 
their  direct  origin  from  the  axis,  that  is,  from  the  stem,  was  denied.  Even  those 
ovules  which  are  situated  on  the  apex  of  the  axis,  projecting  into  the  centre  of 
the  ovarian  cavity,  were  regarded  as  outgrowths  of  the  carpels,  and  it  was  sup- 
posed that  a  freely-ascending,  ovule-bearing  column  projecting  into  the  cavity 
of  the  ovary  rose  up  from  the  base  of  the  united  carpels.  Various  other  forced 
•explanations  have  been  given,  but  it  is  hardly  suitable  to  consider  them  here. 

These  false  interpretations  are  corrected  when  we  no  longer  lay  that  stress 
on  the  difference  between  stem  and  leaf,  which  was  asserted  by  the  advocates 
of  the  two  views  quoted  above,  and  when  we  remember  that  really  all  leaves 
are  produced  from  a  stem,  and  that  it  is  by  no  means  easy  to  settle  where  the 
stem  ceases  and  the  leaf  begins.  If  we  rigidly  adhere  to  the  history  of  develop- 
ment and  to  the  actual  fact  rather  than  to  those  speculations  on  which  is  based 
the  conception  of  an  "ideal  plant",  and  if  at  the  same  time  we  set  ourselves 
against  the  attempt  to  refer  all  plans  of  construction  to  a  single  fundamental 
ground-plan,  we  arrive  at  this  result,  that  in  many  cases  the  ovules  proceed 
directly  from  the  apex  of  the  stem,  and  that  even  in  the  earliest  stages  of  develop- 
ment they  have  no  organic  connection  with  the  carpels.  They  stand  in  the 
.same  relation  to  the  stem  as  carpels  do,  and  there  is  no  reason  why  they  should 
not,  like  them,  be  regarded  as  peculiarly  metamorphosed  leaves.  They  form 
the  last  uppermost  leaves  originating  from  the  axis,  become  subsequently  a  con- 
stituent of  the  fruit,  and  may  also  be  looked  upon,  in  consequence,  as  upper 
•carpels.  In  such  instances  as  these,  two  successive  whorls  of  carpels  are  developed, 
one  situated  below,  whose  members  develop  no  ovules,  and  one  placed  above, 
whose  members  are  only  formed  of  ovules  and  their  supports.  The  lower  carpels, 
without  themselves  developing  ovules,  form  the  capsule  arching  over  the  upper 
carpels  which  have  been  reduced  to  ovules.  This  view  receives  the  more  justi- 
fication from  the  fact  that  similar  conditions  are  observed  in  the  stamens ;  that 


644  SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES. 

is  to  say,  there  are  flowers  in  which  the  outer  lower  stamens  are  flat,  leafy  ex- 
pansions, whilst  the  upper  are  reduced  to  anthers  and  filamentous  supports.  Of 
course  this  is  only  a  view  which  it  would  be  unwise  to  insist  upon  after  the 
foregoing  strictures. 

This  supposition  does  not  exclude  the  fact  that  in  many  instances  a  single 
whorl  of  carpels  is  developed,  and  that  the  carpels  of  this  whorl  not  only  form 
the  capsule,  but  that  at  the  same  time  ovules  may  arise  from  them.  At  one 
time  the  teeth  of  the  margins  of  these  carpels  become  ovules;  at  another,  whole 
segments  of  a  leaf  are  metamorphosed  into  ovules;  again,  in  another  instance, 
groups  of  cells  have  given  rise  to  ovules  over  the  midribs  of  the  carpels;  and 
lastly,  innumerable  ovules  may  have  developed  from  the  whole  inner  surface  of 
the  carpels. 

The  internal  structure  of  the  cavity  of  the  ovary  is  still  further  complicated 
by  the  fact  that  the  end  of  the  axis  in  one  case  rises  up  like  a  hemisphere  or 
truncated  column  in  the  centre  of  the  capsule,  while  in  other  instances  the  end 
of  the  axis  is  hollowed  into  a  pit,  and  sometimes  even  deeply  excavated.  In 
consequence  of  these  manifold  arrangements,  very  different  relations  between  carpels 
and  axis  naturally  follow,  and  the  most  various  constructive  plans  result,  which, 
however,  will  be  more  suitably  discussed  in  the  second  volume  when  considering 
the  individual  families,  especially  the  Primulaceae  and  Onagracese. 

In  whatever  way  the  ovules  may  be  explained,  they  exhibit  a  great  agreement 
in  structure.  In  them  may  be  distinguished  the  nucellus  (nucleus),  surrounded 
by  two,  or  less  frequently  by  only  one  coat  (integumentum),  and  also  the  portion 
by  which  the  ovule  is  connected  with  its  substratum,  the  placenta.  Usually  this 
has  the  form  of  a  stalk  or  filament  (funiculus),  and  then  the  ovules  appear,  as  it 
were,  suspended  in  the  interior  of  the  ovary.  When  the  ovule  is  straight,  and  is 
a  direct  continuation  of  the  funicle,  it  is  called  orthotropous]  if  the  straight  ovule 
is  hung  on  a  thread-like  support,  but  reversed,  and  more  or  less  fused  with  the 
support,  it  is  said  to  be  inverted  or  anatropous',  when  it  is  curved,  the  designation 
campylotropous  is  used.  The  coats  do  not  completely  inclose  the  ovules,  but  at 
one  pole  a  spot  which  bears  the  name  of  micropyle  is  left  uncovered. 

As  already  remarked,  the  style  corresponds  to  a  leaf-stalk  as  regards  its 
position  and  relation  to  the  other  portions  of  the  pistil.  In  the  one-leaved  pistil 
its  form  frequently  resembles  a  leaf -stalk,  especially  in  papilionaceous  plants. 
If  the  ovary  of  a  one-leaved  pistil  be  regarded  as  arising  from  the  sheathing 
portion,  and  the  style  from  the  stalk  of  a  leaf,  it  will  be  easily  conceived  that 
the  style  appears  to  be  affixed  to  one  side  of  the  ovary.  The  lateral  position 
of  the  style  can  be  clearly  understood  if  we  imagine  that  the  sheathing  portion 
of  the  ovary  is  swollen  up  like  a  vesicle,  as  it  is  on  the  foliage-leaves  of  Umbel- 
liferae,  or  that  it  bears  large  stipules  as  in  the  Cinquefoil  (Potentilla).  In  the 
one-leaved  ovary  of  the  cinquefoils  the  style  in  fact  is  not  seen  to  spring  from 
the  apex  of  the  ovary,  but  looks  as  if  it  had  grown  out  laterally  from  its  capsule. 
In  pistils  which  are  built  up  of  many  carpels  arranged  in  a  whorl,  and  having 


SCALE-LEAVES,    FOLIAGE-LEAVES,   FLORAL-LEAVES.  645 

only  their  sheathing  portions  fused,  as,  for  example,  in  the  Meadow  Saffron 
(Colchicum),  or  in  the  much-cultivated  "Love  in  a  Mist"  (Nigella  Damascena), 
the  styles  are  separate  and  always  fixed  at  one  side  of  the  compartment  of  the 
ovary  corresponding  to  them.  But  when  several  whorled  carpels  are  completely 
united  with  one  another  as  far  as  the  stigma,  only  a  single  style  is  to  be  seen. 
This  style,  which  may  be  considered  as  a  combination  of  several  grooved  leaf- 
stalks, then  rises  up  above  the  centre  of  the  many-chambered  ovary.  Just  as 
the  leaf-stalk  may  be  frequently  absent  from  foliage-leaves,  so  sometimes  the 
pistil  has  no  style,  and  the  stigma  is  sessile  or  seated  immediately  upon  the  ovary. 

The  stigma  corresponds  to  the  blade  portion  of  a  leaf,  but  is  expanded  in  only  a 
few  families  of  plants,  amongst  which  the  irises  are  the  best  known.  It  has  to 
receive  and  hold  the  pollen-grains,  and  its  form  varies  according  as  to  whether  these 
are  carried  by  the  wind  as  flower-dust  or  are  brought  to  the  flowers  by  insects  in 
cohering  masses.  In  the  former  case  the  stigmas  are  brush-like  or  feathery,  often 
extended  like  a  cobweb  or  spread  out  like  a  plume;  in  the  latter  case,  projecting 
papillae,  knobs,  ridges  and  bands  are  found  on  them,  against  which  the  insects 
knock  off  the  pollen  as  they  enter  the  flower. 

If  we  consider  now  the  functions  of  the  various  floral  structures  rather  than 
the  position  and  succession  of  the  individual  members,  we  arrive  at  the  following 
result.  Of  all  the  structures  known  as  floral-leaves  the  ovules  and  pollen-grains 
(i.e.  those  parts  of  the  flower  on  which  these  structures  are  produced)  alone  are 
indispensable.  These  portions  of  the  flower,  however,  must  be  protected  not  only 
during  their  development  and  at  the  moment  of  fertilization  against  possible  external 
injurious  influences,  but  the  union  of  pollen-grains  with  ovules  must  be  brought 
about  by  a  suitability  in  the  form  of  the  floral -leaves  in  addition  to  the  mere 
production  of  these  bodies.  In  order  to  be  able  to  fulfil  these  tasks  the  floral-leaves 
which  develop  ovules  or  pollen  are  themselves  often  suitably  equipped  and  adapted, 
or  a  division  of  labour  takes  place,  so  that  only  one  portion  of  the  floral-leaves 
develops  ovules  or  pollen,  while  the  other  exists  for  protection  and  as  a  means  of 
ensuring  fertilization.  In  many  plants,  for  example,  the  carpels  are  not  only  the 
bearers  of  the  ovules,  but  also  at  the  same  time  their  protectors,  and  by  the  pecu- 
liarity of  their  structure  they  conduct  the  pollen  to  the  ovules  they  bear.  In 
numerous  other  plants,  on  the  contrary,  a  division  of  labour  has  occurred;  the 
ovules  spring  from  the  axis  as  independent  structures,  and  the  carpels  proper 
surround  and  protect  them,  and  receive  the  pollen  for  them,  as  may  be  seen  typically 
in  the  flowers  of  primulas.  In  the  American  Pachysandra,  in  the  Persian  Hali- 
mocnemis,  and  in  many  other  plants,  the  stamens  produce  pollen  in  coherent 
masses,  but  some  of  them  are  also  provided  with  allurements  for  those  insects  which 
carry  the  pollen  from  flower  to  flower,  and  distribute  it  to  the  suitable  stigmas. 
A  division  of  labour  is  met  with  in  most  of  these  plants  which  have  coherent  pollen, 
two,  three,  or  more  whorls  of  stamens  are  developed,  the  upper  bear  anthers  and 
produce  pollen,  the  lower  are  without  pollen,  but  assume  the  function  of  attracting 
insects  and  of  protecting  the  upper  anther-bearing  stamens.  Regarded  from  this 


646 


SCALE-LEAVES,    FOLIAGE-LEAVES,    FLORAL-LEAVES. 


stand-point  the  perianth-leaves  are,  as  it  were,  only  antherless  stamens,  and  this  view 
is  supported  by  the  fact  that  in  the  so-called  double  flowers  the  anther-bearing 
stamens  regularly  change  into  antherless  perianth-leaves.  In  the  flowers  of  water- 
lilies  as  a  rule  no  sharp  limit  can  be  drawn  between  stamens  and  perianth-leaves, 
but  a  gradual  transition  from  one  to  the  other  may  be  plainly  noted.  The  flowers 
of  certain  limes  (Tilia  Americana,  alba,  argentea),  as  well  as  those  of  the  arrow- 
grass  (Triglochin),  of  which  an  illustration  is  given  below,  are  very  instructive  in 
this  respect.  In  the  Silver  Lime  (Tilia  argentea,  figs.  151  1  and  151 2)  a  whorl  of 


Fig.  151.— Flowers  of  the  Silver  Lime  (Tilia  argentea),  and  of  a  species  of  Arrow-grass  (Triglochin  BareUieri). 

i  Inflorescence  of  the  Silver  Lime,  natural  size.  2  Longitudinal  section  through  a  single  flower.  3  Flower  of  the  Arrow-grass, 
in  the  first  stage  of  blossoming.  *  The  same  flower  in  a  later  stage  of  development ;  one  of  the  upper  perianth-leaves  cut 
away,  a,  s,  and  *  are  enlarged, 

stamens  with  anthers  is  first  formed  below  the  pistil,  followed  by  a  whorl  of  leaves 
without  anthers,  which,  however,  secrete  honey  to  allure  insects;  then  again  comes 
a  whorl  of  leaves  with,  and  below  these  again  two  whorls  of  leaves  without 

o 

anthers.  The  same  is  the  case  in  Triglochin,  whose  flowers  look  as  if  they  were 
composed  of  two  stories  standing  one  above  the  other,  quite  similarly  arranged  (see 
figs.  151  3  and  151 4).  The  flower  commences  below  with  a  whorl  of  three  hollowed 
antherless  leaves;  above  these  comes  a  whorl  of  three  leaves  with  anthers,  and  the 
large  anthers  are  surrounded  and  protected  during  development  by  the  hollowed 
leaves  as  if  by  a  hood;  then  again  follows  a  whorl  of  three  hollowed  antherless 
leaves,  and  above  these  yet  again  a  whorl  of  three  stamens  with  large  anthers,  an 


DEFINITION   AND   CLASSIFICATION   OF   STEMS.  647 

arrangement  resembling  that  of  the  lower  story.  When  the  powdery  pollen  falls 
from  the  anthers  it  is  not  immediately  carried  away  by  the  wind,  but  falls  first 
of  all  into  the  hollow  cavities  of  the  leaves  below  the  anthers,  where  it  remains 
deposited  until  the  proper  time  has  arrived  for  its  transmission  to  the  stigma 
of  another  flower.  These  hollowed  leaves,  although  themselves  antherless,  are 
therefore  filled  with  pollen  for  a  time,  and  look  like  anthers  which  have  just 
dehisced.  They  are  of  the  greatest  importance  for  the  timely  distribution  of  the 
pollen  and  for  the  accomplishment  of  fertilization,  and  may  be  regarded  with 
respect  to  the  part  which  they  have  to  play  as  antherless  stamens. 

Usually  all  these  leaf-structures  of  the  flower  originating  from  the  axis  below 
the  pistil  and  bearing  no  anthers  are  designated  as  perianth-leaves — as  calyx-  and 
corolla-leaves,  or,  lastly,  as  staminodes.  What  descriptive  botanists  understand  by 
perianth,  calyx,  and  corolla  has  been  already  described  on  p.  641 ;  with  regard  to  the 
term  staminode,  it  should  be  mentioned  that  it  is  applied  to  all  such  antherless 
leaves  as  are  inserted  between  the  whorls  of  perianth  or  corolla  leaves  on  the  one 
hand,  and  the  carpels  on  the  other;  i.e.  they  occur  where  in  most  instances  the 
anther-bearing  stamens  are  placed.  Staminodes  resemble  the  stamens  very  much 
in  shape,  but  are  distinguished  from  them  by  the  fact  that  they  develop  no  pollen. 
They,  however,  make  themselves  useful  in  other  ways.  Thus  in  the  transmission  of 
the  pollen,  they  secrete  honey  and  allure  insects;  or  they  may  serve  as  protective 
agents  for  their  neighbours,  the  anther-bearing  stamens,  against  various  external 
injuries.  A  detailed  description  of  the  part  performed  in  the  process  of  fertilization 
by  all  these  floral  leaves  which  are  so  differently  shaped  and  are  arranged  in  such 
manifold  ways  with  regard  to  one  another,  is  reserved  for  the  second  volume  of  this 
work. 


3.   FORMS  OF  STEM  STRUCTURES. 

Definition  and  Classification  of   Stems.  — The  Hypocotyl.  —  Stems  bearing  Scale-leaves.  —  Stems 
bearing  Foliage-leaves.— Procumbent  and  Floating  Stems.— Climbing  Stems.— 
Stems.— Itesistance  of  Upright  Stems  to  Strain,  Pressure,  and  Bending.— Floral-Stems. 

DEFINITION  AND  CLASSIFICATION  OF  STEMS.    THE  HYPOCOTYL.    STEMS 

BEARING  SCALE-LEAVES. 

In  certain  seeds  consisting  of  rounded  or  ellipsoidal  masses  of  tissue,  the  embryo 
shows  no  obvious  division  into  stem  and  leaf ;  nor  can  any  distinction  be  recognized 
between  the  embryo  and  the  surrounding  seed-coat.     When  such  seeds  begin  t 
germinate,  as,  for  example,  those  of  orchids,  their  cells  become  partitioned 
multiply,  and  the  whole  tissue-body  increases  in  size,  but  for  a  long  time  no  trace  i 
visible  of  a  division  into  stem  and  leaf.     It  is  shown  by  the  development  of  the  see. 
of  Cuscuta,  described  and  figured  on  p.  173,  that  the  embryo,  the  seed-coat  and  I 
reserve  tissue  which  nourishes  the  embryo  for  a  time  and  provides  it  wit 


(548  DEFINITION   AND   CLASSIFICATION    OF   STEMS. 

necesscary  building  materials,  may  all  be  distinguished  from  each  other;  but  the 
embryo  itself  shows  no  segmentation  into  axis  and  leaves.  It  looks  to  the  naked 
eye  like  a  filamentous,  spirally-rolled  structure,  which  breaks  through  the  envelope 
of  the  seed-coat  on  germination,  extends  and  elongates,  then  grows  up  straight  and 
afterwards  twists  and  winds  and  seeks  for  a  resting-place  from  which  it  can  derive 
nourishment.  This  thread  may,  without  further  discussion  be  considered  as  a  stem 
although  it  bears  no  leaves,  and  indeed  never  presents  even  the  rudiments  of  leaves. 
Not  until  later,  when  this  thread-like  stem  has  developed  haustoria  at  the  spots 
where  it  is  in  contact  with  the  host-plant,  and  has  grown  still  longer  by  the  help 
of  the  absorbed  nourishment,  do  small  scales  which  must  be  interpreted  as  leaves 
arise  below  the  growing  point  (cf.  fig.  35 l  on  p.  175).  Projections  are  then  developed 
above  the  scales  which  grow  out  into  lateral  shoots. 

The  fact  that  stems  exist,  which,  in  their  young  condition  exhibit  neither  leaves 
nor  even  the  rudiments  of  leaves,  is  specially  emphasized  here,  because  it  has  been 
repeatedly  denied  that  the  stem  is  a  special  member  of  the  plant.  This  may  of 
course  seem  strange  to  the  uninitiated,  and  it  will  be  asked,  how  then  are  we  to 
regard  the  stem  if  it  has  not  the  value  of  an  independent  morphological  member? 
Although  this  theme  is  so  delicate,  and  its  treatment  so  difficult  for  those  who  are 
not  initiated  into  the  details  of  the  speculative  science  of  form,  yet  I  will  try  to 
briefly  state  the  grounds  which  have  led  to  the  opinion  stated  above 

At  the  free  extremity  of  a  growing  leafy  shoot  a  slight  difference  may  indeed  be 
recognized  between  the  cells  of  the  periphery  and  those  of  the  interior,  but  no  clear 
boundary  can  be  fixed  between  these  parts,  and  the  end  looks  like  an  undifferen- 
tiated  conical  or  hemispherical  mass  of  tissue.  On  observing  more  narrowly  the 
growth  and  further  development  of  the  mass,  it  will  be  noticed  that  cushions  or 
protuberances  arise  on  the  periphery  of  the  cone  and  form  leaves,  while  the  inner 
portion  above  these  leaf -rudiments  continues  to  elongate  as  an  undifferentiated 
mass.  Soon,  however,  fresh  rudiments  of  leaves  arise  from  it,  and  so  as  the  process 
continues  quite  a  large  number  of  the  cells  are  grouped  together  and  form  in  their 
turn  the  starting-points  of  leaves.  If  we  examine  the  tissue  of  a  leaf  as  it  arises 
thus  below  the  tip  of  the  shoot,  we  shall  seek  in  vain  for  the  place  where  the 
substance  of  the  leaf  ceases  and  that  of  the  stem  begins.  It  is  on  such  grounds  as 
these  that  the  view  has  been  formulated  that  the  whole  stem  is  really  nothing  else 
than  a  collection  of  leaves,  standing  one  above  the  other,  whose  basal  portions 
remain  united,  while  the  peripheral  parts  according  to  need  rise  up  and  project 
more  or  less.  Against  this  view,  of  course,  there  is  apparently  the  fact  that  not 
only  leaves  but  also  lateral  shoots  appear  on  the  circumference  of  a  growing  shoot, 
from  which  it  follows  that  the  whole  of  the  tissue  is  not  employed  in  the  formation 
of  leaves,  but  that  a  part  remains  over  from  which  the  commencements  of  lateral 
stems  are  produced,  and  that  it  is  this  part  which  does  not  form  leaves  which  repre- 
sents the  tissue  of  the  main  stem.  It  has  also  been  proved  that  the  rudiments  of 
leaves  arise  on  the  growing  shoot-cone  from  cells  lying  nearer  the  periphery  than 
those  from  which  the  rudiments  of  lateral  stems  develop.  This  different  origin  has 


DEFINITION    AND   CLASSIFICATION   OF   STEMS.  649 

been  used  as  a  mark  for  distinguishing  between  leaf  and  stem,  and  the  peripheral 
tissue  has  been  explained  as  the  basis  of  the  leaves,  and  the  tissue  lying  below  it 
as  that  of  the  stem-structures.  The  outer  layer  of  cells  of  the  growing  cone,  called 
dermatogen,  never  forms  the  starting-point  for  lateral  stems,  although  it  may 
occasionally  give  rise  to  leaves.  The  two  or  three  outer  layers  of  cells  of  the  tissue 
below,  called  the  periblem,  usually  form  the  leaves,  but  lateral  stems  often  originate 
from  the  second  to  the  fourth  layers  of  the  periblem.  However,  these  possible 
differences  of  origin  are  insignificant,  and  no  sharp  limit  can  be  drawn  between  the 
tissues  from  which  originate  the  rudiments  of  leaves  and  lateral  stems;  consequently 
in  this  particular  point  there  is  no  essential  difference  between  leaf  and  stem. 

In  the  stem  the  vascular  bundles  form  a  ring  round  the  axis,  but  in  the  leaf- 
stalk, in  other  respects  often  very  like  the  stem,  they  are  grouped  in  a  semicircle  or 
in  a  plane.  This,  however,  does  not  invariably  occur.  Leaf-stalks  which  bear 
peltate  blades,  as  well  as  those  which  pass  into  blades  with  pinnate  or  palmate 
strands,  as,  for  example,  those  of  Solanum  jasminoides,  Anamirta  Coccidus,  Meni- 
spermum  Carolinianum  and  of  many  other  Menispermaceae  exhibit  circles  of 
vascular  bundles  and  an  actual  ring  of  wood,  so  that  they  cannot  in  their  internal 
structure  be  distinguished  from  stems.  All  other  differences  between  leaf  and  stem 
which  have  been  brought  forward  at  different  times  and  by  different  investigators 
apply  indeed  to  a  number,  often  to  a  very  great  number  of  plants,  but  unfortunately 
not  to  all.  The  following  have  been  suggested  as  relatively  the  best  marks  of 
distinction,  viz.  that  the  leaf  shows  a  limited  growth,  and  that  no  new  leaves  spring 
directly  from  it,  while  the  stem  grows  indefinitely  and  produces  leaves  laterally 
below  its  growing  point.  I  say  expressly  the  relatively  best  marks  of  distinction, 
because  structures  exist  which  cannot  be  forced  into  the  limits  of  this  definition. 
The  flower-bearing  as  well  as  the  flowerless  phylloclades  of  the  Smilacinese  (which 
are  really  reduced  axes)  have  always  a  limited  growth;  and,  on  the  other  hand, 
there  are  plants  from  whose  leaves  other  leaves  grow  out.  In  the  leaf -blades  of  the 
American  twining  plant  Aristolochia  Sipho,  which  is  often  met  with  in  gardens  as 
a  covering  for  arbours  and  trellis-work,  green  projecting  bands  and  lobes,  which  can 
indeed  only  be  explained  as  leaf -structures,  sometimes  arise  on  the  lower  side  of  the 
blade,  especially  in  those  places  where  the  finer  strands  form  delicate  anastomoses. 
This  is  a  case  where  leaf -like  structures  actually  spring  directly  from  leaves,  and 
the  only  difference  is  that  the  places  of  origin  of  the  leaflets  are  not  arranged  in 
geometrical  succession. 

On  reviewing  the  results  of  the  developmental  and  morphological  researches, 
here  only  briefly  touched  upon,  we  are  forced  to  confess  that  it  is  very  difficult  to 
state  absolute  distinctions  between  leaf  and  stem,  and  that,  moreover,  the  view 
already  mentioned,  viz.  that  the  stem  does  not  form  an  independent  member  of  the 
plant,  is  not  really  contradicted.  The  single  fact  opposed  to  this  view  is  the  occur- 
rence of  stems  without  leaves;  those,  for  example,  which  spring  from  the  seeds  of 
Cuscuta.  But  here  also  it  may  be  objected  that  this  stem  in  its  further  development 
forms  small  leaves  below  the  growing-point,  and  that  its  tissue  is  nothing  more 


650  DEFINITION   AND   CLASSIFICATION   OF   STEMS. 

than  the  continuation  of  the  basal  portion  of  these  leaves.  As  in  so  many  similar 
instances,  the  whole  matter  finally  ends  in  an  unfruitful  strife  of  words  where 
everyone  is  in  the  right.  The  simplest  way  is  to  regard  as  a  stem  every  axis  of  a 
plant  which,  when  developed,  always  bears  geometrically-arranged  leaves,  and  to 
avoid  speculations  as  to  whether  this  stem  is  to  be  considered  as  an  independent 
structure  apart  from  leaves,  or  as  a  combination  of  their  basal  portions. 

Whatever  theory  we  may  hold  of  these  relations,  not  only  the  form  but  also 
the  function  of  the  leaves  borne  by  the  part  of  the  stem  in  question  must  be 
regarded  as  the  predominating  factor  in  the  portrayal  of  the  stem  structure — 
especially  when  the  peculiar  construction  of  a  given  stem  is  to  be  explained  by  the 
special  duties  assigned  to  it. 

There  is  no  plant  in  which  the  stem  is  developed  quite  uniformly  from  the  base 
to  the  apex.  We  can  always  distinguish  in  it  stories  following  one  above  the  other, 
each  of  which  is  fashioned  in  accordance  with  the  work  it  has  to  perform.  Just  as 
in  buildings  the  underground  walls,  which  serve  as  the  foundation  of  the  whole  and 
usually  also  as  a  store-room  for  food,  &c.,  exhibit  quite  a  different  kind  of  structure 
from  the  upper  stories  which  are  inhabited,  and  where  kitchen,  bed-rooms,  airy 
parlours  and  passages  are  found,  so,  in  one  and  the  same  plant,  different  plans  of 
construction  are  realized  according  as  to  whether  the  part  in  question  bears  coty- 
ledons, scale-leaves,  foliage-leaves,  or  floral-leaves,  the  functions  of  which  are  so 
extremely  various.  It  therefore  seems  most  natural  to  classify  stems  as  hypocotyls, 
scale-leaf  stems,  foliage-stems,  and  floral-stems. 

There  is  not  much  to  be  said  about  the  hypocotyl  (fundamentum).  The  little 
that  is  of  interest  has  been  stated  already  in  describing  the  cotyledons.  After  it 
has  drawn  the  cotyledons  from  their  envelope  and  has  straightened  itself,  the 
hypocotyl  undergoes  no  alterations  worth  mentioning  and  is  only  of  importance  in 
that  the  bud  of  the  main  shoot  is  developed  from  its  apex,  and  the  food  absorbed 
by  the  radicle  is  conducted  by  its  means  to  this  bud. 

The  stem  bearing  scale-leaves  (subex)  is  usually  so  short  in  its  first  stages  that 
its  leaves  lie  close  packed  above  one  another,  the  upper  ones  being  wholly  or  for  the 
most  part  covered  by  the  lower.  In  many  instances  it  remains  very  short  throughout 
life,  and  is  then  termed  a  reduced  axis  or  "  short  branch  ".  In  others  it  extends  and 
elongates  so  that  its  leaves  are  separated,  and  it  is  then  called  a  "  long  branch  ".  It 
may  happen  that  one  of  these  scaly  stems  is  at  intervals  sometimes  a  long  and  some- 
times a  reduced  axis;  it  may  then  be  compared  to  a  string,  in  which  knots  have 
been  tied  at  certain  distances.  In  the  case  of  a  scaly  stem  passing  over  into  a 
foliage-stem  beset  with  green  leaves,  the  former  usually  has  the  form  of  a  reduced 
axis.  It  is  then  either  flattened  or  disc-like,  or  it  may  be  of  a  shortly  cylindrical  or 
conical  form.  If  it  is  beset  with  large  scale-leaves  and  is  considerably  thicker  than 
the  leafy  foliage-stem  into  which  it  almost  directly  passes,  we  speak  of  it  as  an 
abbreviated  stem.  This,  together  with  its  large  and  hollowed  scale-leaves,  is  termed 
a  bulb  (bulbus)',  it  is  almost  always  underground,  and  its  axis  is  vertical,  as,  for 
example,  in  lilies,  tulips,  hyacinths,  and  stars  of  Bethlehem. 


DEFINITION    AND   CLASSIFICATION   OF   STEMS.  65 J 

A  scale-leaf  stem  which  remains  short,  which  is  clothed  with  membraneous  scales 
and  does  not  exceed  in  thickness  the  foliage  or  floral  stem  which  often  proceeds  from 
it,  is  called  a  sucker  (surculus).  The  sucker,  beset  with  scale-leaves,  appears  as  a  bud 
(gemma)  so  long  as  the  foliage  or  floral  stem  has  not  grown  out  from  it;  later  it 
forms  to  some  extent  the  basis  of  the  foliage  or  floral  stem,  and  is  not  very  remark- 
able, especially  after  its  hollowed  scale-leaves,  as  is  almost  always  the  case,  become 
detached  and  fall  off.  The  scaly  stem  is  but  seldom  developed  at  the  base  of  the 
first  shoot  (plumule)  arising  between  the  cotyledons  (e.g.  in  the  Moschatel,  Adoxa 
Moschatellina).  On  the  other  hand,  it  is  scarcely  ever  absent  from  the  base  of  the 
lateral  shoots  of  woody  plants,  those  bearing  leaves  as  well  as  those  which  are 
terminated  by  flowers.  In  the  subterranean  buds  of  undershrubs  the  stem  is  occa- 
sionally very  thick,  and  such  buds  have  almost  the  appearance  of  bulbs.  The 
subterranean  buds,  especially  those  of  shrubs  and  trees,  always  possess,  on  the  other 
hand,  a  short  cylindrical  or  conical  stem. 

The  tuber  (tuber)  seems  to  be  to  some  extent  a  link  between  the  reduced  and 
the  long  axes  formed  by  scale-leaf  stems.  It  is  always  thicker  than  the  shoots 
arising  from  it;  its  scale-leaves  are  situated  so  far  apart  that  a  clear  space  is  visible 
between  them,  and  they  never  cover  and  envelop  one  another.  The  scale-leaves  of 
the  tuber  are  insignificant;  they  only  appear  as  narrow  horizontal  bands,  or  they 
are  merely  indicated  by  ridges  and  protuberances.  In  old  tubers  the  scale-leaves 
are  often  scarcely  recognizable  externally.  Most  tubers  are,  moreover,  very  perish- 
able structures;  all  those  which  appear  as  local  thickenings  of  an  underground 
shoot,  of  which  the  Potato  (Solanum  tuberosum)  may  serve  as  a  type,  grow  very 
quickly,  and  have  a  resting-period  of  about  half  a  year,  but  perish  completely  after 
they  have  developed  shoots  from  their  buds  (the  so-called  "  eyes ")  which  unfold 
their  green  foliage  above  ground  in  the  sunlight.  Perennial  tubers,  whose  lower 
half  only  is  often  embedded  in  the  earth,  or  which  are  only  covered  with  a  thin 
layer  of  soil,  are  much  less  common.  From  these  spring  up  every  year  a  few  shoots 
which,  however,  do  not  completely  exhaust  the  tuber,  but,  on  the  contrary,  supply 
it  with  materials  manufactured  by  the  green  foliage  in  the  sunlight,  by  which 
means  the  tissue  of  the  tuber  is  actually  enlarged.  These  perennial  tubers  fre- 
quently look  like  tuberous  leafy  stems,  and  the  whole  history  of  development 
must  be  known  in  order  to  be  able  to  determine  and  prove  that  they  really  are  scaly 
stems.  Tubers  are  generally  subterranean.  More  rarely  they  are  formed  above 
the  soil  in  the  axils  of  foliage-leaves,  as,  for  example,  in  the  Lesser  Celandine 
(Ranunculus  Ficaria),  where  those  remarkable  little  tubers  arise,  which  become 
detached  after  the  withering  of  the  plant;  they  afterwards  lie  on  the  ground,  and 
have  often,  where  they  have  been  produced  in  great  quantities,  given  ris 
myth  of  "  potato  rain  ". 

Whilst  some  of  the  stems  which  bear  scale-leaves  are  green,  others 
of  chlorophyll,  and  of  these  latter  the  following  types  may  be  distinguished  :- 
the  aerial,  thread-like,  twining  and  parasitic  stems  of  the  genus  Cuscuto',  second, 
the  thin  subterranean  shoots  of  the  Couch-grass  (Triticum  repens)  and  of  numerous 


652  DEFINITION   AND   CLASSIFICATION   OF    STEMS. 

allied  grasses,  clothed  with  sheathing,  membraneous  scales;  third,  the  erect  and 
fleshy  stems  of  the  Balanophoreae  and  Orobanchacese,  covered  with  dry  scales  (cf. 
figs.  41  and  42);  fourth,  the  branched  stems  of  Lathrcea,  lying  embedded  in  the  earth, 
covered  with  large  fleshy  scales  (cf.  fig.  37);  fifth,  the  coral-like  scaly  stems  branch- 
ing in  all  directions,  which  have  no  roots  and  are  only  covered  with  delicate  scale- 
leaves,  as  shown  by  Epipogium  (cf.  p.  Ill)  and  by  the  Coral-root  (Corallorhiza 
innata)',  sixth,  the  stems  of  the  Tooth-cress  (Dentaria),  creeping  underground  with 
thick,  fleshy  scale-leaves  and  clearly  defined  roots;  seventh  and  last,  the  cylindrical, 
subterranean  stems  with  weak  membraneous  scale-leaves  and  many  roots,  as  in 
Solomon's  Seal  (Convallaria  Polygonatum),  the  Sweet  Spurge  (Euphorbia  dulcis), 
and  numerous  other  perennial  undershrubs.  Subterranean  scale-leaf  stems  developed 
as  elongated  shoots  are  classed  together  in  botanical  terminology  under  the  name 
"root-stock"  or  "rhizome"  (rhizoma);  the  term  "creeping  stem"  (soboles)  is  applied 
to  the  thin,  branching  scaly  stems  which  often  creep  for  a  considerable  distance 
under  the  ground. 

In  the  forms  belonging  to  the  first,  third,  fourth,  and  fifth  groups,  just  enumer- 
ated, the  scaly  stem  passes  directly  into  a  floral  stem,  i.e.  on  the  same  stem  below 
are  to  be  seen  scale-leaves  which  stand  in  no  direct  connection  with  the  processes 
of  fertilization,  and  above  them  perianth  leaves,  as  in  the  Rafflesiacese  (cf.  figs.  44 
and  45),  or  bracts,  as  in  the  Broom-rape  and  Toothwort  (cf.  fig.  37).  In  these  plants 
no  green  foliage  leaves  are  developed;  they  are  unnecessary,  because  these  plants 
are  all  parasites  or  saprophytes,  and  do  not  require  to  manufacture  organic  com- 
pounds for  themselves,  but  derive  the  material  necessary  for  their  further  growth 
from  their  host,  or  from  the  humus  of  the  forest  ground.  In  plants  of  the  other 
groups,  of  which  Dentaria,  Couch-grass,  and  Solomon's  Seal  may  be  taken  as  types, 
two  kinds  of  shoots  are  developed: — Shoots  whose  stem  is  beset  only  with  scale- 
leaves  without  chlorophyll,  and  those  which  branch  off  from  these  grow  up  above 
the  ground,  and  there  unfold  green  foliage-leaves.  Here,  too,  must  be  mentioned 
those  strange  plants  whose  perennial  underground  stems  develop  two  kinds  of 
shoots  which  appear  above  ground; — first,  shoots  whose  stem  is  covered  below  with 
scale-leaves,  but  which  bears  flowers  above  and  later  on  when  these  first  shoots 
begin  to  wither,  leafly,  flowerless  shoots  whose  green  leaf-blades  unfold  in  the 
sunlight.  This  remarkable  division  of  labour  is  observed  in  many  Alpine  plants, 
in  species  of  Butter-bur  (Petasites),  and  in  the  widely-distributed  and  well-known 
Colt's-foot  (Tussilago  Farfara). 

Green  scaly  stems  which  develop  as  elongated  shoots  are  obviously  all  aerial, 
or  rather,  they  grow  above  the  ground  and  the  cortex  of  their  stems  becomes 
green  so  far  as  the  light  can  influence  them.  That  part  of  the  shoot  which  remains 
hidden  in  the  dark  earth  does  not  become  green,  and  many  such  shoots,  e.g.  those 
of  Asparagus,  are  white  and  without  chlorophyll  in  the  lower  half,  their  upper 
portions  alone  being  green,  viz.  the  small  needle-shaped  branches  (phyllocladia) 
growing  out  from  the  axils  of  the  small  scale-leaves.  Amongst  the  green  scale-leaf 
stems  must  be  included  the  cactiform  plants,  the  switch  plants,  and  the  plants  with 


DEFINITION   AND   CLASSIFICATION   OF   STEMS.  653 

flattened  shoots,  which  have  been  fully  described  on  p.  333.  The  Horsetails  (Equi- 
setaceaB)  belong  also  to  this  group,  and  in  one  group  of  these  (Equisetum  arvense, 
E.  Telmateja)  the  division  of  labour  is  similar  to  that  in  the  Colt's-foot.  The  first 
pale  shoots  which  emerge  from  the  ground  are  terminated  by  a  spike  of  sporangia- 
bearing  scales,  and  not  until  later,  after  the  spores  have  been  scattered  by  the  wind 
and  the  pale  primary  shoots  have  withered,  do  the  summer  shoots  appear  whose 
stems  develop  green  tissue  in  the  cortex. 

The  inner  structure  of  the  green  scaly  stems,  whose  duty  is  to  manufacture 
organic  materials,  agrees  essentially  with  that  of  the  foliage-stem.  In  these  plants, 
indeed,  the  functions  have  only  been  transposed  in  this  way,  viz.  that  normal  green 
leaves  are  not  produced,  but  only  small  colourless  scales,  whilst  the  work  usually 
allotted  to  the  leaves  has  been  assumed  by  the  cortex.  Green  scaly  stems  are  just 
as  much  exposed  to  wind  and  sunlight  as  leafy  stems  are;  they  must,  like  them, 
direct  and  establish  themselves  in  accordance  with  the  particular  conditions  of  their 
habitat  and  offer  the  same  resistance  to  the  wind;  they  must  be  just  as  elastic  and 
flexible,  and  consequently  present  a  similar  arrangement  of  their  tissues  rendering 
it  possible  for  them  to  maintain  the  favourable  position  once  assumed.  The  sub- 
terranean scaly  stems  have  no  need  of  such  contrivances;  no  winds  press  against 
them  and  their  tissue  does  not  require  to  be  strengthened  against  bending.  The 
stems  of  Balanophorese  require  only  a  slight  elasticity,  the  part  which  rises  above 
the  ground  is  relatively  very  thick  and  almost  reminds  one  of  the  stalks  of  the 
cap-fungi.  Many  of  these  scale-leaf  stems  under  the  ground  or  rising  only  a  little 
above  it,  are  very  brittle,  and  when  stems  of  Dentaria,  embedded  in  the  humus  of 
the  forest  soil,  are  dug  up,  the  greatest  care  must  be  taken  to  prevent  their  break- 
ing. The  same  is  true  of  underground  tubers  and  bulbs;  they  need  none  of  those 
contrivances  by  means  of  which  a  definite  position  with  regard  to  the  light,  or  a 
great  capacity  of  resistance  to  wind  is  obtained.  Protective  measures  against 
excessive  transpiration  are  likewise  unnecessary,  and  this  accounts  for  the  lack  of 
cuticle  to  the  epidermal  cells,  and  for  the  absence  of  hair-like  structures  and  var- 
nish-like coatings.  When  dry,  tough  scales  occur  as  envelopes  to  bulbs,  they  are 
probably  of  a  protective  nature — not  against  transpiration  or  over-illumination,  but 
against  subterranean  animals  which  might  come  and  nibble  them  for  their  food- 
reserves. 

These  subterranean  shoots  excavate  their  own  bed  by  the  pressure  which  their 
turgescent  tissues  exert  on  the  surrounding  earth  during  growth.  Growing  bulbs 
and  tubers  in  this  way  widen  out  a  bed,  often  of  considerable  size,  and  the  pressure 
exerted  is  so  great  that  the  loose  earth  in  their  neighbourhood  becomes  compressed, 
and  sometimes  transformed  into  hard  cakes.  It  has  already  been  mentioned  that 
not  only  stiff  soil  but  even  bits  of  wood  and  other  objects  may  be  bored  through  by 
the  stiff,  pointed  scale-leaves  of  the  Couch-grass.  A  most  important  function  falling 
to  the  lot  of  underground  shoots,  and  especially  to  tubers  and  bulbs,  is  the  storage 
of  reserve  materials.  These  are  manufactured  during  the  summer  by  the  green 
tissues  in  the  sunlight  above  ground  and  are  then  conducted  down  into  the 


(J54  DEFINITION   AND   CLASSIFICATION    OF   STEMS. 

subterranean  reservoirs.  Here  they  remain  quietly  deposited  during  the  winter 
and  are  not  brought  into  requisition  until  the  plant,  at  the  beginning  of  the  next 
vegetative  period,  sends  up  new  shoots  which  manufacture  organic  materials 
afresh.  It  is  in  the  production  of  these  shoots  which  are  to  be  sunned  above  the 
ground  that  material  is  always  employed  which  was  conducted  down  into  the  store- 
houses during  the  preceding  year. 

We  cannot  help  surmising  that  this  remarkable  alternation  between  rest  and 
vigorous  activity,  together  with  the  temporary  disappearance  of  all  the  aerial  por- 
tions of  the  plant,  is  connected  with  the  peculiar  conditions  of  the  habitat.  This 
opinion  is  confirmed  by  the  actual  distribution  of  tuberous  and  bulbous  plants. 
Most  of  these  plants  are  found  in  those  regions  where  all  the  succulent  tissues 
exposed  to  the  air  would  be  liable  to  the  danger  of  shrivelling  up  in  consequence  of 
months  of  drought,  and  where  also  the  superficial  layers  of  soil  in  which  the  tubers 
and  bulbs  are  embedded  dry  up  so  much  that  they  would  not  be  able  to  replace  the 
water  evaporated  from  the  leaves.  But  when  the  soil  has  lost  all  its  water,  it  forms 
an  excellent  protection  to  the  tubers  and  bulbs;  the  earth  forms  an  actual  crust 
round  the  succulent  structure,  and  in  many  regions  the  clay  soil,  coloured  red  by 
iron  oxide,  is  hardened  into  a  mass  which  resembles  brick.  Embedded  in  this  mass 
the  tubers  and  bulbs  can  survive  the  dry  period  which  lasts  over  seven  or  eight 
months  with  impunity.  When  the  rainy  season  comes  and  the  hard  crust  is 
moistened,  a  wonderful  life  stirs  everywhere  through  it.  Innumerable  tuberous 
and  bulbous  plants  spring  from  the  softened  clay  and  unfold  their  flowers  and 
green  foliage-leaves  during  the  brief  wet  period.  This  is  what  occurs  in  the  clay 
steppes  of  Central  Asia,  in  the  mountainous  districts  of  Asia  Minor,  in  Greece,  and 
generally  all  the  countries  bordering  the  Mediterranean  Sea.  In  especial  degree  is 
the  Cape  celebrated  for  its  almost  inexhaustible  wealth  of  bulbous  and  tuberous 
plants  ("cape  bulbs").  In  Central  Europe,  where  the  activity  of  vegetation  is 
interrupted  not  by  dryness  but  by  frost,  the  number  of  these  plants  is  strikingly 
less  than  in  the  districts  previously  enumerated,  whilst  the  ground  in  which  the 
few  species  occur  exhibits  quite  different  conditions.  Here  the  soil  is  never  exposed 
to  severe  drought,  indeed,  strangely  enough,  the  majority  of  tuberous  and  bulbous 
plants  in  the  depths  of  the  Central  European  forests  are  found  in  loose  and  dampish 
earth,  rich  in  humus.  It  is  well  known  that  in  such  places  as  these  snowdrops  and 
yellow  Gagea,  the  Two-leaved  Squill,  the  purple  Martagon  Lily,  the  Cuckoo-pint, 
the  Broad-leaved  Garlic,  and  the  various  species  of  Corydalis  (Galanthus  nivalis, 
Gagea  lutea  and  G.  minima,  Scilla  bifolia,  Lilium  martagon,  Arum  maculatuwi, 
Allium  ursinum,  Gorydalis  fabacea,  C.  solida,  C.  cava),  flaunt  themselves  with  a 
luxuriant  and  vigorous  growth;  and,  what  is  especially  worth  noticing,  their  flowers 
blossom  in  the  first  part  of  the  year,  their  green  foliage  unfolds  early  in  spring  and 
at  midsummer  is  already  yellow  and  withered,  although,  as  stated,  the  necessary 
moisture  would  not  be  lacking  at  this  season. 

This  peculiar  phenomenon  demands  a  reason,  and  we  shall  not  be  far  wrong  if 
we  explain  the  preference  of  our  early-flowering  bulbous  and  tuberous  plants  for 


STEMS   BEARING   FOLIAGE-LEAVES.  655 

the  ground  of  forests  somewhat  as  follows.  The  leaf -covered  forest  floor,  sheltered 
as  it  is  by  the  trees,  gives  out  but  little  heat,  and  the  frost  only  penetrates  it  to  a 
slight  depth  in  the  winter,  thus  the  tubers  and  bulbs  are  far  less  exposed  to  the 
danger  of  freezing  than  in  the  open  country.  The  early  flowering  and  the  quick 
fading  of  the  leaves  are  caused  by  the  fact  that  the  light  required  for  the  activity 
of  the  green  foliage  can  only  penetrate  to  the  forest  ground  while  the  crowns  of 
the  trees  are  bare.  Later,  when  a  leafy  canopy  and  shady  roof  is  spread  out  above, 
only  a  sunbeam  here  and  there  can  steal  through  the  chinks  to  reach  the  damp, 
cool  soil  of  the  forest  ground.  But  this  scanty  light  would  no  longer  suffice  for  the 
work  to  be  done  by  the  green  leaves  of  the  bulbous  plants,  and  they  must  therefore 
achieve  this  before  the  leafly  roof  has  developed.  The  weak  light  is,  however,  quite 
sufficient  for  parasites  and  saprophytes,  and  it  is  worthy  of  notice  that  in  the 
summer  in  place  of  the  green  leaves  of  bulbous  and  tuberous  plants,  which  even  in 
June  have  turned  yellow  and  disappeared,  the  Monotropa  without  chlorophyll,  the 
leafless  Epipogium,  and  a  host  of  pale  fungi  spring  up  from  the  deep  humus  in  the 
gloom  of  the  forest. 

o 

STEMS  BEARING  FOLIAGE-LEAVES. 

The  foliage-stem  (stirps1)  is  characterized  by  the  fact  that  the  leaves  borne  upon 
it  are  provided  with  green  blades,  and  realize  the  popular  idea  of  leaves.  This 
portion  of  the  stem  might  indeed  be  called  "foliage-leaf  stem",  and  its  essential 
characteristic  would  be  expressed  in  the  term,  but  since  the  cotyledons  frequently 
assume  the  form  of  foliage-leaves,  it  is  perhaps  better,  in  order  to  avoid  confusion, 
to  keep  to  the  term  "foliage-stem".  No  part  of  the  plant  is  so  striking  to  the  eye 
as  the  foliage-stem.  The  rhizomes,  tubers,  bulbs,  and  other  forms  of  scale-bearing 
stems  are  hidden  from  view  in  the  earth,  just  like  roots.  The  flowers  borne  by  the 
floral  stems  are  ephemeral  structures,  the  leafy  stems  alone  retain  their  character 
during  the  whole  vegetative  period  as  the  most  important  portion  of  the  plant. 
When  one  attempts  to  reproduce  the  character  of  the  vegetation  of  any  region 
either  in  words  or  in  the  form  of  a  picture,  it  is  to  the  leafy  portions  of  grasses, 
shrubs  and  trees  that  one  confines  oneself;  these,  blended  in  infinite  variety,  com- 
pose the  carpet  of  the  meadow,  the  bush,  the  thicket,  the  woods  and  forests.  It  is 
the  style  of  architecture  of  the  foliage-stem,  so  to  speak,  which  expresses  the  style 
of  the  whole  plant-body. 

This  peculiar  style  of  architecture,  and  the  habit  of  the  whole  plant  subservient 
to  it,  depends  primarily  upon  the  size,  length  and  thickness  of  the  foliage-stem, 
is  evident  that  in  this  respect  conditions  obtain  quite  analogous  to  those  in  the 

stem  (stirps) ;  (4)  the  floral  stem  (thalamus). 


656 


STEMS   BEARING   FOLIAGE-LEAVES. 


already-described  scaly  stems;  here  only  are  the  differences  in  size  more  marked. 
Contrasts,  like  that  between  filamentous  leafy  stems,  barely  a  centimetre  long,  and 
the  giant  trees  of  North  America  and  Australia,  have  not  their  like  in  the  whole 
vegetable  kingdom.  In  those  plants  which  germinate,  grow,  blossom,  and  fruit  and, 
after  the  distribution  of  their  seeds,  perish,  all  in  a  single  year — in  these  short- 
lived annuals the  foliage-stem  seldom  attains  to  a  considerable  diameter.  In  many 

small  Cruciferse,  e.g.  in  the  small-flowered  Shepherd's  Purse  (Capsella  pauciflora) 
and  in  the  tiny  Chaffweed  (Centunculus  minimum),  the  diameter  of  the  stem  often 
scarcely  amounts  to  half  a  millimetre.  The  largest  dimensions  in  annuals  are  found 


****?&^- 

^^v? 

Fig.  152.— Cotton  Trees  (Cavanillesia  tuberculata)  of  the  Brazilian  catingas.     (After  Martius.) 

in  the  Castor-oil  plant  (Ricinus  communis),  many  of  the  stems  attaining  to  a 
diameter  of  7  centimetres,  and  in  the  balsams  of  the  Himalayas  (Impatiens  tri- 
coniis  and  glanduligera)  which  sometimes  have  a  diameter  of  4  centimetres.  ID 
these  annual  plants  the  stem  which  bears  the  leaves  perishes  with  them  every  year. 
It  is  otherwise  with  plants  whose  stem  remains  alive  for  more  than  one  period  of 
vegetation,  and  which  have  been  called  perennial.  When  these  throw  off  their 
foliage,  they  do  not  die,  but  fashioning  themselves  into  supports  for  the  leafy  shoots 
which  arise  from  their  buds,  attain  a  circumference  in  just  proportion  to  the  new 
burden  to  be  borne.  The  structure  of  such  foliage-stems  then  becomes  altered. 
The  stems  of  annuals  and  those  of  the  young  new  shoots  of  perennial  plants  have 
a  green  succulent  cortex  with  a  peculiarly-developed  epidermis;  such  a  shoot  we 
call  "  herbaceous"  (stirps  herbacea).  In  the  leafless  stems  of  perennial  plants,  now 
transformed  into  columns,  a  dried  crust  or  bark  replaces  the  succulent  green  cortex 


STEMS   BEARING    FOLIAGE-LEAVES 


657 


whilst  within  masses  of  wood  are  continually  formed  and  are  deposited  on  the 
bundles  of  woody  cells  and  vessels  produced  in  the  first  year— thus  increasing  the 
circumference  of  the  stem.  Such  a  stem  is  said  to  be  "woody"  (stirps  lignea). 
Woody  stems  which  have  been  thickened  continuously  in  this  manner  for  centuries 
sometimes  attain  a  circumference  of  50  metres;  that  of  the  Mexican  conifer 


Fig.  153. — Agaves  of  the  Mexican  uplands  (from  a  photograph). 

(Taxodium  mucronatum)  has  even  been  found  with  a  girth  of  5T88  metres;  this 
circumference  exceeds  that  of  the  above-mentioned  stem  of  Centunculus  more  than 
a  hundred  thousand  times.  The  thickness  of  the  stem  is  in  general  greatest  at  the 
base  and  gradually  tapers  off  above;  only  a  few  palms  are  thicker  immediately 
below  their  crown  of  green  leaves  than  at  the  base,  and  in  the  strange  cotton-trees 
of  the  Brazilian  catingas  (Cavanillesia  tuberculata)  of  which  an  illustration  is 
inserted  opposite,  the  stem  forms  a  swollen,  barrel-shaped  mass  attaining  its  maxi- 

VOL.  I.  *2 


(J58  STEMS   BEARING   FOLIAGE-LEAVES. 

mum  about  half-way  up.  Very  often  an  unequal  thickening  may  be  observed  in 
the  foliage-stem;  this  is  due  to  the  fact  that  at  the  places  where  leaves  arise  from 
the  stem  knotty  swellings  are  developed,  while  those  portions  of  the  stem  which 
come  between  successive  leaf -insertions  (or  nodes),  and  which  are  called  internodes, 
are  cylindrical  or  prismatic  in  form.  A  foliage-stem  which  has  this  peculiarity  is 
said  to  be  "nodose"  (nodosus).  Sometimes  the  internodes  of  such  nodose  stems 
adjoin  one  another  at  obtuse  angles,  and  such  a  stem  is  then  called  in  botanical 
terminology  "  zigzag  "  (flexuosus). 

The  fully-developed  internodes  of  which  the  foliage-stem  is  built  up,  are  only 
rarely,  and  then  only  for  short  distances,  of  precisely  equal  length.  Sometimes 
longer  and  shorter  internodes  alternate,  and  quite  as  often  it  happens  that  a  single 
much-elongated  internode  succeeds  several  short  ones.  If  such  an  elongated  inter- 
node  passes  over  into  the  region  of  the  flowers,  it  is  known  as  a  "  scape  "  (scapus). 
As  in  the  scaly  stems,  where  short  and  long  axes  can  be  distinguished,  so  is  it  with 
the  foliage-stem.  The  leaves  are  usually  so  crowded  on  these  short  axes  that  they 
form  rosettes  or  fascicles  which  quite  cover  the  stem  which  bears  them.  On  the 
other  hand,  on  many  long  axes  the  leaves  are  developed  scantily  and  at  long 
intervals  and  we  are  tempted  at  first  glance  to  take  such  an  elongated  shoot  for 
the  leafless  stem  of  a  switch-plant.  A  large  number  of  plants  develop  in  one  year 
only  short  axes  with  rosette-like  radical  foliage-leaves;  in  the  following  year 
the  apex  of  the  short  axis  grows  up  into  a  slender,  elongated  shoot  which  passes 
above  into  a  floral  stem.  This  is  the  case  in  most  plants  whose  stem  is  said  to  be 
"  biennial "  (stirps  biennis).  Similar  conditions  are  observed,  however,  in  many 
perennial  species  of  house-leek  (Sempervivum),  Aloe,  and  various  other  plants  with 
fleshy,  succulent  leaves,  only  in  these  the  alternation  of  long  and  short  axes  extends 
over  several,  often  very  many  years.  A  very  noticeable  form  of  this  kind  is 
Agave  Americana,  known  by  the  name  of  the  "Century  Plant",  illustrated  in 
fig.  153.  Often  20,  30,  even,  it  is  alleged,  100  years  pass  by,  during  which  long 
period  the  plant  produces  only  a  short  stumpy  axis  beset  with  leaves  grouped 
in  a  rosette.  At  length  a  long  axis  arises  from  the  centre  of  the  rosette  and 
terminates  in  a  voluminous  inflorescence.  As  soon  as  the  fruits  have  been  pro- 
duced from  the  flowers,  and  the  seeds  have  escaped,  not  only  the  long  axis,  as 
in  biennial  plants,  but  also  the  short  axis  with  its  large,  stiff  and  spiny  rosette- 
leaves  entirely  dies  away.  In  water-plants  this  type  is  also  met  with  in  the 
remarkable  Water  Soldier  (Stratiotes  aloides),  to  which  allusion  has  been  so 
frequently  made.  In  this  plant,  as  in  the  house-leeks  and  saxifrages,  long  axes 
which  continue  to  grow  until  they  have  arrived  beyond  the  circle  of  the  whole 
rosette,  arise  from  the  axils  of  the  lower  leaves;  when  this  has  happened,  the 
young  horizontally-projecting  shoot  stops  extending,  and  at  its  tip  again  forms  a 
short  axis,  i.e.  a  rosette,  which,  in  the  following  year,  sends  up  a  fresh  long  axis. 
A  similar  alternation  of  long  and  short  axes  is  also  observable  in  numerous  other 
plants,  in  the  shrubby  spiraeas,  and  in  roses,  hawthorn,  sea-buckthorn,  barberry,  and 
Astragalus,  which  we  shall  encounter  later  on  as  hedge-forming  shrubs.  Some- 


STEMS  BEARING   FOLIAGE-LEAVES. 


659 


times  long  and  sometimes  short  axes  develop  from  the  same  shoot.  Also  in  many 
conifers,  e.g.  cedars  and  larches,  the  branches  proceeding  from  a  shoot  are  for  the 
most  part  short  with  needle-like  leaves  arranged  in  fascicles,  and  only  a  few  of 


' 


Fig.  154.— Yucca  glon 


them  become  long  axes.  In  Pines,  on  the  other  hand,  all  the  leaf-bearing  twigs 
are  short  axes,  and  here  we  have  also  the  remarkable  circumstance  that  in  several 
species,  e.g.  the  Scotch  Pine  (Pinus  sylvestris)  a  lateral  twig  bears  only  two  such 
needle-leaves.  Tree-ferns,  Cycads,  Pandaneae,  Grass-trees  (Xanthorrhcea),  many 


660  STEMS   BEARING   FOLIAGE-LEAVES. 

palins,  dracsenas,  and  species  of  Yucca,  of  which  the  Yucca  gloriosa,  illustrated  in 
fig.  154,  may  serve  as  a  type,  exhibit  a  very  peculiar  structure.  The  yearly 
increase  in  length  of  the  stem  is  comparatively  small,  the  leaves  which  project  all 
round  from  this  portion  of  the  stem  are  consequently  crowded  together  and  form  a 
rosette  which  cannot  be  distinguished  as  regards  the  arrangement  of  the  individual 
parts  from  the  radical  rosettes  of  Agaves  and  species  of  house-leek,  and,  like  these, 
must  be  regarded  as  a  short  axis.  In  the  following  year  the  stem  continues  this 
curious,  abbreviated  growth,  the  foliage-leaves  of  the  previous  year  gradually  die 
off,  and  only  the  hardened  remnants  of  their  leaf -bases  are  left  behind,  thus  the 
rosette  or  head  of  fresh  green  leaves  is  now  seen  borne  by  a  naked  columnar  stem. 
This  continues  for  many  years,  and  the  gigantic  crown  of  leaves  rises  higher  and 
higher  above  the  ground.  Plants  with  this  manner  of  growth,  moreover,  never 
attain  even  in  many  years  to  anything  like  the  height  which  is  attained  by  foliage- 
stems  terminating  in  or  branching  out  into  long  axes.  Even  the  tallest  palm 
terminating  in  a  short  axis  is  a  dwarf  in  comparison  with  the  rotangs  or  climb- 
ing palms,  continually  shooting  out  long  axes.  Rotang  stems  are  known  to  extend 
to  almost  200  metres.  The  length  of  200  metres  is  perhaps  the  extreme  limit 
reached  by  a  foliage-stem,  and  if  we  again  contrast  the  extreme  cases,  and  compare 
with  these  climbing  palms  the  stems  of  the  minute  Gentiana  nana  growing  on  the 
high  Alps,  it  is  seen  that  the  shortest  known  of  all  foliage-stems  is  exceeded  by  the 
longest  about  twenty  thousand  times,  in  round  numbers. 

The  ramification  and  facies  of  foliage-stems  is  in  main  part  governed  by  the 
light-requirement  of  the  leaves  they  bear.  Necessarily  the  foliage-stem  as  the 
bearer  of  organs  which  have  to  prepare  organic  materials  in  the  sunlight  is  chiefly 
influenced  in  its  growth,  and  as  to  the  position  which  its  branches  assume  by  the 
conditions  of  illumination.  In  order  that  all  the  green  leaf -blades  of  a  plant 
may  be  suitably  illuminated,  it  is  necessary  that  all  these  foliar  axes  should  be 
grouped  conformably,  and  should  divide  up  the  space  most  economically.  Where 
foliage  is  chiefly  borne  on  short  branches,  even  under  the  most  favourable  condi- 
tions, only  a  relatively  circumscribed  space  can  be  utilized.  But  when  the  reverse 
is  the  case  and  foliage  is  produced  on  long  branches,  the  plant  is  much  more 
favourably  circumstanced.  Such  plants  can  unfold  their  leaves  gradually  above 
one  another,  and  display  them  at  appropriate  intervals  and  distances  to  the  sun- 
light. This  elevation  of  the  leafage  above  the  ground  is  rendered  possible  either 
by  the  possession  of  a  specially-contrived  stem,  or  through  the  employment  by  the 
stem  of  some  strong  substratum  or  support  up  which  it  climbs  to  the  light. 
Again,  long  axes,  which  have  not  the  capacity  of  rising  above  the  ground  in 
either  of  these  ways  can  elongate  while  embedded  in  the  soil  or  extended  on  it, 
and,  running  out  in  all  directions,  can  arrange  their  green  leaves  in  a  mosaic-like 
carpet.  Lastly  the  foliage-stems  can  be  sustained  in  the  position  most  suitable  to 
their  leaves  by  means  of  the  surrounding  water.  According  to  the  circumstances, 
foliage-stems  may  be  broadly  classed  in  four  groups,  viz.  those  which  lie  on  the 
ground  (stirpes  procumbentes),  those  which  float  in  water  (stirpes  fluctuantes), 


PROCUMBENT   AND    FLOATING   STEMS.  661 

those  which  climb   (stirpes   scandentes),  and  the   erect   columnar  stems  (stirpes 
palares). 

PROCUMBENT  AND  FLOATING  STEMS. 

If  we  review  the  plants  whose  characteristic  appearance  is  chiefly  due  to  their 
procumbent  foliage -stem,  we  notice  that  most  of  them  take  root  in  turf,  boggy 
ground,  on  the  stony  plateaus  of  hilly  districts,  in  the  rocky  clefts  of  wind-swept 
mountain  heights,  or,  lastly,  in  the  sandy  plains  of  the  lowlands;  in  general  they 
inhabit  an  infertile  soil,  on  which  the  storm  has  free  play,  and  where  erect  plants 
would  find  it  difficult  to  maintain  themselves.  The  leaves  of  such  stems  are  usually 
undivided  and  small,  and  are  present  in  large  numbers  on  each  year's  growth. 
Where  their  number  is  small,  and  where  correspondingly  the  internodes  of  the 
annual  shoot  are  more  elongated,  the  leaves  are  often  divided,  but  then  the  indi- 
vidual segments  are  of  the  shape  exhibited  by  the  leaves  of  the  short-membered 
shoots.  The  leaves  always  appear  in  two  or  three  rows  on  the  fully-formed  pro- 
cumbent stem,  whether  they  are  decussate  or  spirally  arranged  (cf.  p.  417).  Where 
no  local  insurmountable  obstacles  exist,  the  procumbent  stems  spread  out  in  all 
directions  from  the  spot  where  the  plant  first  took  root,  and  when  the  species  in 
question  are  sociable  weave  a  close  carpet  over  the  ground  in  a  relatively  short  time. 
In  the  earliest  stages  of  development  the  shoots  are  not  extended  over  the  ground, 
that  is  to  say,  the  primary  shoots,  originating  directly  above  the  hypocotyl,  are  at 
first  erect.  Soon,  however,  as  it  elongates,  the  stem  inclines  to  one  side  and  nestles 
to  the  ground,  or  it  arches  over  so  that  its  free  end  reaches  the  soil.  The  apex  of 
course  is  always  more  or  less  erect,  and  most  young,  procumbent  shoots  have  the 
shape  of  an  co .  As  the  stem  elongates,  the  part  immediately  behind  the  growing- 
point  always  nestles  to  the  ground.  In  many  instances  these  stems  have  not  the 
strength  to  hold  themselves  erect;  the  soil  on  which  they  lie  is  their  actual  bed  or 
support.  If  stems  like  these  are  held  up  above  the  ground,  they  hang  limply  down, 
as  may  be  seen  in  the  Periwinkle  (Vinca),  Strawberry  (Fragaria),  and  in  the 
Japanese  Saxifrage  (Saxifraga  sarmentosa)  so  often  grown  in  hanging  baskets.  But 
in  all  cases  it  is  not  their  weight  merely  which  causes  many  shoots  to  assume  this 
manner  of  growth,  in  other  words,  that  the  shoots  do  not  sink  to  the  ground 
under  the  burden  of  their  leaves,  can  be  seen  plainly  enough  in  the  procumbent 
stems  of  hawk  weeds  which  produce  runners  (e.g.  Hieracium  Pilosella)\  these, 
when  gathered  and  placed  upright,  remain  quite  stiff  and  straight,  and  do  not 
show  the  slightest  bending.  When  the  stems  of  Globularia  cordifolia  or  those  of 
the  Hairy  Genista  (Genista  pilosa),  growing  on  a  rocky  ledge,  reach  over  the  edge, 
they  do  not  hang  down  vertically,  as  would  be  the  case  if  their  own  weight  were 
exclusively  the  cause  of  the  direction  taken,  but  they  skirt  along  the  face  of  the 
overhanging  rock  and  remain  closely  pressed  against  it. 

The  first  group  of  plants  with  procumbent  foliage-stems  is  perennial;  the 
growing- points  of  their  stems  advance  over  the  substratum  a  little  every  year, 
and  the  new-formed  shoot  is  the  continuation  of  the  older  portion  of  the  already 


PROCUMBENT   AND   FLOATING   STEMS. 

existing  stem.  At  first  the  new  portion  of  the  stem  is  directed  upwards,  but  after 
a  year  it  lies  flat  on  the  ground  or  is  actually  pressed  to  it.  It  then  sends  out 
lateral  branches  which  repeat  the  method  of  growth  just  described,  but  it  always 
remains  fresh  and  vigorous,  serving  for  years  after  it  has  thrown  off  its  leaves  for 
the  conduction  of  food  from  the  ground  and  only  dies  off  very  gradually  and 
slowly  from  behind. 

In  many  forms  belonging  to  this  first  group  the  older  portions  of  the  stem 
become  lignified,  and  persist  for  a  very  long  time.  They  may  also  increase  in  thick- 
ness, exhibiting  numerous  annual  rings,  as,  for  example,  the  stems  of  procumbent 
willows  clinging  to  the  rock  terraces  of  the  high  Alps,  as  illustrated  on  p.  524.  The 
elongating  stems  do  not  often  throw  out  additional  roots,  as  may  easily  be  shown 
by  raising  the  stems  from  their  procumbent  position.  When  such  stems  branch, 
and  the  branches  have  spread  far  and  wide  over  the  soil,  they  form  an  actual 
carpet,  which  can  be  raised  from  the  ground  or  from  the  rock  terrace  as  a  coherent 
mass,  as,  for  example,  in  the  red  Bearberry  (Arctostaphylos  Uva  ursi)  and  the 
white  Dryas  (Dryas  octopetala).  Many  members  of  this  group  possess  evergreen 
foliage,  as  we  see  in  the  Trailing  Azalea  (Azalea  procumbens)  and  Globularia 
cordifolia.  The  Cinquefoils  with  trailing  woody  stems  (e.g.  Potentilla  nitida 
and  Clusiana),  Sibbaldia  (Sibbaldia  procumbens)  and  several  valerians  (e.g.  Vale- 
riana  tripteris  and  montana),  similarly  provided,  possess,  however,  no  evergreen 
foliage,  and  may  be  distinguished  from  those  named  earlier  by  the  fact  that  the 
annual  increase  of  their  stems  is  very  slight,  in  consequence  of  which  the  older  plants 
have  usually  a  turf -like  appearance.  Many  species  of  Thyme  (Thymus)  are,  on 
the  contrary,  characterized  by  the  fact  that  they  every  year  develop  fairly  long  and 
thin  whip-like  shoots  which  weave  over  the  mossy  substratum,  or,  like  Dryas,  form 
a  carpet  on  the  rocky  bed.  The  stem  of  the  forms  hitherto  brought  forward  is 
termed  "  prostrate  "  (stirps  prostrata),  from  which  is  distinguished  the  "  creeping  " 
stem  (stirps  repens).  Even  when  it  has  lost  its  leaves,  the  creeping  stem  is  not 
lignified,  but  develops  abundant  root-fibres  close  behind  the  growing -point,  which 
penetrate  into  the  ground,  and  often  draw  the  stem  down  into  the  soil  or  mud. 
The  growths  of  former  years  do  not  here  persist  so  long  as  in  plants  with  woody 
prostrate  stems;  they  usually  die  off  after  three  or  four  years,  and  decay  and  vanish 
away  altogether.  Thus  one  might  almost  imagine  the  stem  had  been  shifted  en 
masse,  that  it  had  crept  forward  in  the  direction  of  the  growing  tip.  Sometimes 
on  the  older  portions  of  these  stems,  the  situations  where  leaves  were  formerly 
inserted  are  marked  by  transverse  scars  and  bands — reminding  one  very  much  of 
creeping  worms  and  caterpillars.  The  umber  stems  of  the  Californian  Saxifraga 
peltata  which  creep  over  damp  rocks  by  the  sides  of  streams  are  very  striking 
in  this  respect.  A  likeness  to  worms  crawling  over  the  soil  is  also  possessed  by 
the  stems  of  the  European  and  American  Asarabacca  (Asarum  Europceum  and 
Canadense),  by  those  of  the  marsh-inhabiting  Buckbean  (Menyanthes  trifoliata), 
of  the  Snake-root  (Calla  palustris),  of  the  purple  Marsh  Cinquefoil  (Comarum 
palustre),  and  of  several  species  of  clover  (e.g.  Trifolium  repens  and  fragiferum). 


PROCUMBENT  AND  FLOATING  STEMS.  663 

In  addition  and  in  contrast  to  this  first  group  of  plants  with  procumbent  foliage- 
stems  there  is  a  second,  characterized  by  the  fact  that  only  the  buds  arising  on  the 
new  shoots  remain  throughout  the  year,  strike  root,  and  grow  out  into  new  plants, 
while  the  shoots  themselves — the  axes  from  which  the  buds  have  been  developed — 
soon  perish,  thus  severing  the  connection  with  the  parent  plant.     These  shoots  are 
always  thin,  frequently  quite  thread-like.     Little  building-material  is  wasted  upon 
them,  since  they  are  but  ephemeral  structures.     Two  distinct  types  of  stem  may  be 
distinguished  in  this  second  group.     These  are  known  as  the  stolon  and  runner. 
By  "  stolon  "  (stolo)  we  understand  a  procumbent  stem  which  dies  off  after  a  year, 
and  is  abundantly  beset  with  leaves  not  very  far  apart.     In  the  axils  of  many  of 
these  leaves  no  buds  are  produced,  and  often  only  at  the  ends  of  the  stolons  do  buds 
arise  from  the  axils  of  very  minute  leaves;  these  buds  take  root.    This  is  especially 
the  case  in  the  arched  stolons,  as,  for  example,  in  the  well-known  Periwinkle  (  Vinca), 
and  the  purple  Gromwell  (Lithospermum  purpureo-cceruleum).    The  shoots  arising 
from  old  plants  of  these  species  form  flat  arches  abundantly  beset  with  pairs  of 
leaves.    Their  free  ends  lie  on  the  ground,  swell  and  grow  down  into  some  dark  chink 
or  into  the  black  humus  itself,  striking  root  and  thus  being  drawn  still  deeper  into 
the  ground.     The  end  of  the  stolon,  thus  embedded,  finds  itself  next  year,  so  to 
speak,  on  its  own  feet;  it  grows  up  into  a  new  plant,  while  the  arched  or  connect- 
ing portion  dies  off  sooner  or  later,  and  in  the  following  year,  or  the  year  after 
that,  vanishes,  leaving   no   trace.      The   stolons  of  the   Pennywort  (Lysimachia 
Nummularia)  are  similarly  constructed,  but  in  this  plant  the  shoots  lie  flat  on  the 
soil,  and  the  tip  does  not  thicken,  nor  do  the  apices  avoid  the  light,  or  become  drawn 
far  into  the  earth.     Rooting  buds  arise  in  the  axils  of  small  leaves  close  to  the 
up-bent  apex  of  the  stolon,  and  in  the  following  year  become  starting-points  for 
new  plants.     Several  species  of  saxifrage  and  house-leek  (Saxifraga  and  Semper- 
vivum),  the   Common   Bugle  (Ajuga  reptans),  some  hawkweeds  (e.g.  Hieracium 
Pilosella  and  Auricula),  and  numerous  other  plants  develop  richly-leaved  stolons 
which  produce  at  their  free  ends  short  axes  which  root.     The  leaves  on  these  short 
axes  are   grouped   in   rosettes;   the  short  axes  grow  next  year  into  new  plants, 
the  intervening  stolon  perishing.     A  peculiar  modification  of  this  method  of  growth 
is  found  in  certain  house-leeks  (Sempervivum  arenarium  and  Soboliferum).     Here, 
as  before,  the  tip  of  the  thread-like  stolon  develops   a   short   axis   with  leaves 
arranged  in  rosettes,  but  as  soon  as  this  is  fully  formed,  the  stolon  withers,  the 
spherical  rosette  becomes  detached  from  it  and  rolls  down  over  the  steep  ground 
where  it  had  developed.     Since  these  species  of  house-leek  grow  as  a  rule  on  the 
narrow  ledges  of  precipitous  rock-faces,  it  happens  that  the  rosette  thus  detached 
falls  from  ledge  to  ledge,  often  to  a  depth  of  many  metres,  truly  a  remarkable  method 
of  distribution,  which  we  shall  allude  to  again  in  the  second  volume  (cf.  vol. 

fig.  425). 

=  The  "runner"  (sarmentum)  is  distinguished  from  the  stolon  by  the  fact  that 
its  internodes  are  much  elongated,  and  that  leaves  and  buds  which  stnke  root 
and  form  the  starting-points  of  new  plants,  are  only  formed  at  wide  intervals 


(5(34  PROCUMBENT   AND    FLOATING    STEMS. 

on  them.  The  long  bare  internodes  are  always  thin  and  thread-like,  and  perish 
in  the  course  of  a  year.  One  portion  of  the  buds  developing  at  the  nodes  of  the 
runner  forms  short  axes;  another  part  may  form  even  in  the  first  year  long  axes 
which  again  assume  the  form  of  a  runner.  Since  each  plant  sends  out  simul- 
taneously several  runners  extended  on  the  ground  on  every  side,  it.  comes  to  pass 
that  in  a  very  short  time  considerable  areas  are  spun  over  in  all  directions  with 
filamentous  runners  and  a  host  of  new  plants  produced.  Well-known  examples 
of  this  form  of  procumbent  stem  are  furnished  by  the  strawberries  (e.g.  Fragaria 
vesca,  grandiflora,  Indica),  several  cinquefoils  (e.g.  Potentilla  reptans  and 
Anserina),  the  Creeping  Avens  (Geum  reptans),  the  Stone  Blackberry  (Rubus 
saxatilis),  the  Ground  Ivy  (Glechoma  hederacea),  and  the  Japanese  Saxifrage 
(Saxifraga  sarmentosa).  A  very  peculiar  appearance  is  presented  by  Androsace 
sarmentosa,  which  grows  in  the  Himalayas.  All  its  leaves  are  crowded  together 
into  a  beautiful  rosette  on  an  erect  short  axis.  From  the  axils  of  several  of  these 
rosette-leaves,  long,  thin  runners,  red  in  colour,  radiate  out  during  the  summer; 
they  extend  themselves  on  the  rocky  soil,  and  each  runner  forms  at  its  end  only  a 
single  rooting  bud  or  rosette.  The  red  filaments  perish  in  the  second  year,  but 
by  this  time  five  or  six  freshly-rooted  rosettes  may  generally  be  seen  standing  in 
a  circle  round  the  older  one. 

In  a  third  group  of  plants  the  whole  procumbent  foliage-stem  with  all  its 
branches  dies  off  every  year  at  the  close  of  the  vegetative  period.  The  plants 
belonging  to  this  group  are  either  annuals  and  maintain  themselves  only  by 
seeds,  or  they  possess  perennial  subterranean  scaly  stems,  in  which  case  each 
year  new  leafy  stems  arise.  The  foliage-stem  of  these  plants  is  said  to  be 
"  prostrate "  (stirps  humifusa).  The  following  may  serve  as  examples  of  such 
annual,  prostrate  shoots:  —  The  Caltrops  (Tribulus),  the  Strapwort  (Corrigiola), 
lllecebrum,  the  Pimpernel  (Anagallis),  the  Ivy -leaved  Speedwell  (Veronica  hederi- 
folia),  the  Portulaca  (Portulaca  oleracea),  and  numerous  species  of  Polygonum, 
trefoil,  and  medick  (Polygonum,  Trifolium,  Medicago);  as  examples  of  perennial 
prostrate  plants  —  the  Bird's  -  foot  Trefoil  (Lotus  corniculatus),  the  variegated 
Coronilla  (Coronilla  varia),  and  several  caryophyllaceous  plants  (e.g.  Saponaria 
ocymoides,  Telephium  Imperati). 

When  the  leafy  shoot  lies  on  the  soil,  it  can  easily  dispense  with  the  develop- 
ment of  those  cells  which  would  otherwise  be  required  to  give  to  its  stem  strength 
for  support  and  resistance  to  bending.  Thus  plants  with  procumbent  stems 
have  an  advantage  in  this  respect  over  such  as  stand  erect,  in  that  they  can 
economize  so  much  building  material.  On  the  other  hand,  however,  the  pro- 
cumbent form  has  the  disadvantage  of  being  able  to  expose  to  the  light  relatively 
little  green  tissue;  only  those  of  its  leaves  can  be  well  illumined  which  are 
arranged  like  a  mosaic  in  a  plane  parallel  to  the  substratum.  The  development 
of  a  second  such  layer  of  leaves  higher  up  would  be  a  decided  disadvantage,  for 
it  would  cause  the  lower  stratum  of  foliage-leaves  to  turn  yellow  and  pine  away. 
Consequently,  any  upward  extension  of  the  green  tissues  in  procumbent  shoots  is 


PROCUMBENT   AND   FLOATING   STEMS.  665 

necessarily  limited.  Again,  the  earth  offers  an  insuperable  barrier  to  the  develop- 
ment of  foliage  in  a  downward  direction.  In  the  dark  bosom  of  the  earth  a  green 
leaf  would  be  quite  useless,  and,  as  a  matter  of  fact,  there  is  not  a  single  plant 
whose  green  tissue  is  situated  in  the  depths  of  the  soil. 

With  water  it  is  otherwise.  In  it  green  cells  and  tissues  can  function  as 
far  down  as  the  light  can  penetrate.  Since  the  water  also  maintains  the  stem 
and  leaves  in  a  definite  position,  and  the  plants  consequently  are  spared  the 
development  of  wood  and  bast  and,  generally,  of  masses  of  tissue  for  strength 
and  resistance  to  bending,  and  since,  finally,  a  saving  of  material  and  work  is 
effected  inasmuch  as  water-plants  do  not  require  to  construct  organs  for  con- 
duction of  water  and  for  transpiration,  it  might  be  supposed  that  water  would 
be  an  extremely  favourable  medium  for  green  vegetation,  and  that,  consequently, 
stretches  of  water  all  over  the  world  would  be  quite  crowded  with  green 
plants.  That  this  is  not  the  case  is  explained  by  the  fact  that  light  does  not 
penetrate  far  enough  into  the  water.  In  the  deep  gloom,  200  metres  below  the 
surface,  green  plant-life  is  as  impossible  in  water  as  in  the  dark  bosom  of  the 
earth,  and  the  bottom  of  the  ocean  over  an  enormous  area  is  a  plantless  waste 
shrouded  in  gloom.  But  as  far  as  the  water  is  illuminated,  in  all  places  where 
it  fills  shallow  basins,  and  also  in  a  comparatively  narrow  girdle  around  the 
coasts,  an  inexhaustible  wealth  of  plants  is  to  be  found.  Of  course,  spore-bearing 
plants,  which  are  built  up  of  rows,  nets,  and  plates  of  cells,  have  the  preponderance, 
whilst  seed-plants  are  markedly  in  abeyance  in  relative  number  of  species.  But 
the  latter  species  are  just  the  ones  which  claim  our  interest  in  a  special  degree 
on  account  of  the  very  peculiar  conditions  under  which  they  live. 

The  floating  stems  of  water  and  marsh  plants,  as  already  repeatedly  stated, 
have  no  wood  or  bast,  while,  on  the  other  hand,  they  are  penetrated  by  remarkably 
large  air-canals,  and  are,  in  consequence,  exceedingly  light  and  buoyant.  If  the 
erect  stem  of  a  water-plant  growing  at  the  bottom  of  a  lake  is  cut  through 
close  above  its  roots,  it  rises  immediately  to  the  surface  of  the  water,  there 
assumes  a  horizontal  position,  and  remains  floating;  under  certain  circumstances 
it  may  continue  to  grow  and  may  perhaps  take  root  should  it  drift  to  a  shallow 
place.  On  the  other  hand,  if  a  pond  filled  with  Water  Crowfoot,  Myriophyllum, 
Elodea,  &c.,  be  emptied,  all  these  plants  sink  limp  and  withered  on  to  the  mud, 
as  their  stems  have  not  the  strength  to  hold  themselves  erect.  The  water  in 
which  they  float  supports  and  bears  them,  and  in  this  respect  they  may  be 
likened  to  climbing  stems  which  also  require  a  support  to  enable  them  to  rise 
above  the  ground.  The  analogy  between  these  plants  is  evident  in  so  far  as 
the  need  for  "more  light"  influences  the  direction  of  growth  in  both  cases— in 
the  one  case  the  stem  grows  out  from  the  gloom  of  the  forest  floor  up  to  the 
sunny  tops  of  the  trees,  in  the  other,  from  the  subdued  light  at  the  bottom  of 
the  lake  up  to  the  surface  of  the  water.  In  many  cases,  of  course,  the  stem  of 
water-plants  remains  so  short  that  it  scarcely  rises  above  the  mud  at  the  bottom 
of  the  pond,  but  the  leaves  arising  from  it  are  shaped  into  long  ribbons,  whose 


QQQ  PROCUMBENT  AND  FLOATING  STEMS. 

freely  floating  ends  ascend  into  the  better  illuminated  upper  layers  of  water, 
or  leaves  with  large  blades  and  elongated  stalks  spring  from  the  short  stems, 
and  the  stalks  continue  to  grow  until  the  plate-like  blades  have  reached  the 
surface,  where,  floating,  they  can  enjoy  the  full  sunlight.  There  are  also  some 
plants  not  fixed  but  swimming  close  to  the  surface.  These  sink  down  to  the 
bottom  only  when  the  activity  of  their  leaves  is  suspended  and  here  for  a  time 
they  pass  a  dormant  period. 

We  mention  here  the  most  noticeable  variations  which  are  made  use  of  for 
dividing  the  stem-forming  water-plants  into  architectural  groups.  First  of  all  is 
a  group  of  plants,  of  which  the  Grass  Wracks  (Zostera)  may  be  taken  as  a  type. 
These  have  stems  embedded  in  the  mud,  creeping,  and  anchored  by  root  fibres. 
The  leaves  arising  from  these  steins  are  erect,  very  long  and  narrow,  looking 
like  thin  limp  ribbons,  which  are  only  kept  in  their  erect  position  by  the  water. 
The  Zostera  grows  in  large  patches  on  the  shore  between  tide-levels.  Its  leaves 
are  collected  and  dried  and  under  the  name  of  Sea -grass  are  used  as  stuffing 
for  cushions.  To  this  group  belongs  also  Vallisneria  spiralis,  which  is  figured 
opposite,  and  to  the  flowers  of  which  we  shall  return  in  detail  later  on;  lastly, 
we  may  mention  certain  species  of  Sparganium.  In  addition  to  this  group  is 
a  second,  as  a  representative  of  which  may  be  named  the  curious  Lattice -leaf 
plant  (Aponogeton  fenestrale  or  Ouvirandra  fenestralis)  inhabiting  the  waters 
of  Madagascar.  Its  short  stems  are  buried  in  the  mud;  the  leaves  have  short 
stalks,  and  are  not  erect,  but  distributed  in  rosettes  over  the  muddy  bottom. 
The  green  colour  of  their  chlorophyll  is  almost  entirely  obscured  by  a  reddish- 
brown  pigment;  the  parenchyma,  which  usually  fills  the  meshes  of  the  net- work 
of  strands,  is  absent,  and  the  strands  forming  the  framework  of  the  leaf-blade 
are  covered  only  with  a  thin  layer  of  chlorophyll-bearing  cells,  so  that  the  whole 
structure  reminds  one  of  a  leaf  which  has  fallen  from  a  tree  in  autumn  and  has 
been  macerated  under  water,  of  which,  after  the  falling  away  of  the  easily  de- 
composed parenchyma,  only  the  net-work  of  strands  remains.  The  Water-lilies 
may  serve  for  a  type  of  the  third  group.  Their  stems  are  short,  rooted  in  the 
mud,  and  send  out  leaves  whose  broad  blades,  often  circular  in  outline,  are  borne 
on  very  long  stalks.  The  disc-shaped  leaf-blades  lie  with  their  under  side  on 
the  surface  of  the  water,  while  their  upper  surface  is  exposed  to  the  air.  The 
leaf-stalks  thus  traverse  the  whole  depth  of  the  water,  and  look  like  ropes  by 
which  the  floating  leaf-discs  are  anchored  in  the  muddy  bottom.  The  long  scapes, 
terminating  in  floating  flowers,  serve  a  similar  purpose.  Here  also  must  be 
included  the  aquatic  fern-like  plant— Marsilea.  Its  leaves  remind  one  of  those  of 
the  Wood  Sorrel.  The  Frog-bit  (Hydrocharis)  and  the  Villarsia  (Limnanthemum) 
form  a  fourth  group,  not  unlike  water-lilies  on  a  small  scale.  Their  leaves  and 
flowers,  however,  do  not  arise  directly  from  the  main  stem  (as  in  the  last  group), 
but  from  long  lateral  shoots,  quite  bare  of  leaves,  till  just  close  to  the  surface  (cf. 
vol.  II.,  fig.  419).  Our  fifth  group  includes  forms  transitional  between  the  groups 
already  described  and  the  sixth  and  largest  group.  They  include  forms  with 


PROCUMBENT   AND    FLOATING   STEMS. 


607 


Fig.  155.  —  Vallisneria  spiralis. 


668  PROCUMBENT  AND  FLOATING  STEMS. 

finely-divided,  submerged  leaves,  in  addition  to  orbicular  floating  ones  as  in  the 
water-lilies,  &c.  Such  plants  are  known  as  heterophyllous  (plantce  heterophyllce). 
Examples  are  furnished  by  several  potamogetons  (Potamogeton  heterophyllus, 
rufescens,  spathulatus),  some  water-crowfoots  (Ranunculus  aquatilis,  Baudotii, 
hololeucus),  the  Cabomba  (Cabomba  aquatica)  and  the  Water-chestnut  (Trapa).  In 
the  sixth  group  the  plants  are  firmly  rooted  in  the  mud  like  those  of  the  former 
group,  but  the  shoots  rising  from  them  bear  only  submerged,  thin  and  limp  leaves. 
These  plants  in  descriptive  botany  are  called  "  submerged "  (plantce  submersce). 
Their  leaves  —  arising  from  the  much-branched  filamentous  stems  —  exhibit  an 
endless  variety  of  form.  They  are  sometimes  decussate,  sometimes  spirally 
arranged,  often  broad  and  embracing  the  stem,  and  then  again  fall  into  the 
opposite  extreme,  and  form  long  very  narrow  ribbons  or  threads.  Frequently  they 
are  reduced  to  mere  bristles;  in  other  cases  they  are  entire  and  undivided;  again,  in 
other  instances,  they  have  finely  indented  and  sinuous  margins  (cf.  fig.  136,  p.  551). 
All  these  various  forms  of  leaf  are  connected  with  the  peculiarities  of  the  habitat, 
with  the  attacks  of  animals  to  which  they  are  liable,  with  the  conditions  of 
illumination  at  different  depths  of  water,  but  chiefly  with  the  direction  of  the 
foliage-stem.  The  long  thin  stems  can  only  maintain  a  vertical  position  in  still 
water,  and  only  in  the  calm  inlets  of  lakes  and  in  the  deep  pools  where  an  active 
movement  of  the  water  is  impossible  are  to  be  found  species  whose  submerged 
leaves,  arranged  at  definite  intervals,  exhibit  a  circular  form.  In  running  water, 
especially  in  quickly-flowing  streams,  the  leaves  are  always  long  drawn  out, 
ribbon-shaped,  filamentous,  or  divided  into  thread-like  lobes.  They  adapt  them- 
selves exactly  to  the  current,  and  follow  it  in  all  its  movements  uninjured.  These 
leaves  of  running  water  are  always  fairly  tough;  their  cell- walls  are  corre- 
spondingly thickened;  the  stems  from  which  they  arise  are  protected  against 
rupture  by  bundles  of  bast  deposited  in  the  cortex,  and  are  strengthened  against 
strains  by  various  other  contrivances  to  be  presently  described. 

While  the  foliage  stems  of  the  water  and  marsh  plants  hitherto  described  are 
anchored  fast  by  roots  to  the  muddy  bottoms  of  lakes,  pools,  and  streams,  those  of 
the  Aldrovandia,  figured  on  p.  151,  and  also  of  the  bladder- worts  described  and 
figured  on  pp.  120,  121,  float  in  the  water  without  a  trace  of  root  formation.  Since 
the  leaves  require  light,  it  is  clear  that  they  will  take  up  their  position  near  the 
surface.  At  any  rate  at  the  time  when  they  are  actively  engaged  in  the  manufac- 
ture of  organic  matter  under  the  influence  of  light,  they  are  obliged  to  seek  such 
illuminated  places.  The  bud-like  tips  of  the  shoots  can,  of  course,  in  many  species 
sink  to  the  bottom  for  the  winter  rest,  but  at  the  commencement  of  the  favourable 
season  next  spring,  they  again  ascend  and  produce  their  flowering  axes  above  the 
surface  of  the  water.  A  horizontal,  or  obliquely  ascending  position  is  the  most 
advantageous  for  the  stems  of  these  floating  plants  as  regards  the  illumination  of 
their  leaves,  and,  as  a  matter  of  fact,  this  direction  is  observed  in  them.  Running 
water  would  form  a  bad  environment  for  such  rootless,  freely  oscillating  plants; 
they  are  found  exclusively  in  the  calm  inlets  of  ponds  and  lakes  and  in  pools  and 


CLIMBING   STEMS.  669 

ditches  amid  reeds  and  rushes,  where  a  great  agitation  of  the  water  has  never  to 
be  provided  against. 

In  similar  habitats  other  species  of  the  last  group  of  plants  with  floating  stems 
are  also  found,  viz.  those  known  as  "  swimming "  plants  (plantce  natantes).  They 
are  distinguished  from  floating  plants  especially  by  the  fact  that  their  green  foliage 
and  in  part  their  stems  also  lie  on  the  surface  of  the  water,  and  are  in  contact  on 
the  upper  side  with  the  air,  or  even  rise  above  the  water,  when  they  are  completely 
surrounded  by  air.  The  stem  rests  and  moves  on  the  surface  of  the  water,  and  is 
never  held  fast  in  the  muddy  bottom — even  when  roots  are  present.  Amongst  the 
well-known  forms  belonging  to  this  group  are  several  Duckweeds  (e.g.  Lemnapolyr- 
rhiza,  gibba,  minor)  with  stems  curiously  flattened  and  leaf -like.  Besides  these, 
there  are  Salvinia  and  Azolla,  belonging  to  the  vascular  cryptogams,  and  finally 
several  species  of  Pistia,  Pontederia,  and  Desmanthus,  belonging  to  tropical 
waters.  It  has  already  been  mentioned  (p.  638)  that  the  floating  capacity  of 
Pontederia  crassipes  is  increased  by  the  possession  of  a  vesicular,  air-containing 
tissue  in  its  swollen  leaf -stalks.  Moreover,  in  Desmanthus  natans  an  actual  swim- 
ming apparatus  is  developed,  not  in  the  leaf -stalks,  but  in  the  stem  itself.  It  takes 
the  form  of  a  large-celled,  spongy,  air-containing  mantle,  arising  here  below  the 
epidermis  of  the  internodes  which  renders  sinking  impossible.  The  mimosa-like 
foliage-leaves  rise  up  from  the  nodes  of  these  floating  stems  like  masts  with  flags. 
When  the  leaves  turn  yellow,  the  stems  rid  themselves  of  their  swimming  organs 
which  are  no  longer  needed,  and  indeed  it  appears  to  be  an  advantage  to  the  leafless 
stems  to  be  able  to  sink  down  and  to  obtain  a  period  of  rest  at  the  bottom. 

Several  species  of  the  last  group  of  plants  with  floating  stems  strongly  remind 
us  of  plants  with  procumbent  stems.  At  the  stem-nodes  they  develop  roots  which 
sink  into  the  depths,  and  green  leaves  which  rise  up  to  the  sunlight,  and  the  only 
difference  consists  in  the  fact  that  in  the  one  case  the  water,  and  in  the  other  the 
soil,  forms  the  bed,  and  even  this  distinction  is  sometimes  obliterated.  When  the 
level  of  the  water  sinks,  the  floating  plants  sink  with  it,  till  finally  they  lie  on  the 
mud,  and  then,  as  a  matter  of  fact,  they  are  scarcely  distinguishable  in  habit  from 
plants  with  procumbent  stems  which  grow  on  the  soil  of  the  moor. 

CLIMBING  STEMS. 

Often  it  happens  that  the  name  of  a  plant  affects  our  imagination  by  its  pleasing 
or  harmonious  sound.  One  associates  with  the  name  not  merely  the  idea  of  the 
form  of  a  certain  plant,  but  more  than  this,  its  whole  surroundings,  framed  in  which 
it  grows  and  flourishes.  One  conjures  up  a  picture  of  a  flowery  meadow  or  scented 
wood  with  which  the  plant  with  pleasing  name  can  only  harmonize.  It  may  be 
some  far-back  reminiscence  is  bound  up  with  the  pretty  name,  or  we  have  read  a 
vivid  description  in  a  book  long  ago.  Thus  idealized,  one  shrinks  from  approaching 
it  with  critical  eye,  from  examining  it  with  knife  and  microscope,  and  from  classify- 
ing and  describing  it  in  the  dry  language  of  the  specialist. 


67Q  CLIMBING   STEMS. 

I  am  thinking  here  especially  of  the  word  "  liane".  When  this  beautiful  word 
is  sounded  a  whole  series  of  splendid  pictures  stand  out  in  strong  relief  from  the 
twilight  of  youthful  recollections.  I  see  a  dense  leafy  canopy,  lit  by  a  stray  sun- 
beam here  and  there,  arching  over  the  gigantic  stems  of  the  primeval  forest — stems 
which  rise  up  like  the  columns  of  a  spacious  hall.  On  the  forest  floor  the  scanty 
green  of  shade-loving  ferns  covers  the  remains  of  fallen  trees.  Further  on  a 
confused  brown  mass  of  tangled  roots  renders  progress  over  the  still  dark  ground 
almost  impossible.  In  contrast  to  these  gloomy  depths  how  brilliant  is  the  picture 
in  the  glades  and  on  the  margin  of  the  primeval  forest!  Plant  forms  in  indescribable 
confusion  piled  up  into  the  thickest  of  hedges  rise  higher  and  higher  to  the  very 
crowns  of  the  giant-trees,  so  that  it  is  impossible  to  obtain  even  a  glimpse  into  the 
pillared  hall  of  the  interior  of  the  forest.  This  is  the  true  and  proper  home  of  the 
liane.  Everything  climbs,  winds,  and  twines  with  everything  else,  and  the  eye  in 
vain  attempts  to  ascertain  which  stems,  which  foliage,  which  flowers  and  fruits, 
belong  to  which.  Here  the  lianes  weave  and  work  green  draperies  and  carpets  in 
front  of  the  stems  of  the  forest  border,  there  they  appear  as  swaying  garlands,  or 
hanging  down  as  ample  curtains  from  the  branches  of  the  trees.  In  other  places 
they  stretch  in  luxuriant  festoons  from  bough  to  bough  and  from  tree  to  tree, 
forming  suspension  bridges,  even  actual  arcades  with  pointed  and  rounded  arches. 
Isolated  tree- trunks  are  transformed  into  emerald  pillars  by  the  covering  of  woven 
lianes,  or  more  frequently  become  the  centres  of  green  pyramids  over  the  summit 
of  which  the  crown  spreads  out  in  verdant  plumes.  Where  the  lianes  have  grown 
old  with  the  trees  on  which  they  cling,  and  the  older  portions  of  their  stems  have 
been  long  stripped  of  foliage,  they  resemble  ropes  stretched  between  the  ground 
and  the  tree-summits,  and  often  assume  peculiar  and  characteristic  forms.  Some- 
times drawn  out  tightly,  sometimes  limp  and  swaying,  they  rise  up  from  the  under- 
growth of  the  forest  ground,  and  become  entangled  and  lost  far  above  among  the 
boughs.  Many  are  twisted  like  the  strands  of  a  cable,  others  are  wound  like  a 
corkscrew;  and  others  again  are  flattened  like  ribbons,  hollowed  in  pits,  or  shaped 
into  elegant  steps — the  celebrated  monkey -ladders. 

The  green  garlands,  bowers,  and  festoons  of  lianes  are  adorned  with  the  gayest 
flowers.  Here  a  truss  glows  with  flame-like  brilliancy,  there  a  large  blue  raceme 
sways  in  the  sunshine,  and  here  again  is  a  dusky  curtain  studded  with  hundreds  of 
bright  star-like  passion-flowers.  And  where  flowers  flaunt  themselves  and  fruits 
ripen,  guests  are  not  wanting.  The  gay  assemblage  of  butterflies  and  the  joyous 
songsters  of  the  wood  regard  the  forest  border  interwoven  with  lianes  as  their 
favourite  rendezvous. 

From  what  has  been  hitherto  said  about  lianes,  one  might  think  that  this  par- 
ticular plant  formation  belonged  only  to  the  tropics.  This  would,  however,  be 
incorrect.  In  the  neighbourhood  of  the  Canadian  lakes,  and  in  the  districts  of  the 
large  central  European  rivers,  the  Danube  and  the  Rhine,  various  species  of  Clematis, 
wild  vines,  climbing  roses,  honeysuckle,  bramble,  many  Menispermaceae,  &c.,  climb 
up  to  the  summits  of  the  trees;  and  even  the  woods  of  our  lower  Alps  contain  one 


CLIMBING   STEMS.  671 

of  the  most  charming  lianes,  the  Alpine  Vine  (Atragene  alpina),  adorned  with 
large,  blue,  bell-shaped  flowers.  Of  course  the  number  of  species  increases  immensely 
as  we  approach  the  torrid  zone,  and  we  shall  not  be  far  wrong  if  we  estimate  the 
number  of  lianes  in  the  tropics  at  2000,  those  in  the  temperate  zones  at  200  species. 
Lianes  are  foreign  to  the  Arctic  regions  and  to  the  treeless  mountain  heights;  nor 
are  they  found  on  treeless  steppes.  It  is  remarkable  that  tropical  America  contains 
almost  twice  as  many  climbing  plants  as  tropical  Asia.  Brazil  and  the  Antilles 
exhibit  the  greatest  wealth  of  these  plants. 

The  sweet  word  "  liane  "  originated  in  the  French  Antilles,  and  has  now  found 
its  way  into  most  languages.  It  seems  strange  that  this  word  should  never  have 
been  introduced  into  botanical  terminology;  we  use  the  expression  indeed  in  general 
descriptions  of  the  vegetation  of  a  district,  but  in  that  of  individual  species  it  is 
avoided.  This  is  explained  by  the  fact  that  we  understand  by  lianes  in  the  original 
sense  of  the  word  only  climbing  plants  with  woody  perennial  stems,  and  that  there 
are  many  twining,  creeping,  and  climbing  plants  possessing  herbaceous  stems  to 
which  the  name  liane  is  not  properly  applicable.  On  the  other  hand,  the  climbing 
plants  are  so  much  alike  in  their  manner  of  life  that  they  can  only  be  treated 
together,  and  are  therefore  conveniently  designated  by  a  common  name.  We  now 
name  all  inclusively  "climbing  plants",  whether  woody  or  herbaceous,  and  define  the 
"  climbing  "  stem  (stirps  scandens)  as  that  which  is  able  to  obtain  for  its  free  end 
a  resting  position  at  a  great  height  above  the  nourishing  earth  only  by  the  aid  of 
foreign  supports.  If,  where  climbing  stems  grow,  there  are  no  elevated  objects 
which  might  serve  for  support,  the  earth  itself  is  used  by  the  free  end  as  a  resting- 
place;  the  stem  then  spreads  its  whole  length  upon  the  ground,  or  forms  an  arch, 
having  at  any  rate  its  free  end  supported  on  the  ground.  Such  a  stem  shows  all 
the  characteristics  of  a  prostrate  stem.  In  the  earliest  stages  of  its  development, 
on  the  other  hand,  every  climbing  stem  resembles  an  erect  plant;  it  is  difficult  to 
name  external  characteristics  by  which  young  shoots  of  the  one  can  be  distinguished 
from  those  of  the  other.  The  shoots  at  first  are  erect  and  able  to  maintain  them- 
selves in  a  vertical  position  by  their  inner  structure,  and  especially  by  the  turgidity 
of  certain  groups  of  cells.  Not  until  they  have  become  older,  and  have  reached  a 
certain  height  does  indication  of  a  climbing  habit  appear,  when  the  shoot  seeks  to 
obtain  a  hold  for  its  free  end.  It  curves  over  foreign  bodies  in  the  vicinity,  thrusts 
out  horizontal  branches  over  projecting  edges  of  rocks  or  in  the  forks  of  boughs  of 
trees  which  serve  as  supports;  its  tip  revolves  like  the  hand  of  a  watch,  and  winds 
round  an  erect  post,  or  it  develops  special  organs,  by  which  it  becomes  connected 
and  entwined  with  adjacent  objects.  In  respect  of  their  varying  behaviour,  climbing 
stems  may  be  divided  into  five  groups,  viz.  weaving,  lattice-forming,  twining,  creep- 
ing, and  climbing,  of  which  classification,  of  course,  as  in  so  many  similar  cases,  it 
must  be  noted,  that  it  is  purely  artificial,  and  is  only  used  with  the  object  of  dis- 
tinctness, and  that  intermediate  and  transitional  forms  between  the  several  groups 
occur  in  abundance. 

The  weaving  stem  (stirps  plectens)  obtains  a  resting-place  for  its  branches  and 


672  CLIMBING   PLANTS. 

foliage  in  the  following  manner: — As  a  young  shoot  it  grows  first  of  all  vertically 
erect;  it  has  as  yet  no  lateral  branches,  and  its  leaves  at  the  free-growing  end  are 
still  small,  furled,  and  crowded  closely  together  into  a  cone.  These  young  turgescent 
shoots  readily  pass  through  the  forks  of  the  boughs,  even  through  narrow  chinks 
and  meshes  of  the  net- work  of  twigs  and  branches  in  the  thickets,  without  suffering 
injury.  When  its  growth  in  length  is  terminated,  the  shoot  unfolds  its  leaves  and 
sends  out  lateral  branches  which  project  at  right  angles  in  all  directions.  These 
reflexed  leaves  and  the  lateral  branches  which  have  been  produced  above  the  gaps 
in  the  matted  undergrowth,  now  get  a  good  purchase  on  the  rough  boughs  of  the 
underwood;  the  slender  upgrowing  shoot  is  suspended  by  them  as  if  by  barbs,  and 
it  is  frequently  also  actually  woven  into  the  underwood. 

These  forms  of  weaving  stems  may  be  distinguished  according  to  the  character 
of  the  support.  First,  that  of  the  hedge-forming  shrubs,  of  which  Lycium  may 
serve  as  type.  It  is  astonishing  how  its  long  whip-like  shoots,  as  they  grow  up 
from  the  ground  on  the  edge  of  a  wood,  find  their  way  between  the  spar-like 
branches  of  other  growths,  and  then  perhaps  at  the  height  of  the  lowest  boughs 
of  the  crown  of  one  of  the  trees,  the  free  end  projects  as  if  from  an  opening  in  a 
roof.  In  the  course  of  the  summer  the  thin  slender  stem  lignifies,  and  leafy 
lateral  shoots  spring  from  the  axils  of  the  upper  leaves  at  about  a  right  angle. 
These  end  in  stiff  spines.  Meanwhile  the  highest  portion  of  the  shoot  becomes 
bent  over  some  bough,  so  that  the  whole  shoot  is  so  interwoven  with  the  under- 
growth, that  in  attempting  to  extricate  it  we  tear  innumerable  supporting  branches 
and  twigs,  and  set  the  whole  neighbourhood  in  motion.  The  lignified  shoot  of  the 
first  year  survives  the  winter;  next  spring  those  portions  of  it  which  rest  horizon- 
tally on  the  branches  produce  new  shoots  in  pairs,  close  to  the  thorny  lateral 
branches.  Of  these  one  usually  remains  small;  the  other,  slender  and  vigorous, 
pushes  up  into  the  crown  and  repeats  the  method  of  growth  of  the  former  shoot. 
As  this  is  repeated  from  year  to  year  the  whole  crown  of  the  tree  becomes  densely 
interwoven  with  the  Lycium  shoots.  Often  it  happens  that  shoots  are  produced, 
which  hang  down  from  the  tree-crown  like  branches  of  a  weeping- willow  draping 
the  supporting  tree  as  with  a  curtain,  or  forming  an  actual  hedge  in  front  of  it. 

The  following  well-known  plants  develop  in  accordance  with  this  Lycium 
type: — Numerous  roses  (Rosa),  brambles  (Rubus),  barberry  (Berberis),  spiraeas 
(Spircea),  sea-buckthorn  (Hippophae),  jessamine  (Jasminum),  Celastrus  scandens, 
and  numerous  other  woody  hedge-formers  which  grow  preferably  on  the  borders 
of  forests.  Many  roses,  as,  for  example,  the  Rosa  sempervirens,  abundant  in 
the  Mediterranean  floral  district,  not  only  weave  through  the  undergrowth,  but 
often  reach  the  tops  of  the  highest  oaks.  Also  many  brambles  (Rubus)  reach  far 
up  into  the  boughs  of  the  tree-crown,  and  then  not  unfrequently  depend  their  long 
shoots  in  arching  curves.  I  measured  the  length  of  a  stem,  \  centimetre  thick  in 
the  middle,  of  a  species  of  bramble  (Rubus  amosnus)  which  had  interwoven  with 
the  tree-crown,  and  found  it  to  be  six  and  a  half  metres.  The  long  whip-like  shoots 
of  Jasminum  nudiflorum  and  Celastrus  scandens  also  reach  the  tops  of  high  trees 


CLIMBING    PLANTS.  673 

in  the  same  manner.  If  these  hedge-shrubs  have  not  the  opportunity  of  inter- 
weaving in  the  branches  of  trees,  &c.,  they  are  obliged  themselves  to  form  a  scaffold- 
ing. Their  manner  of  growth  resembles  that  already  described,  except  that  the 
shoots  usually  remain  shorter,  and  the  whole  plant  consequently  appears  more 
compressed.  The  erect  shoots  at  first  mounting  vigorously  upwards  form,  as 
they  become  lignified,  flat  arches,  bent  over  so  that  their  apices  almost  trail  upon 
the  ground.  The  upper  portions  of  these  arches  give  rise  next  year  to  short  flower- 
ing branches  and  to  long  vigorous  shoots,  which  give  rise  to  new  arches.  The  free 
ends  of  the  old  arches  dry  up,  and  fresh  arches  come  to  lie  above  the  dried  remains. 
In  the  following  year  new  arch-like  shoots  proceed  from  the  last-formed  ones. 
This  being  repeated  year  after  year,  an  impenetrable  natural  hedge  gradually  rises, 
which  grows  continually  higher  and  higher,  since  the  stumps  of  the  old,  dried-up 
branches,  whose  ends  have  stopped  growing,  form  supports  for  the  younger  shoots. 
It  is  also  a  very  common  occurrence  for  these  hedge-shrubs,  when  they  have  become 
old,  to  develop  suckers  from  their  roots,  which  grow  up,  thin  and  slender,  between 
the  undergrowth  formed  of  the  old,  dried-up  arches,  which  they  use  as  a  support. 
This  may  be  seen  especially  in  the  barberry,  sea-buckthorn,  mock-orange,  roses, 
jessamine  and  the  elm-leaved  spiraeas. 

This  property  of  forming  hedges  has  long  been  familiar  to  agriculturalists,  who 
are  close  observers  of  nature.  Several  such  plants  are  used  for  the  purpose  of 
inclosing  portions  of  land;  thorny  species  are  especially  suited  for  the  purpose, 
the  so-called  "quickset  hedges".  Gardeners,  too,  make  use  of  this  peculiarity 
of  hedge-weaving  shrubs  when  they  plant  species  with  beautiful  flowers  close 
against  a  trellis- work,  which  is  soon  quite  overgrown  with  their  vigorous  shoots. 
The  so-called  climbing-roses  in  particular  are  used  with  the  best  results  for  covering 
trellis-work  against  the  fronts  of  buildings,  and  it  is  remarkable  how  quickly 
they  grow  without  assistance  right  up  to  the  gables  of  the  houses.  Some  climbing 
roses  (e.g.  Rosa  setigera)  have  this  remarkable  peculiarity,  that  their  new  shoots  at 
first  seek  the  darkest  places,  turning  their  apices  away  from  the  bright  sunshine, 
growing  into  the  shaded  nooks  behind  the  trellis-work,  and  not  inclining  again 
towards  the  light  until  they  are  fully  grown.  In  this  way  the  advantage  is  obtained 
that  the  shoots  originally  turning  from  the  light  enter  the  gaps  of  the  undergrowth 
and  of  the  trellis-work,  while  later  on,  when  lateral  branches  arise  from  them,  they 
are  excellently  supported. 

Generally  resembling  the  woody  stems  observed  in  hedge-builders  are  those  of 
several  undershrubs  which  do  not  become  lignified.  The  shoot  growing  up  annually 
at  the  beginning  of  the  vegetative  period  from  the  underground  portion  of  the  stein 
always  dies  off  again  in  the  autumn,  whilst  the  dried  remains  still  above  the  ground 
decay  so  quickly  that  only  in  rare  cases  can  they  be  used  as  supports  for  the  shoots 
which  grow  up  fresh  from  the  soil  in  the  following  year.  As  a  type  of  weaving 
undershrubs  the  widely-distributed  Marsh  Crane's-bill  (Geranium  palustre)  may  be 
taken.  The  young  shoots  grow  erectly  among  the  bushes  scattered  over  damp  mea- 
dows or  on  the  edge  of  a  forest,  but  they  do  not  become  woody;  their  upper  ends 

VOL.  I. 


674  CLIMBING   PLANTS. 

do  not  bend  over  the  supporting  branches,  but,  having  once  attained  a  certain 
height,  develop  stiff  lateral  branches,  projecting  like  spars,  and  long-stalked  leaves 
which  push  their  way  between  the  stiff,  dry  branches  of  the  supporting  bushes ; 
in  this  way  the  whole  shoot  is  held  fast  so  that  it  cannot  be  displaced.     When  this 
same  Crane's-bill  grows  in  a  meadow  between  low  herbs  which  can  afford  it  no 
support,  the  stem  bends  and  the  whole  shoot  lies  with  its  lower  internodes  on  the 
ground.     The  ends  of  the  internodes  are  thickened,  and  a  turgescent  cell-tissue  is 
formed  at   these  places  by  means  of  which  the  younger  parts  of  the  shoot  are 
brought  again  into  an  erect  position,  appearing  at  right  angles  to  the  older  inter- 
nodes lying  on  the  ground.      The  advantage  obtained  by  this  arrangement,  is  that 
plants  of  Crane's-bill  thus  extended  over  the  ground  are  able,  should  they  encounter 
a  firm  shrubby  undergrowth  not  too  far  removed  from  the  place  where  they  are 
rooted,  to  use  it  as  a  support  and  to  weave  themselves  over  it.      As  a  matter  of 
fact,  plants  of  Geranium  palustre  are  often  seen  with  their  lowest  internodes  lying 
on  the  ground,  while  the  upper  internodes  as  well  as  numerous  lateral  branches  are 
interwoven  into  some  bush  growing  in  the  meadow  near  by,  and  their  red  flowers 
are  displayed  more  than  a  metre  high  above  the  soil  from  between  the  branches  of 
the  bush  serving  as  a  support.     Several  other  species  of  crane's-bill  resemble  this 
one  in  habit  (e.g.  Geranium  nodosum,  divaricatum,  &c.),  also  several  species  of 
bedstraw  and  woodruff  (e.g.  Galium  mollugo,  Asperula  aparine),  the  berry-forming 
Cucubalus   (Cucubalus   baccifer),  and,  finally,   the  remarkable    Marsh    Speedwell 
(Veronica  scutellata).     Here,  too,  belong  several  species  of  asparagus  with  pro- 
jecting, spar-like  branches  and   filamentous  or  needle-shaped   phylloclades.      The 
annual  shoots  of  these  asparaguses  attain  an  astonishing  length  and  push  their  way 
into  the  forkings  of  the  boughs  of  erect-growing  stems.     In  this  respect  the  Aspa- 
ragus acutifolius,  very  common  in  the  region  of  the  Mediterranean  flora,  is  parti- 
cularly worth  mentioning,  and  also  the  Asparagus  verticillatus  growing  in  Asia 
Minor,  the  stems  of  which  not  infrequently  attain  a  length  of  3  metres,  climbing 
up  to  the  crowns  of  the  lower  oaks  and   there  interweaving  their  horizontally 
disposed  branches  with  the  boughs. 

The  third  group  of  plants  with  weaving-stems  includes  the  rotangs,  those  pecu- 
liar palms  celebrated  for  the  fabulous  length  of  their  almost  uniformly  thickened 
stems.  A  species  of  rotang,  drawn  from  nature  in  Java  by  Selleny,  is  given  on 
the  opposite  page.  The  stem  of  all  young  rotang  plants  is  erect,  and  the  yet 
folded  leaves  grow  vertically  upwards  in  the  same  direction  as  the  young  axis. 
Later,  when  the  leaves  unfold  and  expand,  they  arch  outwards  and  spread  them- 
selves over  the  confused  mass  of  other  growths,  amongst  which  the  rotang  plant 
has  germinated  and  grown  up.  If  the  vegetation  in  the  immediate  neighbourhood 
consists  only  of  low  herbs  and  bushes,  the  elongating  rotang  stem  does  not  find  a 
support  sufficient  to  enable  it  to  grow  up  in  the  original  vertical  direction.  So  it 
trails  on  the  ground  like  a  runner,  often  forming  snake-like  coils,  as  shown  in 
fig.  156;  still  always  bending  up  at  the  free  end,  and  continually  pushing  up  new 
leaves.  If  the  rotang  plant  has  developed  amongst  tall  shrubs  and  trees,  or  if  after 


CLIMBING   PLANTS. 


675 


HI   • 


Fig.  156.—  Rotangs  in  Java.    (From  a  drawing  by  Selleny.) 


676  CLIMBING   PLANTS. 

trailing,  it  has  come  within  the  range  of  a  wood,  it  pushes  its  stiff,  folded,  spire-like 
leaves  between  the  lower  branches  of  the  trees,  and  as  these  leaves  unfold  and  bend 
outwards,  they  form  strong  supports  or  barbs  by  which  the  cord-like  stem  is 
anchored  above  in  the  branches  of  the  tree  (cf.  fig.  94,  p.  363).  Under  favourable 
conditions  the  stem  can  grow  up  to  the  tops  of  the  trees,  its  new  leaves  always 
anchoring  thus  in  the  branches  above.  Frequently  the  free  end  of  a  rotang  shoot 


Fig.  157.— Shoot-apices  of  three  species  of  Rotang. 
1  Dcemonorops  hygrophilus.    *  Calamus  extensus ;  with  inflorescence,    a  Desmoncus  polyacanthus ;  much  reduced. 

grows  from  tree  to  tree — now  ascending,  now  descending.  It  is  shoots  of  this  kind 
which  attain  to  lengths  unequalled  by  any  other  plant.  There  are  credible  state- 
ments according  to  which  such  rotang  stems,  with  an  almost  uniform  thickness  of 
only  2-4  cm.,  have  reached  a  length  of  200  metres. 

We  must  not  omit  to  mention  that  most,  if  not  all,  plants  which  weave  into  the 
thicket  of  other  plants  are  equipped  with  barbed  spines,  prickles,  and  bristles,  which 
assist  them  in  maintaining  themselves  at  the  heights  once  reached.  The  goat's 
thorn  is  provided  with  horizontally-projecting  spines;  in  the  roses  and  brambles 


CLIMBING   PLANTS.  677 

both  the  internodes  and  the  ribs  of  the  leaves  are  beset  with  sharp,  backwardly- 
directed  prickles;  several  bedstraws  (e.g.  Galium  uliginosum  and  aparine)  bear 
short,  stiff,  reversed  bristles  on  the  ridges  of  the  stem  and  on  the  leaf-margins  and 
ribs,  whilst  the  midrib  of  the  pinnate  rotang  leaves  is  continued  beyond  the  blade  as 
a  long  whip-like  structure  beset  with  barbs  of  the  most  varied  description.  The 
illustration  of  three  species  of  rotangs  inserted  opposite  shows  the  most  striking  forms 
of  these  peculiar  leaves.  In  one  species  (fig.  157  l)  the  leaf-rachis  is  beset  at  equal 
intervals  with  groups  of  small  but  very  pointed  barbs;  in  a  second  species  (fig.  157  2) 
the  uppermost  leaves  are  wholly  devoid  of  green  pinnae,  and  bear  only  numerous 
claw-like  barbs;  while  in  the  third  (fig.  157  8),  very  long,  pointed,  reversed  spines  are 
found  on  the  foremost  portion  of  the  leaf,  with  little  teeth  between,  so  that  this 


Fig.  158.— Branches  of  the  New  Zealand  Bramble  (Rubus  squarrosus). 

portion  resembles  a  harpoon.  When  we  look  at  these  barbed  structures  and 
consider  that  the  rotang  leaves  are  exceedingly  rough,  we  can  understand  how 
firmly  the  rotangs  anchor  themselves  in  the  crowns  of  the  tree-summits,  and  how 
difficult  it  must  be  to  disentangle  these  climbers,  fastened  as  they  are  with  har- 
poons, from  the  trees  they  interweave. 

A  plant  distinguished  by  its  unusually  rich  development  of  barb-like  spines,  and 
deserving  especial  mention  here,  is  the  New  Zealand  bramble,  Rubus  squarros^, 
Ulustrated  in  fig.  158.      Each  of  its  leaves  is  divided  into  three  portions,  each  being 
provided  with  a  tiny  blade  at  its  apex;  these  three  portion  as  well  as  the  leaf-stalk 
are  green  throughout  their  entire  length  and  beset  with  yellow,  pointed  prickles 
which  anchor  so  firmly  in  the  intertwined  bushes  «md  shrubs  that  a  whoUy  inextri- 
cable tangle  is  the  result.    Finally  those  plants  still  remain  to  be  considered  in  wh, 
the  support  is  afforded  by  the  pointed  teeth  of  the  leaf-margin.     To  these  belc 
especially  several  tropical  Pandanace*,  with  long  thin  stems  resembling  rot 


678  CLIMBING   PLANTS. 

and  also  an  insignificant  little  speedwell  which  grows  in  damp  meadows  in  Great 
Britain,  and  rises  above  the  ground  by  sprawling  over  its  erect  and  stronger 
neighbours.  This  speedwell  (Veronica  scutellata)  has  long,  narrow  leaves  which 
in  section  almost  resemble  those  of  the  tropical  Pandanus.  Like  these  they  are 
erect  when  young,  and  are  inserted  in  pairs  over  the  vertically-growing  apex.  By 
the  further  growth  of  the  stem  they  are  pushed  in  between  the  gaps  in  the  con- 
fusion of  herbage.  By  and  by  the  leaves  are  reflexed  and  afford  the  plant  useful 
support.  While  the  serrated  teeth  of  the  leaf -margins  in  other  species  of  speedwell 
have  their  apexes  directed  forwards,  in  this  they  are  strangely  directed  backwards,. 
i.e.  downwards  towards  the  ground;  by  this  means  the  support  which  these  leaves- 
obtain  is  materially  increased.  In  this  speedwell  the  retro-serrate  teeth  of  the  leaf- 
margin  have  undoubtedly  no  other  significance  than  that  of  firm  anchorage,  though, 
in  many  of  the  other  above-named  instances,  the  pointed  teeth,  prickles,  and  spines 
have  the  additional  task  of  protecting  the  foliage,  and  perhaps  also  the  flowers  and 
fruit,  against  animals  which  might  climb  up  over  the  stem  in  their  search  for  food. 

The  lattice-forming  stem  (stirps  clathrans)  does  not  twine,  nor  indeed  has  it 
any  special  climbing  organs,  and  yet  leaning  against  rock-faces  or  tree-trunks  it 
gradually  attains  to  heights  which  it  would  be  unable  to  reach  without  these  sup- 
ports. It  clothes  its  supports  with  branches,  which,  in  the  aggregate,  constitute  a 
solid  lattice -work,  reminding  one  of  certain  interweaving  climbers,  from  which, 
however,  it  is  distinguished  by  the  fact  that  its  elevation  is  achieved  neither  by 
lateral  branches  projecting  like  spars,  nor  by  arched  shoots,  nor  even  by  reflexed 
foliage-leaves.  Lattice-forming  stems  occur  comparatively  seldom  in  the  floras  of 
the  temperate  zones;  the  most  striking  example  in  these  regions  is  the  small  and 
dainty  species  of  buckthorn  known  as  Rhamnus  pumila,  whose  lattice-work  clothes 
the  steep  limestone  rocks  here  and  there  in  the  outlying  Alps  between  Switzerland 
and  Styria,  and  in  the  Jura.  Anyone  looking  from  a  little  distance  at  a  pre- 
cipitous rocky  face  overgrown  with  this  buckthorn,  might  think  that  it  was  ivy 
which  had  spread  out  its  stems,  climbing  by  means  of  clinging  roots.  The  foliage 
shows,  indeed,  the  same  dark  green  and  is  about  the  same  size  as  that  of  ivy,  but 
it  is  easily  recognized  on  a  nearer  view  that  the  shape  of  the  leaf,  the  distribution 
of  the  strands  in  the  blade,  and  finally  the  character  of  the  flowers  and  fruits  are 
quite  different,  and,  what  is  especially  important  here,  that  the  much-branched 
woody  stems  adhering  to  the  steep  rocks  have  no  clinging  roots.  It  is  also  an 
interesting  fact  that  the  older  stems  are  actually  wedged  into  the  crevices  of  the 
rock,  and  that  the  branches  are  exceeding  brittle.  With  careless  handling  they 
break  and  fall  to  the  ground,  and  only  by  proceeding  very  carefully  can  one  succeed 
in  detaching  a  complete  stem  with  all  its  branches  from  the  rocky  face.  We  may 
conclude  that  this  plant  would  necessarily  perish  without  the  supporting  back- 
ground, since  its  brittle  branches  would  break  off  at  the  first  violent  burst  of  storm, 
and  the  bush  would  be  mutilated  by  every  tempest. 

The  peculiar  structure  and  method  of  growth  of  this  buckthorn  explain  all  these 
striking  phenonema.  Here  there  are  no  strands  of  hard  and  fibrous  bast  deposited 


CLIMBING   PLANTS.  679 

outside  the  soft  bast,  such  as  enable  the  young  branches  of  other  trees  to  resist  flexion 
or  to  resume  their  position  after  bending  by  the  wind.  In  the  centre  of  the  branch 
is  seen  a  woody  cylinder,  surrounded  by  strands  of  soft  bast;  and  beyond  this  a 
very  voluminous  parenchyma,  but  only  a  very  few  hard  tenacious  bast-fibres.  It 
is  evident,  therefore,  why  the  branches  break  away  so  readily.  And  that  they  split 
up  most  easily  at  their  places  of  origin,  i.e.  where  they  arise  from  an  older  branch, 
is  explained  by  the  fact  that  the  woody  cylinder  is  weakest  at  these  points.  The 
method  of  growth  of  the  branch  is  just  as  remarkable  as  its  structure.  When  in 
the  spring  leafy  shoots  proceed  from  the  foliage-buds,  they  do  not  grow  towards 
the  light,  as  in  the  greater  number  of  plants,  especially  woody  plants,  but  they  turn 
away  from  the  light  and  seek  the  darkness,  and  even  curve  round  projecting  angles 
into  shady  corners,  growing  into  the  dark  crevices  and  clefts  in  the  stone  wall 
If  the  face  is  not  cracked  for  a  wide  distance,  but  is  smooth  and  even,  the  longer, 
growing  shoots  always  hug  it  closely,  and  take  a  straight  course;  but,  as  soon  as  a 
fissure  is  reached,  the  shoot  immediately  bends  round  into  it,  much  in  the  manner 
characteristic  of  roots  (cf.  p.  88).  While  in  other  shrubs  the  young,  growing  shoots 
arising  from  an  old  woody  branch  are  directed  upwards,  it  here  frequently  happens 
that  a  downward  course  is  followed.  The  burden  of  the  foliage  unfolding  on  the 
shoots,  and  the  consequent  increase  of  weight  cannot  be  regarded  as  the  cause  of  this 
bending,  for  not  infrequently  from  one  and  the  same  branch,  as  it  runs  horizontally 
over  the  rock  wall,  shoots  arise  side  by  side  of  similar  shape,  similarly  leaved,  and 
of  about  equal  weight,  some  of  which  grow  downwards  and  others  upwards. 

In  this  manner  of  growth  it  is  unavoidable  that  the  branches  should  sometimes 
cross  one  another,  forming  a  lattice- work  which  adheres  to  the  rock.  I  have  never 
observed  actual  fusions  of  the  intersecting  branches  in  this  buckthorn,  but  it  often 
happens  that  the  younger  branches  which  lie  across  the  older  are  so  firmly  attached 
to  them  that  they  still  remain  connected  when  large  portions  of  the  plant  are  re- 
moved from  the  rocky  wall. 

Such  extensive  lattice-branches  have  quite  the  appearance  of  a  root-plexus  which 
has  extended  over  a  boulder,  and  we  are  reminded  of  the  remarkable  latticed  root- 
formation  of  certain  tropical  fig-trees,  which  will  be  discussed  later  on.  There  is 
also  a  temptation  to  take  the  older  stems  of  Ehamnus  pumila  for  roots,  inasmuch 
as  they  are  frequently  seen  embedded  in  the  clefts  and  crevices  of  rocks,  a 
phenomenon  brought  about  in  the  following  way.  When  the  apex  of  the  develop- 
ing, light-avoiding  shoot  reaches  a  dark  cleft,  it  continues  to  grow  in  a  manner 
readily  intelligible  in  the  direction  of  the  crevice,  into  which  it  nestles  so  far  as  its 
foliage  permits.  Later  on  it  becomes  lignified  and  loses  its  foliage;  in  the  following 
year  it  sends  up  new  shoots,  but  itself  remains  growing  in  diameter  by  the  addition 
of  wood  and  bast,  till  sooner  or  later  it  becomes  so  thick  that  it  is  jammed  tight  in 
the  cleft,  and  resembles  a  root  which  has  forced  its  way  in. 

The  lattice  formation  in  tropical  Clusiaceae,  of  which  an  illustration  is  gr 
on  page  681,  is  effected  in  a  manner  quite  different  from  that  obtaining 
buckthorn.      The  young  stems  of  Clusiace*  grow  erect,  and  prefer  to  make  use  c 


CLIMBING   PLANTS. 

tree-trunks  as  supports,  particularly  those  of  palms.  At  first  they  adhere  very 
slightly,  and  lean  on  them  only  to  a  certain  extent.  All  the  shoots  of  these 
Clusiacese  are  thick  and  beset  with  opposite  leathery  leaves;  they  remain  green 
for  a  very  long  time,  and  are  still  unlignified  when  they  develop  lateral  shoots 
from  the  leaf -axils  of  their  erect  branches,  and  when  the  cortex  is  wounded,  a  thick 
adhesive  juice  like  gamboge  makes  its  appearance.  The  leaves  are  so  heavy  that 
the  outstretched  lateral  branches  are  bowed  under  their  burden,  and  sometimes 
even  hang  downwards.  It  is  therefore  unavoidable  that  many  of  these  lateral 
branches  should  intersect  and  come  into  contact  with  each  other,  and  that  at  the 
places  of  contact  the  epidermis  should  be  wounded  by  the  friction.  But  at  such 
places  an  actual  fusion  of  the  branches  occurs,  and  since  this  process  is  many  times 
repeated,  a  lattice- work  results,  as  shown  in  fig.  159.  The  individual  portions  of 
the  latticed  stem  remain  soft  and  pliant,  and  thus  mutually  supporting  one  another, 
the  whole  possesses  a  bearing  capacity  adequate  to  enable  the  erect  shoots  to  rise 
higher  and  higher  from  this  scaffolding.  The  lattice- work  is  additionally  strengthened 
by  the  production  of  aerial  roots  from  the  older  internodes.  These,  like  the  stems, 
fuse  where  they  intersect.  From  their  general  external  similarity  it  is  often  diffi- 
cult to  distinguish  between  the  two  sets  of  elements  comprised  in  an  old  lattice- work. 
In  cases  where  the  inclosed  stem  increases  in  thickness,  the  latticed  sheath  becomes 
tightly  stretched.  Often,  in  Clusia,  many  of  the  branches  die  in  consequence  of  this 
tension.  Still,  new  shoots  generally  arise  from  the  stumps,  and  repeating  the  already- 
described  method  of  growth,  become  interlaced  into  a  lattice-work.  Sometimes  the 
adherent  stems  become  flattened  and  girdle-like,  whilst  aerial  roots  developing  at 
many  points  become  inextricably  interwoven  in  the  lattice-work  till  it  is  impossible 
to  see  the  original  palm  stem.  On  the  banks  of  the  Rio  Guama  in  Brazil,  Martius 
saw  whole  groves  of  the  Macaw  tree  (Acrocomia  sclerocarpa)  covered  with  Clusia 
alba.  The  Clusia  formed  an  absolutely  closed  sheath  bearing  flowers  and  foliage, 
whilst  10  metres  above  the  stately  crown  of  the  palm-tree  projected. 

The  twining  stem  (stirps  volubilis)  is  able  to  reach  considerable  heights  by 
attaching  itself  to  various  objects  and  twisting  spirally  around  them.  In  a  state 
of  nature,  erect  stems  or  even  those  of  other  climbing  plants  may  serve  as  supports, 
whilst  in  gardens,  sticks,  strings  and  wires  are  utilized  in  this  way  when  it 
is  desired  that  twining  plants  shall  cover  walls,  arbours,  &c.  We  find  by  experi- 
ence that  even  very  fine  threads  form  excellent  supports,  while  thick  posts  and 
bulky  tree-trunks  are  less  adapted  for  this  purpose.  In  the  case  of  many  annual 
twining  stems,  props  of  even  20-25  cm.  diameter  are  too  thick  for  the  plants 
to  twist  around.  Those  perennial  and  lignifying  stems  called  lianes  are  sometimes 
found  round  pillars  of  30-40  cm.  diameter,  e.g.  those  of  Glycine  Chinensis  in  the 
avenues  of  the  park  at  Miramare,  near  Trieste.  In  the  tropics,  twining  plants 
are  seen  embracing  the  trunks  of  trees  as  much  as  40-50  cm.  thick,  but  in  such 
instances  it  is  probable  that  the  trunk  did  not  possess  this  thickness  when  it 
was  first  entwined,  and  only  attained  it  later  on.  Of  course  this  can  only  happen 
under  particularly  favourable  circumstances,  for  most  perennial,  twining  stems 


CLIMBING   PLANTS. 


681 


cannot  stand  the  severe  strain  involved  —  a  strain  which  must  occur  whenever 
the  tree,  around  whose  trunk  a  perennial  twiner  has  wound,  increases  much  in 
thickness.  The  twining  stems  of  the  Lonicera,  figured  on  p.  160,  certainly  do 
not  increase  in  length  after  lignifying,  and  must,  therefore,  act  as  constricting 
coils  on  the  young  actively-thickening  tree-stems,  which  they  often  strangle  to 


Fig.  159.-Palm-8tem  iTsed  as  a  support  by  the  lattice-forming  stem,  of  one  of  the  Clusiacea,  (Fragroa  obovatt). 

death.      Sometimes  one  finds  the  hard  basal  parts  of  a  liane  stem  twisted  and 
coiled   apparently  around   nothing.      This   is   due   to  the   fact  that  the  original 
support  has  been  killed,  and  then  slowly  rotting  into  dust,  haa  been  denude 
away  by  the  wind  and  rain.      Thus  many  a  liane  of  the  tropical  forest  seems 
to  have  made  use,  when  young,  of  some  living   plant  with  a  fairly  thick  erec 
stem  as  its  first  support,  up  which  it  has  climbed  into  the  crowns  of  higher 


682  CLIMBING   PLANTS. 

Subsequently,  the  first,  lower  prop  has  perished,  while  the  branches  supporting 
the  upper  portion  of  the  liane  remain  still  vigorous  and  afford  a  good  hold.  An 
erect  corkscrew-like  liane  stem  is  then  seen  hanging  from  the  upper  branches; 
it  has  a  very  odd  appearance,  and  is  only  surpassed  in  the  peculiarity  of  its 
form  by  the  twined  stems  of  bauhinias  and  monkey  ladders,  to  be  described 

presently. 

But  if  the  erect,  young  stem  is  stronger  and  more  vigorous  than  the  twiner 
which  encircles  it,  which  has  been  used  as  a  prop,  it  does  not  allow  itself  to  be 
strangled;  the  twiner  is  destroyed  when  they  both  increase  in  thickness.  The 
coils  of  the  climber  are  gradually  stretched  tighter  and  tighter,  and  many 
are  the  contrivances  which  exist  for  preventing  the  tension  from  immediately 
acting  injuriously  on  the  movement  of  the  sap  in  the  interior  of  the  twining 
liane  stem.  As  this  thickening  continues,  the  pull  on  the  coils  becomes  so 
great  that  the  death  of  the  liane  results.  With  its  decay,  the  coils  of  the  liane 
offer  no  further  resistance  to  the  enlargement  of  the  stem  within;  but  become 
ruptured  and  unravelled.  It  is  clear  from  this  that  it  is  not  always  advantageous 
for  perennial  and  lignifying  twining  stems  to  make  use  of  active  stems  as  sup- 
ports, and  it  is  also  obvious  why  old  and  very  thick  tree-trunks  are  never  seen — 
even  in  tropical  forests — encircled  by  twining  stems.  But  those  growths  whose 
twining  stems  persist  only  through  a  single  summer,  and  either  perish  entirely 
after  the  ripening  of  seed,  like  those  of  the  twining  Polygonum  (Polygonum 
Convolvulus),  or  else  die  down  to  the  ground  like  those  of  the  Hop  (Humulus 
Lupulus),  would  suffer  no  injury  even  if  they  were  to  twine  round  thick  tree 
trunks.  Such  plants  which  have  to  develop  stems  and  leaves  in  the  course  of 
a  short  summer,  and  to  manufacture  by  the  help  of  their  green  foliage  the 
materials  necessary  for  the  formation  of  flowers  and  fruit,  must  spring  up  from 
the  soil  to  the  sunny  heights  as  quickly  as  possible  and  by  the  most  direct  path. 
This  they  can  best  do  by  using  a  thin  thread  as  a  support,  certainly  not  by 
twining  round  a  thick  tree-trunk.  The  path  round  a  thick  trunk  would  be  much 
too  long;  the  material  necessary  for  the  building  up  of  such  lengthy  coils  would 
be  needlessly  expended,  and  such  a  waste  would  be  entirely  opposed  to  the 
economy  of  plant-life.  Of  course  this  does  not  imply  that  twining  plants  have 
the  capacity  of  seeking  out  the  most  suitable  supports,  or  of  selecting  the  most 
desirable  from  amongst  many.  The  capacity  of  selection  is  at  all  times  only 
apparent,  and  if  hop  stems  never  twine  round  props  of  more  than  10  cm.  diameter, 
it  is  not  because  the  hop  shoots  are  able  to  recognize  beforehand  the  unsuitability 
of  large  coils,  but  because  with  such  extensive  spirals  they  lose  the  power  of 
firmly  adhering  to  the  stem.  And  with  this  we  come  to  the  description  of  the 
processes  of  adhesion  and  torsion  of  stems,  so  far  as  they  are  accessible  to  ob- 
servation. 

Like  interweaving  and  lattice-forming  stems,  twining  stems  at  first  grow 
directly  upwards.  The  lowest  internodes  still  remain  e^-ect  whatever  may  be 
the  fate  of  those  developing  above  them.  After  a  sufficient  number  of  successive 


CLIMBING   PLANTS.  683 

internodes  have  been  formed,  the  number  varying  according  to  the  species,  those 
uppermost  bend  over  laterally,  and  the  whole  shoot  now  consists  of  a  lower  erect 
portion  fixed  in  the  soil,  and  an  upper  overhanging  portion  which  ends  freely. 
The  lower  part  forms  a  firm  and  reliable  support,  the  upper  bent  portion,  waving 
in  the  air,  undergoes  movements  the  aim  of  which  is  to  revolve  the  free  end 
round  in  a  circle  or  an  ellipse.  This  movement  of  the  nutating  portion  of  the 
shoot  has  been  compared  to  that  of  the  hand  of  a  clock;  still  better,  it  may  be 
likened  to  the  movement  of  a  pliant  switch  or  whip  which  is  held  in  the  hand 
above  the  head  and  its  end  swung  round  in  a  circle.  The  nutation  of  the 
climber  is  not  so  quick  as  that  of  the  revolving  part  of  the  switch,  but  is  accom- 
plished with  a  rapidity  which  astonishes  the  observer.  In  warm  weather  the 
waving,  revolving  end  of  the  Hop  (Humulus  Lupulus)  makes  a  complete  revolu- 
tion on  an  average  in  2  hours  and  8  minutes;  the  French  Bean  (Phaseolus  vul- 
garis)  in  1  hour  and  57  minutes;  the  Bindweed  (Convolvulus)  in  1  hour  and 
42  minutes;  the  Japanese  Akebia  quinata  in  1  hour  38  minutes;  and  the  Chilian 
climber,  Grammatocarpus  volubilis,  in  1  hour  and  17  minutes.  Since  these 
revolutions  are  performed  by  fairly  long  portions  of  the  shoot,  they  may,  like 
those  of  the  clock-hand,  be  seen  with  the  naked  eye,  especially  when  a  collar 
of  white  paper  is  placed  on  the  shoot  in  the  sunlight  below  the  overarched 
portion.  The  shadow  of  the  moving  part,  like  the  hand  on  the  dial-plate,  is 
then  seen  slowly  but  plainly  advancing  over  the  surface  of  the  paper.  In  other 
twining  plants  the  motion  is  of  course  much  slower,  and  many  of  them  occupy 
24,  or  even  48  hours  in  a  revolution. 

Since  in  most  twining  stems  a  twisting  of  the  extended  fibrous  bundles  on 
the  periphery  of  the  stem  occurs  simultaneously  with  the  circling  of  the  free 
end,  it  was  formerly  supposed  that  this  revolving  movement  was  actually  pro- 
duced by  this  torsion  of  strands  of  fibres  there  situated.  Very  careful  investi- 
gations in  recent  times  have,  however,  demonstrated  that  this  is  not  the  case. 
The  circling  is  produced  independently  of  the  torsion,  and  twining  stems  exist 
in  which  no  torsion  whatever  of  these  fibrous  bundles  takes  place. 

We  shall  obtain  the  most  accurate  conception  of  the  revolving  movement 
of  the  tip  of  a  shoot  if  we  still  retain  our  illustration  of  the  movement  of  a 
switch  swung  round  in  a  circle.  When  the  switch,  which  may  be  best  considered 
as  a  cylindrical  body  whose  periphery  is  striped  longitudinally  with  numerous 
straight  lines  running  parallel  to  the  axis  of  the  cylinder,  begins  its  motion, 
there  is  first  of  all  an  outward  lateral  bending.  The  side  which  becomes  concave 
experiences  a  contraction,  the  convex  side  an  elongation.  Thus  on  the  concave 
side  a  pressure,  and  on  the  convex  a  tension  is  set  up.  At  any  given  moment 
these  opposed  strains  are  greatest  along  two  opposite  lines  running  along 
periphery  of  the  switch;  in  the  next  moment,  however,  the  greatest  strain  passes 
over  to  the  adjacent  opposed  lines,  and  since  the  greatest  strain  on  the  periphery 
of  the  switch  moves  in  this  way,  and  touches  all  the  lines  in  succession,  that 
remarkable  circular  movement  of  the  free  end  of  the  switch  results  winch 


684  CLIMBING   PLANTS. 

entirely  resembles  a  torsion,  but  which,  however,  as  a  matter  of  fact,  is 
connected  only  with  a  successive  bending  to  all  the  points  of  the  compass,  and 
with  no  actual  spiral  twisting  whatever.  This  movement  may  also  be  seen  on  a 
switch  fixed  in  the  ground,  and,  generally,  in  any  pliant  shoot,  by  bending  down 
the  top  in  all  directions  successively,  so  that  the  point  describes  a  circle;  thus 
it  can  be  easily  demonstrated  that  no  spiral  torsion  in  the  tissue  of  the  shoot 
is  caused  by  the  successive  bending  on  all  sides.  This  movement  has  received 
the  name  of  circumnutation. 

We  may  now  proceed  to  inquire  into  the  series  of  changes  within  the  stem 
which  cause  it  thus  to  bend  in  all  directions,  what  must  go  on  in  the  cells  along 
one  line  in  this  stem  to  cause  it  elongate,  along  another  to  make  it  contract, 
and  to  bring  about  this  successive  elongation  and  contraction  in  all  the  peripheral 
longitudinal  rows.  Here  unilateral  pressure  from  outside,  which  so  often  causes 
curvature  in  other  cases,  is  shown  to  be  just  as  little  the  reason  as  unilateral 
illumination,  which  it  is  known  also  produces  a  curving  of  leafy  stems  towards 
the  incident  sunlight.  When  we  see  that  the  young  branches  of  beeches  are 
overhanging  under  their  burden  of  leaves,  we  may  think  of  explaining  the 
matter  by  gravity;  but  how  can  we  thus  explain  the  enigmatical  advance  of 
the  inclination  towards  all  the  points  of  the  compass,  which  is  the  point  at 
issue  here,  and  which  has  to  be  accounted  for?  The  phenomenon  has  also  been 
referred  to  growth,  and  it  has  been  said  that  it  was  caused  by  the  various 
longitudinal  lines  on  the  circumference  of  the  shoot  successively  growing  more 
actively  than  the  sides  opposite  to  them.  But  even  supposing  that  the  whole 
matter  was  only  a  phenomenon  of  growth  (which  is  certainly  not  the  case,  since 
many  shoots  make  these  revolutions  without  showing  the  slightest  increase  in 
length),  the  question  why  it  happens  that  the  stronger  growth  is  transferred  from 
one  longitudinal  row  to  another,  would  still  remain  to  be  answered. 

The  first  step  in  an  attempt  at  explanation  is  to  consider  similar  phenomena 
where  the  conditions  are  much  simpler  and  where  the  investigation  is  hindered 
neither  by  simultaneous  growth  nor  by  simultaneous  torsion.  As  such  phenomena 
we  may  regard  the  rotating  movements  of  protoplasmic  threads  in  swimming 
swarm-spores,  the  circular  movements  of  the  threads  of  Oscillatoriese  composed 
of  disc-shaped  cells  like  rolls  of  coins,  and  the  similar  movements  of  the  whip- 
like  filaments  of  numerous  species  of  Dasyactis  and  Euactis.  Here  we  may 
disregard  the  question  of  the  end  to  be  attained  by  these  movements.  This 
much  is  certain  (1)  that  in  the  one  case  protoplasmic  threads,  and  in  the  other 
simple  rows  of  cells,  exhibit  in  their  revolving  movements  those  advancing,  opposed 
strains  which  we  have  just  noted  in  the  rotating  switch;  (2)  that  the  elongation 
on  the  one  side  and  the  contraction  on  the  other  in  all  these  filamentous  structures 
are  not  produced  by  a  direct  external  stimulus.  This  elongation  and  contraction, 
this  enigmatical  advance  of  inclination  towards  all  the  points  of  the  compass  can 
therefore  be  caused  only  by  internal  forces,  and  we  must  suppose  that  the  living 
protoplasm  of  the  whip-like  thread  spontaneously  elongates  and  contracts,  bends 


CLIMBING   PLANTS.  685 

and  revolves  in  the  manner  described  above.  That  which  is  performed  by  the 
naked  protoplasm  of  a  cilium  may  also  be  accomplished  by  the  association  of 
protoplasmic  masses  in  the  simple  cell-filament  of  an  oscillatoria-thread,  and 
nothing  contradicts  the  supposition  that  also  in  those  extensive  cell-aggregates 
which  compose  the  shoot  of  a  twining  plant,  the  progressing,  opposed  strains, 
which  appear  as  revolving  movements  in  the  shoot,  occur  in  like  manner.  Why 
should  not  one  portion  of  the  masses  of  protoplasm,  associated  together  and 
co-operating  harmoniously  for  the  welfare  of  the  whole  plant,  perform  that  work 
which  is  accomplished  in  minute  unicellular  plant-organisms  by  an  extended 
protoplasmic  thread?  Is  it  not  simplest  to  suppose  that  the  living  protoplasm 
of  certain  rows  of  cells  on  the  circumference  of  the  shoot  should  effect  the 
elongation  and  contraction,  the  advancing  opposed  strains  above  described,  in  a 
word,  the  twining  movement  of  the  whole  shoot-apex?  What  it  is  that  impels 
the  protoplasm  to  this  work  is  just  as  puzzling  as  the  stimulus  to  the  production 
of  partition- walls  in  the  interior  of  a  cell,  or  the  motive  to  those  wonderful  ac- 
cumulative and  divisional  processes  in  the  protoplasm  of  the  Myxomycetes  described 
on  p.  572.  We  know,  indeed,  that  these  processes,  which  depend  on  the  dis- 
placement of  the  ultimate  particles  of  the  protoplasm,  are  possible  only  under 
certain  external  conditions,  but  it  cannot  be  asserted  that  external  conditions 
definitely  shape  and  direct  the  work  done  by  the  protoplasm. 

In  a  number  of  twining  plants,  e.g.  the  Hop,  Honeysuckle,  and  the  twining 
Polygonum  (Humulus  Lupulus,  Lonicera  caprifolium,  Polygonum  convolvulus), 
the  shoots  turn  round  from  the  east  through  the  south  towards  the  west, 
which  is  termed  turning  to  the  right  (dextrorse  or  clockwise).  Others,  again,  as, 
for  example,  the  Scarlet-runner,  the  bindweeds,  and  various  species  of  birthwort 
(Phaseolus  multiflorus,  Convolvulus  sepium,  Aristolochia  sipho),  turn  round 
from  the  east  through  the  north  towards  the  west,  and  this  is  termed  turning 
to  the  left  (sinistrorse  or  counter-clockwise).  External  conditions  have  no 
influence  on  the  maintenance  of  these  directions.  It  is  a  matter  of  indifference 
to  the  direction  of  these  movements  whether  we  allow  light,  warmth,  and 
humidity  to  operate  on  this  side  or  that;  the  particular  species  always  twists 
in  the  same  direction,  the  Hop  towards  the  right,  the  Scarlet-runner  towards  the 
left.  More  than  this,  even  if  the  twining  portion  is  continuously  bound  in  an 
opposite  direction,  the  result  is  all  the  same;  the  plant  cannot  be  coerced  into 
any  other  path,  and  will  not  depart  from  the  direction  peculiar  to  it.  It  continues 
to  twist  and  twine  according  to  an  innate  tendency  inherited  from  generation  to 
generation,  and  we  can  only  refer  the  different  directions  of  twisting  to  internal 
causes,  to  the  peculiar  constitution  of  the  living  protoplasm  in  each  particular 

plant. 

However  puzzling  the  ultimate  causes  of  this  torsion  may  be,  the  end 
attained  by  these  revolutions  of  growing  shoots  is  patent  enough.     That  it  may 
twine  upwards  a  shoot  requires  an  erect  support,  with  which  it  must  come  into 
contact  almost  at  a  right  angle.     If  such  a  support  exists  in  the  immediate  neigh- 


(586  CLIMBING   PLANTS. 

bourhood,  this  contact  occurs  at  the  very  beginning  of  circumnutation,  but  when 
there  are  no  erect  stems  close  by,  the  shoot  in  its  search  bends  its  apex  to  all  the 
points  of  the  compass,  and  describes  wider  and  wider  circles  with  its  increasing 
length.  If  in  the  space  so  traversed  it  finds  no  suitable  support,  the  lower 
portion  of  the  shoot  falls  on  the  ground  and  becomes  a  procumbent  stem;  but  the 
middle  portion  again  rises  up,  and  the  free  end  twists  round  in  a  circle  afresh.  The 
place  where  the  nutation  now  occurs  is  removed  some  distance  away  from  the  spot 
where  it  first  began,  and  perhaps  the  revolving  shoot  in  its  new  position  may  strike 
against  something  which  may  serve  it  as  a  support.  But  if  here  also  no  suitable 
support  is  encountered,  a  further  migration  may  occur;  thus  a  comparatively  exten- 
sive area  is  explored  by  the  circling  shoot  in  its  quest  for  something  to  climb 
around.  The  phenomena  just  detailed  gave  rise  to  the  view  formerly  held,  that 
twining  plants  possessed  the  power  of  searching  for  a  support,  indeed,  the  idea  was 
favoured  that  the  twining  stem  was  positively  attracted  by  such  support.  But  such 
a  notion  is  disposed  of  by  the  actual  facts.  The  meeting  of  the  nutating  shoot 
with  an  erect  stem  must  be  held  to  be  quite  accidental,  still  it  is  certain  that  this 
meeting  is  facilitated  by  the  movements  described  above,  and  the  probability  of  an 
erect  stem  being  encountered  is  obviously  greater  the  more  extensive  the  space 
traversed  by  the  shoot-apex. 

As  soon  as  the  revolving  end  of  the  shoot  comes  into  contact  with  an  erect 
support  of  suitable  thickness,  it  embraces  the  support,  and  adhering  to  it,  twists 
round  it  spirally  and  assumes  the  form  of  an  elongated  spiral  wound  around  it. 
This  process  may  be  illustrated  by  comparing  it  with  the  movement  of  a  rope 
swung  in  a  circle  coming  in  contact  with  a  post,  that  is  to  say,  when  one  swings  a 
long  rope  or  a  long  whip  horizontally  with  the  hands  raised  above  the  head,  and  at 
the  same  time  approaches  so  near  to  an  erect  post  that  the  revolving  rope  must 
reach  it,  then  that  portion  of  the  rope  beyond  the  point  of  contact  twines  spirally 
round  the  post. 

It  has  been  shown  by  manifold  observations  and  experiments  that  erect  props 
are  most  easily  embraced  by  twining  stems.  When  the  inclination  of  the  prop 
amounts  to  not  less  than  45°  with  the  horizon,  the  twining  shoot  still  forms  a  spiral 
round  it;  but  horizontal  sticks  are  very  seldom,  though  occasionally,  entwined.  It 
has  also  been  ascertained  that  the  revolutions  made  by  the  twining  stem  become 
both  higher  and  steeper  with  increasing  age.  The  coils  formed  by  the  youngest 
and  uppermost  portions  of  the  shoot  are  often  very  close  together  and  almost 
horizontal,  but  lower  down  the  spiral  appears  more  drawn  out,  and  the  newly- 
formed  upper  flat  coils  are  gradually  pushed  passively  upwards.  Thus  the  advan- 
tage is  obtained  that  the  lower  portion,  as  it  assumes  a  steeper  position,  gets  a  better 
grip  of  the  support.  In  most  cases  of  twining  the  stem  is  to  some  extent  twisted 
on  itself,  i.e.  undergoes  torsion.  This  torsion  of  the  axis  must  not  be  confused  with 
its  twisting  around  the  support.  The  two  things  are  distinct.  We  can  take  two 
ropes,  in  one  of  which  the  strands  are  twisted,  whilst  in  the  other  they  are  straight. 
Each  of  these  may  be  wound  round  a  support.  The  former  (i.e.  the  twisted  rope) 


CLIMBING   PLANTS.  687 

will  have  the  best  grip,  as  it  is  stiffer  and  its  obliquely-running  strands  admit  of  a 
better  hold.  So  it  is  with  the  climbing  stem.  By  the  torsion  of  its  own  axis  it  gets 
a  better  hold.  The  longitudinal  ridges  on  its  surface — due  to  its  bundles — corre- 
spond to  the  strands  of  the  rope.  When  these,  by  torsion,  run  obliquely,  more 
purchase  on  the  support  is  obtained. 

Not  infrequently  the  attachment  of  the  twining  stem  is  also  strengthened  by 
stiff,  backwardly-directed  bristles,  and  by  barbs  which  are  developed  on  the  ridges, 
as  is  the  case,  for  example,  in  the  twining  Polygonum,  and  in  bean-plants.  These 
reversed  prickles  are  comparatively  large  in  Ipomoea  muricata,  a  species  of  bind- 
weed. Hops  also  possess  prickles  of  a  remarkable  form.  In  the  Hop,  as  may  be 
seen  in  fig.  160,  they  have  the  shape  of  an  anvil;  that  is  to  say,  a  cell  which  is  much 
extended  in  the  longitudinal  direction,  and  tapers  to  a  point  at  either  end,  is 
developed  on  a  peg-shaped  or  conical  base.  Its  wall  is  silicified  and  very  hard,  and 
the  points  hook  into  softer  tissue  like  claws.  Such  climbing  hooks  are  found  in 
regular  rows  on  the  six  ridges  of  the  twining  hop  stem,  and  are  a  great  assistance 
in  attaching  it  to  the  entwined  support. 

In  Hoya  carnosa,  known  for  its  waxen  flowers,  and  often  cultivated  in  green- 
houses, the  young  twining  stems  are  thickly  beset  with  reversed  hairs  which  under 
certain  circumstances  contribute  materially  to  the  adhesion  to  rugged  substrata. 
Moreover,  the  stems  of  this  plant,  as  soon  as  they  have  ceased  to  nutate,  develop 
light-avoiding,  climbing  roots  which  nestle  to  the  substratum  and  unite  with  it, 
thus  adding  to  the  security  of  the  stem.  The  stems  of  Hoya,  like  those  of  Ca&aytha 
and  Cuscuta,  described  on  p.  171,  are  thus,  in  a  way,  intermediate  between  those 
of  twining  plants,  in  the  strict  sense,  and  climbing  plants  provided  with  clinging 
roots,  which  latter  will  be  discussed  presently. 

When  the  nutating  end  of  a  twining  stem  has  found  no  erect  support  in  its 
neighbourhood,  the  older  portions  of  its  stem  which  no  longer  revolve  take  on,  even 
without  a  support,  a  spiral  twisting  and  a  torsion  of  the  axis.  Just  as  a  rope 
becomes  more  rigid  when  twisted,  so  the  stiffness  of  these  twisted  stems,  though 
they  have  no  support,  is  increased  in  comparison  with  untwisted  stems.  Such  a 
twisted  stem  may  even  rise  a  little  above  the  ground,  and  in  many  instances  the 
still  nutating  free  end  is  enabled  to  reach  some  bough  of  a  neighbouring  tree  or 
bush,  and  winding  round  it,  to  attain  to  the  tree-crown.  Many  twining  plants,  as, 
for  example,  hops,  frequently  send  up  above  the  ground  from  their  subterranean 
perennial  portions  several  shoots.  If  these  find  no  support  in  the  ordinary  way, 
they  wind  round  one  another,  and  a  regular  coil  or  cable  is  produced  (cf.  p.  364). 
These  cables  often  rise  without  any  foreign  support  to  a  considerable  height  above 
the  ground,  and  thus  single  nutating  apices  are  afforded  the  possibility  of  grasping 
a  support  which  otherwise  might  have  been  denied  them. 

Should  all  these  methods  prove  of  no  avail  the  twisted  stem  takes  up  its 
position  on  the  ground;  its  growth  is  retarded,  and  it  has  the  appearance  of  a 
stunted,  sickly  plant.  This  fact  is  in  so  far  interesting  because  it  seems  to  indicate 
that  the  pressure  experienced  by  a  twining  stem  adhering  to  a  supporting  prop  has 


688 


CLIMBING   PLANTS. 


a  favourable  influence  on  the  growth  of  the  shoot  as  a  whole.  This  pressure  must 
be  regarded  as  a  stimulus,  just  like  the  pressure  which  incites  the  tendrils,  to  be 
described  below,  to  luxuriant  growth.  We  may  therefore  conclude  that  twining 


Fig.  160.— Twining  Hop  (Humulus  Lupulus). 

i  Free  end  of  a  shoot  recently  emerged  above  the  ground.  2  Shoot  of  Hop  twining  round  an  elder-stem ;  natural  size.  8  A 
portion  of  the  Hop  stem  magnified.  *,  *  Single,  anvil-shaped  climbing-hooks  detached  from  the  stem ;  more  highly 
magnified. 

stems  are  irritable,  although  the  irritability  in  this  case  is  not  so  conspicuous  as  in 
tendril-forming  structures. 

In  the  temperate  zones  the  majority  of  twining  stems  have  only  a  short  life. 
The  twining  Polygonum  is  an  annual;  hops  and  bindweeds  are  indeed  perennial, 
but  their  stems  sent  up  fresh  each  year  from  the  underground  stock  always  perish 


CLIMBING   PLANTS. 


689 


in  the  following  autumn.  Only  the  Bitter-sweet  (Solanum  dulcamara)  and  several 
species  of  honeysuckle  (e.g.  Lonicera  caprifolium  and  Periclymenum),  which  exist 
in  comparatively  inclement  regions,  possess  twining  stems  which  increase  in  thick- 
ness from  year  to  year.  But  in  many  of  these  species  the  twining  is  not  very 
conspicuous,  and  the  Bitter-sweet  forms,  so  to  speak,  a  link  between  plants  with 
twining  and  those  with  interweaving  stems.  In  tropical  regions,  on  the  other 
hand,  long-lived  twining  stems  are  by  no  means  rare.  Obviously  the  coils  of  a  stem, 
firmly  attached  round  a  thin  support  and  increasing  in  thickness,  must  approach  one 
another  very  closely;  thus  arise  those  strange  lianes  which  excite  the  astonishment 
of  all  visitors  to  tropical  forests.  Stems  are  quite  common  of  a  diameter  of  4  cm., 


Fig.  161.— Portion  of  a  Liane  stem,  twisted  like  a  corkscrew,  from  a  tropical  forest;  natural  size. 

wound  like  a  corkscrew  round  the  thin  stems  of  other  lianes,  and  sometimes  such 
structures — of  which  a  small  portion  is  represented  in  natural  size  in  fig.  161 — are 
seen  stretching  right  up  to  the  summits  of  the  trees  in  hundreds  of  uniform  twists, 
like  a  thick  ship's  cable  many  metres  long. 

The  tendril-bearing  stem  (stirps  cvrrhosa)  climbs  up  into  the  sunlight  by  the 
help  of  special  organs  known  as  tendrils.  The  tendrils  are  filamentous  structures 
when  young;  sometimes  of  exceeding  delicacy,  sometimes  thick  and  stiff.  In  some 
cases  they  are  simple,  in  others  forked,  but  always  sensitive,  and  so  constructed 
that  they  can  grasp  any  body  with  which  they  come  in  contact,  hold  it  fast,  and 
use  it  as  a  support.  Before  the  tendril  adheres  to  a  support  it  is  straight,  and 
extends  in  the  direction  in  which  there  is  the  greatest  probability  of  reaching  a 
support.  It  also  performs  movements  the  aim  of  which  is  to  strike  against  some 
firm  object.  If  this  end  is  attained,  the  support  which  it  has  encountered  is 
firmly  gripped  by  the  tip  of  the  tendril,  whilst  the  part  lying  immediately  behind 
the  point  of  attachment  contracts  together  spirally.  By  this  spiral  contraction  the 

VOL.  I. 


690 


CLIMBING   PLANTS. 


stem  from  which  the  tendril  arose  is  drawn  towards  the  support,  and  is,  as  it  were, 
attached  to  it  by  a  spiral  spring. 

Tendrils  are  always  produced  in  numbers  from  the  stem.  Usually  one,  some- 
times two  tendrils  arise  from  each  of  the  upper  nodes,  and  with  the  exception  of 
the  lowest  portion,  which  is  usually  quite  without  them,  the  stem  is  very  regularly 


Fig.  162. — Stipular  tendrils  of  the  common  Smilax  (Smilax  aspera). 

beset  with  tendrils  along  its  whole  length.  The  advantage  of  this  is  that  in  case 
one  tendril  should  fail  or  find  no  support,  a  neighbouring  one  can  always  take  its 
place.  Generally  plants  with  tendril-bearing  stems  are  at  a  decided  advantage  in 
comparison  with  all  other  forms  of  climbing  growths,  which  explains  the  fact  that 
their  number  is  in  considerable  excess  of  the  others.  In  climbing  over  a  shattered 
rock -face  or  thick  tree -trunk  they  have  a  great  advantage  over  plants  with 
twining  stems.  In  some  cases  the  tips  of  the  tendrils  fasten  on  even  to  the  smoothest 
rocks  by  peculiar  discs,  or  they  grip  and  hold  fast  to  small  projecting  portions 


CLIMBING  PLANTS.  -„, 

of  bark  and  the  stumps  of  broken  twigs,  things  which  are  impossible  to  twining 
stems.  Tendrils  preferably  twine  round  horizontal  twigs  and  leaf -stalks  and 
frequently  round  old  tendril -bearing  stems  which  have  previously  climbed  up  to 
the  crown  of  a  tree.  When  they  have  reached  up  to  the  branches,  they  can  pass 
over  from  one  bough  to  another,  fasten  themselves  firmly  above  and  below,  and  so 
gradually  mvest  the  whole  of  the  crown.  From  the  summit  fresh  shoots  arise 


Fig.  163.— Leaf-stalk  tendrils  of  Air agent  alpina. 

which  curve  downwards  and  are  swayed  by  the  lightest  breath  of  wind;  from 
them  new  tendrils  project,  like  the  tentacles  of  some  sea -monster,  and  if  one  of 
them  but  touches  a  leaf -stalk  or  twig  of  a  neighbouring  tree,  it  curves  round  it 
and  grasps  it  firmly.  Very  soon  a  second,  third,  and  fourth  tendril  will  similarly 
become  attached,  and,  contracting  spirally,  will  pull  the  pendent  shoot  up  to  the 
neighbouring  tree-crown.  The  bridge  so  formed  is  again  used  as  a  means  of  transit 
by  other  climbing  stems,  and  thus  arise  garlands  and  festoons,  which  hang  from 
tree  to  tree;  whilst  not  infrequently  an  actual  arcade  is  formed  whose  roof, 


(592  CLIMBING   PLANTS. 

formed  of  tendril -bearing  stems,  is  borne  by  two  adjacent  trees  or  thickets  as 
though  by  two  gigantic  piers.  Another  advantage  which  tendril-bearing  stems 
have  over  twiners  consists  in  the  fact  that  they  can  reach  the  same  height  above 
the  ground  with  less  expenditure  of  material.  The  twining  stem  of  the  Scarlet- 
runner,  which  has  climbed  a  metre  above  the  ground,  shows,  when  unrolled,  a 
length  of  1J  metres.  The  pea,  which  climbs  with  tendrils  to  the  same  height,  is 
little  more  than  a  metre  long.  Of  course  in  the  production  of  tendrils  building 
material  is  expended,  but  this  bears  but  a  small  proportion  to  that  which  is  required 
for  the  extra  half-metre  of  stem. 

Now  as  to  the  nature  of  tendrils,  are  they  leaf,  stem,  or  root?  They  may  be 
each  of  these  according  to  the  species  in  question.  A  tendril  may  even  be  formed 
by  metamorphosis  from  each  of  the  different  sections  of  a  leaf  independently,  and 
the  leaf -blade,  the  mid-rib,  the  leaf -stalk,  even  the  stipules  themselves  may  become 
tendrils.  From  the  standpoint  of  development  and  with  regard  to  the  origin  and 
mutual  relation  of  individual  plant  -  members,  the  exceedingly  manifold  tendril- 
structures  have  been  classed  generally  in  the  following  groups.  First  of  all  the 
stipule-tendrils  (cirrhus  stipularis),  of  which  species  of  smilax  (Smilax)  afford  an 
excellent  example.  As  may  be  seen  in  Smilax  aspera  (see  fig.  162),  so  common  in 
the  region  of  the  Mediterranean  flora,  the  leaves  are  divided  into  lamina,  leaf -stalk, 
sheath,  and  stipules,  and  the  two  stipules  arising  from  the  sheath  are  transformed 
into  rather  long  tendrils  which  surround  the  branches  of  other  plants,  and  even 
their  own  branches. 

More  common  than  this  rather  rare  form  is  iheleaf-stalk  tendril (cirrhus petiola/ris), 
which  itself  again  shows  numerous  modifications  according  as  to  whether  the  whole 
leaf -stalk  of  an  undivided  leaf,  or  the  stalks  of  single  leaf -segments  play  the  part  of 
tendrils.  The  former  is  seen  very  beautifully  in  the  numerous  species  of  Nasturtium 
(Tropceolum)  and  in  the  tendril-bearing  snap-dragon  (Antirrhinum  cirrhosum); 
the  latter  in  many  species  of  fumitory  (Fumaria),  in  the  Traveller's  Joy  (Clematis), 
and  in  the  only  liane  of  the  European  Alps,  the  Atragene  alpina,  illustrated  on 
the  last  page  (fig.  163).  In  pitcher -plants  (Nepenthes)  a  portion  of  the  leaf- 
rachis  is  transformed  into  a  tendril,  and  by  it  the  pitchers  are  suspended  on  the 
branches  of  supporting  plants  (cf.  fig.  24,  p.  133).  When  the  midrib  of  a  foliage- 
leaf  projects  far  beyond  the  green  tissue  of  the  blade,  as  a  filament  which  grasps 
and  surrounds  firm  supports  and  attaches  the  whole  plant  to  them,  this  structure 
is  known  as  a  midrib-tendril  (cirrhus  costalis).  To  this  class  belong  the  strange 
South  American  mutisias  (e.g.  Mutisia  ilicifolia,  hastata,  subspinosa,  decurrens), 
the  Indian  Flagellaria  Indica  and  Gloriosa  superba,  and  several  fritillaries 
(Fritillaria  cirrhosa,  verticillata,  and  Ruthenica),  attaching  themselves  to  stiff 
culms  and  .leaves  of  neighbouring  grasses.  The  leaf-tendril  (cirrhus  foliaris)  is 
also  interpreted  as  the  midrib  of  a  leaf -blade  or  of  a  leaf -segment,  but  here  none  of 
the  green  tissue  of  the  blade  is  developed,  and  only  the  midribs  are  seen  to  form 
filaments  which  curve  and  fasten  as  soon  as  they  come  into  contact  with  a  prop. 
This  form  of  tendril  is  the  commonest  of  all,  and  is  found  particularly  in  Papilion- 


CLIMBING   PLANTS. 


693 


«»»  m  great  variety.  Sometimes  the  whole  leaf-blade  is  metamorphosed  into  a 
sing  e  tendril,  as  in  the  Yellow  Vetchling  (Lathyru*  Aphaca);  but  usually  tendrils 
are  formed  only  in  the  place  of  the  terminal  leaflet  and  of  the  upper  leaflets  of  the 
pinnate  leaves,  as  may  be  seen  especially  in  vetches,  peas,  and  lentils  ( Vicia,  Pisum 
It  should  be  mentioned  here  that  in  proportion  as  the  green  tissue  of  the 
leai-blade  is  reduced  in  consequence  of  the  formation  of  tendrils,  the  amount  of 


Fig.  164.— Branch-tendrils  of  Serjania  gramatophora. 

green  tissue  of  the  lowest  leaflets,  leaf -stalks,  and  stipules  increases;  in  other  words, 
that  when  tendrils  appear  in  place  of  the  upper  leaflets,  the  lowest  pair  of  leaflets 
and  the  stipules  form  large  green  laminae.  In  many  vetches  even  the  stem  and 
leaf -stalks  are  beset  with  green  leaf -like  bands  and  wings. 

By  a  stem-tendril  (cirrhus  capreolus)  is  meant  one  which  can  be  interpreted  as 
a  stem-structure,  and  a  distinction  is  drawn  particularly  between  branch-tendrils 
{cirrhus  rameaneus)  and  flower-stalk  tendrils  (cirrhus  peduncularis)  according  as 
to  whether  the  tendril  is  to  be  regarded  as  a  metamorphosed  flower-bearing  or 


(594  CLIMBING   PLANTS. 

foliage-shoot.  Flower-stalk  tendrils  are  found  in  particular  in  grape-vines  and  in 
species  of  Cissus,  in  Passiflora  cirrhiflora,  in  several  species  of  the  genera  Paullinia 
and  Cardiospermum',  branch  -  tendrils  in  Fumaria  claviculata,  and  in  numerous 
gourd-like  plants.  These  tendrils,  of  which  the  Serjania  gramatophora  (cf.  fig. 
164)  may  be  taken  as  an  example,  arise  usually  not  from  the  true  axil  of  a 
foliage-leaf,  but  are  displaced,  pushed  to  the  side  of  or  below  the  subtending  leaf; 
frequently  even  opposed  to  leaves  which  one  might  think  really  subtended  them. 
This  displacement  is  particularly  striking  in  vines  and  gourd-like  plants,  for  which 
reason  these  tendrils  were  formerly  explained  not  as  stems  but  as  leaf -tendrils. 
Finally  we  must  consider  here  the  root-tendrils  (cirrhus  radicalis),  which  really 
are  roots  arising  from  the  foliage-stem,  but  in  regard  to  their  activity  behave 
exactly  like  tendrils,  and  are  chiefly  observed  in  climbing,  delicate-stemmed  lyco- 
podiums. 

This  classification  of  the  manifold  tendril-developments,  useful  for  the  speculative 
doctrine  of  form,  and  also  to  the  descriptive  botanist,  has  only  a  secondary  value  for 
the  questions  which  are  discussed  in  this  book.  It  gives  no  conclusion  concerning 
the  significance  which  the  different  forms  have  with  regard  to  the  habitats  of  climb- 
ing plants,  and  it  offers  not  the  slightest  assistance  to  our  understanding  how  the 
stem  is  fastened  to  the  support  by  the  tendril  arising  from  it.  Tendril-bearing  stems 
are  extremely  wonderful  in  this  respect,  and  the  different  methods  require  a  detailed 
description.  For  the  purpose  of  this  description  we  classify  tendril-bearing  stems 
into  three  groups,  viz.  into  those  with  ringed  tendrils,  with  nutating  tendrils,  and 
with  light-avoiding  tendrils. 

Stems  with  ringed  tendrils  are  especially  adapted  for  climbing  up  between  the 
erect  and  much-branched  growth  of  dense  hedges,  copses,  and  low  woods.  Some  of 
them  are  annual  and  do  not  rise  far  above  the  low  underwood  and  shrubs,  e.g.  various 
species  of  fumitory  and  nasturtium  (Fumaria  and  Tropceolum).  Others,  e.g.  the 
Traveller's  Joy  and  Atragene  (Clematis  and  Atragene)  are  perennial;  their  stems 
become  woody,  often  attaining  to  a  considerable  age,  and  the  youngest  branches  of 
the  old  stems  may  climb  up  to  the  tops  of  trees.  When  one  sees  these  plants  hanging 
rope-like  from  the  summits  of  tall,  unbranched  forest-trees,  one  may  conclude 
that  they  first  became  fastened  to  them  at  a  remote  period,  when  the  trees  were  still 
quite  small,  and  that  they  have  ever  since  kept  pace  with  them  in  their  growth. 
The  young  shoots  of  such  climbers  with  their  leaves  still  small,  erect,  and  folded  to 
the  stem,  appear  capable  of  pushing  through  even  very  small  gaps  in  the  thickest 
undergrowth,  thus  reminding  us  strongly  of  the  manner  of  growth  of  interweaving 
stems.  They  also  agree  with  interweaving  stems  inasmuch  as  they  form  actual 
anchor-arms  by  extending  and  reflecting  their  leaves  and  leaf -stalks  by  whose  help 
they  suspend  themselves  on  the  horizontal  branches  of  the  supporting  undergrowth. 
This  is  the  case  in  Clematis  and  in  the  Atragene  illustrated  in  fig.  163, —  these 
plants  having  opposite  leaves  whose  stalks  project  from  the  stem  almost  at  a  right 
angle.  The  stalks  and  blades  of  the  leaflets,  also,  complete  the  semblance  to  the 
arms  of  an  anchor,  since  the  former  sink  down  at  an  obtuse  angle  with  the  main 


CLIMBING   PLANTS.  695 

stalk,  whilst  the  latter,  after  the  laminae  are  unfolded,  curve  like  an  arch,  forming 
an  actual  loop. 

As  already  stated,  no  distinction  can  be  recognized  in  the  earlier  stages  between 
interweaving  stems  and  those  with  ringed  tendrils.  The  difference  first  appears  as 
soon  as  the  lower  side  of  the  leaf-stalks  comes  in  contact  with  a  branch  of  the 
undergrowth.  This  contact,  if  it  is  not  of  too  transient  duration,  acts  as  a  stimulus 
on  the  leaf -stalk,  and  the  result  is  that  it  curves  round  the  branch  and  grips  it  like  a 
ring.  The  stalks  always  curve  towards  the  side  which  has  been  touched,  or  pressed. 
Since  the  leaf-stalks  are  equally  sensitive  on  all  sides,  the  curvature  may  take  place 
above  or  below,  or  laterally,  according  to  whichever  part  has  been  stimulated.  Even 
continuous  contact  with  flower-stalks  of  hair-like  delicacy  is  sufficient  to  produce 
the  ring- formation,  and  it  has  been  shown  by  experiment  that  the  continued  pressure 
of  a  thread  weighted  up  to  four  milligrams  is  followed  by  a  curvature.  The  stimu- 
lated leaf -stalk  usually  forms  one  or  two,  less  often  several  annular  coils  on  the 
branch  embraced,  as  shown  in  fig.  163.  It  also  frequently  happens  that  neighbour- 
ing stems  of  the  same  plant  are  connected  together  by  their  tendrils  and  twined 
into  inextricable  knots.  The  conversion  of  the  irritable  leaf -stalk  into  an  effective, 
gripping  tendril  in  many  of  the  plants  in  this  group  is  materially  assisted  by  the 
fact  that  the  younger  portions  of  the  shoot  revolve  in  circles  like  those  of  twining 
stems,  though  less  regularly.  Thus  a  much-increased  number  of  suitable  objects  in 
the  environment  become  possible  as  supports.  These  leaf -stalks,  which  become 
tendrils,  do  not,  however,  themselves  nutate,  consequently  they  are  clearly  dis- 
tinguished from  those  of  the  following  group,  which  are  called  nutating  tendrils. 

Stems  with  nutating  tendrils  have  not  the  power  of  climbing  up  rocky  walls 
or  the  bark  of  thick  tree-trunks,  and,  like  the  foregoing,  are  only  able  to  use  as 
supports  culms,  leaves,  and  thin  branches  of  other  erect  plants,  to  which  they  adhere 
and  up  which  they  are  drawn  by  means  of  the  spiral  curvature  of  the  attached 
tendrils.  Plants  equipped  with  this  class  of  tendril  require  far  more  light  than 
those  with  ringed  tendrils,  and  they  find  their  best  and  most  favourable  habitat  in 
the  open  country  dotted  with  isolated  groups  of  trees,  or  on  the  edges  of  a  forest 
bordered  with  bushes,  and  in  sunny  meadows  studded  with  shrubs.  They  have  not 
to  interweave  through  the  interlacing  branches  of  an  underwood;  ringed  tendrils 
are  suitable  there,  but  not  tendrils  with  long  nutating  filaments  which  could  either 
not  accomplish  their  movements  in  the  midst  of  the  thick  brushwood,  or  if  they  did, 
would  not  attain  the  desired  end,  viz.  the  subsequent  elevation  of  the  stem. 

The  lowest  portions  of  the  young  shoot  possess  no  tendrils,  and  they  are  kept 
erect  solely  by  the  turgescence  of  their  tissues.  In  many  species  the  stiff,  spreading 
leaf -stalks,  or  the  peculiar  barbed  leaf-blades  help  to  support  the  young  shoots  on 
the  neighbouring  plants  and  to  keep  them  erect.  But  these  supports  are  but  tem- 
porary measures,  and  the  upper  portion  of  the  shoot  soon  develops  tendrils.  These 
elongate  quickly  and  get  to  work.  The  filaments  of  these  tendrils  elongate  with 
extraordinary  rapidity,  straighten  out,  and  then  project  like  tentacles  far  beyond 
the  foliage-leaves.  At  their  tips  only  do  they  exhibit  a  more  or  less  hook-like 


(596  CLIMBING   PLANTS. 

curvature  (cf.  fig.  165).  When  they  have  reached  their  full  length  they  begin  to 
move  round  in  a  circle  just  like  the  apices  of  twining  stems.  If  by  this  movement 
they  meet  with  an  object  suitable  for  a  support,  they  grasp  and  embrace  it  by  their 
hooked  ends.  That  is  to  say,  contact  with  a  foreign  body  acts  like  a  stimulus  on 
the  tendril;  it  loops  itself  over  the  object  with  which  it  is  in  contact,  and  then  rolls 
up  in  a  spiral,  thus  drawing  the  stem,  which  bears  it,  obliquely  upwards.  Now 


Fig.  165.— Tendrils  of  the  Bryony  (Bryonia). 

comes  the  turn  of  the  tendril  inserted  next  above.  This  behaves  exactly  in  the 
same  way  as  the  first,  and  in  a  very  short  time  is  succeeded  by  a  third,  fourth,  &c. 
It  does  not  much  matter  if  in  its  nutation  one  of  these  tendrils  should  have  found 
no  support,  since  the  successive  tendrils  are  placed  so  close  to  one  another,  and 
replace  each  other  so  quickly,  that  the  shoot  is  still  drawn  up  uniformly,  and  is 
prevented  from  falling.  When  whole  series  of  tendrils  find  no  places  of  attachment, 
the  shoot  of  course  falls  down,  under  which  circumstances  possibly  one  of  its  tendrils 
may  encounter  a  distant  branch  to  which  it  can  fasten,  and  which  it  can  use  as  a 
support.  If  this  should  fail,  the  tip  of  the  pendent  shoot  again  rises  up,  sends  out 


CLIMBING    PLANTS.  697 

fresh,  nutating  tendrils,  and  so  may  still  succeed  in  grasping  some  projecting  twig 
in  the  neighbourhood  upon  which  it  can  climb.  The  paths  traversed  by  such 
tendril-bearing  stems  are  therefore  often  wound  oddly  hither  and  thither,  but  the 
stem  always  follows  the  periphery  of  the  bush  or  tree-crown  which  it  has  selected, 
and  the  inner  branches  of  these  supports  are  never  interwoven  by  it.  Plants 
whose  tendril-bearing  stems  ramify  strongly  may  invest  the  whole  tree  over  which 
they  grow  with  an  actual  mantle,  and  if  the  climber  in  question  has  large  leaves, 
it  may  be  quite  impossible  to  determine  from  outside  what  species  of  plant  has 
become  thus  enveloped. 

The  account  given  above  deals  only  with  such  phenomena  as  are  displayed  by 
all  tendril-climbers  in  common;  but  in  individual  cases  there  are  innumerable  special 
contrivances,  which  it  would  be  impossible  to  describe  in  detail  in  the  limits  of  this 
book,  and  I  must  therefore  be  content  with  enumerating  some  of  the  most  striking 
that  have  been  observed. 

First,  it  has  been  pointed  out  that  in  many  cases,  for  example,  in  the  tropical 
passion-flowers,  not  only  the  young,  extended  tendrils,  but  also  the  whole  shoot-apex 
revolve  in  circles  thus  widening  the  space  traversed  by  the  tendrils,  and  increasing 
the  probability  of  meeting  with  a  support.  If  the  tendrils  are  forked,  each  of  the 
two  branches  performs  its  particular  oscillations,  as  can  be  seen,  for  instance,  in  the 
tendrils  of  the  grape-vine.  The  period  of  revolution,  taken  by  a  nutating  tendril, 
varies  very  much  according  to  the  species.  Cobcea  scandens  takes  only  25  minutes, 
Passiflora  sicyoides  30-46  minutes,  and  Vitis  vinifera  67  minutes  for  a  revolution. 
The  rapidity  with  which  the  tendrils  curve  in  consequence  of  the  pressure  exercised 
on  them  by  a  foreign  body  which  acts  as  a  stimulus  also  varies  very  much  with  the 
species.  In  Cyclanthera  pedata  the  curvature  commences  20  seconds  after  contact 
with  a  hard  stick;  in  passion-flowers  (e.g.  Passiflora  gracilis  and  P.  sicyoides)  after 
the  lapse  of  about  half  a  minute,  in  Cissus  discolor  after  4-5  minutes.  If  the  stick 
with  which  the  tendril  is  in  contact  is  removed,  the  curved  portion  straightens  out 
again.  If  it  is  left  in  contact,  the  curvature  proceeds  uniformly.  In  Cyclanthera 
pedata  the  first  complete  coil  around  the  support  is  accomplished  in  4  minutes,  in 
others,  on  the  other  hand,  it  may  take  several  hours,  or  even  1-2  days.  Usually 
the  tendril  is  not  content  with  a  single  coil,  but  forms  several  of  them.  The  coils 
are  closely  pressed  to  the  prop,  and  in  their  growth  adapt  themselves  like  a  plastic 
mass  to  all  its  projections  and  depressions;  the  substance  of  the  tendril  even  pene- 
trates into  the  small  clefts  and  crevices,  and  when  the  tendril  is  detached  from  its 
substratum,  an  actual  cast  of  all  the  inequalities  of  the  support  can  be  seen  on  its 
contact-surface.  In  many  species,  e.g.  in  Hanbwrya  mexicana,  peculiar  callus-like 
growths  arise  here.  The  ends  of  the  tendrils,  as  already  stated,  are  curved  like  a 
hook  so  as  to  more  easily  grasp  the  object  to  which  their  circling  movement  brings 
them.  In  many  species  the  tendrils  terminate  in  actual  claws.  The  tendrils  of 
Cobcea  scandens,  a  native  of  Mexico,  but  frequently  grown  as  a  decorative  plant  in 
our  conservatories,  are  specially  elegant.  They  are  leaf-  or  midrib-tendrils,  and 
divide  repeatedly  in  the  most  beautiful  manner.  Each  of  the  ultimate  branches 


CLIMBING   PLANTS. 

bears  a  double  claw  whose  points  immediately  fasten  into  any  object  at  the  slightest 
touch,  and  will  even  remain  suspended  in  the  skin  of  the  hand.  The  three  delicate 
branches  of  the  tendril  of  Bignonia  venusta  also  end  in  pointed  claws  which 
resemble  those  of  insects'  feet.  The  majority  of  tendrils  are  branched,  whilst  simple 
undivided  filaments,  as  shown  in  the  Bryonia  (fig.  165),  are  comparatively  rare. 
Passion-flowers  and  gourd-plants  have  the  longest  tendrils,  those  of  the  common 
gourd  (Cucurbita  pepo)  often  measuring  more  than  30  cm.  in  length.  The  spiral 
contraction  of  the  part  of  the  tendril  not  wound  round  the  support  begins,  according 
to  the  species,  half  a  day,  or  one  or  two  days  after  the  apex  has  formed  the  first 
coil  round  the  support,  but  it  is  very  quickly  accomplished  when  it  has  once  begun. 
This  torsion  is  sometimes  towards  the  right,  sometimes  towards  the  left,  and  fre- 
quently it  is  accomplished  partly  in  one  direction  and  partly  in  the  other,  by  the 
same  tendril.  The  number  of  twists  formed  in  this  spiral  contraction  is  very  vari- 
able. In  the  long  tendrils  of  some  gourds  as  many  as  30  or  40  are  produced.  By 
these  spiral  springs  the  fastening  of  the  stem  to  the  support  is  excellently  accom- 
plished; it  is  at  once  adequately  attached  to  the  support,  but  not  pressed  to  it,  conse- 
quently unnecessary  friction  is  avoided.  During  a  blast  of  wind  there  is  a  certain 
amount  of  "give",  but  directly  the  gust  subsides,  the  climber — thanks  to  its  elastic 
tendrils — resumes  its  former  position.  This  spiral  twisting  occurs  also  in  tendrils 
which  have  not  succeeded  in  grasping  a  support,  but  strangely  enough,  they  become 
stunted,  shrivel,  and  wither,  sometimes  becoming  detached  from  the  stem  like 
autumn  leaves.  Those  tendrils,  on  the  other  hand,  which  have  embraced  a 
support  become  much  stronger  and  thicker,  and  also  undergo  a  series  of  changes 
in  their  inner  structure  which  adapt  them  excellently  to  the  part  they  have  to 
perform. 

Stems  with  light-avoiding  tendrils  remind  us  of  the  light-avoiding  interweaving 
and  lattice-forming  stems,  and,  like  these,  are  found  in  plants  which  have  to  climb 
up  steep  rock  faces  and  over  the  bark  of  large  trees.  In  these  cases  the  more  or 
less  plane  surface  of  the  rock  or  tree-trunk  is  the  only  support  afforded  for  climb- 
ing. The  stem  on  such  a  substratum  would  extend  its  tendrils  in  vain  on  the  side 
where  there  is  only  the  air  to  be  met  with;  here  there  is  no  resting-place  or 
support  which  can  be  reached  by  circling  movements.  The  best  the  tendril  can  do 
under  the  circumstances  is  to  seek  out  the  solid  wall  along  which  the  stem  has  to 
climb  as  quickly  as  possible.  In  such  cases  the  desired  support  is  on  the  side 
turned  away  from  the  light,  and  as  a  matter  of  fact,  the  tendrils  of  these  plants 
turn  towards  this  side  with  great  persistence.  According  to  the  position  of  the 
point  at  which  the  tendril  springs  from  the  stem,  it  curves  at  an  angle  of  90-180° 
in  less  than  24  hours,  and  grows  towards  the  background  without  digression  and 
without  wasting  its  energy  in  revolving  movements.  The  leaves  of  the  same  plant, 
for  exposure  to  light  and  air,  are  extended  in  a  direction  away  from  the  wall,  and 
try  to  assume  the  position  most  favourable  for  this  purpose.  The  path  it  has  taken 
soon  brings  the  tendril  in  direct  contact  with  the  wall,  with  which  it  now  has  to 
obtain  a  firm  hold.  This  it  does  either  by  peculiar  adhesive  discs,  or  by  wedging  itself 


CLIMBING   PLANTS. 


699 


into  the  dark  clefts  and  crevices  existing  in  the  supporting  wall.  Several  species  of 
the  genera  Ois^B,  Vitis,  and  Ampelopsis  develop  adhesive  discs.  In  the  Vitis 
mconstans,  a  native  of  Japan  and  China,  and  known  among  gardeners  by  the  name 
of  Oissus  VeitMi  (figured  on  the  right-hand  side  of  fig.  166),  as  soon  as  the  tips  of 
the  tendrils,  which  are  provided  with  tiny  knobs,  come  in  contact  with  a  hard  wall 
they  spread  out,  just  like  the  toes  of  a  tree-frog.  In  a  very  short  time  disc-like 
pads  are  formed  from  the  knobs,  and  these  become  cemented  to  the  substratum  by 
means  of  a  sticky  fluid  mass  secreted  from  the  cells  of  the  disc.  This  cement  now 
holds  so  fast  that  on  trying  to  separate  the  tendril  from  the  substratum  it  is  much 


Fig.  166. -Light-avoiding  Tendrils 
1  Vitis  (Ampelopsis)  inserta.    »  Vitit  inconstant. 

more  likely  that  the  tendril -filament  will  be  torn  than  that  the  disc  will  be 
detached.  Vitis  Royleana  and  Ampelopsis  hederacea  (the  Virginian  Creeper)  also 
develop  these  adhesive  discs,  but  here  they  are  not  prefigured  by  knobs  on  the 
branches  of  the  tendrils  as  in  Cissus  Veitchii]  the  ends  are  curved  like  hooks,  and 
are  barely  thickened.  As  soon  as  they  reach  the  hard  wall  the  tendril-branches 
diverge,  spread  out  on  it  laterally,  and  arrange  themselves  at  definite  intervals 
in  the  most  advantageous  manner.  Within  two  days  the  curved  apices  thicken  and 
turn  crimson,  and  in  another  two  days  the  discs  are  complete,  and  the  tendrils  are 
cemented  by  them  to  the  wall.  These  plants  can  climb  up  smooth  walls,  and  even 
planed  wood,  glass,  and  smooth,  polished  iron  are  not  rejected  as  substrata. 

Bignonia  capreolata,  and  Vitis  (Ampelopsis)  inserta  (whose  tendrils  are  repre- 
sented in  fig.  1661)  behave  differently  from  the  three  tendril-plants  just  mentioned. 
Here  the  curved  tips  of  the  tendrils,  growing  towards  the  wall,  seek  the  crevices 


700  CLIMBING   PLANTS. 

and  crannies  of  stone  or  bark  and  actually  creep  into  them,  or  when  only  shallow 
grooves  are  to  be  found  in  the  substratum,  bury  themselves  in  them.  Smooth 
surfaces  are  avoided  as  far  as  possible  since  they  afford  no  suitable  hold  to  this  class 
of  tendril.  When  established  in  the  chinks  and  crevices,  the  ends,  which  until  now 
have  been  hooked,  swell  out  like  a  club  or  ball,  and  in  a  short  time  thicken  so  much 
that  they  occupy  the  entire  crack.  It  looks  as  if  melted  wax  had  been  poured  into 
the  crevice  and  had  then  solidified  and  fitted  itself  to  all  its  inequalities.  The 
growth  of  the  tissue  extends,  according  to  the  depth  of  the  crack  and  the  nature 
of  the  contact-surface,  over  a  sometimes  larger,  sometimes  smaller  portion  of  the 
embedded  part  of  the  tendril,  and  sometimes  a  callus-thickening  is  seen  to  arise 
even  behind  the  apex,  at  places  where  the  tendril  has  adhered  closely  to  a  small 
projection  of  stone.  The  thickened  end  of  the  tendril  clings  so  firmly  to  the  depres- 
sion into  which  it  has  wedged  itself,  that  it  is  very  difficult  to  remove  it;  and  here 
also  the  attachment  seems  to  be  completed  by  means  of  a  secreted  cement.  It  is 
seen  on  examining  the  parts  of  the  adhesive  disc  or  of  the  wedged  callus-thickening 
which  adhere  firmly  to  the  substratum,  that  the  epidermis  in  particular  has  under- 
gone a  remarkable  change.  The  epidermal  cells  are  enlarged,  drawn  out  as  wart- 
like  protuberances  or  conical  projections,  and  adapt  themselves  to  all  the  elevations 
and  depressions  of  the  substratum,  grasping  even  microscopic  projections,  so  that  the 
contact-surface,  after  being  detached  by  chemical  agents,  resembles  sealing-wax 
against  which  a  seal  has  been  pressed  while  it  was  in  a  plastic  condition. 

It  is  remarkable  that  these  adhesive  discs  and  growths  of  callus  are  only 
developed  after  contact  with  a  solid  body.  If  from  any  cause  the  tendril  is  pre- 
vented from  coming  in  contact  with  a  solid  substratum,  the  growth  of  tissue,  the 
development  of  papillae  on  the  epidermis  and  the  secretion  of  a  cement-substance 
do  not  occur,  and  the  end  of  the  tendril  dries  up  and  dies.  This  process  reminds  us 
strongly  of  the  formation  of  weals  on  human  skin,  and,  like  this,  is  dependent  upon 
stimulus,  friction,  and  pressure. 

A  spiral  torsion  occurs  in  the  light-shunning  tendril  as  soon  as  it  has  become 
attached  in  one  way  or  the  other  to  the  substratum.  The  attached  tendrils  now 
become  much  stronger,  and  always  much  more  vigorous  than  those  whose  apices 
have  not  found  a  resting-place.  The  stem  is  now  fastened  by  the  elastic  tendril  to  the 
steep  rock  face  or  fissured  back  of  an  old  tree-trunk.  Strong  winds  may  drag  the 
stem  somewhat  away  from  the  wall,  but  when  they  subside  it  again  resumes  its 
normal  position,  as  in  the  cases  previously  described,  by  means  of  the  elastic  tendrils. 
If  the  stem  subsequently  grows  in  thickness  the  spiral  springs  holding  it  are  drawn 
out  just  as  far  as  is  required.  Very  old  stems  no  longer  need  their  clinging  organs; 
they  stand,  as  strong  erect  stems,  in  front  of  the  wall  up  which  they  had  years  ago 
clambered  as  young  shoots,  although  their  tendrils  have  now  been  long  dried  up;  the 
young  shoots  alone,  always  striving  higher  and  higher,  still  continue  to  climb  up  the 
substratum  in  the  manner  described. 

The  climbing  stem  in  the  restricted  sense  (stirps  radicans)  holds  itself  in  the 
normal  position  attained  by  growth  by  means  of  climbing  roots,  and  uses  as  supports 


CLIMBING   PLANTS.  701 

the  trunks  of  old  trees,  steep  walls  of  rock,  and  under  cultivation  often  wooden  planks 
and  palings.  All  these  climbing  stems  have  two  kinds  of  roots— absorbent  roots,  by 
means  of  which  they  suck  up  fluid  food,  and  climbing  roots,  which  serve  to  maintain 
them  on  their  supports.  In  most  instances  the  functions  of  these  two  kinds  of  roots 
are  kept  distinct,  so  that  a  climbing  stem  soon  withers  and  dies  when  it  is  cut  across 
above  the  absorbent  roots,  although  affixed  to  a  rock  or  to  the  bark  of  a  tree  by  a 
thousand  climbing  roots.  But  in  a  few  cases  the  climbing  roots  will  under  these 
circumstances  begin  to  absorb,  provided,  of  course,  that  the  substratum  to  which 
they  adhere  is  able  to  afford  them  the  necessary  food. 

In  many  respects  climbing  stems  agree  with  the  group  of  tendril-bearing  stems 
just  described,  especially  in  the  fact  that  the  organs  which  furnish  the  adhesion  to 
the  support  turn  from  the  light,  and  also  inasmuch  as  the  adhesion  to  the  support 
is  brought  about  by  a  viscous  substance  either  secreted  by  the  cells  in  contact  or 
produced  by  the  breaking  down  into  mucilage  of  the  outer  layers  of  the  walls  of 
these  cells.  The  avoidance  of  light  by  all  climbing  roots  is  an  extremely  interesting 
fact.  Whether  the  stem  which  forms  climbing  roots  nestles  close  to  its  substratum, 
or  some  spans  distant  from  it,  whether  it  grows  upwards  along  a  stone  wall  or 
is  deflected  to  one  side  by  some  obstacle — in  all  cases  the  first  rudiments  of  the 
climbing  roots  make  their  appearance  on  the  side  of  the  stem  turned  away  from 
the  light.  And  when  these  small  cushions  develop  into  root-fibres,  the  direction 
assumed  by  their  growth  is  always  away  from  the  light  and  towards  the  dark  back- 
ground. The  darker  the  place,  the  more  vigorous  do  the  root-fibres  become.  When 
the  climbing  roots  developed  by  Tecoma  radicans  (figured  on  p.  479)  in  the  darkest 
places  under  a  projecting  ridge  are  compared  with  those  which  have  been  formed  in 
less  shaded  places  below,  it  is  seen  that  the  former  are  always  much  more  luxuriant 
and  longer  than  the  latter.  If  by  chance  a  shoot  which  has  already  begun  to 
develop  climbing  roots  is  moved  from  its  position  so  that  the  hitherto  shaded  side 
is  exposed  to  the  light,  it  twists  round  until  the  side  with  the  rudiments  of  aerial 
roots  is  again  turned  from  the  light.  If  obstacles  lie  in  the  way  of  this  torsion,  the 
young  climbing  roots  thus  exposed  remain  undeveloped  and  grow  no  further;  ulti- 
mately they  wither  and  dry  up. 

As  soon  as  the  climbing  roots  originating  from  the  shady  side  of  the  stem  come 
into  contact  with  the  substratum  below  them  their  growth  is  noticeably  increased, 
and  in  a  very  short  time  they  become  firmly  united  to  it.  Not  only  do  the  rootlets 
grow  into  all  the  crevices  of  the  substratum  and  adapt  themselves  most  accurately 
to  its  larger  inequalities,  but  each  single  epidermal  cell  of  the  growing  rootlet 
exhibits  a  like  behaviour,  fits  itself  to  the  smallest  projections  and  depressions, 
and  spreads  out  on  entirely  smooth  surfaces  like  a  plastic  mass.  Sometimes  the 
epidermal  cells  are  drawn  out  like  tubes  and  form  so-called  root-hairs,  these  penetrate 
into  the  smallest  clefts  of  the  substratum  and  spread  out  like  a  hand  whose  palm 
and  outspread  fingers  press  against  the  soil.  These  epidermal  cells  of 
roots  also  unite  with  the  supports  against  which  they  have  placed  themselves 
the  absorbent  cells  described  on  p.  87,  and  the  union  is  so  firm  that  the  roots  are 


702  CLIMBING   PLANTS. 

much  more  likely  to  break  off  at  the  base  than  the  united  surfaces  to  separate  when 
the  roots  are  forcibly  displaced. 

The  following  types  of  climbing  roots  may  be  distinguished  according  to  their 
shapes.  First  densely  crowded,  simply  or  only  shortly  branched,  filamentous  roots, 
arising  in  groups,  but  each  separately  from  the  stem;  these  are  increased  in 
number  by  the  production  of  new  batches  as  the  lignifying  stem  becomes  older 
and  thicker,  and  they  sometimes  grow  together  and  border  the  stem,  adhering  to 
the  substratum  in  irregular,  membrane-like  rows.  On  older  stems  the  climbing 
roots  are  usually  for  the  most  part  dried  up,  and  those  which  have  not  united  with 
the  support  then  project  from  the  sides,  often  forming  shaggy  beards,  and  giving  a 
very  odd  appearance  to  the  stem.  The  Ivy  (Hedera  Helix,  of  which  old  stems  are 
shown  growing  up  on  an  oak  in  fig.  167)  may  be  taken  as  an  example  of  this  type. 

The  second  form  presents  a  wholly  different  aspect;  as  the  type  we  may  select 
the  Tecoma  radicans,  a  native  of  the  southern  United  States,  often  used  for 
covering  garden -walls.  The  climbing  roots  here  are  strictly  localized.  At  each 
node  of  the  shoot  below  the  point  of  insertion  of  the  leaves  a  paired  cushion-like 
structure  arises,  and  from  each  of  these  cushions  several  rows  of  protuberances, 
which  grow  out  into  as  many  rows  of  unbranched  or  shortly -branched,  fringing 
fibres,  1-5  cm.  long  (see  fig.  on  p.  479).  The  epidermal  cells  of  this  fringe,  which 
come  into  contact  with  a  firm  substratum,  elongate  and  form  root -hairs,  that  is, 
papillae  and  tubes,  which  in  a  very  short  time  fasten  to  the  support;  after  this  they 
turn  brown  and  die,  thus  never  functioning  as  absorbent  organs. 

A  form  materially  differing  from  these  is  shown  by  the  climbing  roots  of  the 
cactus  Cereus  nycticalus,  known  as  the  "Queen  of  the  Night",  a  native  of 
Mexico  and  the  West  Indies,  and  also  of  several  tropical  Bignoniaceae,  and 
especially  in  Ficus  stipulata,  so  often  used  in  greenhouses  for  covering  the  walls. 
In  the  last-named  plant  the  climbing  roots  arise  in  fascicles  in  the  shade  of  the 
green  leaves;  they  are  filamentous  and  terminate  in  many  hair -like,  spreading 
rootlets.  They  adhere  by  root-hairs  to  the  substratum,  and  thus  fasten  to  it  the 
tender,  pliant  stems.  These  roots  are  not  very  long  and  soon  dry  up,  but  close 
behind  them  much  stronger  roots  arise  from  the  stem,  which  has  meanwhile  become 
thicker,  and  these  traverse  the  walls  like  cords,  repeatedly  branching  and  intersect- 
ing, and  form  actual  net-works,  often  becoming  several  metres  long.  These  latter 
roots  do  not  help  much  in  fastening  the  stem  to  the  supporting  wall;  they  are 
absorbent  roots,  and  take  up  the  atmospheric  water,  with  its  abundance  of  food 
materials,  which  has  condensed  or  trickled  down  the  bark  of  trees  and  rock  walls. 

The  clasping  roots  borne  by  the  stems  of  Wightia,  a  genus  of  Scrophulariacese 
growing  in  the  mountainous  regions  of  the  Himalayas,  and  of  several  species  of 
fig  in  the  same  district,  may  be  regarded  as  a  fourth  type.  The  attachment  of  the 
young  shoots  is  brought  about  here  as  in  the  form  just  described  by  the  finely- 
branched  but  not  much  elongated  roots,  which  soon  dry  up.  But  when  the  climbing 
stem  begins  to  thicken  much  stronger  roots  arise  which  surround  the  supporting 
tree-trunk  like  clamps  and  actually  engirdle  it.  These  girdle-like  clasping  roots 


CLIMBING   PLANTS. 


Fig.  167.— Ivy  (Hedera  Uelix)  fastened  by  climbing  roots  to  the  trunk  of  an  Oak 


704  CLIMBING   PLANTS. 

often  fuse  at  the  places  where  they  adjoin  one  another  and  increase  in  circumfer- 
ence, frequently  becoming  as  thick  as  a  man's  arm.  The  illustration  on  the  next 
page  (fig.  168),  taken  from  a  photograph  at  Darjeeling  in  the  Himalayas,  shows 
these  stems,  which  look  as  if  they  had  been  actually  tied  on  to  the  smooth  trunks 
of  tall  trees,  and  which  bend  away  somewhat  from  the  support,  and  then  ramify 
and  develop  abundant  leafy  branches. 

Many  tropical  species  of  fig,  which  may  serve  as  representatives  of  a  fifth  type, 
exhibit  the  following  peculiarities :  —  their  climbing  roots,  nestling  to  the  sub- 
stratum, flatten  and  spread  out  like  a  doughy  plastic  mass;  the  adjacent  roots 
fuse  together,  and  in  this  way  irregular  lattice-works,  or  incrusting  mantles,  only 
interrupted  here  and  there  by  gaps,  are  formed,  which  lie  on  the  supporting  trunks 
and  are  firmly  fastened  and  cemented  to  them  without  fusing  with  it  or  deriving 
nourishment  from  it.  Frequently  not  the  trunk  only  but  the  branches  of  a  tree 
serving  as  support  are  incrusted  with  the  flattened  clamping  roots  of  the  climber. 
Sometimes  the  climbing  Ficus  sends  columnar  aerial  roots  down  to  the  ground, 
whilst  its  leafy  branches  intersect  those  of  the  supporting  tree;  so  complete  is  the 
entanglement  that  at  first  sight  it  is  hardly  possible  to  distinguish  what  belongs 
to  the  support  and  what  to  the  climbing  plant.  Fig.  169  is  a  faithful  reproduc- 
tion of  a  sketch  by  Selleny  drawn  at  Kondul,  one  of  the  small  Nicobar  Islands, 
showing  one  of  these  remarkable  climbers  with  flattened  roots  incrusting  the 
support,  i.e.  Ficus  Benjamina  on  a  supporting  myrtaceous  tree,  the  latter  obviously 
suffering  under  the  burden  of  its  oppressor,  and  already  in  a  dying  condition. 

These  "  tree  constrictors",  as  one  might  call  them,  although  they  do  not  absorb 
materials  from  their  supports,  as  was  formerly  supposed,  are  certainly  not  indifferent 
to  them,  and  may  injure  and  even  kill  them  like  the  constricting,  twining  stems 
described  and  figured  on  pp.  159  and  160.  The  entwined  tree  decays  and  its  wood 
disintegrates,  perhaps  termites  assist  in  carrying  away  the  remains  of  the  dead 
trunk,  but  the  climbing  stem  with  the  flattened,  climbing  roots  remains  still 
vigorous.  It  has  meanwhile  created  a  sufficient  support  for  itself  by  its  prop-like 
aerial  roots,  and  these  prevent  it  from  falling.  As  Hooker  says  in  his  Himalayan 
Journals: — "We  found  great  scandent  trees  twisting  around  the  trunks  of  others 
and  strangling  them:  the  latter  gradually  decay,  leaving  the  sheath  of  climbers  as  one 
of  the  most  remarkable  vegetable  phenomena  of  these  mountains".  When  at  length 
the  climber,  deprived  of  its  original  support,  also  dies,  its  roots  and  stem-structures 
become  bleached,  and  its  curious  forms,  in  which  to  speak  with  Martius,  "the 
excited  imagination  fancies  it  recognizes  fantastic  spectres  and  gigantic  voracious 
monsters",  rise  up  weirdly  against  the  dusky  background  of  the  primeval  tropical 
forest. 

The  manner  in  which  climbing  roots  become  fixed  upon  their  supports  is  not 
less  varied  than  their  manifold  structural  modifications.  It  has  already  been  stated 
that  the  climbing  roots  are  light  -  avoiding,  and  that  their  growing  points  are 
directed  towards  the  rocky  faces  and  boughless  tree  -  trunks  upon  which  they 
climb  Should  the  distance  between  the  stem  and  the  wall  be  not  great,  the 


CLIMBING    PLANTS. 


705 


Fig.  168,-Ficus  with  girdle-like  clasping  roots,  at  Darjeeling  in  the  Sikkim  Himalayas.    (Fron.  a  photograph.) 


climbing  roots  grow  out  directly  to  the  wall.  This  is  usually  the  case  with 
climbing  plants.  Several  Aroideae  and  species  of  Ficus,  and  especially  our  ivy 
(Hedera  Helix),  the  shoots  of  which  occur  anywhere  at  the  foot  of  a  tree^rujik  or 

VOL.1. 


706  CLIMBING   PLANTS. 

of  a  rocky  wall,  develop  climbing  roots  close  below  the  growing  shoot-apex.  These 
roots  speedily  reach  the  wall  and  fasten  the  portion  of  the  stem  from  which  they 
arise  to  it.  This  continues  slowly,  the  shoot-apex  continually  creeping  higher  up 
over  the  substratum.  This  is  the  simplest  way  in  which  climbing  stems  become 
attached.  The  process  by  which  those  of  the  of  ten -cited  Tecoma  radicans  are 
affixed  is  much  more  complex.  These  stems  shun  the  light  in  a  marked  degree. 
If  Tecoma  radicans  is  planted  in  front  of  a  wall  covered  with  trellis -work,  the 
actively  growing  shoots  turn  away  from  the  light,  slip  behind  the  trellis- work,  and 
adhere  closely  to  the  wall  by  those  portions  of  the  stem  at  which  climbing  roots 
appear.  So  soon  as  they  come  in  contact  with  the  hard  substratum  the  small 
pale  rootlets  grow  out  from  the  cushions  as  a  fringe  of  threads  which  cling  very 
firmly  to  the  wall.  The  growing  shoot  never  leaves  the  wall,  but  keeps  close  to 
it,  always  seeking  the  darkest  places  under  projecting  tiles,  ledges,  and  cornices, 
attaching  itself  at  intervals  by  fresh  clamp-roots. 

The  most  remarkable  method  by  which  shoots  destined  for  climbing  reach  a 
wall  which  will  afford  them  a  support  is  observed  in  several  tropical  Bignoniacese 
related  to  Bignonia  unguis,  one  of  which,  Bignonia  argyro-violacea,  growing  by 
the  Rio  Negro  in  Brazil,  is  represented  in  fig.  170.  This  plant  bears  two  kinds 
of  leaves:  simple  leaves,  the  blades  of  which  attain  to  a  considerable  size,  and 
others  which,  like  the  leaves  of  the  vetch  (Lathy rus),  bear  two  opposite  leaflets 
on  one  stalk  and  end  in  a  structure  which  divides  into  three  limbs  with  pointed 
hooked  claws,  and  which  is  not  unlike  the  foot  of  a  bird  of  prey. 

The  development  of  this  clawed,  grasping  organ  always  precedes  that  of  the 
leaflets,  so.  that  in  the  youngest  stages  the  green  leaflets  only  appear  as  minute 
scales.  Leaves  ending  in  claws  are  only  found  on  stems  which,  so  to  speak,  have 
to  seek  a  firm,  safe  support  for  the  flowering  and  fruit  -  bearing  shoots  to  be 
developed  later  on.  These  stems,  however,  are  thin,  much  elongated,  and  are  always 
pushing  out  new  internodes.  They  hang  down  as  long  threads  from  the  tree,  whose 
bark  is  already  quite  covered,  and  which  offers  no  space  for  a  new  settlement,  and 
are  easily  set  in  motion  by  the  action  of  the  wind.  At  the  end  of  each  thread  can 
be  seen  two  young  leaves  placed  opposite  one  another,  on  each  of  which  only  the 
three  hooked  limbs  are  at  first  developed,  appearing  to  be  extended  for  prehensile 
purposes,  just  as  in  a  bird  of  prey.  If  the  shoot  oscillating  in  the  air  fails  to 
encounter  an  object  which  it  can  seize  with  its  claws,  the  latter  fold  back,  and  the 
leaflets  are  developed.  Before  the  following  day  the  filamentous  stem  has  produced 
a  new  portion  similarly  equipped.  These  fresh  claws  are  again  extended,  and  the 
supple  stem  again  sways  in  the  wind,  in  the  hope  of  being  able  to  catch  hold  of 
a  firm  object.  The  same  thing  is  repeated  day  after  day  until  some  suitable 
anchorage  comes  within  reach  of  the  elongating  shoot.  Now  is  the  time  for  the 
development  of  the  clamp-roots,  which  have  to  fix  the  stem  to  the  substratum 
still  more  firmly  than  the  claws  could  do.  The  climbing  roots  are  really  already 
present  at  each  node  of  the  filamentous  stem  as  small  protuberances,  but  they 
remain  quite  short  until  such  time  as  the  swaying  shoot  effects  its  attachment.  Ther 


CLIMBING   PLANTS. 


707 


Fig.  m.-Ficus  Benjamina  with  incrusting  climbing  roots.    (After  a  drawing  from  nature,  by  Selleny.) 


708  CLIMBING   PLANTS. 

they  grow  out,  elongate,  and  produce  lateral  branches,  as  may  be  seen  in  fig.  170. 
Under  favourable  conditions,  i.e.  when  these  swaying  shoots  reach  an  unoccupied 
support  and  become  permanently  attached  there  by  their  scandent  roots,  these 
anchoring  shoots  exhibit  a  marked  change  of  habit.  They  give  rise  to  vigorous 
and  compact  shoots  with  simple  leaves  destitute  of  claws,  and  may  unfold  flowers 
and  ripen  fruit.  In  due  time,  when  the  space  has  become  occupied,  pendent  shoots 
are  again  produced  which  explore  the  neighbourhood  for  a  new  anchorage  in  the 
manner  already  described. 

The  group  of  root-climbers  as  a  whole  undoubtedly  presents  many  points 
of  resemblance  to  forms  with  stems  prostrate  on  the  ground.  The  climbing 
stems  of  Ivy  remind  one  of  the  stems  of  Periwinkle,  the  climbing  stems  of  species 
of  Pothos  of  the  creeping  stems  of  the  Snake-root  (Calla  palustris),  the  climbing 
stems  of  Tecoma  radicans  of  the  runners  of  strawberry  plants.  The  only  real 
difference  is  that  in  one  case  the  substratum  is  the  surface  of  the  soil,  while  in 
that  of  climbing  stems  it  is  the  abruptly-ascending  surface  of  rocks  and  tree- 
trunks.  And  this  distinction  is  wanting  in  the  Ivy.  Ivy  stems  which  grow 
over  stony  ground,  fix  on  to  the  horizontal  blocks  of  stone  by  climbing  roots 
exactly  as  on  vertical  walls  of  rock.  If  mould  is  present  in  the  crevices  of 
these  stone  blocks,  the  climbing  roots  become  true  absorbent  roots,  not  only 
fastening  the  stem  to  its  substratum,  but  also  taking  up  food.  But  ivy  stems 
climbing  up  steep  rocky  walls  also  behave  in  this  way.  The  roots  which  proceed 
from  the  portions  of  the  stem  growing  over  the  bare  stone  wall  are  climbing 
roots,  but  as  soon  as  the  stem  in  its  growth  comes  to  a  crevice  filled  with  earth, 
the  roots  developing  at  that  point  become  absorbent  like  those  which  it  produces 
when  creeping  on  the  ground. 

Thus  it  is  clearly  impossible  to  draw  a  sharp  line  between  climbing  and 
creeping  stems.  Similarly,  on  the  other  hand,  there  are  some  climbing  stems 
transitional  between  this  condition  and  an  erect  habit.  Ivy,  Tecoma  radicans, 
the  climbing  species  of  Ficus,  even  several  tropical  aroids,  and  the  Brazilian 
Marcgravia  umbellata,  exhibit  this  peculiarity.  In  the  last-named,  so  soon 
as  it  has  climbed  up  a  tree-trunk  or  steep  rocky  wall  into  an  illuminated  place, 
it  alters  its  growth  completely.  The  shoots  now  formed  up  above  no  longer 
avoid  the  light;  they  no  longer  develop  climbing  roots  for  attachment  to  the 
substratum,  their  wood  becomes  more  extensive,  the  hard  bast  surrounding  the 
wood  is  much  more  strongly  developed,  the  shoots  now  not  only  stand  erect 
without  support,  but  are  even  able  to  withstand  flexion,  and  develop  flowers 
with  abundant  honey  and  ripe  fruits.  The  erect  shoots  of  Ivy  and  of  the 
climbing  species  of  Ficus,  bathed  in  sunshine,  also  unfold  foliage-leaves,  which 
are  strikingly  different  from  those  of  the  climbing  shoots  in  size  and  shape, 
and  even  in  their  internal  structure.  Anyone  knowing  only  the  long  fila- 
mentous shoots  of  Ficus  stipulata,  used  for  covering  the  walls  in  green-houses, 
happening  to  see  the  vigorous  shoots  of  this  plant  with  large  leaves  and  figs, 
would  think  it  impossible  that  both  should  belong  to  one  and  the  same  plant. 


CLIMBING   PLANTS. 


709 


The  erect  stems  of  the  Ivy,  adorned  with  cordate  shining  foliage-leaves  when 
treated  as  slips  or  cuttings,  send  absorbent  roots  into  the  ground  and  ramify; 
but,  strangely  enough,  the  shoots  which  they  develop,  although  they  now  spring 
close  upon  the  ground,  do  not  become  climbing  stems,  but  exhibit  exactly  the 
same  structure,  the  same  erect  position,  and  the  same  foliage  as  the  shoots  on 
the  top  of  a  wall  or  on  the  summit  of  a  tree.  Anyone  seeing  for  the  first  time 


Fig.  170.—  Signonia  argyro-violacea,  from  the  banks  of  the  Rio  Negro  in  Brazil. 


such  Ivy  grown  in  pots,  is  tempted  to  mistake  it  for  some  erect  tropical  Aralia, 
and  even  experienced  gardeners  and  botanists  may  be  misled  by  these  plants. 
We  are  involuntarily  reminded  by  these  successive  shoot-structures,  which  differ 
so  much  in  their  outer  form  and  internal  construction,  of  the  alternation  of 
generations  occurring  in  Vascular  Cryptogams,  and  so  much  the  more  since  the 
climbing  shoots  which  precede  the  erect  flowering  shoots  do  not  develop  £ 
and  fruits,  and  thus  to  some  extent  resemble  an  asexual  generation. 

Several  Indian  species  of  fig,  the  stems  of  which  climb  up  rocky  walls  and 


710  ERECT   FOLIAGE   STEMS. 

adhere  to  them  by  girdle-shaped,  flattened,  and  in  part  lattice-forming  roots, 
send  up  an  erect  stem  with  large  foliage  when  they  have  reached  the  top  of  the 
wall  or  the  summit  of  the  block  of  stone.  By  this  time  the  leaves  of  the  climbing 
parts  of  the  stem  have  fallen  away.  Generally,  this  climbing  stem,  the  first 
stage,  as  it  were,  can  no  longer  be  recognized  as  such;  only  the  clamp-roots 
which  proceeded  from  it,  which  have  meanwhile  become  much  thickened  and 
wide-meshed,  appear  in  a  most  remarkable  manner  like  a  lattice-work  spread 
out  over  the  stone.  Any  one  not  knowing  the  history  of  development  of  these 
species  of  fig,  would  think  that  the  stems  rising  erectly  from  the  top  of  a  block 
of  stone  or  in  the  cleft  of  a  rocky  wall,  had  germinated  in  the  place  where 
they  rise  up  into  the  air,  and  had  sent  down  from  thence  a  net-work  of  aerial 
roots  enveloping  the  stone.  This  idea,  which  at  first  occurs  to  everyone  who 
looks  at  the  two  fig-trees  faithfully  represented  on  the  left-hand  side  of  fig.  171, 
does  not,  however,  correspond  with  the  actual  process  of  development.  The 
lattice-forming  roots  adhering  to  the  stone  are  not  sent  out  by  the  small  trees 
rising  above  them,  but  have  been  developed  by  the  climbing  stem  which  had 
mounted  up  by  their  help,  and  then  became  transformed  into  an  erect  stem 
growing  freely  up  into  the  air.  We  must  also  guard  against  generalizing  and 
regarding  all  root-structures  of  this  kind  as  climbing  roots.  In  the  tropics 
there  are  plants  whose  erect  stems  do  send  down  aerial  roots  which  continually 
ramify,  and  then  look  deceptively  like  lattice-forming  climbing  roots. 

[For  further  details  as  to  climbing  plants  the  reader  is  referred  to  H.  Schenck's 
masterly  Beitrage  zur  Biologie  und  Anatomie  der  Lianen.  Jena,  1892.  ED.] 

ERECT  FOLIAGE  STEMS. 

Plants  with  procumbent  and  subterranean  stems  preponderate  in  high  mountain 
and  in  arctic  regions,  whilst  in  these  places  the  majority  of  woody  stems  cling 
closely  to  the  substratum,  or  are  embedded  in  the  soil.  Lateral  shoots  rising  erect 
from  the  ground,  of  course,  often  spring  from  these  main  stems,  but  they  bear 
no  foliage,  or  possess  green  leaves  only  at  the  base,  and  terminate  in  flowers. 
They  are  essentially  of  the  nature  of  flower-stalks  or  scapes,  and  are  for  the  most 
part  to  be  regarded  as  floral-stems.  The  few  flowerless,  erect  foliage -stems 
which  are  met  with  in  these  frosty  districts  are  all  very  short,  usually  closely 
crowded  together  into  a  carpet,  or  have  the  form  of  numerous  erect  branchlets; 
they  seldom  rise  more  than  a  span-high  from  the  ground.  The  only  noticeable 
erect  stems  besides  the  type  of  low,  woody  shrubs  are  the  culm  and  the  her- 
baceous stem.  On  passing  from  elevated  regions  down  into  the  valley,  and  from 
the  arctic  zone  southwards,  besides  these  forms,  we  meet  with  reeds,  high  shrubs 
and  trees,  and  still  nearer  the  equator  we  £63  the  erect  stems  of  cactiform  plants, 
bamboos,  and  palms. 

In  this   connection   the   terms   caudex,   culm,   stalk,  and   trunk  are  used   to 
indicate  the  forms  of   erect  foliage-stems   standing  out  in  the  landscape,  terms 


ERECT   FOLIAGE   STEMS. 


11 


7]  O  ERECT   FOLIAGE    STEMS. 

which  have  arisen  in  the  popular  tongue,  and  of  which  everyone  thinks  he  knows 
the  meaning;  these  words  have  also  been  admitted  into  scientific  terminology, 
although,  when  more  closely  examined,  they  are  seen  to  be  ill-adapted  for  the 
nomenclature  of  erect  stems.  Thus  there  are  procumbent  culms,  procumbent 
caules,  and  procumbent  tree-trunks,  and  it  is  therefore  not  correct  to  use  these 
terms  for  erect  forms  only.  It  has  been  proposed  to  designate  the  erect  stem, 
which  may  be  compared  to  a  post,  a  standard -stem  (stirps  palaris),  prefixing 
the  word  "standard"  to  the  names  of  the  various  sorts  of  erect  stem.  The 
names  resulting  from  this  combination  would  prevent  any  confusion,  but, 
unfortunately,  they  are  cumbrous  and  unusual,  and  on  the  whole  unsuited  to 
this  book.  For  these  reasons  the  current  expressions  will  be  still  employed, 
with,  of  course,  the  proviso  that  in  this  case  they  refer  only  to  standard  stems. 

The  cactiform  stem,  especially  those  gigantic  specimens  which  are  natives  of 
the  Mexican  plains,  and  attain  to  a  height  of  some  15  metres,  might  have 
been  taken  as  a  type  of  a  standard-stem.  In  their  erect  habit,  together  with 
their  lack  of  branches,  they  look  like  posts  which  have  been  driven  into  the 
ground  to  form  the  foundation  for  a  scaffolding.  But  since  these  stems  have 
no  foliage-leaves,  or  rather,  since  their  leaves  have  been  transformed  into  spines, 
so  that  the  formation  of  organic  materials,  which  is  usually  performed  by 
foliage,  has  to  be  done  by  the  green  cortex,  they  cannot  really  be  reckoned  as 
foliage-stems,  and  can  only  be  mentioned  here  incidentally. 

The  caudex  (cauloma,  caudex)  has  the  greatest  claim  of  all  the  series  of 
erect  foliage -bearing  stems  to  be  compared  to  a  standard.  The  form  seen 
in  slender  palms,  to  which  the  term  Caudex  columnaris  has  been  applied, 
stands  foremost  in  this  respect.  The  Palmyra  Palm,  one  of  the  most  beautiful 
of  all  palms,  and  so  common  a  feature  along  portions  of  the  coast-line  of 
the  island  of  Ceylon,  gives  a  clear  idea  of  this  form  of  caudex.  As  a 
rule,  the  height  of  palms  is  much  exaggerated;  there  is  a  great  temptation, 
especially  in  the  case  of  isolated  stems,  to  estimate  them  as  much  higher  than 
they  really  are.  This  is  on  account  of  an  optical  illusion  which  comes  into 
play  just  as  in  the  estimation  of  the  heights  of  mountains.  An  isolated  mountain 
peak  rising  up  abruptly  is,  at  first  sight,  always  thought  to  be  higher  than  a 
continuous  ridge  which  gradually  ascends  in  gentle  slopes,  although  both  may 
have  exactly  the  same  elevation;  and  the  same  thing  occurs  in  estimating  the 
height  of  stems.  An  isolated  Palmyra  Palm  rising  from  among  low  shrubs 
appears  to  be  much  higher  than  one  which  is  actually  taller,  but  which  grows  in 
the  midst  of  a  group  of  trees  and  whose  summit  only  rises  a  little  above  the 
other  tree-crowns.  The  highest  columnar  caudex  is  shown  by  Ceroxylon  andicola, 
a  palm  growing  in  the  Andes,  of  which  stems  are  known  57  metres  in  length. 
The  caudex  of  the  Cocoa-nut  Palm  (Cocos  nucifera)  attains  a  height  of  32  metres, 
and  that  of  the  Palmyra  Palm  (Borassus  flabelliformis),  not  far  behind  the 
last,  30  metres.  Most  other  palms  are  lower  than  this,  the  great  majority 
never  exceeding  30  metres.  The  so-called  Dwarf  Palm  (Chamcerops  humilis)  is 


ERECT   FOLIAGE   STEMS. 


713 


Fi~  172,-BambooB  in  Java.    (From  a  photograph.) 


714  ERECT   FOLIAGE   STEMS. 

only  4  metres  high,  and  there  even  exist  palms  whose  caudex  barely  rises  above 
the  ground. 

The  caudex  of  tree-ferns  and  cycads  also  remains  comparatively  short.  When 
travellers  speak  of  the  gigantic  trunks  of  tree-ferns,  they  only  mean  gigantic  in 
comparison  with  the  stems  of  the  ferns  growing  in  our  European  forests,  which 
either  never  appear  above  the  ground,  or  like  those  of  the  ostrich  fern  (Struthiop- 
teris  germanica),  only  10  cm.  above  the  soil.  The  New  Zealand  tree-fern  Dicksonia 
antarctica,  with  a  diameter  of  40  cm.  reaches  a  height  of  15  metres,  and  the  caudex 
of  Alsophila  excelsa,  with  a  thickness  of  60  cm.,  is  22  metres  high.  The  cycads 
scarcely  ever  reach  this  height,  nor  do  the  various  other  flowering  plants  possessing 
a  caudex,  such  as  the  species  of  the  genera  Yucca,  Dracaena,  Urania,  Pandanus, 
Aloe,  and  Xanthorrhosa.  The  celebrated  Dragon-tree  (Dracaena  Draco)  of  Orotava, 
whose  age  is  estimated  to  be  6000  years,  has  a  circumference  of  14,  and  a  height  of 
22  metres. 

The  caudex  is  in  most  cases  simple,  but  several  Pandanese  and  dragon-trees,  and 
among  the  palms,  the  Doum  Palm  (Hyphcene  thebaica)  growing  in  the  Nile  valley 
and  Hyphcene  coriacea,  fork  and  develop  a  few  short  branches  when  their  main 
caudex  has  attained  a  great  age.  Many  caudices,  e.g.  those  of  the  tree-ferns  Dick- 
sonia antarctica  and  Todea  barbata  are  completely  covered  with  short  aerial  roots, 
in  consequence  of  which  their  surface  has  a  peculiar  bristling  appearance.  Many 
caudices  are  also  abundantly  provided  with  thorns.  For  the  appearance  of  most 
of  them  it  is  of  importance  whether  the  dead  leaves  break  off  above  the  base,  so 
that  the  leaf -sheaths  persist,  or  whether  the  leaf-sheaths  are  detached  with  them, 
only  a  scar  being  left  on  the  caudex.  In  the  former  case  the  stem  is  clothed  some- 
times with  ridges  or  scales,  sometimes  with  a  fibrous  integument,  or  even  with  dry 
stumps.  In  the  latter  case  it  is  covered  with  circular  or  shield-like  scars.  The 
caudex  of  Caryota  (cf.  fig.  74,  p.  311)  becomes  quite  smooth  after  the  leaves  have 
fallen  off,  and  looks  like  a  gigantic  culm;  indeed,  it  forms  a  link  between  the  caudex 
and  that  kind  of  stem  which  is  termed  a  culm. 

The  stem-structures  which  are  comprehended  under  the  name  culm  (culmus) 
differ  in  size  even  more  than  does  the  caudex.  They  may  be  classified  in  the  follow- 
ing groups,  which,  of  course,  are  not  sharply  marked  off  from  one  another.  First, 
the  culm  in  the  narrow  sense  of  the  word,  which  embraces  those  forms  whose  stem 
does  not  exceed  a  diameter  of  J  cm.;  then  the  reed,  which  is  not  branched,  whose 
internodes  are  always  surrounded  by  long  sheaths,  and  whose  stem  has  a  diameter 
of  J-5  cm. ;  and,  further,  the  bamboo,  which  divides  into  numerous  branches,  having 
short  leaf-sheaths  and  a  very  peculiar  anatomical  structure;  this  will  again  be 
referred  to  in  the  next  chapter.  The  culm  exhibits  its  highest  proportions  in 
bamboos,  especially  in  the  species  represented  in  fig.  172,  which  attains  to  a  height 
of  25  metres  and  a  thickness  of  more  than  half  a  metre.  From  this  extreme,  on  the 
one  hand,  to  the  delicate  little  culm  2-3  cm.  long,  of  many  annual  grasses  of  the 
Mediterranean  flora,  there  exists  an  unbroken  series  about  the  middle  of  which  comes 
the  Southern  Reed  (Arundo  Donax)  with  a  height  of  4  m.  and  a  diameter  of  5  cm. 


ERECT   FOLIAGE  STEMS.  7]  5 

The  stalk  (caulis)  does  not  become  woody,  but  remains  green;  it  persists  only  for 
a  single  period  of  vegetation,  and  then  dies  down.  The  stem  of  annual  and  bien- 
nial plants  classed  as  herbs  (herbce)  is  called  a  "  herbaceous  "  stem  (caulis  herbaceus), 
and  that  of  perennial  plants  a  "  suffruticose  "  stem  (caulis  su/ruticosus).  By  the 
term  "undershrub"  (su/rutex)  we  understand  especially  those  perennial  plants 
whose  underground  stem  annually  sends  up  shoots  which  do  not  become  woody, 
but  which  die  off  at  the  beginning  of  the  winter,  e.g.  the  Dwarf  Elder  (Sambucus 
ebulus),  the  common  Avens  (Geum  urbanum),  and  the  Meadow  Sage  (Salvia  pra- 
tensis).  Whilst  the  caudex  and  culm  are  generally  circular  in  cross-section,  the 
caulis  is  usually  3-,  4-,  and  5-ribbed.  Longitudinal  furrows  traverse  its  exterior,  and 
the  relation  of  these  to  the  nearest  leaves  will  be  described  more  in  detail  subse- 
quently. The  extreme  limits  of  size  of  the  caulis  have  already  been  discussed  on  p.  656. 

The  woody  stem  (truncus)  either  remains  without  branches  until  it  has  attained 
a  considerable  height,  when  it  is  called  "  arborescent "  (truncus  arborescens),  or  it 
is  very  short,  and  its  branches  spring  from  near  the  ground,  in  which  case  it  is 
called  "  shrubby  "  (truncus  frutescens).  A  distinction  is  also  drawn  in  descriptive 
botany  with  regard  to  size  between  the  "tree"  (arbor)  in  the  narrower  sense,  and 
the  "  small  tree  "  (arbuscula),  the  "  shrub  "  (frutex)  and  the  "  small  shrub  "  (fruti- 
culus).  The  expression  "semi-shrub"  (semifrutex)  may  be  employed  to  denote 
shrubs  whose  yearly  shoots  only  become  woody  at  the  base  before  the  next  period 
of  vegetation,  and  which  wither  and  die  off  above  this.  These  form  a  transition  to 
the  undershrubs  mentioned  above. 

Of  all  these  forms  of  woody  stem  the  tree,  especially  prominent  on  account  of 
its  mass,  naturally  claims  most  interest.  Nor  is  this  interest  limited  to  the  botanist; 
it  is  shared  with  him  by  the  landscape-painter,  forester,  gardener,  indeed,  by  every 
lover  of  nature,  each  from  his  own  particular  point  of  view.  Among  all  the  forms 
of  vegetation  trees  are  the  best  known;  they  have  received  a  special  name  in  all 
languages,  different  nations  have  chosen  certain  species  of  their  country  as  favourites, 
and  have  extolled  them  as  national  trees  in  their  songs,  and  even  in  the  religious 
observances  and  customs  of  ancient  and  modern  times  trees  played  and  yet  play  a 
prominent  part.  Many  persons  who  have  never  occupied  themselves  with  botany, 
and  have  never  observed  plants  closely,  but  who  have  a  strongly-developed  sense  of 
form,  are  able  to  distinguish  and  recognize  the  various  kinds  of  trees  at  the  first 
glance  and  at  considerable  distances.  How  is  this  possible?  The  explanation  is 
very  simple.  The  aspect  of  every  tree,  like  the  face  of  every  man,  presents  certain 
features  which  are  peculiar  to  it  alone;  these  features  imprint  themselves  almost 
unconsciously  on  the  memory  of  anyone  who  is  occupied  much  in  the  open,  and  the 
species  is  recognized  again  by  him,  even  at  a  distance,  like  an  old  friend  one  meets 
in  the  street.  To  the  landscape-painter  these  features,  which,  taken  together,  form 
what  has  been  termed  the  habit  of  the  tree,  are  particularly  important,  for  he  has 
to  seize  them  and  give  them  artistic  expression.  Our  task  here,  however,  is  to 
detail  and  to  interpret  these  features  in  the  appearance  of  the  trees,  or  let  us  put  it, 
to  give  a  scientific  basis  to  the  "  habit ". 


716 


ERECT   FOLIAGE   STEMS. 


Fig.  173.— The  Oak. 


ERECT   FOLIAGE   STEMS. 


717 


Fig.  174.— The  Silver  Fir. 


718  ERECT  FOLIAGE  STEMS. 

The  limits  of  this  book  do  not  allow  me  to  treat  this  theme  as  fully  as  my 
inclination  and  predilection  for  the  relations  between  art  and  science  would  prompt 
me  to  do,  but  as  a  tree  may  be  sketched  on  a  wall  with  a  few  strokes,  so  I  will 
endeavour  to  represent  the  principles  of  the  "  habit "  in  a  few  words. 

In  every  tree  the  position  of  the  buds  depends  upon  the  position  of  the  foliage- 
leaves,  and  it  is  evident  that  the  distribution  of  the  lateral  twigs  proceeding  from 
a  branch  is  also  dependent  upon  the  position  of  the  leaves.  The  correlation  between 
the  arrangements  of  leaves  and  of  branches  is  therefore  the  first  which  has  to  be 
considered  in  explaining  the  "habit".  Like  leaves,  the  branches  are  either  whorled 
or  decussate,  or  arranged  along  a  spiral  line,  and  it  may  therefore  be  said  that  the 
branches  also  exhibit  the  definite  geometrical  relations  which  were  described  in 
detail  for  leaves  on  pp.  396-407 ;  even  this  fact  gives  a  characteristic  stamp 
to  every  tree.  How  very  different  are  maples  and  ashes  with  their  decussating 
branches,  in  comparison  with  elms,  limes,  and  alders,  with  leaves  arranged  on  the 
one-half  and  one-third  system,  and  with  beeches,  oaks,  and  poplars  characterized  by 
the  two-fifths  and  three-eighths  arrangement;  they  differ  not  only  in  detail,  but 
also  in  the  grosser  features  of  the  whole  tree-crown.  Not  only  are  the  bare  trees 
in  winter-time  readily  recognizable  at  a  distance  by  their  ramification,  but  every 
portion  of  the  leafy  crown  derives  its  particular  contour  in  consequence  of  this 
branching.  Then,  again,  the  size  and  shape  of  the  foliage-leaves  have  to  be  con- 
sidered in  interpreting  the  habit.  This  does  not  imply  that  the  painter  should 
represent  the  individual  leaves,  so  that  they  could  be  recognized,  for  that  would 
in  a  picture  be  undesirable.  The  significance  of  the  configuration  of  the  single 
leaves  lies  rather  in  the  fact  that  they  regulate  the  form  of  the  whole  tree.  The 
boughs  and  branches  of  trees  with  narrow,  linear,  or  needle-shaped  leaves  have  far 
less  to  support  than  those  which  are  adorned  with  large,  flat,  extended  leaf -blades. 
Trees  of  the  former  class  are  characterized  by  their  height,  of  the  latter  by  their 
width,  a  difference  which  appears  in  the  trees  of  all  parts  of  the  world.  For 
example,  the  difference  in  the  architecture  of  slender,  narrow-leaved  eucalyptuses 
and  willows,  and  the  broad-leaved  paulownias,  catalpas,  and  planes,  with  their  wide- 
spreading  boughs,  is  very  striking.  If  we  compare  the  illustrations  of  oak  and  fir 
placed  opposite  one  another  on  the  preceding  pages,  we  notice  that  the  needle- 
bearing  boughs  and  branches  borne  by  the  slender  stems  of  the  fir-tree  scarcely 
occupy  a  third  of  the  space  of  that  taken  up  by  the  thick,  heavy  trunk  of  the  oak, 
the  leaves  of  which  are  so  much  broader. 

A  third  point  which  comes  under  consideration  is  the  light  required  by  the  leaves 
on  the  lower  boughs  of  older  trees.  The  thicker  and  more  abundant  the  foliage 
on  the  summit  or  top  of  the  crown,  the  deeper  becomes  the  shade  around  the  lower 
part  of  the  main  trunk.  If  the  lower  boughs  are  not  able  to  elongate  continually 
by  means  of  new  additions  they  die,  together  with  their  shaded  foliage,  withering 
up  and  breaking  off  either  wholly  or  in  part  at  the  first  opportunity  and  falling 
to  the  ground;  but  if  they  have  this  capacity  of  elongating,  they  push  and  extend 
their  leafy  branches  as  far  as  possible  out  of  the  circle  of  the  shadow  into  the 


ERECT   FOLIAGE   STEMS.  719 

sunlight,  and  frequently  curve  up  towards  the  light,  as  may  be  well  seen  in  ash 
and  chestnut-trees,  and  also  in  the  spruce  firs  represented  on  p.  415. 

The  lower  portion  of  the  stem  which  has  lost  its  boughs  increases  in  circum- 
ference as  the  burden  it  has  to  bear  becomes  greater,  and  its  thickness  and  strength 
in  every  species  bears  a  definite  relation  to  the  weight  of  the  crown.  The  increase 
of  circumference  is  brought  about  by  the  addition  each  year  of  new  masses  of  wood 
to  those  already  present.  In  very  young  stems  the  wood  appears  in  the  form  of 
strands,  symmetrically  arranged  round  the  central  pith,  closely  adjoining  one 
another  and  forming  a  cylinder  which  is  only  interrupted  by  the  medullary  rays. 
The  annual  increments  of  wood,  deposited  on  the  periphery  of  this  primary  ring, 
also  have  the  form  of  rings  in  cross  section;  each  is  known  as  an  annual  ring. 
The  age  of  a  felled  tree  can  be  reckoned  from  the  number  of  these  annual  rings, 
and  obviously  the  girth  of  the  stem  increases  with  their  increasing  number.  The 
enlargement  of  the  circumference  is,  however,  not  without  its  effect  on  the  external 
appearance  of  the  stem.  While  still  quite  young,  the  stem  possesses  a  covering 
skin  (epidermis)  which  closely  surrounds  the  green  tissue  of  the  cortex.  This 
epidermis,  however,  only  keeps  pace  with  the  development  of  the  interior  of  the 
stem  as  long  as  this  particular  part  continues  to  grow  in  length.  When  it  stops, 
and  increase  in  thickness  commences,  the  first  skin  perishes,  and  is  replaced  by  a 
second,  the  so-called  periderm.  This  usually  begins  to  develop  even  at  the  end 
of  the  first  period  of  vegetation.  The  most  important  constituent  of  periderm  is 
cork,  a  tissue  of  cells  impervious  to  water  and  almost  impervious  to  air,  and  there- 
fore excellently  fitted  as  a  covering  for  the  inner  sap-conducting  portions  of  the 
stem.  Whatever  lies  outside  this  cork,  or  is  secreted  through  it  from  the  inner 
sap-containing  portions,  dries  up  and  dies.  If  the  periderm  is  developed  immediately 
beneath  the  epidermis,  this  alone  perishes;  but  if  the  periderm  arises  in  the  deeper 
layers  of  the  cortex,  a  considerable  thickness  of  cortex  also  dies  and  remains  outside 
the  cork  as  a  dead  dry  crust.  This  inner  periderm  with  the  dead  adhering  parts 
of  the  cortex  is  called  the  bark. 

The  development  of  the  periderm  keeps  pace  with  the  development  of  the  stem. 
As  soon  as  the  wood  of  the  stem  becomes  thicker,  by  the  intercalation  of  a  new 
annual  ring,  the  mantle  of  periderm  stretches,  and  consequently  the  whole  envelope 
of  bark.  In  many  trees  this  bark  remains  year  after  year  on  the  periphery  of  the 
stem;  it  becomes  fissured  by  the  continuous  increase  in  thickness,  but  new  bark  is 
as  continuously  produced  from  within  closing  up  the  fissures.  In  other  instances 
a  part  of  the  bark  falls  off  on  to  the  ground  in  consequence  of  the  thickening  of  the 
stem,  and  is  again  replaced  by  new  bark  from  within. 

Since  every  kind  of  tree  has  its  own  special  bark,  the  texture  and  colour  of 
this  structure  contributes  not  a  little  to  the  appearance  of  the  whole  tree;  it 
forms  one  of  the  characteristic  features  which  must  not  be  overlooked  when 
describing  the  habit  of  the  tree.  The  following  are  the  most  important  forms  of 
bark.  First  the  scale  bark,  which  is  detached  annually  in  the  form  of  shields 
and  plates,  to  be  seen  especially  well  in  the  stems  of  planes,  almond  willows, 


720  ERECT   FOLIAGE   STEMS. 

and  many  species  of  Australian  eucalyptus.  Then  the  membraneous  bark, 
which  separates  as  dry  films  and  ribbons;  this  form  of  bark  is  shown  in  the 
Common  Birch  (Betula  alba),  illustrated  opposite.  Many  species  of  the 
Australian  genus  Melaleuca  exhibit  a  bark  which,  when  stripped  from  the  stem, 
looks  deceptively  like  a  thin  silky  material.  A  third  form  is  the  ringed  bark, 
which  is  detached  from  the  stem  in  the  form  of  thin,  irregularly-fissured  tubes, 
and  is  especially  developed  in  the  Mock  Orange  (Philadelphus).  A  fourth  form, 
of  which  the  Vine  (Vitis  vinifera)  may  serve  as  an  example,  is  the  fibrous  bark 
which  is  detached  as  numerous  stiff  threads.  Finally  there  is  the  fissured  bark, 
which  is  produced  on  the  stems  of  the  oak,  lime,  ash,  and  numerous  other  leafy 
trees.  In  this  form  the  bark  is  not  detached  in  large  pieces,  but  is  ruptured  by 
the  increasing  thickness  of  the  stem,  causing  longitudinal  fissures  with  a  sinuous 
or  zigzag  course,  by  which  in  one  case  only  narrow  ridges  and  grooves,  and  in 
other  cases  broad  angular  patches  are  outlined.  Epiphytes,  especially  mosses  and 
lichens,  prefer  to  settle  on  fissured  bark,  and  older  stems  with  this  kind  of  bark 
are  in  temperate  regions  usually  overgrown  with  cushions  of  moss,  in  the  tropics 
with  ferns,  bromeliads,  and  orchids.  Such  a  colonization  would  be  impossible  in 
bark  which  falls  off  annually,  and  the  stems  of  plane  trees  are  not  only  free 
from  epiphytes,  but  always  look  as  if  they  had  been  scraped  or  peeled. 

The  form  of  the  bark  is  so  characteristic  that  by  it  alone  the  species  of  the 
tree  can  be  recognized;  it  therefore  constitutes  an  important  feature  in  the  picture 
of  a  tree,  nor  can  it  be  altered  according  to  fancy.  It  is  inadmissible  that  artists 
should  combine  the  studies  they  have  made  of  various  trees  as  they  please, 
perhaps  putting  the  crown  of  an  oak  on  the  trunk  of  a  plane.  That  the  colour 
of  the  bark  is  as  important  in  the  habit  as  the  tint  of  the  foliage  goes  without 
saying,  and  it  is  evident  that  the  relative  sizes  of  the  various  trees  round  about 
must  also  be  considered. 

The  height  and  age  of  trees  cannot  be  represented  in  definite  figures,  but  this 
much  is  certain,  that  every  species  of  tree,  just  like  every  species  of  animal,  is 
limited  to  a  certain  size  and  age  which  is  but  rarely  exceeded.  The  records  of 
age  which  have  come  down  to  us  are  for  the  most  part  too  great.  When  trees  of 
primeval  forests  are  said  to  be  a  thousand  years  old,  the  estimates  are  based  upon 
conjecture,  and  only  in  rare  cases  on  actual  measurements.  The  celebrated  Baobab 
(Adansonia  digitata)  was  reckoned  by  Adanson  on  the  ground  of  the  thickness 
of  the  annual  growth  to  be  about  5000  years  old,  but  whether  a  miscalculation 
has  not  crept  in  must  remain  uncertain.  The  age  of  the  celebrated  Dragon  Tree 
of  Orotava,  already  mentioned  once  before,  has  even  been  estimated  at  6000  years; 
the  Plane  of  Bujukdere,  on  the  Bosphorus,  at  4000;  and  the  so-called  Mexican 
Cedar  (Taxodium  Mexicanum)  was  estimated  by  Humboldt  at  4000  years.  I 
would  not  like  to  stand  security  for  these  numbers  either.  On  the  other  hand, 
the  following  extreme  limits  of  age  are  calculated  with  fair  accuracy: — For  the 
Cypress  (Cupressus  fastigiata),  3000  years;  the  Yew  (Taxus  baccata),  3000;  the 
Chestnut  (Castanea  vulgaris),  2000,  the  Oak  (Quercus  pedunculata\  2000;  the 


ERECT   FOLIAGE   STEMS. 


721 


VOL.  I. 


722 


ERECT   FOLIAGE   STEMS. 


Cedar  of  Lebanon  (Cedrus  Libani),  2000 ;  the  Spruce  Fir  (Abies  excelsa),  1200 ; 
the  Broad-leaved  Lime  (Tilia  grandifolia),  1000;  the  Arolla  Pine  (Pinus  Cembra), 
500-700;  the  Larch  (Larix  Europcea),  600;  the  Scotch  Pine  (Pinus  sylvestris), 
570;  the  Abele  (Populus  alba),  500;  the  Beech  (Fagus  sylvatica),  300;  the  Ash 
(Fraxinus  excelsior),  200-300;  the  Hornbeam  (Carpinus  Betulus),  150  years. 

The  certified  estimates  of  the  heights  of  trees  are  of   such   general  interest 
that  they  are  included  below  in  the  following  table: — 


Name. 

Height  in 
metres. 

Name. 

Height  in 
metres. 

Peppermint    Tree    (Eucalyptus   amyg- 

100-130 

Mexican  Cedar  (Taxodium  Mexicanum) 
Durmast  (Quercus  sessiliflora)  

387. 
35 

Mammoth  Tree  (Sequoia  gigantea)  

79-142. 

Plane  (Platanus  Orientalis)  

30. 

Silver  Fir  (Abies  %)ecti?iata) 

75 

Ash  (Fraxinus  excelsior) 

30 

Spruce  Fir  (Abies  excelsa)                

60 

Baobab  (Adansonia  digitata)  

23-1. 

Larch  (Larix  Europcea)    

537. 

Arolla  Pine  (Pinus  Cembra)  

22'7. 

Cypress  (Cupressus  fastigiata) 

52 

Tree  of  Heaven  (Ailanthus  glandulosa) 

22 

Scotch  Pine  (Pinus  sylvestris)      .  .  . 

48. 

Oak  (  Quercus  pedunculata)  

20. 

Beech  (Fagus  sylvatica)    

44. 

Hornbeam  (Carpinus  Betulus)  

20. 

Cedar  of  Lebanon  (Cedrus  Libani) 

40 

Yew  (Taxus  baccata)    

15. 

Abele  (Populus  alba).    .          

40. 

Eucalyptus  amygdalina  (represented  in  fig.  176,  after  a  drawing  by  Selleny), 
is  amongst  the  giants  of  the  vegetable  kingdom.  The  highest  of  these  stems 
placed  beside  St.  Paul's  Cathedral  would  tower  about  20  metres  above  the  cross, 
and  would  be  only  a  little  lower  than  Cologne  Cathedral. 

That  the  height  and  girth  of  trees  do  not  increase  proportionately  will  be 
seen  by  comparing  the  following  table  with  the  previous  one: — 


Name. 

Diameter 
of  trunk  in 
metres. 

Name. 

Diameter 
of  trunk  in 
metres. 

Chestnut  (  Castanea  vulgaris)    

20. 

Cypress  (Cupressus  fdstigiata) 

3*2 

Mexican  Cedar  (Taxodium  Mexicanum) 

16'5. 

Elm  (  Ulmus  campestris)    

3. 

Plane  (Platanus  Orientalis) 

15'4 

Silver  Fir  (Abies  vectinata) 

3 

DeciduousC  vpress(  Taxodium  distichum) 

11'9. 

Abele  (Populus  alba)  

2'8 

Mammoth  Tree  (Sequoia  gigantea)  

11. 

Beech  (Fagus  sylvatica)    

2. 

Baobab  (Adansonia  digit  ata)  

9-5. 

Spruce  Fir  (Abies  excelsa) 

2 

Broad-leaved  Lime  (Tilia  grandifolia) 

9. 

Arolla  Pine  (Pinus  Cembra)    

1'7 

Peppermint    Tree   (Eucalyptus  amyg- 

Ash  (Fraxinus  excelsior)  

17. 

dalina)  

8 

Larch  (Larix  Europcpa) 

1*6 

Oak  (  Quercus  pedunculata)  

7. 

Cornel  (Cornus  mas)  

1-4. 

Yew  (Taxus  baccata)    

4'9. 

Scotch  Pine  (Pinus  sylvestris) 

1 

Oak  (Quercus  sessiliftora)  

4'2 

Hornbeam  (Carpinus  Betulus) 

1 

Tree  of  Heaven  (Ailanthus  glandulosa) 

0-9. 

According  to  these  certified  estimates  there  actually  exist  plants  whose  stems 
attain  a  diameter  of  20  metres,  and  others  whose  stems  rise  to  a  height  of 
142  metres  above  the  ground. 


ERECT   FOLIAGE   STEMS. 


23 


176.— Eucalyptus  trees  iu  Australia.    (After  a  drawing  by  Selleny.) 


724         RESISTANCE   OF   FOLIAGE-STEMS  TO   STRAIN,   PRESSURE,   AND   BENDING. 


RESISTANCE  OF  FOLIAGE-STEMS  TO  STRAIN,  PRESSURE,  AND  BENDING. 

When  the  weight  of  the  individual  parts  of  these  huge  trees  is  considered,  it 
is  difficult  to  understand  how  their  comparatively  slender  main  stems  are  able  to 
support  a  crown  weighing  many  thousand  kilogrammes,  and  how  it  is  that  the 
boughs  extending  far  out  from  the  trunk  horizontally  do  not  crack  and  break 
under  the  weight  of  the  branches  and  leaves  they  carry.  The  culms  of  grasses 
and  the  stems  of  bushes  and  herbs  are  also  so  loaded  as  to  astonish  us,  and  we 
cannot  help  asking  how  it  is  they  are  able  to  keep  erect,  and  how,  when  their 
equilibrium  is  disturbed,  they  can  resume  their  normal  resting  position  almost 
at  once.  If  we  wish  to  investigate  the  mechanisms  which  make  it  possible 
for  these  plants  to  maintain  their  stems  in  this  position  without  assistance,  we 
must  in  the  first  place  consider  the  lowest  portion  of  the  erect  main  stem,  since 
it  is  that  part  which  would  naturally  have  the  heaviest  burden  to  carry.  Given 
that  the  pressure  caused  by  the  loading  operates  in  the  direction  of  the  axis,  the 
main  stem  must  exhibit  contrivances  enabling  it  to  resist  the  vertical  pressure; 
in  other  words,  it  must  possess  what  is  known  as  columnar  strength.  With  the 
exception  of  some  palms  whose  erect  stems  rise  up  like  pillars  from  the  ground, 
and  whose  leaves  project  equally  in  all  directions,  such  a  pressure,  acting  exactly 
in  the  direction  of  the  axis  of  the  stem,  is  but  rarely  found.  As  a  rule  some 
inequality  in  the  stem  or  crown,  although  perhaps  but  slight,  causes  the  pressure 
to  be  diverted  from  the  central  axis;  the  stem  is  bent  by  the  one-sided  burden, 
and  has  need  not  only  of  columnar  strength,  but  of  resistance  to  flexion  as  well. 
Winds  also  will  effect  a  bending,  not  only  by  direct  impact,  but  also  inasmuch  as 
they  displace  the  centre  of  gravity  of  the  load  sustained  by  the  lower  part  of  the 
stem.  Observation  shows  us  that  this  bending  is  only  rarely  followed  by  the 
fracture  of  the  stem.  Not  only  grasses  and  reed  culms,  but  also  the  thinner 
erect  branches  of  trees,  shrubs,  and  bushes,  and  even  palm  caudices  may  be  bent 
down  a  considerable  distance,  but,  when  the  wind  subsides,  quickly  return  to  their 
erect  position  without  having  suffered  the  least  harm. 

Formerly  but  little  attention  was  given  to  these  phenomena,  perhaps  because 
they  were  so  common  and  frequent,  or  perhaps  because  it  was  thought  to  be 
impossible  to  give  a  scientific  explanation  and  reason  for  the  swaying  of  branches 
in  the  wind.  It  was  reserved  for  modern  times  to  explain  the  mechanisms 
underlying  the  returning  of  bent  stems  to  a  definite  position  of  rest,  and  the 
contrivances  which  permit  such  stems  to  bend  but  not  break,  even  when  con- 
siderably loaded  and  under  strong  pressure.  Investigations  into  this  subject 
have  demonstrated  that  the  bearing  capacity  and  power  of  resisting  bending 
moment  in  plant  stems  are  obtained  by  structures  exactly  similar  to  those 
used  by  man  in  spanning  a  river  with  bridges,  in  fixing  the  supports  of  a  roof, 
of  wooden  partitions,  &c.  Further  that  the  principle  so  important  to  every 
builder,  the  obtaining  of  the  greatest  strength  possible  with  the  smallest  outlay 


RESISTANCE   OF   FOLIAGE-STEMS  TO  STRAIN,  PRESSURE,   AND   BENDING.          725 

of  material,  also  finds  expression  in  the  construction  of  the  stem.  In  one  case  we 
are  reminded  of  the  system  of  tubular  bridges,  in  other  cases  of  that  of  lattice- 
bridges;  here  of  a  massive  pillar-like  structure  with  architrave  and  flattened  top, 
there  of  a  Gothic  building  with  pointed  arches,  buttresses,  and  steep  gables;  but 
the  special  conditions  of  the  habitat  are  always  taken  into  consideration,  and 
the  whole  structure  for  this  reason  always  exhibits  the  greatest  adaptability  of 
the  means  to  the  end. 

The  framework  which  gives  the  desired  strength  to  the  whole  structure  is 
made  up  of  parts  which  would  be  called  by  builders  "constructive  pieces",  and 
these  are  in  turn  made  up  of  special  cells,  termed  mechanical  cells.  Mechanical 
cells  have  already  been  alluded  to  in  the  description  of  the  conducting-apparatus 
(cf.  p.  474),  although  only  very  briefly.  It  was  pointed  out  that  the  tubes  and 
cells  which  serve  for  the  transport  of  fluid  materials  up  and  down  the  plant  are 
usually  united  into  a  bundle,  the  so-called  vascular  or  conducting  bundle,  and 
that  when  the  constituent  parts  of  this  bundle  occur  in  organs  which  are 
exposed  to  the  danger  of  being  broken,  mechanical  cells  always  make  their 
appearance  alongside  the  conducting  cells  and  vessels.  The  delicate  vascular 
bundles  then  usually  lie  embedded  in  a  channel  of  hard  bast,  or  are  protected 
laterally  by  a  strand  of  hard  bast,  or  more  rarely  they  are  interposed  between 
two  bands  of  this  tissue.  These  strands  and  bands  of  hard  bast  are  frequently 
of  merely  local  importance  for  the  vascular  bundle,  and  may  be  likened  to  the 
strengthening  appliances  of  gas  and  water-pipes  in  human  dwellings,  which  are 
very  important  in  their  special  use,  but  do  not  help  to  strengthen  the  whole 
house.  Very  often,  however,  these  special  supporting  agents  of  vascular  bundles 
are  absent,  and  then  the  conducting  tissues  are  affixed  to  the  groups  of  mechanical 
cells  which  form  the  foundation-framework  of  the  whole  structure. 

The  hard  bast  is  the  mechanical  tissue  most  often  employed  in  both  cases.  To 
the  naked  eye  the  cells  of  hard  bast  look  like  tiny  threads.  They  are  elongated, 
fusiform,  pointed  at  both  ends,  and  interlaced  and  dovetailed  with  one  another  as 
shown  in  fig.  125  5  (p.  469).  They  are  generally  about  1-2  mm.  long,  but  in  certain 
cases  attain  a  much  greater  length;  those  of  the  Hemp  are  10,  those  of  Flax  20-40, 
of  the  Nettle  77,  and  of  Boehmeria  nivea  even  220  mm.  long.  The  walls  of  hard 
bast  cells  are  always  very  much  thickened,  and  the  cell-cavity  is  very  narrow,  often 
being  reduced  to  an  exceedingly  fine  canal,  in  some  cases,  e.g.  in  the  cells  of  hard 
bast  of  Gorchorus  olitorius  (known  as  Jute),  the  canal  here  and  there  is  quite 
obliterated,  so  that  the  cell  is  transformed  into  a  solid  fibre.  It  is  concluded  from 
the  direction  of  pores  which  sometimes  appear  in  the  walls  that  the  micellae  which 
build  up  the  walls  of  these  thick  bast-cells  are  arranged  in  left-handed  spiral  lines, 
and  this  spiral  torsion  is  supposed  to  be  connected  with  the  strength  of  the  whole 
hard  bast  cell.  It  is  known  that  bundles  of  straight  threads  are  not  as  strong  as 
bundles  twisted  into  a  string,  and  we  are  justified  in  supposing  that  this  is  also  the 
case  with  the  rows  of  micellae  forming  the  extremely  fine  fibrillse  in  the  walls  of 
the  hard  bast  cell.  When  a  cell  of  hard  bast  is  fully  developed,  the  living  proto 


726         RESISTANCE   OF   FOLIAGE-STEMS  TO   STRAIN,   PRESSURE,   AND   BENDING. 

plasm  disappears  from  its  interior,  and  the  narrow  space  of  the  cell-cavity  becomes 
filled  with  air,  or  less  often  with  a  watery  fluid.  The  cell  can  then  no  longer 
continue  its  growth,  neither  can  it  serve  to  take  up  and  conduct  food  nor  to 
manufacture  organic  compounds;  it  cannot  be  employed  in  transformations  and 
transmission  of  materials,  and  has  exclusively  an  architectural  significance.  It  is 
excellently  adapted,  however,  to  the  task  thus  assigned  to  it.  Its  strength  and 
elasticity  are  indeed  extraordinary.  It  has  been  estimated  that  the  bearing  capacity 
of  hard  bast  amounts  to  between  15  and  20  kg.  to  the  sq.  mm.  in  cross  section,  and 
is  therefore  equal  to  that  of  wrought  iron;  indeed  the  bearing  capacity  of  many 
species  of  plants  is  even  equal  to  that  of  steel.  Hard  bast  has  this  advantage  over 
iron,  that  it  is  far  more  extensible  and  consequently  less  subject  to  breaking.  From 
the  consideration  of  all  these  properties  it  becomes  evident  why  the  hard  bast  of 
many  plants  has  been  used  by  man  to  such  advantage  in  the  manufacture  of  fabrics, 
string,  ropes,  and  the  like,  since  very  remote  times. 

Woody  fibres,  also  known  as  libriform  cells,  differ  very  little  from  hard  bast 
cells.  Whilst  hard  bast  forms  one  of  the  most  important  constituents  of  the  cortex, 
the  woody  fibres  form  an  essential  element  in  the  wood  of  those  stems  which 
annually  add  a  new  layer  to  the  already  existing  wood,  thus  increasing  in 
circumference  and  exhibiting  annual  rings  in  cross  section.  Their  length  varies 
between  0'3  and  T3  mm.,  so  that  they  are  somewhat  shorter  than  the  fibres  of  the 
bast.  Their  walls  are  as  a  rule  strongly  lignified,  but  in  other  respects  it  is 
impossible  to  draw  a  sharp  line  between  the  two  forms  of  cells.  When  a  woody 
stem  has  grown  in  thickness  and  has  developed  bark  on  its  periphery,  the  role 
played  by  the  hard  bast  in  the  cortex  is  evidently  at  an  end;  the  woody  fibres 
then  assume  the  tasks  which  in  the  young  shoots  are  allotted  to  the  hard  bast, 
and  they  might  therefore  be  called  the  hard  bast  cells  of  the  wood. 

In  many  plants  a  special  form  of  mechanical  cell-tissue  is  developed,  known 
as  collenchyma.  The  cells  which  compose  it  are  elongated  and  connected  with  one 
another  just  like  hard  bast  cells,  but  they  differ  from  these  and  from  the  woody 
fibres  in  the  fact  that  their  walls  are  unequally  thickened.  Where  three  or  four 
of  these  cells  adjoin  one  another  by  their  long  sides  the  walls  are  very  thick,  but 
in  places  the  wall  common  to  two  neighbouring  cells  remains  thin;  the  whole  of 
the  tissue  may  be  compared  to  a  building  in  which  thick  main  walls  alternate 
with  thin  partitions  which  are  strengthened  here  and  there  with  quartering,  and 
attain  a  great  supporting  capacity.  A  further  distinction  from  hard  bast  cells  and 
woody  fibres  consists  in  the  fact  that  living  protoplasm  remains  in  the  interior  of 
collenchymatous  cells  in  which  chlorophyll-corpuscles  are  often  embedded;  more- 
over, this  protoplasm  can  draw  some  of  the  materials  necessary  for  growth  through 
the  thin  places  in  the  walls  from  the  surronding  tissue,  and  can  employ  these  as 
building  materials; — in  a  word,  the  collenchyma  is  capable  of  further  growth. 
This  explains  the  advantage  of  collenchyma  over  hard  bast  cells,  and  woody  fibres 
or  libriform  cells.  The  hard  bast  and  libriform  cells  when  once  fully  formed  lose 
their  capacity  of  further  development,  and  would  therefore  be  of  little  use  as 


RESISTANCE   OF   FOLIAGE-STEMS   TO   STRAIN,   PRESSURE,   AND   BENDING.          727 

architectural  elements  in  a  still  growing  portion  of  the  stem;  they  would  either 
prevent  the  lengthening  of  the  other  tissues,  or  would  be  ruptured  by  the  force 
of  the  elongating  cells,  and  in  both  instances  would  be  injurious.  The  collenchy- 
matous  cells,  on  the  contrary,  are  able  to  continue  developing,  they  can  elongate 
and  grow  with  the  other  tissues,  and  may  be  compared  with  the  scaffolding  of  a 
several-storied  building,  which  is  constantly  being  raised  as  the  work  progresses. 
The  collenchyma,  of  course,  has  this  disadvantage  when  compared  with  the  hard 
bast  and  libriform  fibres,  that  its  absolute  strength  is  somewhat  less;  its  bearing 
capacity  is  only  10-12  kg.  to  the  sq.  mm.  in  cross-section.  The  limits  of  elasticity 
of  the  collenchyma  are  also  considerably  less,  but  where  hard  bast  or  libriform  cells 
would  be  unsuitable,  from  the  reasons  stated  above,  collenchyma  replaces  it.  It 
cannot  be  said  that  hard  bast  and  libriform  fibres  are  more  important  than 
collenchyma;  each  in  its  own  way  has  an  especial  architectural  value,  and  some- 
times the  one,  sometimes  the  other,  is  the  more  advantageous. 

The  hard  bast,  libriform  cells,  and  collenchyma  which  are  comprehended  under 
the  common  term  mechanical  tissue  are  usually  arranged  in  strands  running 
parallel  to  the  long  axis  of  the  stem.  If  they  were  confined  to  the  centre  it 
would  be  anything  but  a  suitable  arrangement,  for  an  erect  stem;  they  would 
contribute  almost  nothing  to  the  resistance  to  flexion  as  will  be  seen  from  the 
following  considerations.  Let  us  imagine  a  horizontal,  cylindrical  stem  resting 
on  solid  supports  at  either  end  and  loaded  in  the  middle;  it  will  bend  downwards 
in  proportion  to  the  load  laid  on  it,  and  in  doing  so  the  concave  side  will  be 
shortened  and  the  convex  side  lengthened;  the  shortened  side  will  be  subjected 
to  compression  and  the  elongated  side  to  tension.  These  forces  will  be  greatest 
at  the  periphery,  on  the  upper  and  under  limiting  surfaces,  of  the  bent  stem.  The 
opposed  forces  diminish  towards  the  middle  of  the  stem,  and  completely  vanish  at 
the  centre,  therefore,  in  order  that  the  stem  should  resist  bending  as  much  as 
possible,  it  is  obvious  that  the  strengthening  material  is  best  applied  when 
wholly  used  in  the  form  of  flat  plates  where  the  forces  are  greatest.  These 
particular  constructive  pieces  are  known  technically  as  flanges,  and  a  flange  is 
fixed  at  either  side  of  a  beam  which  requires  to  be  strengthened  against  flexion. 
The  mass  lying  between  the  two  flanges  is  called  the  web,  and  the  whole  beam 
so  constructed  is  termed  a  girder.  Fig.  177  1  gives  a  diagrammatic  representation 
of  such  a  girder  in  cross-section.  The  web  may  be  composed  of  much  softer 
material  than  the  flanges;  it  may  consist  of  a  lattice-  or  merely  of  a  frame-work. 
Where  these  girders  are  developed  in  plants,  the  web  consists  of  vascular  bundles 
or  of  parenchymatous  cells,  while  the  flanges  are  always  built  up  of  mechanical 
tissue.  In  flat,  extended  foliage-leaves  the  girders  are  fitted  in  so  that  their  flanges 
are  parallel  to  the  upper  and  lower  surfaces  of  the  leaf,  but  these  leaves  only  resist 
bending  in  one  plane.  This  construction,  which  can  be  seen  in  the  leaf-sections 
given  in  figs.  86  1  and  87  3  (pp.  342-343),  would  be  ill-adapted  to  stems.  ^  An  erect 
stem  which  is  struck  by  the  wind,  sometimes  from  one  side  and  sometimes  from 
another,  must  be  strengthened  indifferently  on  every  side,  and  in  accordance 


728          RESISTANCE   OF    FOLIAGE-STEMS   TO   STRAIN,    PRESSURE,    AND   BENDING. 

with  this  demand  the  most  different  kinds  of  combinations  of  girders  are  seen 
developed  in  it.  Usually  several,  at  least  two,  but  often  very  many  girders 
are  so  combined  that  they  traverse  the  axis  in  common,  as  shown  in  the  dia- 
grammatic cross-sections  in  figs.  1772'3-4.  In  this  case  all  the  flanges  are  on 
the  periphery  of  the  stem,  and  every  pair — diametrically  opposite  one  another — 
must  be  regarded  as  belonging  to  the  same  girder.  In  many  stems  all  the  flanges 
have  a  parallel  course;  in  other  cases  they  are  bent  in  and  out,  and  so  connected 
together  as  to  form  a  lattice-work  of  the  most  complicated  kind.  In  other  cases 


Fig.  177. — Diagrammatic  representation  of  various  combined  girders. 

i  A  simple  I  (or  double  T)  girder.  *  Two  combined  girders,  arranged  crosswise.  '  Three  combined  girders.  *  Six  combined 
girders;  the  flanges  are  laterally  in  contact  to  form  a  cylindrical  tube.  «  Four  combined  girders;  their  flanges  are 
formed  of  secondary  girders.  In  Figs.  2-*  the  web  of  the  girders  is  indicated  by  dotted  lines. 

all  the  flanges  lying  near  the  periphery  of  the  stem  are  fused  together  (fig.  177  4) 
so  as  to  form  a  cylindrical  tube,  in  which  case  the  web  is  not  required  and  the 
stem  is  either  hollow  inside,  or  is  filled  only  with  a  loose  pith.  Sometimes  each 
separate  flange  is  itself  transformed  into  a  girder,  and  in  this  way  the  flanges  of 
the  chief  girder  become  secondary  girders,  as  represented  in  fig.  177  5.  There  is 
almost  as  great  a  variety  in  this  matter  as  there  is  in  the  arrangement  of  the 
strands  of  leaves,  but  since  researches  into  the  course  and  grouping  of  the  strands 
of  mechanical  tissue  in  stems  are  still  not  far  enough  advanced  for  us  to  be  able 
to  place  the  various  forms  in  well-arranged  series,  we  must  content  ourselves  with 
sketching  the  most  noticeable  cases. 


RESISTANCE    OF    FOLIAGE-STEMS   TO   STRAIN,   PRESSURE,    AND   BENDING.          729 

First  we  will  give  a  general  idea  of  the  distribution  of  mechanical  tissue,  in  as 
far  as  it  enables  erect  stems  to  resist  bending.  We  can  distinguish  three  groups  of 
forms  in  this  respect.  The  first  group  includes  forms  with  simple  girders  whose 
flanges  of  hard  bast  are  placed  as  near  the  periphery  as  possible,  but  are  not  fused 
together  into  a  cylindrical  tube.  The  line  connecting  every  pair  of  flanges  passes 
through  the  axis  of  the  stem.  To  this  group  belong  almost  all  young  stems  of 
woody  plants,  e.g.  those  of  willows,  oaks,  elms,  maples,  and  limes  (cf.  fig.  1781). 
Special  emphasis  must  be  laid  on  the  words  "young  stems",  since  in  the  older  stems 
of  these  trees — when  the  wood  has  become  thickened — the  hard  bast  on  the  outer 
side  of  the  cambium-ring,  and  therefore  outside  the  vascular  bundle,  has  finished  its 
task,  and  its  functions  are  transferred  to  the  wood,  more  especially  to  the  woody 
fibres  (libriform  cells)  (cf.  p.  726). 

In  the  erect  stems  of  undershrubs  belonging  to  this  group  the  simple  girders  are 


Fig.  178.— Transverse  sections  of  erect  foliage-stems  with  simple  girders  not  fused  together  into  a  tube. 

i  One-year-old  branch  of  the  Broad-leaved  Lime  (TUia  grandifolia).  »  White  Dead-nettle  (Lamium  album).  »  Date  Palm 
(Phoenix  dactyl\fera).  In  these  diagrammatic  figures  the  mechanical  tissue  is  grey  and  the  vascular  bundles  black  will 
white  spots. 

very  often  assisted  by  collenchymatous  strands  which  lie  close  to  the  periphery  of 
the  stem,  and  are  arranged  so  that  each  strand  appears  to  strengthen  the  bundle 
of  hard  bast  forming  a  flange.     Fig.  178 2  shows  a  transverse  section  of  a  stem  of 
an  undershrub  belonging  to  this  group,  the  White  Dead-nettle  (Lamium  album), 
in  which  the  further  peculiarity  is  noticeable,  that  the  strengthening,  collenchy- 
matous strands  in  the  corners  of  the  four-sided  stem  are  thick  and  pillar-like, 
while  those  at  the  sides  of  the  stem  are  broad  and  flattened.     This  condition  is  not 
an  uncommon  one.     In  palms,  of  which  the  diagrammatic  cross  section  of  the  Date 
Palm  (Phcenix  dactylifera,  fig.  1783)  may  serve  as  a  type,  the  strands  accessory  t 
the  simple  girders  are  in  the  form  of  numerous  bundles  of  hard  bast  developed  on 
the  periphery  of  the  stem,  but  not  exactly  in  front  of  the  flanges  o 
These  bundles  of  hard  bast  are  always  in  pairs  opposite  one  another,  and 
be  regarded  as  the  flanges  of  special  girders.     In  these  cases  the  number  c 
is  always  very  large,  and  the  flanges  appear  in  two,  three,  or  even  more  circles  in  a 
cross  section  of  the  stem.     Sometimes  also  two  or  three  adjoining  flanges  are  fused 


730         RESISTANCE  OF   FOLIAGE-STEMS  TO   STRAIN,   PRESSURE,   AND   BENDING. 

laterally  with  one  another,  and  form  what  may  be  regarded  as  a  link  connecting 
this  with  the  following  group. 

The  second  group  comprises  all  stems  in  which  the  flanges  of  numerous  simple 
girders  are  fused  laterally  so  as  to  form  a  cylindrical  tube.  This  tube  lies  as  near 
to  the  periphery  as  possible,  and  consists  of  hard  bast  developed  from  the  bast 
portions  of  the  originally  distinct  vascular  bundles.  In  consequence  of  this  the 
vascular  bundle  is  always  in  connection  with  the  hard  bast  tube.  The  various 
methods  of  connection,  and  the  presence  or  absence  of  accessories  to  this  bast  tube 


Fig.  179.— Transverse  sections  of  erect  foliage-stems  with  simple  girders  fused  into  cylindrical  tubes. 

i  Crow  Garlic  (Allium  vineale).  2  Carnation  (Dianthus  CaryopJiyllus).  8  Polygonatum  verticillatum.  *  Purple  Molinia  (Molinto 
co&rulea.  «  Woodruff  (Asperula  odorata).  6  Sumbul  (Euryangium  Sumbul).  In  these  diagrams  the  mechanical  tissue  ia 
represented  grey,  and  the  vascular  bundles  black  with  white  spots. 

in  its  resistance  to  flexion,  give  rise  in  this  group  to  a  great  multiplicity  of  structure. 
Some  of  the  most  interesting  forms  are  represented  in  fig.  179.  In  fig.  1792,  the 
cross  section  of  the  stem  of  a  Carnation  (Dianthus  Caryophyllus),  the  vascular 
bundles  are  situated  on  the  inner  side  of  the  bast  ring;  in  fig.  179  \  a  transverse 
section  of  the  stem  of  a  species  of  garlic  (Allium  vineale),  the  bundles  are  partially 
embedded  in  the  outer  part  of  the  bast  ring,  and  in  179  3,  a  transverse  section  of  the 
stem  of  a  species  of  Solomon's  Seal  (Polygonatum  verticillatum),  the  bundles  are 
wholly  embedded  in  the  ring  of  bast.  The  first  case  is  by  far  the  most  common,  and 
may  be  regarded  as  characteristic  of  most  dicotyledonous  herbs  and  undershrubs;  the 
second  case  obtains  in  many  bulbous  plants;  whilst  the  third,  the  rarest  of  all,  is 
only  found  in  a  few  monocotyledons.  The  accessory  parts  occur  either  as  band-like 


RESISTANCE   OF   FOLIAGE-STEMS  TO   STRAIN,   PRESSURE,   AND   BENDING.          731 

projections  from  the  bast  tube,  e.g.  in  the  grass  Molinia  ccerulea  (fig.  179 4),  or  as 
independent  collenchymatous  strands  in  the  corners  of  the  angular  stem,  as  in  the 
Woodruff  (Asperula  odorata,  fig.  179  5),  or,  again,  a  circle  of  independent  bundles  of 
hard  bast  appears  outside  the  bast  tube,  as  in  the  stately  umbellifer  Euryangium 
Sumbul  (fig.  1796).  In  this  plant  the  strengthening  accessories  are  combined  into 
independent  simple  girders  and  a  canal  filled  with  air  is  situated  on  the  inner  side 
of  each  of  the  flanges  (cf.  fig.  179 6). 

The  third  group  consists  of  all  stems  in  which  the  flanges  are  developed  as 


QBD 


(SB 


Fig.  180.— Transverse  sections  of  erect  foliage-stems  with  flanges  developed  as  secondary  girders. 

i  Tufted  Scirpus  (Seirpus  casspitosus).  *  Perfoliate  Silphium  (Silphium  perfoliatum).  •  Black-stemmed  Bamboo  (Bambuta 
nigra).  *  Hard  Rush  (Juncus  glaucus).  *  Common  Reed  (Phragmites  communis).  •  Sugar-cane  (Saccharum  offlcinarum). 
In  these  diagrammatic  figures  the  mechanical  tissue  is  represented  grey,  and  the  vascular  bundles  black  with  white  spots. 

secondary  girders.  The  web  in  these  secondary  girders  always  consists  of  vascular 
bundles,  and  the  flanges  themselves  of  hard  bast.  Sometimes  the  secondary  girders 
are  arranged  in  a  single  circle,  but  in  most  instances  they  form  several  concentric 
rings.  In  fig.  180  some  of  the  most  striking  forms  of  this  group  are  given  diagram- 
matically.  Fig.  ISO1  represents  a  transverse  section  of  a  stem  of  Scirpus  cces- 
pitosus,  in  which  the  secondary  girders— arranged  in  a  single  circle— alternate  with 
large  air-spaces;  fig.  ISO2  shows  a  similar  section  of  the  stem  of  the  composite  illus- 
trated on  p.  239  (Silphiwn  perfoliatum),  with  its  numerous  series  of  secondary 
girders  parallel  to  the  four  sides;  and  fig.  ISO3  is  the  transverse  section  of  a  bamboo 
(Bambusa  nigra)  in  which  the  secondary  girders  are  grouped  in  several  concei 
rings.  Here,  as  in  the  first  and  second  groups,  accessory  structures  are  pn 


732          RESISTANCE   OF   FOLIAGE-STEMS   TO   STRAIN,   PRESSURE,   AND   BENDING. 

usually  in  the  shape  of  tubes  of  hard  bast,  or  as  collenchymatous  strands  at  the 
circumference  of  the  stem.  In  the  common  Reed  (Phragmites  communis)  this  tube 
is  quite  uninterrupted  and  intact  (fig.  ISO5);  whilst  in  the  Sugar-cane  (Saccharum 
officinarum,  fig.  180  6)  air-canals  and  vascular  bundles  are  embedded  in  it.  Much 
less  frequently  the  strengthening  is  produced  by  bundles  of  bast  which  lie  close 
under  the  epidermis  of  the  stem  and  are  not  fused  into  a  tube,  as,  for  example,  in 
the  Hard  Rush  (Juncus  glaucus),  the  transverse  section  of  whose  stem  is  shown 
in  fig.  ISO4.  This  rush  is  also  characterized  by  the  insertion  of  large  air-spaces 
between  the  accessory  strands  which  form  the  outer  circle.  Some  of  the  erect  stems 
here  cited  which  resist  bending  are  hollow  within,  whilst  others  are  filled  with  a 
loose  pith.  In  the  diagrammatic  figures  the  central  cavity  has  been  marked  off  by 
a  circular  line. 

We  should  naturally  expect  to  find  that  stems  which  are  not  able  to  rise  from 
the  ground  without  external  support  (including  those  numerous  forms  which  are 
comprehended  under  lianes),  would  exhibit  a  structure  different  from  that  of  erect 
stems.  .  In  climbing  plants  the  young  shoots  alone  require  to  resist  bending;  stems 
which  have  found  a  support  can  dispense  with  this  property,  and  consequently  with 
contrivances  designed  for  this  purpose.  On  the  other  hand,  these  plants,  especially 
when  perennial  and  lignified,  must  be  protected  against  strains  which  are  unavoid- 
able in  consequence  of  alterations  occurring  in  their  supports.  Rocky  walls  and  old 
battlements  overgrown  with  climbing  plants,  of  course,  do  not  alter  sufficiently  to 
materially  affect  the  stems  attached  to  them;  but  it  is  otherwise  where  the  climber 
is  supported  by  a  thickening  stem.  This  class  of  support  continues  to  grow,  its 
stem  increases  in  volume,  the  extent  of  the  boughs  and  branches  differs  from  year 
to  year,  and  displacements  and  alterations  in  position  occur  which  cannot  but  influ- 
ence the  plants  climbing  over  them.  Suppose  a  twining  plant  has  embraced  and 
twined  around  the  stem  of  a  young  tree  or  the  branch  of  a  young  shrub;  years  pass 
by  and  the  stem  of  the  tree  has  meanwhile  increased  a  hundredfold  in  diameter, 
and  the  entwined  branch  of  the  shrub  has  been  shifted  about  a  metre;  this 
cannot  be  without  effect  on  the  twining  stem,  and  it  requires  no  further  explanation 
to  see  that  it  will  exert  a  pull  and  lateral  pressure.  Perennial  twining  plants  must 
therefore  be  so  organized  that  their  stem  will  bear  tension  and  lateral  pressure 
without  injury,  in  other  words,  that  their  skin  must  be  constructed  to  resist  tension 
and  compression.  Resistance  to  strain  is  obtained  in  twining  and  interweaving  stems 
in  very  different  ways;  in  many  cases,  such  as  in  the  Rotang  or  Climbing  Palm,  by 
ample  depositions  of  hard  bast  in  the  vascular  bundles  lying  next  to  the  axis  of  the 
stem;  in  other  cases,  e.g.  in  Tamus  and  Dioscorea,  by  a  considerable  thickening  of 
the  cells  of  the  pith,  and  in  others,  again,  e.g.  in  many  species  of  Pepper,  by  the 
development  of  a  ring  of  mechanical  cells  within  the  peripheral  circle  of  vascular 
bundles.  It  is,  of  course,  an  advantage  to  the  twining  stem  which  requires  protec- 
tion against  strain  if  the  tissues  lying  next  its  centre  possess  a  corresponding  firm- 
ness. Thus  we  find  unmistakable  differences  between  these  and  erect  stems;  corre- 
lated with  this  is  the  fact  that  the  pith,  or  the  medullary  cavity,  in  twining  stems 


RESISTANCE    OF   FOLIAGE-STEMS   TO   STRAIN,   PRESSURE,   AND   BENDING.          733 

is  very  much  reduced,  and  that  hollow  twining  stems,  e.g.  that  of  Thunbergia  lauri- 
folia  (cf.  fig.  128  \  p.  477)  are  very  rare.  Perennial  twining  stems  are  usually 
protected  from  lateral  pressure  by  a  layer  of  collenchyma  surrounding  the  conduct- 
ing tissues  like  a  mantle.  Sometimes  the  collenchyma  is  also  connected  with  bundles 
of  bast,  and  there  is  no  doubt  that  the  same  mechanical  cells  which  strengthen  the 
young  twining  stem  protect  it  later  on  against  lateral  pressure. 

Perennial  climbers  which  have  clambered  up  growing  woody  plants  are  exposed 
to  the  same  dangers  as  described  in  the  case  of  twining  and  interweaving  plants, 
but  in  them  tendrils  as  a  rule  afford  a  protection  against  tearing,  and  tissues  pro- 
viding a  resistance  to  strain  are  absent  from  the  stems  themselves.  In  such  plants 
it  is  the  tendrils  especially  which  are  constructed  to  resist 
tension,  as,  for  example,  in  the  Atragene  (Atragene 
alpina,  the  stem  of  which  is  shown  in  cross  section  in 
fig.  181).  Tendrils,  therefore,  are  evidently  of  complex 
structure.  First,  they  must  have  a  great  capacity  of 
resisting  strain,  but  since  they  also  have  other  functions 
to  perform,  and  since  these  functions  are  different  before 
and  after  the  attachment  to  the  support,  very  remarkable 
alterations  in  their  inner  structure  must  occur  during 
development.  At  first  they  are  required  to  resist  flexion,  rig.  isi.-Transverse  section  of  the 

...  .         •      i    j-  •       i         i          j  climbing  stem  of   the  Atragene 

for  which  purpose  mechanical  tissue  is  developed  round       (Atragen*  alpina).    The  tissues 

.    ,  ,    ,  .11  •    i  •  v  •   r  are  represented  in  the  following 

the  periphery;  later  on  they  have  to  resist  tension  which       way:-soft  bast,  entirely  black; 


renders   it   necessary  that  mechanical  tissue  should   be 

developed  nearer  the  axis.      An  abundant  development     .  chanicai  tissues,  obliquely  shaded; 

the  cork  (penderm),    stratified; 

of  mechanical  tissue  is  also  required  on  the  convex  side       the  loose  reticuiar  tissue,  white 

with  dark  reticulations. 

of  the  tendril  bending  round  the  support  so  as  to  increase 

the  resistance  to  strain  at  that  part,  as  also  to  prevent  its  unrolling  from  the  sup- 

port; such  a  development  is  actually  to  be  seen  in  all  tendrils. 

Older  lignified  stems  of  climbing  and  twining  plants  often  exhibit  a  longi- 
tudinal splitting  in  the  wood.  Before  they  obtain  their  split  appearance  the 
narrow  vascular  bundles,  which  consist  for  the  main  part  of  wood,  are  isolated  by 
a  loose,  wide-meshed  tissue,  and  there  is  no  central  pith.  In  transverse  section 
the  narrow  vascular  bundles  of  such  a  stem  resemble  the  spokes  of  a  wheel,  the 
weakly-developed  mechanical  tissue,  which  had  served  to  protect  against  bending 
in  the  one-year-old  stem,  together  with  the  cork  (peridenn),  forming  to  some 
extent  the  rim  of  the  wheel  (cf.  fig.  181). 

When  lateral  pressure  is  brought  to  bear  on  these  old  stems,  the  cork  am 
hard  bast  become  ruptured  at  the  places  acted  on,  but  only  above  the  dead,  large- 
meshed  tissue;  above  the  narrow  vascular  bundles  they  remain  uninjured. 
loose    dead   tissue   also   ruptures   and    crumbles,   and   falls   out   of   the   groove* 
between   the  vascular  bundles.     These  bundles,  which   now  resemble  plates 
lamellae  of  wood,  lie  above  one  another  like  the  leaves  of  a  book  on  the  aid 
the  pressure  is  felt.     The  wood  looks  as  if  it  had  been  divided  or  split 


734         RESISTANCE  OF   FOLIAGE-STEMS  TO   STRAIN,   PRESSURE,   AND   BENDING. 

tudinally  in  this  way.  These  proceedings  have  no  disturbing  influence  on  the 
functions  of  the  vascular  bundles,  on  the  conducting  power  of  the  wood,  or  on 
that  of  the  soft  bast,  though  by  the  compression  of  the  woody  plates  the  shape 
of  the  cross  section  of  the  stem  is  altered.  The  lateral  pressure  exerted  on  the 
broad  side  of  the  plate-shaped  vascular  bundle  is  now  harmless,  and  interrupts  the 
transport  of  the  sap  neither  in  the  wood  nor  in  the  soft  bast. 


Fig.  182. — Undulations  of  old  ribbon-shaped  liane  stems  (Bauhinia  anguina)  from  an  Indian  jungle. 


It  has  already  been  shown  on  p.  477  in  one  example  (Rhynchosia  phaseoloides) 
that  injuries  due  to  lateral  pressure  in  the  conducting  tissues,  especially  in  the 
soft  bast,  are  also  prevented  in  twining  or  climbing  plants  by  the  development  of 
ribbon-shaped  stems,  and  it  need  only  be  added  here  that  with  this  flattening 
and  ribbon-like  shaping  of  the  wood,  and  with  the  development  of  these  wings, 
there  is  combined  an  economy  of  building  materials.  If  the  stem  were  cylindrical, 
an  abundant  mechanical  tissue  would  have  to  be  developed  for  the  protection  of  the 


occur 


RESISTANCE  OF   FOLIAGE-STEMS  TO  STRAIN,  PRESSURE,  AND   BENDING.          735 

soft  bast  against  lateral  pressure.  The  ribbon-shaped  stem,  however,  can  do  very 
well  without  this,  for  the  pressure  along  its  edge  is  scarcely  worth  considering, 
and  the  soft  bast  is  excellently  protected  against  pressure  on  the  broad  side  by 
the  wood,  whlch  is  broken  up  into  a  number  of  detached  masses  with  the  soft  bart 
between. 

There  is  no  doubt  that  the  spiral  torsion  of  ribbon-like  lianes  (which  is  plainly 
shown  m  the  illustration  of  Mhynchvsia  phaseoloides,  fig.  127,  on  p.  475)  increases  the 
resistance  to  strain,  a  matter  of  some  importance  in  all  cases  where  growing  trees 
or  shrubs  serve  as  supports,  and  where  straining  of  the  lianes  clinging  to  them 
is  unavoidable. 

The  undulations  of  ribbon-shaped  liane  stems  in  tropical  forests  may  also  be 
regarded  as  a  protection  for  the  sap-conducting  tissues  against  strain.  They 
in  many  bauhinias  and  in 
the  peculiar  species  of  Cau- 
lotretus  known  as  monkey- 
ladders.  The  central  part 
of  the  ribbon-shaped  stem 
is  alone  strongly  undulated, 
as  may  be  seen  in  the  por- 
tions of  a  Bauhinia  repre- 
sented in  fig.  182;  the  two 
edges  are  much  less  curved 
and  are  of  ten  quite  straight, 
forming  a  framework  for 
the  sinuous  middle  part. 
In  the  case  of  a  longitudinal 
tension,  at  first  only  the 

frame  is  affected,  the  tissues  in  the  centre  can  still  uninterruptedly  conduct  the  sap 
to  and  from  the  branches  which  arise  from  its  broad  surface. 

Stems  of  water-plants  as  well  as  those  embedded  in  the  ground,  and  the  stem- 
structures  which  lie  on  the  surface  of  the  ground,  have,  like  climbing  plants,  little 
need  for  resisting  flexion,  but,  on  the  other  hand,  require  a  greater  resistance  to 
pressure  and  strain.  The  soil  or  the  surrounding  water  forms  the  immediate  sup- 
port for  all  these  stems,  and  the  arrangement  of  tissues  suited  to  erect  aerial  stems 
would  be  useless  here. 

As  a  matter  of  fact  they  do  not  possess  the  peripheral  strands  of  hard  bast 
and  collenchyma  so  characteristic  of  erect  stem -structures;  the  vascular  bundles 
are  placed  together  near  the  centre  of  the  stem,  as  is  most  advantageous  for  organs 
which  have  to  resist  strain,  and  the  bast  strands  belonging  to  these  bundles  are 
relatively  far  removed  from  the  circumference  of  the  stem.  The  central  pith  is 
much  reduced  and  is  often  completely  absent  (cf.  the  diagrammatic  sections  of  a 
runner  of  the  Garden  Strawberry,  Fragaria  grandiflora,  and  of  a  hydrophyte, 
Myriophyllum  spicatum,  in  the  above  figure). 


Fig.  183. 

*  Transverse  section  of  a  runner  of  the  Garden  Strawberry  (Fragaria  grandijlora) 
which  lies  on  the  ground.  »  Transverse  section  of  the  stem  of  the  Water 
Milfoil  (Myriophyllum  spicatum).  In  these  diagrammatic  figures  the  mechani- 
cal tissue  is  represented  grey,  and  the  rascular  bundles  black  with  white 
spots. 


736  THE   FLORAL  STEM. 

The  stems  here  considered  are  protected  against  the  lateral  pressure  by  a  layer 
of  thick-walled  parenchyma  (183 1),  or  by  the  strands  of  tissue  crossing  the  larger 
air-canals  which  run  longitudinally  outside  the  circle  of  vascular  bundles  in  the 
stem  (183 2).  In  the  underground  stems  of  the  Grass  of  Parnassus  (Parnassia 
palustris),  and  of  several  other  herbaceous  plants,  there  is  no  pith,  they  exhibit 
a  central  strand  of  compressed  vascular  bundles  and  their  structure  is  very  similar 
to  that  of  roots  growing  in  the  ground. 

From  this  general  account  it  is  sufficiently  evident  that  the  arrangement  of 
the  tissues  in  stems  does  not  so  much  depend  upon  whether  the  part  in  question 
belongs  to  a  scaly  stem,  a  foliage  stem,  or  a  floral  stem,  but  rather  upon  its 
relations  with  the  outer  world,  and  in  particular  upon  the  influences  exercised  by 
the  surroundings  serving  as  a  support  or  substratum.  The  stem,  as  the  bearer  of 
the  foliage  and  flowers,  must  be  so  constructed  that  the  organs  named  may  be 
raised  into  the  air,  sunned,  exposed  to  the  wind  and  to  the  visits  of  flying  insects 
and  birds,  and  retained  in  the  most  advantageous  posture  in  spite  of  all  opposing 
influences  of  the  environment.  In  such  a  stem  are  comprehended  the  various 
organs  of  food-conduction,  the  conducting  capacity  of  which  must  not  be  im- 
paired by  pressure,  flexion,  or  strain.  All  the  functions  of  the  stem  are  influenced 
and  governed  in  a  variety  of  ways  by  the  varying  circumstances  of  the  habitat, 
and  by  the  forms  of  foliage  and  flowers  peculiar  to  each  species.  These  functions 
are  wonderfully  correlated,  and  the  different  arrangement  of  the  tissues  in  the 
stem  in  each  individual  case  is  nothing  but  the  expression  of  the  relation  of  the 
form  to  the  conditions  under  which  the  plant  lives. 

THE  FLORAL   STEM. 

The  portion  of  the  stem  from  which  floral  leaves  proceed  is  called  the  floral 
stem  (thalamus).  It  has  the  form  of  an  axis,  from  the  upper  part  of  which  project 
the  carpels  and  stamens,  and  below  these  the  perianth  leaves.  The  floral  stem,  like 
every  other,  is  built  up  of  internodes  whose  number  corresponds  to  the  number  of 
leaves  on  its  circumference,  standing  vertically  above  one  another;  but  since  the 
vertical  intervals  are  usually  very  small,  the  articulation  of  the  stem  is  but  seldom 
plainly  visible  to  the  naked  eye.  Below  the  perianth  leaves  only  the  floral  stem 
appears  more  or  less  extended,  and  this  portion  is  distinguished  as  the  "flower- 
stalk"  from  the  part  which  bears  the  perianth  leaves,  which  is  termed  the  "  floral 
receptacle". 

The  flower-stalk  (pedunculus)  originates  only  in  a  few  Rafflesiaceae  immediately 
from  the  tissue  which  represents  the  scaly  stem.  It  is  also  of  comparatively  rare 
occurrence  (restricted  to  a  few  annuals)  that  the  stem  proceeding  from  the  bud 
of  the  hypocotyl  (i.e.  the  main  axis  of  the  whole  plant)  passes'  directly  into  the 
flower-stalk  and  terminates  in  a  floral  receptacle.  The  flower-stalk  often  springs 
as  a  lateral  shoot  from  the  main  axis  of  the  plant,  and  generally  it  proceeds  as  a 
lateral  axis  from  a  stem  structure  which  is  itself  only  a  lateral  axis  of  the  main 


THE   FLORAL   STEM.  737 

stem.  The  flower-stalk  may  originate  from  all  three  regions  of  the  stem.  In-many 
parasites  and  saprophytes  without  chlorophyll  it  arises  from  the  axil  of  a  scale-leaf; 
in  many  annual  plants,  e.g.  the  Pimpernel  and  the  Ivy-leaved  Speedwell  (Anagallis 
arvensis  and  Veronica  hederifolia),  it  springs  from  the  axil  of  a  green  foliage-leaf; 
more  frequently,  however,  it  is  developed  in  the  axil  of  a  so-called  bract,  which  is 
to  be  regarded  as  a  floral-leaf. 

The  flowers  are  seldom  isolated;  in  most  instances  they  are  associated  in  clusters, 
each  cluster  being  termed  an  inflorescence  (inflorescentia).  For  descriptive  purposes 
it  was  found  necessary  to  apply  short  names  to  the  different  inflorescences,  and  a 
special  terminology  was  created  by  the  older  botanists  which  was  most  excellent, 
but  which  in  modern  times  has  become  very  cumbrous  owing  to  the  introduction 
and  substitution  of.  a  host  of  Greek  names  which  sound  very  learned,  but  are  quite 
superfluous.  It  does  not  lie  within  the  scope  of  this  book  to  follow  this  terminology 
in  detail.  It  is  enough  to  bring  forward  the  most  prominent  forms  of  inflorescence. 
I  shall  also  touch  as  shortly  as  possible  on  the  significance  of  these  various  associa- 
tions and  groups  of  flowers  to  the  life  of  the  plant,  since  this  subject  will  be  fully 
discussed  in  the  second  volume  when  describing  the  processes  of  fertilization,  and 
especially  the  crossing  of  neighbouring  flowers. 

In  describing  inflorescences  we  shall  frequently  make  use  of  the  words  "  main 
axis  "  and  "  lateral  axis  ",  and  in  order  to  prevent  misapprehension,  it  is  as  well  to 
point  out  here  that  the  main  axis  of  the  inflorescence,  i.e.  that  part  of  the  stem 
from  which  the  flower-stalks  branch  off,  is  only  in  rare  cases  the  direct  continuation 
of  the  stem  which  proceeds  from  the  bud  of  the  hypocotyl  (i.e.  the  real  main 
axis  of  the  whole  plant).  Even  in  the  Hyacinth  the  green  scape  which  rises  from 
the  ground,  and  branches  off  into  a  wealth  of  flower- stalks  in  its  upper  part,  is 
not  the  original  main  axis,  but  a  side  axis  springing  from  the  axil  of  a  bulb- 
scale.  We  are  accustomed,  however,  to  call  that  stem  the  main  one  which  takes 
the  lead  in  a  certain  region  of  the  plant,  forming  buds  which  become  lateral  shoots 
in  the  axils  of  its  leaves.  The  term  "main  axis"  is  therefore  only  relative;  with 
respect  to  its  lateral  shoots  it  is  a  main  axis,  but  with  regard  to  the  stem  from 
which  it  originates,  it  itself  must  be  looked  upon  as  a  lateral  axis.  In  order  to 
simplify  the  account  and  to  shorten  the  descriptions  of  inflorescences,  it  is  better 
to  call  the  main  axis — round  which  the  individual  flower-stalks  are  grouped  as 
round  a  common  centre,  or  which  has  conspicuously  taken  the  lead  in  the  whole 
system  of  axes — the  "  rachis  ". 

Inflorescences  have  been  classified  into  two  groups,  the  centrifugal  and  centri- 
petal. In  centrifugal  inflorescences  the  rachis  terminates  with  a  flower,  but  is 
retarded  in  growth  and  is  outstripped  by  two,  more  rarely  by  three,  lateral  axes 
springing  from  the  rachis  below  the  first-formed  flower-bud  just  mentioned. 
Secondary  lateral  axes  may  again  spring  from  each  of  these  lateral  shoots,  and 
their  relative  main  axes  may  be  again  overtopped  in  the  manner  described.  The 
flower-bud  by  which  the  rachis  is  terminated  always  opens  first;  then  the  flower- 
buds  on  the  first  series  of  lateral  axes,  then  those  on  the  second  series  of  lateral 
VOL.  I. 


738  THE   FLORAL   STEM. 

axes,  and  so  on  throughout  the  entire  series.  The  unfolding  of  the  flower-buds 
therefore  proceeds  always  from  the  centre  towards  the  circumference  of  the 
inflorescence  in  accordance  with  the  succession  of  age,  and  consequently  such  an 
inflorescence  may  be  termed  centrifugal.  The  simplest  form,  the  type  of  all 
centrifugal  inflorescences,  is  the  simple  cyme  (cyma).  This  presents  only  three 
flower-stalks,  a  central  older  one  (the  rachis)  and  two  younger  lateral  ones.  Since 
the  latter  spring  at  the  same  level  from  the  rachis,  the  simple  cyme  has  the 
appearance  of  a  three-pronged  fork.  It  often  happens  that  the  flower-bud  on 
the  rachis  becomes  stunted  or  does  not  develop  at  all,  and  then  the  inflorescence 
looks  like  a  two-pronged  fork  (e.g.  in  many  species  of  Lonicera).  If  the  lateral 
axes  arising  from  the  rachis  serve  as  starting-points  for  secondary  lateral  axes,  and 
if  the  arrangement  just  described  is  repeated  in  them,  a  compound  cyme  (cyma 
composita)  results.  The  flower-stalks  may  be  arranged  either  as  two  prongs  or 
three  prongs  in  the  compound  cyme,  and  this  branching  may  be  repeated  almost 
indefinitely,  as  is  the  case,  for  example,  in  Gypsophila  paniculata.  When  one 
of  the  opposite  flower-stalks,  or  lateral  axes  of  a  cyme,  does  not  develop,  while 
the  other,  on  the  contrary,  becomes  very  vigorous  "and  projects  beyond  the  rachis, 
this  lateral  looks  like  the  main  axis,  and  at  first  sight  the  rachis  is  mistaken 
for  a  lateral  shoot.  Similarly  on  this  vigorous  lateral  axis,  one  of  the  secondary 
lateral  shoots  does  not  develop,  while  the  other  continues  to  grow  the  more 
strongly.  If  this  happens  continuously,  the  form  of  cymose  inflorescence  called 
scorpioid  (cincinnus)  is  formed,  numerous  modifications  of  which  have  been 
distinguished.  If  the  flower-stalks  of  a  compound  cyme  are  all  plainly  visible 
and  the  whole  inflorescence  bulky  and  diffuse,  it  is  termed  a  panicle  (panicula)] 
if  the  flower-stalks  are  much  shortened  and  the  flowers  consequently  crowded 
thickly  together,  the  inflorescence  is  called  a  fascicle  (fasciculus).  Caryophyl- 
laceae,  Labiatese,  and  Boragineae  exhibit  an  almost  inexhaustible  variety  of  cymose 
inflorescences. 

Centripetal  inflorescences  may  be  recognized  by  the  fact  that  the  rachis 
terminates  in  a  bud  which  is  the  youngest  structure  of  the  whole  inflorescence, 
the  flower-stalks  which  spring  from  the  base  of  the  rachis  being  the  oldest 
lateral  axes.  Looking  down  from  above  on  such  an  inflorescence,  or  observing 
the  points  of  insertion  of  the  individual  flower-stalks  in  horizontal  projection,  the 
lowest,  and  at  the  same  time  the  oldest  flower-stalks,  are  seen  to  stand  at  the 
periphery,  the  youngest  at  the  centre  of  the  inflorescence.  The  flowers  on  the 
oldest  flower-stalks  unfold  first,  those  of  the  youngest  last;  the  blossoming 
therefore  proceeds  in  a  centripetal  direction.  The  rachis  is  terminated  as  a  rule 
by  a  stunted  bud  which  does  not  complete  its  development;  occasionally,  however, 
this  bud  does  develop;  it  assumes  the  form  of  a  foliage-bud  from  which  later  on 
is  formed  a  leafy  shoot,  as  can  be  seen  especially  in  several  Australian  Myrtales 
from  the  section  of  the  Leptospermese  (Callistemon,  Metrosideros,  Melaleuca),  and 
also  in  many  Bromeliacese  (e.g.  the  Pine-apple,  Ananassa  sativa).  Among  centri- 
petal inflorescences  may  be  distinguished  the  raceme  (raccmus)  with  elongated 


THE   FLORAL   STEM.  739 

rachis  and  evident  flower-stalks;  the  spike  (spica)  with  elongated  rachis  and 
extremely  reduced  flower-stalks;  the  umbel  (umbella)  with  an  extremely  reduced 
rachis  and  elongated  flower-stalks;  and  the  capitulum  (capitulum)  with  a  very 
short  thick  rachis  and  exceedingly  reduced  flower-stalks.  All  these  inflorescences 
are  connected  together  by  intermediate  forms,  of  which  the  corymb  (corymbus}- 
^specially  characteristic  of  Cruciferae-forming  a  link  between  the  umbel  and  the 
raceme,  deserves  special  mention.  The  capitulum  exhibits  the  greatest  variety, 
but  this  is  produced  less  by  the  different  forms  of  the  floral  stem  than  by  the 
shape  of  the  floral  leaves,  especially  of  the  numerous  crowded  bracts  which 
collectively  surround  the  flowers  as  a  cup-like  envelope.  A  form  of  spike  with 
thickened  rachis,  called  a  spadix  (spadix),  is  also  worthy  of  note,  and  also  the 
spike  known  by  the  name  of  catkin  (amentum),  the  flowers  of  which  are  devoid 
of  perianth-leaves,  and  spring  from  the  axils  of  scale-like  bracts.  The  whole 
catkin  falls  off  after  flowering,  or  after  the  ripening  of  the  fruit,  a  separation  of 
the  tissue  and  a  detachment  of  the  cells  having  previously  occurred  at  the  base 
•of  the  rachis. 

When  spikes  are  themselves  arranged  in  a  spicate  manner,  the  whole 
inflorescence  is  called  a  compound  spike  (spica  composita)',  racemes  grouped 
into  larger  racemes  form  a  compound  raceme  (racemus  compositus);  and  umbels 
when  arranged  in  larger  umbels  form  a  compound  umbel  (umbella  composita). 
The  first  two  occur  very  often  in  grasses,  the  last  in  umbelliferous  plants.  The 
term  panicle  is  also  often  applied — rather  loosely — to  any  compound  raceme. 

Various  combinations  of  the  above  simple  inflorescences  have  been  distinguished, 
particularly  combinations  of  centripetal  with  centrifugal  inflorescences.  Capitula 
and  compound  umbels  which  are  arranged  in  cymes,  and  cymes  which  succeed 
•one  another  in  a  spicate  or  racemose  manner  are  of  very  common  occurrence.  In 
these  inflorescences  the  order  of  blossoming  becomes  altered.  Of  the  many  umbels 
which  are  grouped  together  in  an  extensive  cyme,  the  central  umbel  is  the  first 
of  the  series,  but  it  is  the  flowers  on  its  periphery  and  not  the  central  flowers 
which  open  first.  If  cymes  are  arranged  like  a  spike,  the  lowest,  i.e.  those  on  the 
periphery  of  the  whole  inflorescence  blossom  first,  though  in  each  individual  cyme 
the  central  flowers  are  always  the  first  to  open. 

The  order  of  blossoming,  which  is  determinate  for  the  flowers  of  every  given 
species,  is  related  to  the  transmission  of  the  flower-dust  or  pollen  to  the  stigma,  and 
therefore  with  the  processes  of  fertilization.  When  in  one  and  the  same  flower 
the  organs  in  which  the  pollen  and  those  in  which  the  ovules  are  developed  stand 
closely  side  by  side,  it  might  be  thought  that  the  pollen  would  be  certain  to  reach 
the  adjoining  stigma.  But  this  opinion  is  not  confirmed  by  experience.  It  has 
been  demonstrated,  on  the  other  hand,  that  it  is  of  advantage  to  the  plant  that 
the  pollen  of  one  flower  should  reach  the  stigma  of  another,  indeed  of  the  flower 
of  quite  another  plant  often  some  distance  away;  thus  we  find  that  cross-fertili- 
zation is  aimed  at,  at  any  rate  at  the  commencement  of  the  flowering  period. 
I  purposely  say  "aimed  at",  and  avoid  saying  that  crossing  of  different  plants 


740  THE    FLORAL   STEM. 

always  takes  place,  because  very  often  the  crossing  is  prevented  from  some  cause 
or  another.  The  event  of  failure  is  also  actually  provided  for;  in  case  the  crossing 
of  different  plants  does  not  succeed,  care  is  taken  that  in  the  second  stage  of 
flowering  the  pollen  should  reach  the  stigmas  of  the  neighbouring  flowers  of  the 
same  plant.  In  most  plants  only  when  this  plan  also  fails,  and  at  the  last 
moment,  so  to  speak,  does  the  pollen  developed  in  the  stamens  of  a  flower  reach 
the  stigma  of  the  same  flower  which  hitherto  has  remained  intact  although  placed 
in  the  closest  proximity.  The  wonderful  and  extremely  complicated  contrivances 
which  are  met  with  for  the  attainment  of  this  threefold  aim  will  be  considered 
fully  in  the  second  volume;  but  they  must  here  be  mentioned  cursorily,  because 
the  peculiar  grouping  and  the  remarkable  order  of  the  opening  of  the  flowers  repre- 
sent contrivances  which  render  possible  the  crossing  of  neighbouring  flowers,  and 
because  the  shape  of  the  inflorescence  can  only  be  comprehended  in  connection  with 
these  contrivances. 

In  thousands  of  different  species  it  can  be  seen  that  in  the  event  of  failure  of 
crossing  between  flowers  of  different  plants,  a  cross-fertilization  between  neigh- 
bouring flowers  is  brought  about  by  elongations,  shortenings,  depressions,  and 
various  other  alterations  of  position,  sometimes  of  the  style,  sometimes  of  the 
stamens,  of  the  floral  receptacle,  or  of  the  flower-stalks.  In  the  racemose  inflores- 
cences of  Eremurus  (a  liliaceous  plant)  the  long  styles  of  the  lower  flowers, 
which  are  directed  towards  the  rachis,  bend  upwards,  towards  the  end  of  the  flower- 
ing period,  in  order  to  obtain  pollen  from  the  younger  flowers  above;  and  the  same 
thing  occurs  in  the  floral  fascicles  of  a  Woodruff  (Asperula  taurina),  in  which  the 
styles  bend  down  laterally  to  the  neighbouring  flowers  in  order  to  come  into  contact 
with  their  pollen-laden  anthers.  The  stamens  of  the  Wayfaring  Tree  ( Viburnum 
lantana)  curve  down  towards  the  neighbouring  flowers  so  that  the  pollen,  falling 
from  their  anthers,  must  alight  on  the  stigmas  of  these  neighbouring  flowers.  The 
same  thing  happens  in  Hacquetia,  Chcerophyllum  hirsutum,  Siler  trilobum,  and 
various  other  umbelliferous  plants.  In  these  we  find  that  the  stamens  of  the 
flowers  in  the  centre  of  the  umbel  stretch  out  so  far  that  their  pollen-laden  anthers 
are  situated  above  the  stigmas  of  the  neighbouring  older  flowers,  already  deprived 
of  stamens,  at  the  periphery  of  the  umbel.  In  Anthriscus  sylvestris  the  younger 
umbels  are  placed  above  the  older  so  that  the  pollen  falling  from  the  former  must 
necessarily  reach  the  latter  standing  below  them. 

In  numerous  composites,  especially  in  asters  and  Golden-rod  ( Aster  and  Solidago), 
as  well  as  in  species  of  Cacalia,  Senecio,  and  Arnica,  the  tubular  florets  are  so 
arranged  in  the  centre  of  the  capitulum  that  the  pollen  expelled  from  the  younger, 
inner  flowers  necessarily  falls  on  the  stigmas  of  the  adjacent,  outer  flowers  without 
the  aid  of  any  special  elongation  or  curvature.  In  those  composites,  on  the  other 
hand,  of  which  the  Chamomile  (Matricaria  chamomilla)  may  be  taken  as  a  type, 
the  stigmas  of  the  older,  peripheral  flowers  are  brought  under  the  pollen  falling 
from  the  inner,  younger  flowers  by  an  elongation  of  the  arched  or  conical  rachis, 
and  by  the  slight  raising  or  displacement  of  the  flowers  of  the  capitulum  so  pro- 


THE   FLORAL   STEM.  741 

duced.  Very  many  composites  with  ligulate  florets,  e.g.  the  species  of  Salsify  and 
Hawkweed  (Tragopogon  and  Hieracium),  periodically  open  and  close  their  capitula, 
i.e.  the  ligulate  portions  of  their  flowers  curve  for  a  time  outwards,  so  that  the  upper 
side  is  turned  towards  the  sky;  they  then  again  become  erect,  curve  inwards,  and 
at  length  close  tightly  together.  In  this  closing  of  the  capitulum  the  stigmas  of 
the  peripheral  flowers  become  pressed  against  the  pollen  of  the  central  ones,  and  in 
this  way  a  crossing  is  necessarily  brought  about  between  neighbouring  florets.  All 
these  crossings,  however,  could  not  occur  if  the  flowers  of  a  plant  were  developed 
at  great  distances  from  each  other  and  all  unfolded  at  the  same  time,  and  there  is 
no  doubt  that  the  formation  of  capitula,  umbels,  close  racemes,  spikes,  and  cymes, 
ranks  as  an  important  contrivance  for  accomplishing  the  cross-fertilization  of  the 
flowers. 

Another  advantage  obtained  by  the  close  grouping  of  the  flowers  consists 
in  the  fact  that  certain  portions  of  one  flower  serve  as  temporary  resting-places  for 
the  pollen  falling  from  an  adjoining  flower,  which  at  the  moment  of  dehiscence  is 
not  yet  ready  for  dispersion  in  the  air.  In  order  to  clearly  explain  this  contrivance, 
which  may  be  observed  in  catkins,  I  will  take  the  case  of  the  flowers  of  the  Walnut 
(Juglans  regia)  figured  on  the  next  page.  As  long  as  the  male  inflorescence  is 
immature,  the  flowers  are  crowded  together  in  a  short,  thick  spike,  the  free  end  of 
the  rachis  being  directed  upwards.  Simultaneously  with  the  development  of  the 
pollen  in  the  anthers,  however,  very  remarkable  changes  are  brought  about  in  a 
short  time  in  the  whole  inflorescence.  Within  a  few  days  the  rachis  elongates  to 
three  or  four  times  its  former  length,  and  becomes  limp  and  pendent;  the  flowers 
are  in  this  way  somewhat  separated  and  brought  into  an  inverted  position,  so  that 
now  the  open  side  of  each  flower  is  directed  downwards,  and  the  lower  side  upwards. 
When  the  wind  is  still,  the  anthers,  hanging  on  thin  short  filaments,  open,  and  the 
pollen  rolls  out  of  them  as  a  powdery  mass.  It  does  not  fall  directly  into  the  air, 
but,  first  of  all,  drops  on  to  the  under  side  of  a  neighbouring  flower  which  previously, 
in  the  erect  spike,  stood  above  the  anthers  in  question,  but  now  that  the  spikes  have 
become  pendent,  is  situated  below  them.  This  under  side  is  plainly  excavated  as 
a  depression,  and  the  pollen  of  the  flower  above  is  deposited  in  it  for  a  time,  as 
shown  in  the  illustration  over  page  (fig.  1842).  The  pollen  has  to  reach  the  stigmas 
of  flowers  developed  a  long  distance  from  the  catkins,  often  on  other  branches  up 
above.  It  would  be  highly  disadvantageous,  if,  after  the  dehiscence  of  the  anthers, 
the  pollen  should  fall  immediately  to  the  ground;  it  would  then  be  lost  and  wasted, 
and  neither  favourable  winds  nor  lightly  hovering  insects  would  be  able  to  carry 
it  from  the  earth  to  the  stigmatic  flowers  on  the  branches  of  the  tree.  But  in  the 
depressions  on  the  under  sides  of  the  flowers,  as  if  in  a  waiting-room,  it  occupies  the 
most  favourable  position  conceivable.  While  there  is  no  wind,the  tassel-like  spikes  are 
undisturbed,  and  the  pollen  remains  quietly  in  its  temporary  resting-places;  but  as 
soon  as  a  gust  of  wind  comes,  the  spikes  oscillate,  swinging  to  and  fro  like  pen- 
dulums, and  the  pollen,  emptied  and  blown  out  of  the  pit-like  cavities,  is  carried  to 
the  neighbouring  branches  and  whirled  round  the  tree-crown  on  to  the  stigmas,  in 


742 


THE   FLOKAL   STEM. 


the  form  of  small  clouds  of  dust.  In  this  instance  the  pollen  is  not  only  prevented 
from  being  wasted  by  the  spikate  arrangement  of  the  flowers,  but  this  further  ad- 
vantage is  obtained,  that  each  flower  shelters  the  pollen  of  the  neighbouring  flower 
in  a  safe  harbour  until  it  can  be  transmitted  by  a  favourable  wind  to  its  desired  goal. 


Fig.  184. 


'  Branch  of  the  Walnut-tree  (Juglans  regia)  with  hanging  male  catkins,  and  a  small  cluster  of  female  flowers;  natural  size 
2  The  tip  of  a  male  catkin ;  enlarged. 

The  grouping  together  of  the  flowers  also  offers  numerous  advantages  with 
regard  to  flower-visiting  insects.  Flies,  bees,  and  humble-bees  do  not  content  them- 
selves when  seeking  honey  with  taking  it  from  single  flowers,  but  climb  from  one 
flower  to  another,  from  below  up  to  the  highest  points  of  the  spikes  and  racemes, 


THE   FLORAL  STEM.  743 

or  walk  from  one  fascicle  and  umbel  to  a  neighbouring  one  as  if  over  a  flower-strewn 
surface,  thus  moving  the  pollen  from  place  to  place  and  effecting  innumerable  cross- 
ings which  would  not  take  place  so  easily  if  the  flowers  were  isolated  and  not 
collected  into  inflorescences  with  a  definite  order  of  blossoming.  The  likelihood  of 
a  crossing  between  different  flowers  is  of  course  increased  with  their  greater  number, 
and  consequently  plants  with  grouped  inflorescences  have  so  far  an  advantage  over 
those  whose  flowers  unfold  singly  at  greater  distances.  Isolated  flowers,  it  is  true, 
possess  large,  brilliantly-coloured  perianth-leaves  which  serve  to  allure  honey- 
seeking  animals  on  the  wing;  but,  on  the  other  hand,  the  same  effect  is  produced 
by  the  accumulation  of  many  small  flowers,  and  an  attraction  is  also  afforded  by 
the  development  of  so-called  ray  florets  on  capitula  and  umbels,  as  well  as  by  the 
brightly-coloured  bracts  forming  a  tuft  on  the  top  of  cymose  and  spiked  inflores- 
cences, which  is  no  less  effective  than  the  largest  corolla.  This  explains  why  90  per 
cent  of  plants  visited  by  winged  insects  bear  inflorescences  and  not  isolated  flowers. 
Large  isolated  flowers  only  serve  the  purpose  of  larger  honey-seeking  animals,  of 
such  butterflies  and  moths,  humming  and  other  honey-seeking  birds,  which  would 
not  be  able  to  obtain  the  honey  from  small,  conglomerated  flowers.  But  it  is  a  well- 
ascertained  fact  that  the  number  of  small  flies,  bees,  wasps,  and  humble-bees  which 
visit  flowers  greatly  exceeds  that  of  larger  animals,  and  this  explains  why  clusters 
of  small  flowers  occur  much  more  frequently  than  large  single  flowers. 

Remarkable  correlations  with  the  animal  world  also  exist  in  other  regions  of 
the  plant,  but  in  no  other  part  of  the  stem  do  they  appear  so  striking  and  so  mani- 
fold as  in  the  floral  region.     Nowhere  else  can  the  harmonious  co-operation  of  the 
members,  the  practical  division  of  labour,  and  the  mutual  aid  for  the  attainment 
of  an  end,  be  seen  so  plainly  and  convincingly  as  in  the  inflorescence.     In  many 
capitula  and  umbels  one  portion  of  the  flowers  forms  the  pollen;  another  develops 
the  ovules;  a  third  allures  insects;  and  a  fourth  prevents  the  depredations  of  unwel- 
come visitors.     Most  remarkable  of  all,  this  practical  division  of  labour  within  a 
single  inflorescence  does  not  terminate  even  with  the  fading  of  the  flowers,  but 
still  continued  in  the  same  parts  during  its  subsequent  passage  into  a  fruiting  state. 
Many  processes  give  us  the  impression  that  the  flowers  collected  in  a  raceme,  umbe 
or  cyme  mutually  understand  one  another;  thus,  for  example,  in  the  Crucifer* 
often  happens  that  older  flowers,  whose  stigmas  have  already  withered,  and  whic; 
have  also  entirely  lost  their  pollen,  allure  insects  to  the  adjoining  younger  f 
since  now,  instead  of  falling,  the  petals  enlarge  and  adorn  themselves  with  c 
spicuous  colours,  visible  at   a  distance.     It  also  frequently  happens  that 
flowers,  whose  time  is  over,  vacate  the  most  advantageous  position  for  bios* 
in  favour  of  neighbouring  younger  flowers.     When  the  flower  of  a  nasturti 
(Tropvolum)  fades,  its  flower-stalk  bends  downwards,  contracts  in  a  spir 
hidef  under  the  green  peltate  foliage-leaves,  while  a  new  bud  pushes  into 
where  the  older  flower  formerly  stood;  this  bud  opens  next  day  and  awaits  ins  - 
visits,  and  hasty  observers  might  think  that  the  same  flower  had  remam ed^he 
for  more  than  a  week.      The  same  thing  occurs  in  L^nar^  cymbalar^,  Led 


744  THE   FLORAL   STEM. 

palustre,  and  numerous  species  of  clover.  In  the  Alsike  Clover  (Trifolium 
hybridum)  growing  abundantly  in  marshy  meadows,  the  older  flowers  not  only 
sink  down  in  order  to  give  the  place  best  adapted  to  insect  visits  to  the  younger 
ones,  but  their  corollas  turn  a  beautiful  red  colour,  contrasting  vividly  with  the 
white  of  the  younger  flowers.  The  contrast  is  visible  at  a  great  distance  and  serves 
to  attract  insects.  In  the  curled  inflorescences  of  the  Comfrey,  Forget-me-not,  and 
Viper's  Bugloss  (Symphytum,  Myosotis,  Echium),  and  many  other  Boraginese,  the 
inflorescence  may  be  seen  to  unfold  and  fix  itself,  so  that  the  flowers  in  turn  are 
placed  in  the  position  in  which  they  are  best  seen  by  and  most  accessible  to  flying 
insects;  meanwhile  the  older  flowers,  whose  time  is  over,  and  to  which  insect- visits 
are  of  no  further  use,  move  out  of  the  way  of  those  which  have  just  opened,  and 
always  choose  their  position  so  as  not  to  obstruct  the  entrance  to  the  new  flowers 
of  the  same  inflorescence.  In  this  process  not  only  the  flower-stalk  but  the  rachis 
of  the  whole  inflorescence  takes  part,  and  it  is  interesting  to  observe  how  even 
widely  distant  parts  of  the  stem  are  sympathetically  affected,  so  to  speak,  and  how 
all  the  different  parts  of  the  system  of  axes  are  extended,  raised,  depressed,  and 
curved  exactly  as  required  for  the  purpose  of  affording  the  most  favourable  position 
to  each  flower  in  turn. 

The  most  remarkable  thing  of  all,  however,  is  that  under  certain  conditions, 
which  only  occur  exceptionally,  the  most  favourable  position  for  the  flowers  is 
striven  after  and  obtained  by  means  of  curvatures  produced  in  the  stem  in  places 
where,  in  the  ordinary  course  of  things,  such  changes  would  not  have  occurred. 
When  the  Wood  Forget-me-not,  Larkspur,  Monkshood,  Adenostyles,  the  Willow 
Herb  (Myostis  silvatica,  Delphinium  elatum,  Aconitum  variegatum,  Adenostyles 
alpina,  Epildbium  angustifolium),  and  numerous  other  undershrubs  whose  stiff, 
erect  stems  are  terminated  by  a  group  of  brilliantly-coloured  flowers  adapted  to 
insect-visits,  are  pressed  down  and  extended  on  the  ground  shortly  before  the 
unfolding  of  the  flowers  by  some  unusual  occurrence,  so  that  the  normally  erect 
inflorescence  lies  on  the  soil,  the  stem  will  be  seen  to  form  a  bend  below  the 
inflorescence,  as  it  is  no  longer  capable  of  raising  up  the  whole  length,  and  the 
portion  bearing  the  flowers  will  be  elevated  until  it  again  becomes  erect,  and  its 
flowers  are  again  placed  in  a  position  favourable  to  insect-visits.  This  curvature  is 
no  ordinary  phenomenon  of  growth,  for  the  portion  of  the  stem  forming  the  bend 
has  already  ceased  growing,  and  the  curvature  does  not  extend  to  the  rachis  of 
the  inflorescence,  but  takes  place  below  it,  being  strictly  localized  there;  the  rachis 
itself,  which  is  raised  up,  always  remaining  straight.  Finally,  no  kind  of  stimulus 
can  be  shown  to  affect  the  internodes  in  which  the  bend  is  formed.  Contact  with 
the  soil  and  illumination  from  above  act  just  in  the  same  way  on  it  as  on  the 
internodes  above  and  below  it.  No  external  causes  whatever  can  be  assigned  to 
this  knee-shaped  bending,  and  only  this  much  is  certain — the  bending  could  not 
take  place  at  a  spot  better  suited  to  the  purpose  of  restoring  the  flowers  once 
more  to  a  favourable  position. 

The  flowers  of  more  than  an  eighth  part  of  all  flowering  plants  are  grouped 


THE    FLORAL   STEM.  745 

together  into  capitula,  this  inflorescence  being  the  commonest  of  all.  Next  to  it 
comes  the  cyme  with  its  numerous  modifications,  and  then  the  umbel,  the  raceme, 
and  the  spike.  Of  all  plants,  perennial  undershrubs  exhibit  the  most  extensive 
inflorescences,  in  comparison  with  the  size  of  the  whole  plant.  Many  of  them  only 
send  up  an  axis  every  year  which  bears  a  few  large  foliage-leaves  at  the  base,  is 
beset  with  scale-like  bracts  further  up,  and  terminates  in  numerous  umbels,  racemes, 
or  cymes,  forming  a  single  gigantic  inflorescence.  As  an  example  of  this  form  found 
in  the  East,  especially  in  the  steppes  of  Persia  and  Turkestan,  may  be  instanced 
Euryangium  Sumbul  This  umbelliferous  plant,  abundant  near  Pentschakend, 
south  of  Samarkand  in  Southern  Turkestan,  develops  at  the  beginning  of  the 
vegetative  period  some  five  radical  foliage-leaves  divided  into  innumerable  lobes, 
and  having  a  musky  odour;  these  leaves  only  retain  their  fresh  green  for  a  few 
weeks  and  then  wither  and  become  bleached,  turning  a  pale  violet  colour  com- 
paratively early.  As  soon  as  these  radical  leaves  have  begun  to  change  colour 
a  leafless,  blue-tinted,  asparagus-like,  slim  shoot,  4-5  cm.  thick,  rises  above  the 
ground  and  branches  repeatedly  in  its  upper  third  into  numerous  umbels.  A  whole 
series  of  oriental  Umbelliferse  behave  like  this  strange  Sumbul  plant,  especially 
those  of  the  genus  Ferula,  as  also  the  Scorodosma  Asa  foetida,  yielding  the 
notorious  asafoetida,  and  several  Cruciferse.  One  of  these  cruciferous  bushes, 
Crambe  cordifolia,  develops  within  a  few  weeks  an  inflorescence  with  long  branches, 
projecting  like  spars,  about  2  metres  high  and  almost  as  much  across.  The  Agave 
Americana,  known  as  the  Century  Plant  (illustrated  on  p.  657),  is  also  similar  to 
these  plants.  The  stem,  5-7  metres  high  and  6—12  cm.  thick,  which  rises  above 
the  rosette  of  thick  fleshy  foliage  -  leaves  with  spinous  margins,  is  covered  only 
with  dry,  scale-like  leaves,  without  chlorophyll,  and  forms  the  rachis  of  an  inflor- 
escence which  is  one  of  the  larges-t  in  the  whole  vegetable  kingdom. 

In  contrast  to  the  undershrubs,  whose  rapidly -growing  stems,  terminated  by 
large  inflorescences,  remain  herbaceous  and  wither  and  die  down  to  the  ground 
without  lignifying  after  the  maturing  of  the  fruit  and  seeds,  woody  plants,  espe- 
cially trees,  produce  as  a  rule  only  small  inflorescences.  It  is  true  that  their 
number  is  correspondingly  large.  The  perianth-leaves  are  frequently  green,  and 
the  inconspicuous  inflorescences  distributed  between  the  foliage-leaves  are  then 
entirely  invisible  at  a  little  distance.  Often,  however,  numerous  small  but  brightly- 
coloured  inflorescences  are  crowded  so  close  together  as  to  be  inseparable;  in  cases 
where  the  flowers  unfold  before  the  green  foliage,  as,  for  example,  in  Almond  and 
Cherry  trees,  each  tree  from  a  distance  resembles  a  gigantic  bouquet  of  flowers. 

Only  a  few  inflorescences  are  found  in  palms,  but  they  are  usually  very  large 
and  many-flowered.  Generally  speaking  palm  inflorescences  are  the  largest  of  all. 
Those  of  the  Doum  Palm  (Hyphcene  thebaica)  and  of  several  species  of  Phoenix 
are  more  than  a  metre,  those  of  Raffia  Ruffii  and  of  Plectocomia  elongata  2  metres 
long,  and  the  Talipot  Palm  (Corypha  vmbraculifera),  illustrated  on  p.  289,  is 
celebrated  as  possessing  the  largest  inflorescence  of  all  plants.  This  remarkable 
dioecious  palm  grows  comparatively  slowly :  its  caudex  often  takes  30-40  years 


746  THE    FLORAL   STEM. 

before  attaining  a  height  of  20  metres,  and  during  this  period  flowers  never 
appear.  Not  until  the  caudex  has  attained  its  full  size  of  22  metres  does  the 
inflorescence  spring  from  its  apex,  the  rachis  reaching  an  additional  height  of  14 
metres.  Twelve  or  thirteen  rounded  branches  are  given  off  from  this  rachis,  the 
longest  of  which  becomes  6  metres  long.  All  the  branches  terminate  in  numerous 
branchlets  and  twigs,  and  are  richly  covered  with  flowers.  The  whole  inflorescence 
when  fully  grown  exhibits  the  fabulous  height  of  14  metres,  with  a  breadth  of  12 
metres.  As  soon  as  the  flowers  open,  the  fan-like  foliage-leaves  below  begin  to 
fade  and  often  all  fall  off  during  the  flowering  period,  so  that  the  shaft  alone 
remains,  bearing  the  inflorescence  at  its  apex.  The  flowering  period  lasts  for  3-4 
weeks.  As  soon  as  it  is  over  and  the  fruits  matured,  the  whole  plant  dies  down, 
as  in  Agave  Americana.  Each  of  these  palms  therefore  only  blossoms  once  in 
its  life. 

With  this,  the  largest  inflorescence,  may  be  contrasted  that  which  is  regarded 
as  the  smallest  of  all,  viz.  the  capitulum  of  Nananthea,  only  2-3  millimetres  in 
diameter,  found  growing  on  the  mountains  of  Corsica. 

The  size  of  the  inflorescence,  and  that  of  the  flowers  composing  it,  do  not  vary 
proportionately.  Extensive  inflorescences  usually  have  very  small  flowers,  and  vice 
versa,  but  a  universal  rule  cannot  be  laid  down  in  this  matter.  The  inflorescence 
of  Paulownia  imperialis  has  100  large  flowers,  and  that  of  Spircea  Aruncus,  equal 
in  extent,  10,000  small  ones.  The  Talipot  Palm  is  said  to  bear  about  100,000 
flowers  in  its  gigantic  bouquet.  In  simple  cymes  it  often  happens  that  the  central 
flower  is  not  developed,  and  the  whole  then  consists  of  a  pair  of  flowers,  usually 
curiously  united,  as  can  be  seen  in  many  species  of  the  genus  Honeysuckle  (Loni- 
cera  Xylosteum,  nigra,  cosrulea,  alpigena).  In  many  Acanthaceae,  bindweeds,  and 
;  labiate  flowers,  on  the  other  hand,  it  is  observed  that  the  two  lateral  flowers  of 
the  three  of  a  simple  cyme  are  suppressed,  and  that  only  the  central  one  attains 
development,  in  which  case  the  whole  inflorescence  is  represented  by  a  single  flower. 

The  floral  receptacle  (podium,  also  torus),  i.e.  that  part  of  the  floral  stem  from 
which  the  perianth  leaves  spring,  is  always  somewhat  thickened  in  comparison 
with  the  flower -stalk,  and  may  be  either  conical  or  disc -shaped.  The  conical 
receptacle  (conopodium)  has  the  form  of  a  cone,  being  sometimes  elongated  and 
peg-shaped,  but  often  short  and  but  slightly  curved;  it  is  always  narrowed  from 
its  base,  the  thickest  part,  up  to  the  apex.  Unlike  the  very  simply-constructed 
conical  receptacle,  the  disc-shaped  receptacle  (discopodium)  presents  a  great  variety 
of  form.  The  apex  of  the  floral  axis  is  retarded  in  growth,  the  tissue  round  it 
thickens  and  becomes  flattened,  or  surrounds  the  apex  with  a  circular  cushion  or 
rampart  often  rising  so  much  above  the  apex  that  the  whole  receptacle  has  a  crater- 
like  or  cup-shaped  appearance.  In  the  first  case,  viz.  when  a  circular  wall  has  been 
formed,  it  surrounds  the  pistil,  developed  in  the  centre  above  the  apex,  without 
overtopping  it,  as,  for  example,  in  the  flowers  of  Orange  and  Lemon  trees.  The 
stamens  and  perianth-leaves  usually  arise  outside,  less  frequently  within  the  ring, 
and  most  rarely  of  all  from  the  edge  of  the  ring  itself.  When  a  cup -shaped 


THE   FLORAL  STEM.  747 

receptacle  has  been  formed,  the  end  of  the  axis  is  overtopped  by  the  edge  of  the 
cup,  and  the  actual  apex  of  the  receptacle  must  be  sought  at  the  bottom  of  the 
cup.  The  stamens  and  perianth-leaves  then  spring  in  most  cases  from  the  edges 
of  the  cup.  In  many  instances  the  carpels  also  arise  from  the  edges  and  cover 
over  the  crater-like  depression  of  the  receptacle.  More  frequently  the  carpels  are 
developed  at  the  bottom  or  on  the  inner  walls  of  the  cup,  and  then  either  a  single 
carpel  is  to  be  seen  in  the  depression,  as,  for  example,  in  cherry  flowers,  or  several 
carpels,  as,  for  example,  in  the  rose.  Sometimes  the  pistil  developed  at  the  bottom 
of  the  cup-shaped  receptacle  is  fused  with  the  inner  wall  of  the  cup,  as,  for 
example,  in  the  flowers  of  apple  and  pear  trees. 

The  disc-shaped  receptacle  is  not,  as  in  the  examples  selected,  always  developed 
symmetrically  all  round.  In  flowers  which  project  laterally  from  an  erect  rachis, 
the  circular  wall  is  often  interrupted,  or  instead  of  the  circular  disc  a  one-sided 
projecting  ridge  or  cushion  is  seen.  The  ring  is  often  replaced  by  a  circle  of  pro- 
tuberances or  papillae,  or  the  receptacle  is  drawn  out  on  one  side,  taking  the  form 
of  a  peg,  a  tongue,  or  a  scale. 

Honey  is  usually  secreted  from  that  tissue  of  the  disc-shaped  receptacle 
which  does  not  pass  over  into  perianth-leaves,  but  which  projects  and  is  inserted 
between  the  whorls  of  perianth-leaves,  stamens,  and  carpels  in  the  form  of  knots, 
warts,  cushions  and  rings;  this  serves  to  attract  insects  whose  visits  are  of  use 
to  the  flowers  in  effecting  fertilization.  The  part  of  the  receptacle  which  is 
developed  as  the  under-structure  or  envelope  of  the  carpels,  on  the  other  hand, 
very  often  becomes  a  part  of  the  fruit.  In  most  cases,  however,  the  signifi- 
cance assigned  to  the  various  developments  of  the  receptacle  in  respect  to  the  life 
and  welfare  of  plants  is  not  yet  rendered  sufficiently  evident.  That  the  rela- 
tions between  receptacle  and  fruit-formation  are  of  the  greatest  importance  is  the 
only  thing  that  can  be  affirmed  with  certainty,  but  why  in  one  instance  this  and 
in  another  that  form  of  receptacle  is  produced  remains  entirely  enigmatical.  The 
opinion  has  been  repeatedly  stated  that  all  the  architectural  conditions  of  plants 
are  not  necessarily  beneficial,  and  that  the  forms  in  which  the  individual  organs 
and  plant  members  appear  fall  into  two  groups — those  whose  use  to  the  species 
in  question  is  obvious,  and  those  in  which  this  is  not  the  case.  The  former  were 
said  to  be  variable,  the  latter  invariable.  This  hypothesis  was  forthwith  raised 
to  a  dogma,  and  it  was  further  concluded  that  only  structures  whose  significance 
in  the  life  of  plants  cannot  be  explained  are  of  use  in  the  limitation  and  systematic 
determination  of  species  and  groups  of  species.  I  cannot  justify  this  notion,  and 
maintain  rather  that  nothing  is  ever  formed  in  a  plant  which  is  not  beneficial,  which 
is  not  even  indispensable  to  it.  Those  organs  which  are  so  often  termed  "  reduced  " 
are  not  without  importance  in  the  life  of  plants;  they  are  rather  developed  in 
this  only  apparently-reduced  form  for  the  welfare  of  the  whole,  and  cannot  be 
dispensed  with  without  injury.  If  they  were  unnecessary,  they  would  also  be 
absent.  The  plant  builds  up  nothing  superfluous,  and  no  hair,  no  cell  even,  i 
developed  without  some  purpose.  It  is  hazardous  and  unwarranted  to  say  that 


748  THE    FLORAL   STEM. 

this  or  that  structure  is  useless,  and  to  be  interpreted  only  as  the  remnant  of  an 
organ  which  was  developed  more  fully  a  long  time  ago  in  some  ancestral  species 
to  which  it  was  indispensable.  When  we  cannot  immediately  see  the  advantage 
of  any  structure,  we  are  not  justified  in  saying  that  in  its  particular  form  it  is 
worthless  or  indifferent  to  the  plant.  The  saying  dies  diem  docet  is  perhaps 
nowhere  more  applicable  than  to  questions  concerning  the  significance  of  forms. 
How  many  structures  which  were  enigmatical  a  century  ago  are  now  recognized 
as  essential  members  of  very  various  contrivances,  and  explained  in  all  their 
details;  their  recognition  being  regarded  as  an  incontestable  scientific  thesis! 
The  tendency  of  our  age,  indeed,  is  not  merely  to  regard  and  describe  forms  as 
mute  puzzles  of  nature,  but  to  comprehend  their  value  as  parts  of  a  living  entity. 
Therefore  I  doubt  not  that  sooner  or  later  the  importance  of  the  different  forms 
of  floral  receptacles  will  find  interpretation  and  explanation  in  the  individual 
species  to  which  they  belong. 

A  peculiarity  which  distinguishes  the  floral  receptacle  from  all  other  stem- 
structures,  which  has  to  be  considered  here  in  conclusion,  is  its  limited  growth.  As 
long  as  the  receptacle  forms  floral-leaves  on  its  periphery  it  always  continues  to 
elongate  to  some  extent,  although  the  increase  in  length  is  inconsiderable;  but 
after  the  production  of  the  highest  floral-leaf  no  further  divisions  take  place  in  the 
cells  of  the  apex,  and  the  elongation  of  the  axis  is  at  an  end,  not  temporarily,  but 
permanently.  This  fact  is  of  importance  inasmuch  as  one  of  the  few  differences 
which  have  been  established  between  stem  and  leaf  undergoes  a  material  restric- 
tion thereby.  But  the  limited  growth  of  the  floral  receptacle  has  also  a  special 
significance  in  regard  to  the  architecture  of  the  whole  plant.  The  portion  of 
the  stem  which  forms  the  floral  receptacle  separates  usually  with  the  flower- 
stalk,  and  not  infrequently  even  with  the  whole  rachis  of  the  inflorescence  from 
the  floral  stem  below,  as  soon  as  the  leaf -structures  proceeding  from  the  receptacle 
have  fulfilled  their  function;  or,  in  other  words,  the  flower  and  fruit-stalks  become 
detached  as  soon  as  the  perianth-leaves  have  withered,  the  stamens  emptied,  and 
the  fruits  matured — a  process  which  reminds  us  of  the  detachment  of  those 
foliage-leaves  which  are  no  longer  able  to  fulfil  their  allotted  tasks.  Just  as  a 
scar  arises,  or  a  withered  stump  remains  behind  where  once  a  leaf  existed,  so  a 
healing  tissue  is  formed  at  the  place  where  a  portion  of  the  floral  stem  has 
separated  off,  and  at  this  spot  no  further  stem  growth  takes  place.  If  the 
shoot  terminates  with  a  single  flower,  or  an  entire  inflorescence,  it  can  no  longer 
elongate  in  a  straight  line  after  the  fruit  has  fallen;  its  apical  growth  is  terminated 
for  ever.  Lateral  shoots,  on  the  contrary,  may  spring  from  the  axils  of  lower 
foliage-leaves  and  may  grow  up  beyond  the  scarred  places,  a  fact  which  of  course 
materially  influences  the  type  of  branching  and  the  architecture  of  the  whole 
stem.  This  influence  is  very  noticeable,  especially  in  tall  woody  shrubs  and  trees. 
Where  for  instance  the  scarred  apex  of  a  branch  is  overtopped  by  two  lateral 
branches  springing  close  under  the  scar,  a  more  or  less  regularly  two-pronged 
fork  results;  and  when  the  process  is  repeated  on  the  prongs  of  this  fork,  a  very 


RELATION   OF   STRUCTURE   TO   FUNCTION    IN    ROOTS.  749 

ornamental  form  of  branching  is  produced  which  may  even  be  recognized  on  the 
older  boughs,  and  gives  a  characteristic  habit  to  the  shrub  or  tree.  Although  the 
annual  growth  in  height  in  woody  plants  branching  in  this  fashion  is  only  slight, 
the  crown  grows  in  breadth  to  a  striking  degree,  and  the  older,  leafless  boughs 
have  usually  the  appearance  of  horns  or  of  an  interwoven  lattice-work  spreading 
out  above,  as  may  be  seen  in  a  remarkable  manner  in  the  Stags-horn  Sumach 
(Rkus  typhina),  and  also  in  several  species  of  Horse  Chestnut  (e.g.  JEsculus  flava 
and  discolor).  In  the  Oleander  (Nerium  Oleander),  and  frequently  in  the 
Mistletoe  (Viscum  album,  c.f.  fig.  46,  p.  206),  the  scarred  apex  of  the  main  shoot 
is  overtopped  by  a  whorl  of  three  lateral  shoots,  which  produces  another  charac- 
teristic modification  of  this  form  of  branching. 

The  internal  structure  of  the  floral  stem,  especially  the  arrangement  of  the 
mechanical  tissue,  is  always  adapted  to  the  tasks  naturally  assigned  to  the  bearer 
of  the  flowers  and  fruits.  When  the  floral  portions  and  the  fruits  proceeding  from 
them  are  to  be  maintained  in  an  erect  position,  the  stalks  and  also  the  rachis  in 
question  are  constructed  so  as  to  resist  flexion.  The  stalk  and  rachis  of  pendent 
flowers,  and  especially  of  heavy  pendent  fruits,  are,  on  the  other  hand,  constructed 
to  resist  tension;  in  both  cases  they  are  provided  with  mechanical  tissue 
suitably  strengthened  and  arranged.  Frequently  the  same  bast  cylinder  which 
afforded  the  resistance  to  flexion  in  the  erect  flower-stalk  at  the  time  of  the 
opening  of  the  flowers  has  subsequently  to  provide  a  resistance  to  strain,  as  when  a 
pendent  fruit  is  produced  from  an  erect  flower.  The  converse  also  happens,  and  not 
infrequently  erect  fruit-stalks,  very  resistant  to  bending,  which  take  part  in  the 
dispersion  of  the  seeds,  are  developed  from  pendent  flower-stalks  with  the  capacity 
of  resisting  strain.  For  the  rest,  in  all  these  alterations  of  position,  the  turgescence 
of  the  parenchymatous  tissue  on  the  periphery  of  the  flower-stalk  plays  a  prominent 
part. 


4.  FORMS   OF   ROOTS. 

Relation  of  external  and  internal  structure  to  function.— Definition  of  the  root 
— Eemarkable  properties  of  roots. 

RELATION   OF   EXTERNAL  AND  INTERNAL  STRUCTURE  TO  FUNCTION. 

Every  seed  is  provided  by  the  parent  plant  with  as  much  starch,  fat,  sugar, 
and  other  materials  as  are  necessary  for  its  further  independent  developn 
The  germinating  seed  respires,  provides  itself  with  water,  dissolves  the  mat 
Btored  up  in  its  cells,  augments  the  number  of  its  cells,  and  increases  in  s 
food-substances  of  the  soil  at  first  take  little  or  no  part  in  these  process 
as  the  seed  germinates  its  aim  is  to  develop  organs  capable  of  laying  1 
substances  contained  in  the  soil  and  air  under  contribution,  and  of  manufacturing 


750  RELATION   OF   STRUCTURE   TO    FUNCTION   IN   ROOTS. 

fresh  building-materials  as  those  with  which  it  was  provided  by  the  parent  become 
exhausted.  The  tissues  of  the  young  seedling  always  contain  cells  for  the  absorp- 
tion of  the  dissolved  food-salts  and  gases;  these  enter  immediately  into  a  close 
union  with  the  substratum,  whether  it  consists  of  inorganic  earth,  of  decaying 
organic  matter,  or  of  a  living  host-plant. 

There  are  plants  in  whose  seeds  no  differentiation  into  various  parts,  no 
separation  into  embryo  and  food-stores,  can  be  recognized,  and  in  the  seeds  of 
many  thousand  species  we  cannot  even  distinguish  an  embryo  with  cotyledons,  in 
which  case  the  whole  of  the  group  of  cells  forming  the  seed  must  be  regarded  as 
embryo.  This  group  of  cells  first  grows  up,  at  the  expense  of  its  own  materials, 
into  a  structure  having  the  form  of  a  small  tubercle,  which  on  one  side  joins  with 
the  substratum  by  absorbent  cells,  and  on  the  other  sends  out  a  shoot,  but  develops 
no  system  of  tissues  which  could  be  called  a  root.  This  occurs,  for  example,  in 
Monotropa  and  in  the  Coral-root  (Corallorhiza),  described  on  p.  Ill,  which  are 
usually  termed  rootless  plants.  In  other  examples  of  this  group,  in  which  the 
undifferentiated  embryo  grows  up  directly  into  a  small  tubercle  or  stem,  warts, 
papillae,  pegs  and  vermiform  structures,  equipped  with  absorbent  cells,  develop 
on  this  tubercle,  and  join  with  the  substratum;  these  are,  therefore,  of  the 
nature  of  roots.  These  structures  always  originate  in  great  numbers  from  the 
tubercle,  i.e.  from  the  enlarged  developing  embryo;  in  many  orchids  living 
epiphytically  on  the  bark  of  trees  they  are  formed  on  the  side  turned  towards 
the  tree;  in  parasitic  Orobanchaceaa  around  the  thickened  lower  end  of  the 
tissue-body  (cf.  figs.  34 n  and  34 12  on  p.  173),  and  in  species  of  Cuscuta  and 
Cassytha  laterally  on  the  thread-like  embryo  where  it  has  attached  itself  to  a 
host-plant. 

In  plants  whose  seed  contains  an  embryo  differentiated  into  stein  and  leaf, 
only  a  single,  wart-like  or  conical  body  arises  at  one  end  of  the  hypocotyl,  opposite 
the  bud  of  the  epicotyl;  it  grows  at  germination  into  a  cylindrical  root  provided 
with  absorbent  cells,  and  later  appears  as  a  straight,  downwardly-directed  con- 
tinuation of  the  hypocotyl. 

Neither  the  abundant  roots  proceeding  from  the  undifferentiated  embryo,  nor 
still  less  the  single  root  springing  from  the  membered  embryo,  suffice  for  the 
requirements  of  the  shoot  arising  from  it.  In  proportion  as  this  increases  in  size, 
forming  one  internode  above  another,  developing  leaves  with  buds  in  their  axils 
which  grow  out  into  lateral  shoots,  the  need  of  water  and  food-salts  becomes 
greater  and  greater.  Fresh  sources  must  be  obtained  for  these  materials,  and 
new  conducting  mechanisms  must  be  established — in  a  word,  new  roots  must  be 
formed.  When  only  a  single  primary  root  is  present  in  the  embryo,  the  new 
roots  frequently  spring  from  this  as  lateral  branches,  and  it  is  customary  to  say 
that  the  primary  or  main  root  has  become  branched,  that  it  has  formed  lateral 
roots.  Of  course  each  branch  can  again  ramify,  and  indeed  the  branching  is  often 
repeated  beyond  measure.  The  branched  root  (radix  ramosa)  is  to  be  seen  espe- 
cially in  annual  land-plants  with  erect  leafy  stems.  Almost  as  often  it  happens 


RELATION   OF   STRUCTURE   TO   FUNCTION   IN   ROOTS.  751 

that  the  root  proceeding  from  the  embryo  perishes  as  soon  as  it  has  emerged 
from  the  seed,  and  that  then  many  new  roots  originate  from  the  hypocotyl, 
close  to  the  place  from  which  sprang  the  dead  primary  root;  or  that  roots  are 
developed  on  the  lower  end  of  the  epicotyl  embedded  in  the  ground — in  which 
case  they  stand  closely  crowded  together  in  great  numbers  forming  a  cluster,  and 
are  then  known  botanically  as  fascicled  roots  (radix  fasciculata).  But  many  roots 
also  arise  further  up  on  the  shoot-axis,  not  only  in  the  region  of  the  scale-leaves, 
but  also,  if  required,  in  the  foliage  portion  of  procumbent,  erect,  and  climbing 
stems,  and  under  certain  conditions  even  on  the  foliage-leaves.  These  structures 
which  may  originate  from  the  stem  at  all  stages  of  age  and  height,  and  even  from 
the  leaves,  are  called  adventitious  roots  (radices  adventicioe). 

When  roots  are  developed  on  a  leafy  stem,  it  is  noticed  that  their  places  of 
origin  are  near  the  points  of  insertion  of  the  leaves.  In  epiphytes,  especially  in 
aroids  and  orchids  living  on  the  bark  of  trees,  they  are  sometimes  seen  to  be  so 
distributed  that  a  single  root,  a  pair  of  roots,  or  a  whole  fascicle  of  roots,  arises  at 
exactly  defined  places  on  the  stem.  Each  internode  in  these  plants  has  its  own 
roots,  and  is  therefore  almost  independent  of  neighbouring  internodes,  so  that, 
supposing  one  or  both  the  adjoining  internodes  should  die  from  some  cause  or 
other,  it  can  maintain  itself  independently  (cf.  fig.  51,  p.  224).  In  stems  which 
lie  on  the  ground,  as  in  runners,  the  roots  always  originate  only  at  the  nodes, 
i.e.  at  the  commencement  of  an  internode.  In  the  underground  stems  known  as 
rhizomes,  the  roots  are  distributed  in  the  same  way.  When  the  older  internodes 
of  these  runners  and  rhizomes  die  off  behind,  the  next  youngest  are  not  injured, 
for  they  are  already  provided  with  roots  of  their  own,  by  whose  help  their 
requirements  of  water  and  food-salts  are  supplied,  and  by  which  they  are  firmly 
fixed  in  the  ground.  The  general  symmetry  and  geometrical  distribution  of  the 
places  of  origin,  as  shown  in  leaves,  is,  however,  absent  in  the  majority  of  roots, 
the  arrangement  being  frequently  quite  irregular,  especially  in  underground,  much- 
branched  roots  where  influences  operate  on  them  which  will  be  spoken  of  later. 

The  functions  assigned  to  the  root  are:  first,  the  absorption  and  transport  of 
water  and  of  food-salts  dissolved  in  water,  and  second,  the  fixing  of  the  whole 
plant  in  the  substratum.  In  most  cases  this  twofold  function  is  performed  by 
the  same  root,  but  occasionally  a  division  of  labour  occurs,  so  that  one  portion 
of  the  root-system  serves  only  for  the  absorption  of  food,  and  another  for  the 
fixing  in  the  substratum.  For  instance,  the  repeatedly  mentioned  Tecoma  radicans 
has  two  kinds  of  roots;  the  first  underground,  absorbing  water  and  food-salts  from 
the  soil,  and  the  second  the  clinging  roots  (figured  on  p.  479),  by  which  the  light- 
avoiding  shoots  are  attached  to  places  from  which  no  fluid  nourishment  could 
possibly  be  absorbed.  When  one  of  these  shoots  is  cut  across  below  the  place 
at  which  it  is  fixed  by  roots  to  a  wall  or  rock-face,  the  part  above  the  section 
forthwith  dries  up,  even  although  these  roots  and  the  substratum  are  kept 

tinually  moistened  and  damp. 

Roots  of  biennial  and  perennial  plants,  in  regions  where  the  vegetative  activity 


752  RELATION   OF   STEUCTUEE   TO    FUNCTION    IN    EOOTS. 

is  temporarily  interrupted  by  drought  or  cold,  frequently  have  a  third  function 
to  perform,  viz.  that  of  storing  up  starch,  fat,  sugar,  and  other  reserve  food- 
materials.  Obviously  the  parts  concealed  in  the  ground  are  protected  in  a  high 
degree  against  aridity  and  frost  in  countries  with  long-continued  summer  drought 
or  with  severe  winters,  and  therefore  the  underground  root-structures  principally, 
together  with  underground  parts  of  stems  and  the  scale-leaves  arising  from  them, 
can  be  used  most  advantageously  as  reservoirs  for  the  materials  formed  in  the 
green  organs  above  ground  during  the  short  period  of  vegetation. 

The  variety  of  functions  assigned  to  roots,  the  diversity  of  the  substrata,  and 
the  peculiar  conditions  of  the  habitat  and  climate  render  necessary  a  large  number 
of  different  forms,  the  most  noticeable  of  which  bear  special  names  in  botanical 
terminology,  and  will  be  briefly  enumerated  here.  According  to  the  substratum 
into  which  the  roots  penetrate,  and  from  which  they  derive  water  and  food,  we 
may  distinguish  between  subterranean,  aquatic,  aerial,  and  parasitic  roots. 

Subterranean  roots  (radices  hypogaece)  push  their  ends,  which  are  beset  with 
root-hairs,  into  the  ground  with  great  energy,  and  are  entirely  covered  over  with 
soil,  or  at  any  rate  in  so  far  as  their  absorbent  portions  are  concerned.  Roots 
proceeding  from  the  radicle  of  the  embryo  are  chiefly  subterranean.  The  roots 
springing  from  the  different  forms  of  scaly  stem  are  almost  all  subterranean,  and 
we  shall  not  be  far  wrong  if  we  estimate  the  roots  of  70  per  cent  of  all  existing 
phanerogams  as  subterranean. 

Aquatic  or  floating  roots  (radices  natantes)  spring  laterally  from  floating  stems 
and  are  generally  arranged  in  clusters,  more  rarely  singly,  and  are  to  a  slight 
extent  spirally  twisted.  They  are  developed  both  from  stems  whose  foliage-leaves 
lie  flat  on  the  surface  of  the  water,  and  also  from  the  floating,  leafless  stem- 
structures  which  have  been  metamorphosed  into  phylloclades  (e.g.  in  Lemna  polyr- 
rhiza,  gibba,  minor).  In  these  plants  the  root-tips  are  also  surrounded  by  water. 
If  they  come  to  lie  on  the  slimy  bottom  in  consequence  of  a  fall  in  the  water- 
level,  they  do  not  penetrate  into  it,  nor  do  they  enter  into  relations  with  the 
particles  of  mud.  Marsh-plants,  on  the  contrary,  send  their  first  roots  right  down 
into  the  mud,  whilst  those  developed  subsequently  from  the  higher  internodes 
are  allowed  to  float  in  the  water.  The  primary  root  produced  from  the  seed 
of  the  Water  Soldier  (Stratiotes  aloides)  is  embedded  in  mud,  and  is  therefore 
really  a  subterranean  root;  after  it  has  died  off  the  whole  plant  rises  up,  remains 
oscillating  below  the  surface  of  the  water,  and  develops  floating  roots  from  its 
abbreviated,  leafy  stem;  later  the  plant  again  sinks  down,  and  the  floating  roots 
again  become  subterranean  (cf.  the  account  on  p.  76).  Conversely  it  often 
happens  that  subterranean  roots  become  transformed  into  aquatic  roots.  In 
alders,  willows,  and  elms  growing  on  the  sides  of  streams,  extensive  net-works 
of  roots  are  often  to  be  seen  which  have  grown  out  from  the  slope  of  the  bank 
and  float  in  the  water.  Indeed,  many  terrestrial  roots,  strangely  enough,  exhibit 
a  much  more  luxuriant  growth  in  flowing  water  than  in  the  ground,  and  it  is 
well  known  that  the  roots  of  the  above-named  trees,  when  they  have  effected  an 


RELATION   OF  STRUCTURE  TO   FUNCTION   IN   ROOTS.  753 

entrance  into  water-pipes,  grow  so  extensively  that  in  a  short  time  the  pipes  are 
entirely  blocked  up  and  the  water-flow  in  them  interrupted.  The  net-works  of  roots 
taken  out  of  these  pipes  resemble  plexuses  of  hair,  so  abundantly  are  their  tresses 
developed.  Hyacinths  and  many  other  bulbous  plants,  and  even  various  foliage- 
trees,  as,  for  example,  maples  and  horse-chestnuts,  whose  roots  usually  grow  in  the 
ground,  can  be  reared  with  the  best  results  if  their  roots  are  allowed  to  grow 
in  water,  provided  that  the  water  contains  the  necessary  food-salts  in  adequate 
amounts. 

Aerial  roots  (radices  aerew)  are  developed  on  the  periphery  of  the  erect  caudices 
of  tree-ferns,  and  in  great  profusion  on  the  stems  of  epiphytes,  especially  of  aroids 
and  orchids.  In  species  of  the  tree-ferns  Todea  and  Dicksonia  the  aerial  roots  are 
all  very  short  but  so  numerous  and  crowded  together  that  they  form  an  actual 
mantle  round  the  caudex.  In  orchids  growing  on  the  bark  of  old  trees  the  aerial 
roots  also  arise  in  great  numbers  close  together,  but  are  always  elongated  and 
filamentous,  and  form  manes,  as  shown,  for  example,  in  Ontidium,  figured  on 
p.  221.  In  other  orchids,  however,  they  may  occur  singly,  and  are  then  usually 
much  thicker,  fairly  stiff,  and  curved  sinuously  in  and  out  or  spirally  twisted  as  in 
the  Sarcanthus  rostratus,  illustrated  on  p.  107.  As  already  stated,  in  many  aroids 
and  orchids  they  appear  arranged  with  great  regularity,  either  singly  or  in  pairs, 
opposite  the  points  of  insertion  of  the  leaves  on  the  stem.  All  these  aerial  roots 
are  excellently  adapted  by  their  structure  (described  on  p.  222)  not  only  for  the 
absorption  of  water  and  solutions  of  food-materials,  but  also  for  the  condensation 
of  aqueous  vapour  from  the  surrounding  air.  In  most  instances  they  are  enveloped 
by  a  papery  covering;  more  rarely  they  are  thickly  beset  with  so-called  root-hairs, 
and  then  have  a  velvety  appearance.  Most  of  those  with  root-hairs  are  rusty 
brown  in  colour,  whilst  the  others  are  white  in  dry  air,  and  greenish  in  wet  weather 
— in  consequence  of  the  abundant  chlorophyll  contained  in  the  tissue  below  their 
papery  envelope. 

We  must  carefully  distinguish  these  condensing  aerial  roots  from  such  as,  whilst 
springing  from  epigeal  stems  and  surrounded  by  air,  are  unable  to  condense  aqueous 
vapour  or  to  absorb  atmospheric  water.  These,  on  the  other  hand,  grow  down  to 
the  ground  and  must  penetrate  into  it  in  order  to  obtain  the  water  and  food-salts 
they  require.  These  root-structures  are  especially  observed  in  climbing  plants  in 
which  the  lowest  internodes  have  died,  and  which  then  no  longer  stand  in  direct 
connection  with  the  soil.  Their  large  foliage-leaves  nevertheless  require  a  much 
greater  amount  of  water  than  could  be  obtained  from  the  tree-trunks  on  which 
they  support  themselves.  The  large-leaved  aroids  illustrated  on  p.  365,  whose 
cord-like  roots,  from  4  to  6  metres  in  length,  descend  to  the  ground,  may  be 
regarded  as  typical  of  this  class.  These  forms  are  indeed  called  aerial  roots,  but 
if  we  adhere  to  the  distinction  given  above,  they  would  be  more  accurately  regard( 
as  a  special  class  of  subterranean  roots.  But  since  it  has  been  repeatedly  obs 
that  the  aerial  roots  of  some  orchids,  when  they  come  in  contact  with  the  ground, 
penetrate  into  it  and  assume  the  character  of  subterranean  roots,  the  boundary 


VOL.  I 


754  RELATION   OF   STRUCTURE   TO    FUNCTION    IN    ROOTS. 

between  subterranean  and  aerial  roots  vanishes,  and,  as  in  other  similar  cases,  it 
becomes  evident  that  all  these  classifications  are  but  artificial. 

Parasitic  roots  (radices  parasiticce)  grow  down  into  the  living  tissue  of  the 
host-plant  and  absorb  from  it  the  materials  needed  by  them,  and  by  the  plant  to 
which  they  belong,  for  further  development.  They  are  sometimes  called  haustoria. 
They  are  either  wart-like,  disc-shaped,  or  spherical  in  outline,  or  assume  the  form 
of  sinkers;  occasionally  they  remind  one  of  a  hyphal  net- work.  Sometimes  they 
spring  laterally  from  an  epigeal,  sometimes  from  an  underground  stem.  They  also 
frequently  proceed  as  lateral  members  from  underground  roots.  Their  structure 
and  various  developments  were  so  fully  described  on  pp.  173-213  that  we  need 
only  now  refer  to  what  was  there  stated. 

Roots,  whose  especial  province  it  is  to  maintain  a  plant  in  the  position  it  has 
once  assumed,  may  be  distinguished  as  clinging  and  as  supporting  roots.  Clinging 
roots  (radices  adligantes)  really  comprise  all  roots  whose  absorbent  cells  are  so 
closely  united  with  the  substratum  that  a  displacement  can  only  be  brought  about 
by  the  exertion  of  considerable  force.  Even  floating  roots,  inasmuch  as  they  adhere 
to  the  water  and  so  give  a  certain  amount  of  stability  to  the  whole  plant,  may  be 
regarded  as  clinging  roots.  The  duckweeds  (Lemna  minor,  polyrrhiza,  gibba), 
whose  long,  spirally  -  twisted,  fascicled  roots  grow  down  into  the  water,  are  not 
easily  moved  by  wind  from  the  position  they  have  taken  up.  Plants  are  of  course 
still  better  fixed  in  the  substratum  by  subterranean  roots  which  adhere  to  the 
solid  particles  of  the  soil  by  means  of  their  root-hairs.  By  this  union  of  roots 
and  earth -particles  we  get  a  compact  mass,  difficult  to  break  up,  and  it  is  well 
enough  known  that  loose  soil  may  be  solidified  by  plants  possessing  much-branched, 
wide-spreading  roots,  and  that  certain  grasses  are  made  use  of  to  bind  shifting 
sands  together.  When  clinging  roots  are  mentioned  in  plant  descriptions,  those 
in  particular  are  referred  to  which  firmly  connect  epigeal  portions  of  stems  to  any 
support,  as,  for  example,  the  short,  climbing  roots  of  the  Ivy,  or  of  Tecoma 
radicans,  the  much-branched  roots  which  cover  stones  and  the  bark  of  trees  with 
their  net-works,  the  adherent  roots  of  numerous  species  of  Bignonia  and  Cereus, 
and  the  ribbon-like  roots  of  certain  tropical  orchids  which  have  fastened  to  the 
bark  of  trees, — especially  those  of  Phalcenopsis  Schilleriana,  described  on  p.  108; 
and  finally  the  girdle-like  roots  of  Ficus  and  Wightia,  figured  on  p.  705. 

Supporting  roots,  as  their  name  implies,  have  the  task  of  supporting  the  stems 
to  which  they  belong.  They  are  always  visible  above-ground,  and  assume  the  form 
of  buttresses  when  they  spring  from  erect  trunks,  of  pillars  when  they  belong  to 
the  projecting  lateral  branches  of  a  stem.  They  may  be  conveniently  divided  into 
tabular,  stilt-like,  and  columnar  roots.  Tabular  roots  (radices  parietiformes)  proceed 
from  the  lower  part  of  an  erect  trunk,  and  have  the  form  of  flanges  placed  on  end. 
They  may  also  be  compared  to  massive  planks  of  wood  used  for  fencing  in  roads. 
They  radiate  out  in  all  directions  and  give  to  the  approaches  to  the  main  trunk  the 
appearance  of  short  precipitous  valleys  which  become  gradually  narrowed  and 
terminate  blindly  in  an  acute  angle.  The  tabular  roots  frequently  resemble  narrow 


RELATION   OF   STRUCTURE   TO   FUNCTION    IN   ROOTS. 


755 


Fig.  185.-Tndia-rubber  Fig  (Ficus  elastica)  ami  Banyan-tree  (Ficu,  Ind 


756  RELATION   OF   STRUCTURE   TO   FUNCTION    IN   ROOTS. 

buttresses,  with  regular  radiating  arrangement  around  the  trunk,  inclosing  small 
niches,  much  sought  after  as  hiding-places  by  various  animals,  and  offering  very 
acceptable  holes  to  foxes,  for  instance.  In  point  of  fact,  these  roots  are  often  called 
"  buttress-roots  ".  Tabular  roots  are  a  peculiarity  of  tropical  trees  with  huge,  heavy 
crowns.  A  particularly  well-defined  form  is  exhibited  by  the  West  Indian  Cotton 
Tree  (Eriodendron  caribceum)  and  by  the  India-rubber  Fig  (Ficus  elastica)  belong- 
ing to  tropical  Asia,  and  yielding  caoutchouc.  The  picture  of  this  tree,  drawn  from 
nature  by  Ransonnet,  fig.  185),  gives  us  a  very  clear  idea  of  these  tabular  or  buttress- 
roots;  the  same  figure,  in  the  background  to  the  right,  also  shows  another  species- 
of  Ficus,  viz.  the  celebrated  Banyan-tree  (Ficus  Indica),  which  will  be  described 
presently. 

Stilt-like  roots  (radices  fulcrantes)  also  arise  in  the  same  way  from  the  erect  or 
oblique  main  trunk,  but  they  are  cylindrical,  and  have  the  form  of  oblique  props. 
Sometimes  the  oldest,  lowest  portion  of  the  erect  trunk  thus  supported  dies  away, 
or  the  disintegration  may  be  continued  some  little  distance  up,  so  that  only  the 
upper  part  of  the  stem  remains  fresh  and  living.  The  first  roots  of  mangrove 
seedlings  (illustrated  on  p.  605),  which  penetrate  the  mud,  have  also  the  power  of 
raising  the  trunk  to  which  they  belong  up  above  the  mire  by  their  growth  in  length. 
These  trunks  then  look  as  if  they  were  on  stilts,  and  are  only  connected  with  the 
ground  by  means  of  the  roots.  On  page  758  we  have  a  figure  of  the  Screw 
Pine  (Pandanus),  and  in  fig.  187,  of  a  species  of  mangrove,  in  both  of  which  these 
odd  root-structures  are  seen.  They  are  also  to  be  found  in  many  other  plants  of 
the  tropics,  viz.  in  palms,  Clusiacese,  and  fig-trees.  In  some  clusias  the  stilt-roots- 
are  thicker  than  the  stem  they  support,  and  in  the  mangroves,  growing  in  crowded 
forests  on  the  sea-shore,  where  they  are  exposed  to  the  ebb  and  flow  of  the  tide, 
they  branch  and  fork  continually,  forming  a  tangled  confusion,  the  strange  appear- 
ance being  heightened  by  the  fact  that  all  the  root-branches  and  stems,  up  to  the 
level  of  the  water  at  high  tide,  are  covered  with  an  armoured  coat  of  various 
molluscs  and  crustaceans. 

Columnar  roots  (radices  columnares)  originate  from  horizontal  or  obliquely 
ascending  branches  of  trees,  and  grow  vertically  down  until  they  reach  the  ground. 
They  then  penetrate  into  it,  unite  with  the  soil,  and  thus  form  pillars  on  which  the 
widely  projecting  boughs  of  the  tree  are  supported.  Trees  whose  erect  trunks  are 
supported  by  tabular  roots  and  those  which  are  provided  with  stilt-roots  may  at 
the  same  time  develop  columnar  roots  from  their  branches.  One  of  the  oblique 
branches  of  the  India-rubber  Fig,  illustrated  in  the  foreground  of  fig.  185,  is- 
seen  to  be  supported  by  a  huge  pillar,  which  gets  thicker  towards  the  base,  whilst 
the  mangroves  figured  on  pp.  605  and  759  also  exhibit  long,  supporting  roots  passing 
down  from  the  lower  horizontal  branches  of  the  crown,  which  push  in  between  the 
stilt-roots,  and  grow  down  into  the  mud.  Not  long  ago  these  mangrove  roots  were 
thought  to  grow  out  of  the  fruits  while  these  were  yet  hanging  on  the  trees,  and 
to  grow  lower  and  lower  until  finally  they  reached  the  swampy  ground.  It  is, 
of  course,  true  that  the  embryo  grows  out  from  the  fruits  while  they  are  hanging 


KELATION   OF   STRUCTURE   TO   FUNCTION   IN    ROOTS.  757 

on  the  branches;  but  they  become  detached,  as  described  on  p.  603,  as  soon  as 
they  are  from  30  to  50  cms.  long,  and  falling  with  considerable  velocity,  bore 
into  the  mud  by  their  lower  thickened  end.  It  never  happens  that  one  of  these 
embryos  grows  down  to  the  ground  from  the  branch,  and  there  is  no  doubt  that 
the  long  roots  extending  from  the  crown  of  the  tree  down  to  the  mud  originate 
from  the  lower  horizontal  branches  of  the  mangroves  just  like  other  columnar 
roots.  Columnar  roots  are  distinguished  from  the  flexible,  cord-like,  aerial  roots 
of  aroids  and  other  epiphytes  (cf.  p.  365)  by  their  great  resistance  to  bending  and 
by  their  possession  of  a  characteristic  mechanical  tissue,  in  consequence  of  which 
they  have  a  totally  different  internal  structure,  which,  however,  will  be  described 
later  on. 

Perhaps  the  most  imposing  cases  of  development  of  columnar  roots  are  exhibited 
by  the  Indian  banyans  (Ficus  nitida,  Tsiela,  and  many  others),  which  are  usually 
comprehended  under  the  name  Ficus  Indica,  one  of  which  is  illustrated  in  the 
background  of  fig.  185.  To  these  also  belongs  the  celebrated  Asvhatta,  the  sacred 
Fig-tree  of  the  Hindoo  (Ficus  religiosa),  beneath  the  shade  of  which  Buddha  is 
said  to  have  learned  the  vanity  of  existence  and  the  mystery  of  the  universe.  In 
proportion  as  the  boughs  which  project  almost  horizontally  from  the  main  trunk 
of  this  tree  become  stronger,  and  give  rise  to  branches  and  increase  in  weight,  they 
send  out  cylindrical  roots  which  grow  down  to  the  ground,  penetrate  into  the  soil, 
strengthen  themselves  by  lateral  roots,  and  serve  as  supports  for  the  branches  in 
question.  These  columnar  roots,  which  continue  to  grow  in  thickness,  resemble 
•erect  stems,  develop  leafy  branches,  and  not  only  function  as  supports,  but  also 
serve  for  the  absorption  and  transmission  of  water  and  dissolved  food-salts  from 
the  ground.  Below  the  crown  of  one  of  these  banyan-trees  we  might  imagine 
ourselves  in  a  spacious  hall  of  which  the  roof  is  supported  on  pillars;  and  since 
the  leafy  covering  of  the  crown  is  almost  impervious  to  rain  and  sun,  a  weird 
twilight  always  pervades  these  halls  even  during  the  daytime.  Tradition  states 
that  an  army  of  5000  men  have  encamped  in  the  halls  of  a  single  banyan-tree. 
Near  the  village  of  Dena  Pitya,  in  Ceylon,  there  stands  an  Asvhatta  under  whose 
shade  a  village  of  a  hundred  huts  is  established,  and  in  a  single  banyan-tree  350 
large  and  3000  smaller  columnar  aerial  roots  have  been  counted.  When  left  en- 
tirely to  themselves  the  banyan-trees  scarcely  ever  assume  such  gigantic  propor- 
tions, because  the  ground  under  the  crown  is  so  dry  and  hard  that  the  supporting 
props  which  grow  down  often  fail  to  penetrate  it  and  are  unable  to  take  root 
there;  but  in  the  trees  held  sacred  by  the  Buddhists  the  rooting  is  assisted  by 
conducting  the  roots  descending  from  the  branches  through  long  bamboo  tubes, 
and  by  breaking  up  and  moistening  the  soil  where  they  would  penetrate  into  the 

ground. 

The  shape  of  roots  differs  materially  according  as  to  whether  the  plants  to 
which  they  belong  are  annual,  biennial,  or  perennial.  Annual  plants  produce  as 
many  seeds  as  possible  in  the  short  period  of  vegetation  allowed  them,  and  provide 
the  embryos  within  the  seeds,  which  have  to  travel  about  the  world,  with  the 


758 


RELATION    OF   STRUCTURE   TO   FUNCTION    IN    ROOTS. 


Fig.  186.— The  Screw  Pine  (Pandanus  utilis).    From  a  photograph. 


RELATION   OF   STRUCTURE   TO    FUNCTION    IN    ROOTS. 


759 


Fig.  187.— Stilt-roots  of  Mangroves  (Rhizophora  cot 


760  RELATION   OF   STRUCTURE   TO   FUNCTION    IN    ROOTS. 

reserves  of  food  necessary  for  the  founding  of  a  new  establishment.  It  would  be 
of  no  use,  and  contrary  to  the  economy  of  plants,  if  reserve  materials  were  de- 
posited in  any  other  parts,  say  in  the  stem  or  roots,  since  these  parts  shrivel  and 
dry  up  as  soon  as  the  seeds  are  dispersed,  and  the  energy  expended  in  the  manu- 
facture and  storage  of  starch,  fat,  sugar,  and  other  reserve  food  would  be  expended 
in  vain.  The  roots  of  annual  plants  are  therefore  satisfied  with  delivering  the 
necessary  water  and  the  required  amount  of  food-salts  to  the  plant  during  its  short 
period  of  vegetation,  and  with  providing  a  suitable  attachment  to  the  substratum; 
they  waste  no  energy  in  founding  subterranean  reservoirs.  In  biennial  and 
perennial  plants  it  is  quite  otherwise.  Biennial  plants — as  well-known  examples 
of  which  may  be  taken  the  various  roots  used  as  vegetables,  the  Carrot  (Daucus 
Garota),  the  Turnip  (Brassica  Rapa  rapacea),  and  the  Beet-root  (Beta  vulgaris 
rapacea) — develop  during  the  first  year  a  very  short  stem  with  foliage-leaves 
crowded  in  a  rosette,  and  a  thick,  fleshy  tap-root  (radix  palaris),  or  turnip-shaped 
root  (radix  napiformis).  When  vegetative  activity  recommences  in  the  second 
year,  an  erect  shoot  with  foliage  and  flowers  is  constructed  at  the  expense  or  at 
any  rate  with  the  help  of  the  materials  stored  up  in  the  thickened  root;  fruits  are 
produced  from  the  flowers,  and  after  the  ripening  of  the  seeds  the  whole  shoot  dies 
off  together  with  the  exhausted  roots.  In  perennial  plants  the  roots,  when  they 
serve  for  the  reception  of  abundant  reserve-materials,  are  usually  considerably 
thickened;  but  in  these  plants  it  is  the  clustered  root-fibres  springing  from  the 
lower  end  of  the  underground  part  of  the  stem,  after  the  primary  root  has  died 
off,  which  undergo  this  development.  When  the  thickening  is  symmetrical  and 
fusiform,  as  in  the  Orpine  (Sedum  Telephium)  and  in  the  white-flowered  Orobus 
Pannonicus,  the  roots  are  called  fusiform  (radices  grumosce)',  when  they  are 
swollen  at  intervals  into  knots,  as  in  the  Dropwort  (Spircea  Filipendula),  and  in 
the  yellow  Day-lily  (Hemerocallis  flava),  they  are  termed  nodose  (radices  nodosce). 
Many  of  our  terrestrial  orchids  have  two  kinds  of  roots  united  in  a  fascicle,  long 
cylindrical  vermiform  roots  and  short  thick  roots  filled  with  reserve-materials 
which  look  very  like  tubers,  and  are  called  tuberous  roots  (radices  tuberosce).  The 
Mediterranean  flora  and  that  of  steppes,  where  in  midsummer  the  vital  activity  of 
plants  is  much  reduced,  are  particularly  rich  in  plants  whose  roots  are  developed 
as  storehouses  for  reserve  materials.  Plants  of  widely  different  families  (e.g. 
Ranunculus  Neapolitan's,  Centaurea  napuligera,  Valeriana  tuberosa,  Rumex 
tuberosus,  Asphodelus  albus)  there  form  thickened  fascicled  roots  crowded  with 
reserve-materials  which  pass  through  the  dry  season  unharmed  underground,  and 
in  the  next  period  of  vegetation  supply  the  materials  for  the  rapid  construction 
of  epigeal  foliage  and  flowering  shoots.  These  thickened  bundles  of  roots  are 
characteristic  of  the  perennial,  parasitic  species  of  the  genus  Pedicularis.  They 
serve  for  the  storage  of  reserve  foods,  for  the  fixing  of  the  plant,  and  for  the  absorp- 
tion of  nourishment,  but  the  latter  function  is  here  carried  on  by  means  of  suckers, 
which  are  developed  at  the  end  of  the  thickened  fusiform  fibres,  and  which  attach 
themselves  to  the  roots  of  the  host  plants  in  the  manner  described  on  p.  179. 


RELATION   OF   STRUCTURE   TO   FUNCTION   IN   ROOTS.  7(J1 

It  would  naturally  be  expected  that  in  accordance  with  the  various  tasks 
assigned  to  roots  there  should  be  a  difference  in  the  arrangement  of  cells  and 
tissues,  and  that,  especially,  supporting  roots  which  exhibit  the  greatest  analogy 
with  erect  stems,  and  subterranean  roots,  which  have  so  much  in  common  with 
procumbent  and  subterranean  stem -structures,  should  resemble  them  in  internal 
structure.  Columnar  roots  cannot  really  be  distinguished  from  upright  stems  in 
their  inner  construction,  and  stilt-roots  also  present  an  arrangement  of  cells  and 
vessels  which  often  agrees  much  better  with  that  of  erect  stems  than  of  under- 
ground rhizomes.  In  Fragrcea  obovata,  belonging  to  the  Clusiacese,  the  cellular 
structure  of  the  erect  stem  is  only  distinguishable  from  that  of  its  supporting  roots 
by  the  somewhat  stronger  development  of  the  medulla  and  woody  portion  of  the 
vascular  bundles,  but  otherwise  there  is  no  sort  of  difference.  The  stilt-roots  of 
the  mangrove  figured  on  p.  759  (Rhizophora  conjugata)  likewise  show  a  stem-like 
internal  structure.  In  the  centre  is  a  thick  pith  surrounded  by  numerous,  con- 
ducting bundles,  which  together  form  a  hollow  cylinder,  and  are  accompanied  by 
mechanical  tissue;  further  outwards  come  the  cork,  hypoderm,  and  a  strongly - 
cuticularized  epidermis — exactly  the  same  arrangement  required  in  an  erect  stem 
as  a  protection  against  bending.  In  these  mangroves  the  strength  is  even  in- 
creased by  a  peculiar  tissue,  viz.  by  so-called  trichoblasts,  peculiarly  interlaced 
fusiform  cells  with  very  thick  walls,  which  are  so  hard  that  even  the  sharpest 
knife  will  scarcely  cut  through  them.  Though  these  adult  roots  are  structurally 
indistinguishable  from  stems,  this  is  not  usually  true  of  them  at  early  stages  in 
their  development.  When  young,  and  as  yet  unthickened,  these  roots,  as  a  rule, 
possess  an  internal  structure  characteristic  of  roots  in  general. 

In  mangroves  and  in  the  earlier  mentioned  Clusiaceae,  the  supporting  roots  are 
thick  and  widely  spread,  and  form  extensive  foundations  which  entirely  replace 
the  comparatively  weak  trunk,  so  far  as  fixing  on  the  substratum  is  concerned; 
they  need  especially  to  be  protected  against  bending.  A  resistance  to  tension 
scarcely  comes  under  consideration  in  these  plants.  It  is  quite  otherwise  in  plants 
whose  stilt-roots  have  to  support  a  stem  bearing  an  extensive  and  richly-leaved 
crown.  The  Pandanus  figured  on  p.  758  may  serve  as  a  type  of  these.  As  soon 
as  wind  sways  the  massive  crown  and  slender  stem  bearing  it,  the  roots  sup- 
porting the  stem  on  every  side  have  alternately  to  resist  bending  and  strain. 
If  the  wind  blows  from  the  north,  the  supporting  roots  springing  from  the  south 
side  experience  a  longitudinal  pressure  as  the  stem  inclines  to  the  southland  are 
pressed  and  curved  down  like  an  arch,  while  the  supporting  roots  springing  from 
the  north  side  are  at  the  same  time  subjected  to  a  powerful  strain.  When  the 
wind  sinks,  the  stem  is  again  brought  into  its  erect  normal  position  by  the  elasticity 
of  the  south  roots.  The  reverse  is  the  case  when  the  wind  attacks  the  crown  and 
stem  from  the  south.  This  form  of  stilt-root  must  therefore  be  construct 
to  resist  strain  as  well  as  bending,  and  accordingly  in  the  aerial  roots  of 
are  formed  two  cylinders  of  mechanical  tissue,  an  outer  one  which  is  formed 
the  hard  bast  of  a  peripheral  ring  of  vascular  bundles,  resembling  the  arrangemer 


762  RELATION   OF   STRUCTURE   TO    FUNCTION   IN    ROOTS. 

occurring  in  the  majority  of  dicotyledons,  and  an  inner  which  is  formed  of  the 
hard  bast  of  a  ring  of  vascular  bundles  lying  near  the  centre  of  the  root.  By  the 
former  the  supporting  roots  are  afforded  the  necessary  resistance  to  bending,  and 
by  the  latter  the  corresponding  resistance  to  strain. 

The  stilt-roots  springing  from  the  lowest  nodes  of  maize -plants  are  adapted 
to  this  double  function  just  as  are  those  of  Pandanus.  Here  also  are  two  cylinders 
of  mechanical  tissue.  The  outer  one,  situated  in  the  cortex,  consists  merely  of 
hard  bast  and  provides  a  resistance  to  bending,  while  the  inner,  in  connection  with 
the  conducting  bundles,  furnishes  a  resistance  to  strain.  In  the  stilt-roots  at  the 
base  of  the  maize-stem  there  is,  however,  a  central  pith  or  wide  medullary  cavity 
which  is  wanting  in  the  roots  of  Pandanus. 

Clinging  roots  adhering  to  the  bark  of  trees,  stones,  or  some  other  hard  sub- 
stratum, as  well  as  the  many  forms  of  subterranean  roots,  are  not  required  to  resist 
bending,  and  in  them  there  is  none  of  the  mechanical  tissue  which  would  be 
necessary  for  this  resistance.  On  the  other  hand,  these  roots  are  unavoidably 
subject  to  a  severe  strain  from  the  pulling  exerted  by  the  stem  and  branches  as 
they  sway  to  and  fro.  For  a  cylindrical  body  which  has  to  resist  a  powerful 
longitudinal  strain  there  is  no  better  contrivance  than  the  fusion  of  the  resisting 
elements  into  a  compact  mass  in  the  axis  of  the  cylinder,  and  this  arrangement  is 
actually  met  with  in  clinging  and  subterranean  roots.  The  conducting  bundles 
and  the  adjoining  mechanical  tissue  form  a  single  central  strand  in  the  cylindrical 
root,  and  the  typical  form  of  a  subterranean  root  is  a  cylindrical  body  of  tissue 
which  has  no  central  pith  and  no  hard  bast  cylinder  near  the  circumference,  but 
whose  vascular  bundles  are  so  crowded  towards  the  axis  that  they  form  there  a 
single,  thick  strand  or  "central  cylinder". 

Roots  embedded  in  the  ground  are  of  course  exposed  to  a  lateral  pressure  from 
their  surroundings,  and  care  must  be  taken  that  the  functions  of  the  conducting 
bundles  are  not  disturbed  by  this  pressure,  that  the  transmission  is  not  interrupted 
or  even  entirely  stopped.  This  is  effected  by  padding  the  central  strand  just 
described,  that  is,  by  surrounding  it  with  a  mantle  of  parenchymatous  cells.  The 
thickness  of  this  coat  varies  according  to  the  extent  of  the  lateral  pressure,  and 
when  the  roots  are  subjected  to  very  great  pressure,  the  walls  of  the  parenchyma- 
tous cells  are  even  thickened  in  a  corresponding  degree. 

Reserve-materials  may  also  be  deposited  in  this  parenchymatous  mantle.  In 
biennial  and  perennial  roots  the  tissue  surrounding  the  sap-conducting  and  strain- 
resisting  strand  is  not  only  thickened  so  as  to  give  the  necessary  support  against 
pressure,  but  also  provides  a  place  for  starch,  fat,  sugar  and  other  supplies  which 
are  to  be  consumed  in  the  next  period  of  vegetation. 

Naturally  these  soft  tissues,  often  filled  with  reserve-food,  are  an  attraction  to 
diverse  animals  living  underground,  and  the  establishment  of  such  a  storehouse 
renders  a  corresponding  protection  against  the  attacks  of  mice  and  insect-larvae 
necessary.  Though  the  protective  agents  and  weapons  by  which  the  green  foliage, 
and  flowers,  and  fruit  are  preserved  from  the  ravages  of  animals  would  not  serve 


RELATION   OF  STRUCTURE  TO   FUNCTION   IN   ROOTS.  763 

here,  still,  by  the  development  of  poisonous  and  disagreeable  substances,  the  sub- 
terranean, burrowing  insects  are,  as  far  as  possible,  kept  away.  It  is  well 
known  that  roots  are  particularly  rich  in  poisonous  alkaloids,  in  resins  which 
are  repulsive  to  animals,  in  bitter  substances  and  the  like;  these  parts  of  plants 
are  well  known  as  providing  many  drugs  of  the  pharmacopeia.  These  do  not 
indeed  afford  an  infallible  protection  against  all  attacks  from  animals,  but  that 
a  partial  safeguard  at  least  is  obtained  by  the  storing  up  of  certain  materials 
seems  very  probable  by  the  following  observations.  The  field-mice  in  a  garden 
at  Innsbruck  once  caused  great  havoc  under  the  winter  coat  of  snow,  and  various 
roots  were  gnawed  by  them;  but  the  roots  and  root-stock  of  the  Soap  wort 
(Saponaria  officinalis),  containing  quantities  of  poisonous  saponin,  were  always 
left  untouched  by  them.  The  bitter  roots  of  gentians  (Gentiana  punctata,  lutea, 
Pannonica),  which  are  very  rich  in  reserve-foods,  and  which  grow  in  deep  alpine 
meadows  riddled  by  mice,  were  never  seen  to  be  attacked  by  a  single  animal. 
This  was  also  the  case  with  the  thick  tap-roots  of  the  poisonous  monkshood,  the 
massive  roots  of  rhubarb-plants  and  of  many  Umbelliferae,  which  are  all  abundantly 
supplied  with  starch  and  other  food-stuffs,  and  therefore  would  afford  an  excellent 
food  for  herbivorous  animals  under  stress  of  hunger. 

When  the  parenchymatous  tissue  surrounding  the  central  strand  of  the  con- 
ducting bundles  in  subterranean  roots  serves  not  only  as  an  agent  for  protecting 
against  lateral  pressure,  but  also  for  the  storing  up  of  food-materials,  and  in 
addition  possesses  contrivances  for  warding  off  voracious  animals,  the  structure  of 
the  roots  is  much  more  complicated  than  in  cases  where  it  affords  protection  against 
lateral  pressure  alone.  There  are  also  very  many  different  developments  of  par- 
enchymatous tissue  on  the  periphery  of  subterranean  roots  in  accordance  with  the 
various  demands  necessitated  by  the  conditions  of  the  habitat  and  the  peculiar 
mode  of  life  of  the  species.  In  aquatic  roots  the  need  for  abundant  ventilation  has 
also  to  be  considered,  and  the  storage  of  reserve-foods  in  these  organs  must  be 
avoided  since  the  increase  in  weight,  due  to  the  massing  of  reserve-food,  might 
draw  the  floating  water-plant  down  into  the  water  at  an  unsuitable  time. 

A  storage  of  food-materials  in  the  special  tissue  developed  at  the  growing  root- 
tip,  and  known  as  the  root-cap,  would  also  be  unsuitable.  In  subterranean  roots 
the  root-cap  only  protects  the  delicate  dividing  and  multiplying  cells  at  the  growing 
end.  The  pressure  to  which  these  continually  dividing  cells  are  exposed  in  their 
penetration  into  the  ground  is  much  greater  than  that  operating  on  the  fully 
formed  parts  behind  the  root-tip.  The  growing  point  of  the  root  has  to  push  on 
one  side  hard  grains  of  sand  and  other  particles  of  earth,  and  to  make  a  hole  like  a 
ground-auger  in  which  later  on  the  fully  developed  root  can  take  up  its  position. 
The  root-cap  may  be  compared  to  a  shield  which  is  formed  by  the  growing  and 
therefore  advancing  cells  in  the  direction  required,  these  constantly  pushing  it  in 
front  of  them.  This  shield  is  always  being  supplemented  and  renewed  by  the 
growing  tissue.  The  half  of  the  root-cap  adjoining  the  growing  tissue  consists  of 
angular,  closely-fitting  cells;  the  outer  half,  directed  towards  the  soil,  consist*  of 


764  DEFINITION    OF   THE   ROOT. 

rounded,  loosely-fitting  cells,  and  on  this  outer  side  of  the  root-cap  the  cells  are 
also  seen  to  be  partially  separated  and  torn  off.  As  the  outer  cell-layers  are  rubbed 
away  by  the  advance  of  the  root,  and  by  unavoidable  contact  with  the  sur- 
rounding soil,  new  cells  are  always  being  pushed  forward  from  within,  and  in  this 
way  the  loss  is  made  good,  and  the  shield  continually  repaired. 

Obviously,  aquatic  roots  do  not  require  a  shield  of  this  kind  at  their  apex,  and 
in  aerial  roots,  at  least  in  the  form  described,  it  would  likewise  be  superfluous. 
Even  roots  which  penetrate  into  mud  do  not  require  it.  Accordingly  many  water- 
plants  and  the  marsh-inhabiting  mangroves  do  not  develop  a  cap  at  their  root-tip. 
The  root-cap  is  also  entirely  absent  in  parasitic  plants  which  it  would  only  hinder 
from  penetrating  into  the  tissue  of  the  host-plants. 

DEFINITION  OF  THE  ROOT. 

In  the  preceding  pages  we  have  continually  spoken  of  roots,  although  we  have 
not  yet  defined  technically  what  a  root  is,  and  now,  contrary  to  the  usual  custom 
in  scientific  works,  the  definitiou  of  this  organ  has  to  come  not  at  the  beginning 
but  in  the  middle  of  the  chapter.  This  alteration  in  position  has  been  caused  by 
the  necessity  of  establishing  the  definition  on  some  peculiarities  in  the  external 
and  internal  structure  of  roots,  with  which  we  could  not  suppose  all  readers  to  be 
familiar,  and  which  therefore  had  to  be  described  beforehand  as  far  as  required. 

But  many  readers  will  ask  if  any  definition  is  required,  if  everyone  does  not 
know  without  it  what  the  root  of  a  plant  is,  and  how  it  can  be  distinguished  from 
a  stem  and  leaves?  The  case  is  exactly  parallel  with  that  of  the  leaf.  Every  one 
who  is  not  a  botanist  thinks  he  knows  what  is  meant  when  he  hears  the  word 
"leaf",  and  cannot  conceal  his  astonishment  or  possibly  his  smile  when  he  is 
informed  that  scientific  men  are  not  agreed  about  such  a  simple  question, 
and  that  they  write  violent  polemics  upon  this  question.  To  the  impartial  reader 
debates  as  to  whether  a  certain  part  of  a  plant  is  to  be  regarded  as  a  root  or  not 
doubtless  appear  hypercritical  and  a  pedantic  splitting  of  hairs,  and  with  regard  to 
many  of  the  discussions  I  would  hardly  venture  to  deny  the  justice  of  his  position. 
The  savant  who  constructs  for  himself  the  picture  of  an  ideal  or  primitive  plant 
from  a  sometimes  larger,  sometimes  smaller  number  of  single  observations,  who 
finds  out  how  the  individual  parts  were  situated  in  their  succession  as  to  time, 
and  in  their  mutual  relations  in  space,  and  who  distinguishes  and  defines  the 
various  parts  accordingly,  is  indeed  very  easily  tempted  to  take  the  abstract  ideal 
he  has  created  as  a  standard  for  the  whole  vegetable  kingdom.  From  his  point 
of  view,  obtained  by  the  consideration  and  comparison  of  so  many  individual  cases, 
all  forms  are  arranged  and  explained,  everything  must  fit  into  the  now  firmly 
established  groundwork,  and  where  it  will  not  coincide,  he  talks  of  exceptions, 
forgetting  that  in  such  a  case  exceptions  are  not  permissible,  but  are  rather  a  proof 
of  inadequate  generalization  from  the  single  cases  observed. 

In  the  comprehension  of  the  results  of  general  comparative  studies  of  this  kind 


DEFINITION    OF   THE   ROOT.  765 

into  the  configuration  of  plants,  it  is  a  matter  of  great  import  how  the  definitions 
of  the  individual  parts  and  members  of  the  plant  are  formulated,  and  whether  the 
author  lays  particular  stress  on  this  or  that  characteristic.  Suppose  that  some 
observer  holds  the  opinion  that  the  presence  or  absence  of  the  root-cap  affords  an 
important  distinction  between  a  root  and  stem;  then  he  would  sp«ak  of  the  supports 
of  mangrove-trunks  as  lateral  stems  which  grow  downwards;  another,  who  lays 
particular  weight  on  the  fact  that  roots  produce  no  leaves  behind  their  growing 
points,  would,  on  the  contrary,  declare  the  supports  of  the  mangrove-trunk  to  be 
roots  devoid  of  root-caps.  It  would  be  the  same  with  the  contradictory  explana- 
tions and  different  appellations  which  would  be  given  to  the  supports  of  Clusiaceae 
and  figs,  to  the  fixing  and  absorbent  apparatus  of  Mistletoe  which  penetrate  into 
the  host-plant,  and  to  so  many  other  hypogeal  and  epigeal  parts  of  the  plant-body. 
These  examples  will  suffice  to  show  how  a  conflict  may  arise  over  such  an  appa- 
rently simple  thing,  how  easily  the  investigators  into  the  region  of  the  speculative 
science  of  form  may  become  one-sided,  what  great  difficulties  are  to  be  encountered 
in  formulating  a  definition,  and  how  in  particular  a  hasty  generalization  must  be 
avoided  about  characteristics  which  it  is  not  at  all  certain  are  really  to  be  met  with 
universally.  Every  definition  is  dependent  upon  the  extent  of  our  knowledge  at 
the  time;  it  may  not  hold  good  as  our  experience  widens,  and  therefore  has  only 
a  relative  value. 

From  the  standpoint  of  our  present  knowledge,  however,  the  following  may  be 
taken  relatively  as  the  best  definition: — A  root  is  a  body  of  tissue  provided  with 
vascular  bundles,  which  originates  from  an  older,  previously-existing  part  of  the 
plant;  its  growth  is  not  limited,  and  it  never  directly  gives  rise  to  leaves. 

In  connection  with  this  definition,  some  remarks  may  be  made  here  by  which 
many  relations  between  the  root  and  other  parts  of  the  plant  will  be  elucidated. 
First  it  should  be  noted  that  in  the  above  definition  the  youngest  developmental 
stage,  viz.  the  embryo,  is  included  under  the  term  "plant".     It  has  further  to  be 
explained  why  the  characteristic  which  is  first  thought  of  in  non-botanical  circles, 
when  speaking  of   roots,  viz.   their  power  of   deriving   fluid   nourishment   from 
another  body,  has  not  been  mentioned  in  the  above  definition.    It  is  perfectly  correct 
to  say  that  an  absorption  of  fluids  is  generally  observed  in  roots,  but  in  reality  it 
is  only  the  root-hairs  proceeding  from  the  roots  which  perform  this  task,  and  these 
absorbent  cells  are  known  to  be  also  developed  on  stems  and  leaves.     The  coty- 
ledon extended  from  the  seed  of  the  Bulrush  (Typha),  penetrates  into  the  soil  with 
absorbent  cells.     The  cavities  of  the  green  leaf-structures  in  insectivorous  plants 
are  also  abundantly  provided  with  them,  and  special  absorbent  cells  are  develop* 
on  the  green  leaves  of  many  saxifrages,  tamarisks,  and  so  forth;  whill 
marsh-plants,  the  leaves  of  which  float  partly  on  the  surface  of  the  water  and 
partly  submerged,  the  epidermal  cells  also  function  as  absorbent  cells.     In  many 
aquatic  plants  (e.g.  Hottoma,  Ceratophyllum,  Naias)  absorption  is  only  earned 
by  means  of  the  epidermal  cells  of  the  foliage-leaves,  and  no  trace  of  roots  i 
be  Found  in  them.     Their  foliage-leaves,  however,  remind  one  very  much  of  root- 


7(56  DEFINITION   OF   THE    ROOT. 

structures,  and  in  a  floating  water  fern  (Salvinia  natans,  cf.  vol.  ii.  fig.  380)  the 
submerged  leaves  have  the  greatest  resemblance  to  roots  in  shape  and  colour.  In 
such  cases,  though  we  can  say  that  the  leaves  are  metamorphosed  into  absorbent 
organs,  we  cannot  assert  that  they  have  become  roots.  This  applies  also  to  plants 
whose  underground  steins  are  provided  with  absorbent  cells  (e.g.  Bartsia,  Epi- 
pogium,  Corallorrhiza),  or  whose  stem-structures,  submerged  in  water,  are  fur- 
nished with  epidermal  cells  functioning  as  root-hairs  (e.g.  Lemna  trisulca).  In 
these  plants  the  stem-structures  are  indeed  metamorphosed  into  absorbent  organs, 
but  they  are  never  transformed  into  roots. 

We  are  accustomed  to  think  of  the  roots  of  plants  as  organs  with  white, 
yellow,  red,  brown,  or  black,  but  never  green,  colour,  because  as  a  matter  of  fact 
by  far  the  greater  number  are  devoid  of  chlorophyll.  But  there  are  plants  whose 
roots  do  contain  chlorophyll,  e.g.  those  of  Lemna  minor,  and  various  aroids  and 
orchids.  In  orchids  with  aerial  roots  but  no  green  foliage-leaves,  the  green  roots 
must  take  on  the  formation  of  organic  compounds  from  food-gases  in  sunlight, 
that  is,  the  function  which  is  performed  by  the  foliage-leaves  in  so  many  other 
cases.  We  should,  therefore,  be  as  little  justified  in  bringing  forward  the  absence 
of  chlorophyll  as  a  characteristic  feature  of  roots  as  in  saying  that  the  roots  had 
become  changed  into  green  leaves.  The  roots  of  the  orchids  mentioned  have  indeed 
become  transformed  into  assimilating  organs,  but  they  remain  roots  nevertheless. 

It  was  formerly  thought  that  roots  and  stems  could  be  distinguished,  the  former 
by  their  inability  to  develop  buds,  and  the  latter  by  their  power  of  forming  them. 
But  although  this  difference  is  actually  observed  in  most  instances,  it  cannot  be 
applied  universally.  The  roots  in  many  plants  develop  buds  which  unfold  into 
leafy  shoots,  and  not  merely  lateral,  but  terminal  buds  also.  When  this  happens 
it  looks  as  if  the  root  were  continued  directly  into  a  leafy  shoot,  and  this  occur- 
rence has  led  to  the  mistaken  idea  that  the  root-tip  may  become  metamorphosed 
into  a  leafy  stem. 

Finally  we  have  to  consider  the  difference  in  the  mode  of  origin  of  roots  and 
stems.  It  cannot  be  denied  that  the  points  of  origin  of  stem-structures  are  usually 
arranged  geometrically,  while  roots  only  exhibit  such  an  arrangement  in  rare 
cases.  But  we  must  again  insert  the  words  "usually"  and  "rare",  for  here  too 
a  universal  distinction  does  not  exist.  The  stem-structures  springing  from  the 
underground  roots  of  the  Aspen  (Populus  tremula),  and  from  old  trunks  of  the 
Black  Poplar  (Populus  nigra)  make  their  appearance  quite  irregularly,  whilst, 
on  the  other  hand,  the  roots  of  many  aroids  originate  with  the  same  regularity 
as  leaves  and  the  lateral  shoots  arising  from  the  axils.  In  most  cases  the  root 
proceeds  from  a  group  of  cells  in  the  interior  of  a  stem  or  older  root,  and  it  used 
to  be  thought  that  this  constituted  a  difference  between  roots,  and  stems  and  leaves, 
since  the  latter  arise  from  cells  near  the  surface  of  the  tissue- body  which  bears 
them.  But  aquatic  roots,  e.g.  those  of  Ruppia  and  Zannichellia,  also  proceed  from 
cells  near  the  surface  of  the  stem,  and  in  the  same  way  roots  arise  from  the  epi- 
dermal cells  of  the  leaves  of  the  Cuckoo  Flower  (Cardamine  pratensis),  and  from 


REMARKABLE   PROPERTIES   OF   ROOTS.  767 

the  parenchyma  lying  immediately  below  the  epidermis,  so  that  this  again  does  not 
furnish  a  universal  distinction. 

But  although  all  those  characters,  which  have  been  used  in  turn  to  characterize 
the  root,  cannot  be  thus  employed  because  they  have  not  a  universal  value,  yet  one 
distinguishing  feature  always  remains,  viz.  that  leaves  are  never  produced  from 
root-tissues,  and  the  greatest  stress  is  to  be  laid  on  this  point.  After  weighing 
everything  carefully  we  come  to  the  conclusion  that  the  plant,  and  even  its 
youngest  developmental  stage,  the  embryo,  begins  with  a  stem,  which  develops 
leaves  and  roots.  Stems,  leaves,  and  roots  may  perform  widely  different  functions, 
may  shape  themselves  accordingly,  and  may  be  metamorphosed  into  widely  differ- 
ent organs.  A  plant  is  comparable  to  a  crustacean  which  is  divided  into  a  body 
and  appendages.  The  appendages  in  most  cases  serve  as  organs  for  locomotion, 
grasping,  and  clinging,  but  are  sometimes  also  metamorphosed  into  respiratory 
organs,  egg-carriers,  &c. 


EEMAEKABLE  PROPERTIES  OF  ROOTS. 

The  small  stem-structures  which  proceed  from  germinating  orchid-seeds  behave 
very  differently  according  to  the  nature  of  their  germinating  bed.  From  the  small 
tubercles  of  species  growing  as  epiphytes  on  the  bark  of  trees  arise,  first  of 
all,  hair-like  absorbent  cells  which  adhere  to  the  substratum;  then  roots  make 
their  appearance,  which  also  unite  firmly  with  the  bark,  though  their  superficial 
cells  are  not  able  to  penetrate  into  it.  The  small  tubercles  of  the  terrestrial 
orchids,  which  inhabit  the  meadows  and  the  humus  of  the  forest  soil,  develop 
roots  which  grow  down  into  the  ground  and  direct  their  growing  tips  towards  the 
centre  of  the  earth.  In  this  way  they  draw  the  stem-structure  from  which  they 
originate  down  with  them,  and  thus  the  tuberous  stem  in  two  years'  time  comes 
to  lie  6-10  cm.  below  that  point  in  the  meadow  where  the  seed  actually  germinated. 
The  same  thing  happens  with  the  embryos  of  many  biennial  and  perennial  plants, 
especially  of  those  whose  underground  roots  and  stems  are  subsequently  used  as 
storehouses  for  reserve  materials,  e.g.  in  Carrots,  Evening  Primroses,  in  the  Monks- 
hood,  Meadow  Clover,  Vincetoxicum,  Dog's  Mercury,  Martagon  Lily,  Bulbous 
Crowfoot  (Daucus,  (Enothera,  Aconitum,  Trifolium  pratense,  Cynanchum  Vince- 
toxicum, Mercurialis  perennis,  Lilium  Martagon,  Ranunculus  bulbosus),  and 
many  others.  In  these  plants  also  the  embryonic  stem  is  drawn  more  or  less 
deeply  under  the  ground,  and  the  scarred  point  of  insertion  of  the  cotyledons  is 
not  infrequently  found  to  be  several  centimetres  lower  down  that  it  was  at  the 
time  of  their  withdrawal  from  the  integument  of  the  seed. 

Many  roots  arising  later  on  from  procumbent  and  from  erect  or  twining  and 
climbing  leafy  stems  have  the  power  of  exercising  a  pull  on  their  stem. 
springing  from  the  stem  nodes  of  runners,  e.g.  from  those  of  strawberry  plants 
draw  the  nodes  a  centimetre  below  the  ground     This  is  also  the  case  with  t 
long  roots  proceeding  from  the  stems  of  perennial  primulas.     When  these  primula 


768  REMARKABLE   PROPERTIES   OF   ROOTS. 

settle  in  the  clefts  and  crevices  of  perpendicular  rock  faces,  a  phenomenon  is 
produced  by  this  down-drawing  which  surprises  anyone  noticing  it  for  the  first 
time,  appearing  at  first  quite  inexplicable.  The  thick  stems  of  these  primulas 
(e.g.  Primula  Auricula,  Clusiana,  hirsuta)  are  terminated  by  a  rosette  of  foliage- 
leaves;  these  turn  yellow,  and  dry  up  in  the  autumn,  and  a  new  rosette  is  laid 
down  in  the  axil  of  one  of  them  for  the  next  year.  Although  the  leaves  of  the 
rosettes  stand  close  above  one  another,  the  portion  of  the  stem  clothed  by  them 
is  quite  a  centimetre  long,  and  the  annual  increase  undergone  by  the  stem  which 
grows  towards  the  light  is  also  a  centimetre.  The  increase  during  ten  years  would 
amount  to  10  cm.,  and  it  would  be  expected  that  the  rosette  of  the  tenth  year 
would  be  about  10  cm.  above  the  level  where  stood  the  first  year's  rosette.  But, 
strange  to  say,  the  rosettes  of  all  the  succeeding  years  always  remain  at  the  same 
place,  that  is,  they  always  cling  to  the  rocky  edges  of  the  crevice  or  cleft  in  which 
the  stock  is  rooted.  The  explanation  of  the  phenomenon  is  that  the  roots  springing 
from  the  rosette-bearing  stem  draw  it  down  every  year  about  a  centimetre  into 
the  soil  or  crevice  filled  with  humus.  But  naturally  this  can  only  occur  if  the 
lower  end  of  the  stem  annually  dies  off  and  decays  to  a  corresponding  extent, 
and  this  is  what  actually  happens.  In  rocky  clefts  which  are  not  well  adapted 
to  this  process  the  primulas  grow  badly,  and  their  stems  project  above  the  edges 
of  the  crevice;  ultimately  the  entire  plant  falls  into  a  slow  decline  and  no  longer 
blossoms,  but  perishes  in  a  few  years.  The  knowledge  of  their  peculiar  mode 
of  growth  is  therefore  of  some  importance  in  the  cultivation  of  these  primulas, 
vsince  care  can  be  taken  to  plant  them  so  that  the  stems  can  be  annually  drawn 
a  certain  amount  into  the  soil  by  the  roots.  It  is  of  course  needless  to  mention 
that  many  other  plants  beside  primulas,  rooted  in  crevices  of  rock,  behave  in  the 
same  way,  e.g.  Phyteuma  comosum,  Qentiana  Clusiana,  Campanula  Zoisii, 
Paederota  Ageria. 

The  ends  of  branches  of  many  species  of  bramble  are  drawn  under  the  ground 
in  a  very  peculiar  way.  One  of  these  species,  Rubus  bifrons,  is  represented  in 
fig.  188,  where  the  roots  and  the  ends  of  the  branches  drawn  under  the  soil 
by  them  are  rendered  evident,  the  earth  in  the  foreground  being  removed  as  if 
dug  away  by  a  spade.  Rubus  bifrons  develops  strong  five-ridged  shoots  beset 
with  reversed  prickles;  they  at  first  grow  directly  upwards,  but  towards  autumn 
hang  in  broad  curves,  so  that  their  tips  approach  the  ground.  Before  they  have 
reached  the  soil,  however,  small  scale-like  protuberances,  looking  like  stunted 
leaves,  are  to  be  noticed  arising  near  the  tip;  these  are  the  commencements  of 
roots.  When  the  apex  of  the  branch  at  length  trails  on  the  ground  the  protuber- 
ances, now  in  contact  with  the  soil,  elongate  into  roots  which  penetrate  the  ground. 
They  lengthen  very  rapidly,  sending  out  numerous  lateral  roots,  and  in  a  short  time 
an  extensive  subterranean  root-system  is  the  result.  But  the  apex  of  the  branch 
which  serves  as  a  starting-point  for  these  roots,  and  which  is  now  considerably 
thickened,  has  also  come  under  the  ground.  It  has  been  drawn  down  by  the 
roots,  and  is  now  embedded  in  the  soil.  In  the  following  spring,  sometimes  even 


REMARKABLE   PROPERTIES   OF   ROOTS. 

in  the  autumn  in  which  the  rooting  has  taken  place,  this  branch  apex,  nourished 
)ts,  grows  up  into  a  shoot,  which  again  appears  above  the  ground.     The 


Fig.  188.— Bramble-bush  in  which  the  branches  have  taken  root 


old  branch,  however,  which  had  arched  down  to  the  soil,  and  whose  apex  had 
been  drawn  into  the  ground  by  the  roots,  dies  off  sooner  or  later,  consequently 
a  new,  independent  plant  results  from  this  action. 

VOL.  I.  49 


770  REMARKABLE   PROPERTIES   OF   ROOTS. 

It  has  been  shown  that  where  stems  are  drawn  into  the  ground,  it  is  by 
means  of  the  roots.  After  the  growth  in  length  of  a  root  is  completed,  it  shortens, 
in  some  instances  only  2-3  per  cent,  in  other  cases  as  much  as  20-30  per  cent, 
i.e.  almost  a  third  of  its  entire  length.  The  shortening  depends  upon  alterations 
in  the  turgidity  of  the  cells  connected  with  an  absorption  of  water.  While  the 
cells  of  that  portion  of  the  root  which  is  still  growing  elongate  by  increased 
turgescence,  those  of  the  fully-formed  root  become  shorter  and  broader  in  con- 
sequence of  the  increase  in  their  turgidity.  The  parenchymatous  cells  in  fully- 
formed  roots  become  broader  at  the  expense  of  their  length  in  consequence  of 
the  increased  turgescence  produced  by  the  absorption  of  water,  and  the  natural 
result  is  a  shortening  of  the  whole  tissue-body.  This  contraction  of  the  mature 
root-portion  exerts  a  tension  on  both  ends.  At  the  lower  end  of  the  fully-formed 
part  of  the  root  is  the  still  immature  portion  growing  downwards,  whilst  at  the 
upper  end  that  part  of  the  stem  from  which  the  root  originated.  Above  the 
downwardly-directed  point  the  immature  part  of  the  root  is  equipped  with  hair- 
like  absorbent  cells,  and  these  are  closely  united  to  the  surrounding  soil.  In  this 
way  a  resistance  is  afforded  which  the  strain  of  the  contracting  part  of  the  root 
cannot  overcome.  And  since,  as  already  stated,  the  cells  at  the  growing  end  of 
the  root  are  lengthened  by  turgescence,  the  tissue  is  extended,  and  the  root-tip, 
in  spite  of  the  strain  operating  from  above,  continues  to  penetrate  into  the 
ground.  The  strain,  therefore,  has  no  effect  in  this  direction.  But  it  is  otherwise 
with  the  pull  exercised  on  the  stem  by  this  contracting  of  the  root.  There  is  no 
powerful  resistance  here  to  be  overcome,  and  consequently  the  part  of  the  stem 
in  question,  whether  the  hypocotyl  of  the  embryo,  the  end  of  the  epicotyl,  or  a 
node  from  the  middle  or  end  of  the  leafy  foliage-stem,  is  drawn  down  into  the  soil. 

This  remarkable  planting  of  course  only  occurs  where  the  roots  grow  down 
vertically  into  the  ground,  and  as  remarked,  it  is  most  noticeably  observed  in 
species  whose  subterranean  stem  and  root-structures  store  up  reserve  materials. 
Roots  which  run  horizontally  below  the  surface  of  the  ground  are  not  adapted 
to  influence  the  stem  in  the  manner  indicated.  On  the  contrary,  in  certain 
circumstances,  these  are  able  to  effect  an  elevation  of  the  stem.  This  happens 
especially  in  trees  with  thick  woody  roots,  e.g.  in  pines  and  firs,  oaks  and  chestnuts, 
and  is  to  be  explained  in  the  following  simple  way.  The  first  embryonic  root 
growing  down  vertically  into  the  ground  soon  dies  off,  or  its  development, 
especially  its  increase  in  length,  is  greatly  retarded,  and  much  more  vigorous 
roots  develop  from  it,  or  from  the  lowest  part  of  the  erect  hypocotyl.  These 
spread  out  in  a  horizontal  direction  under  the  surface  of  the  ground.  They 
usually  radiate  out  in  all  directions  forming  a  whorl  at  the  base  of  the  erect 
stem,  as  can  be  plainly  seen  in  pines  uprooted  by  a  devastating  storm.  These 
horizontal  roots  at  first  have  only  a  slight  thickness,  but  their  diameter  in- 
creases with  age,  and  the  successive  layers  of  wood  in  them  form  annual  rings, 
just  as  in  stems.  Now  these  subterranean  roots,  in  addition  to  resisting  the 
pressure  of  the  surrounding  soil,  actually  exercise  a  considerable  lateral  pressure 


REMARKABLE   PROPERTIES   OF   ROOTS.  771 

by  their  growth  in  thickness.  In  consequence  of  this  the  soil  below  the  cylin- 
drical, horizontal  root  becomes  compressed,  but  that  above  it  is  raised  and  burst 
open.  The  thick,  woody  root  gradually  becomes  visible  on  the  surface,  and  is 
entirely  stripped  of  earth  on  the  upper  side.  The  axis  of  the  horizontal  root 
never  again  assumes  the  position  of  earlier  years;  then  the  roots  were  only  a 
few  millimetres  thick,  but  now  they  have  attained  to  a  diameter  of  20-30  centi- 
metres, and  the  root-axis  has  been  shifted  upwards  through  almost  half  its 
diameter,  i.e.  through  10-15  centimetres.  The  erect  trunk,  which  is  firmly 
united  to  the  horizontal  roots  in  the  way  just  described,  is,  of  course,  raised  up 
to  the  same  extent.  In  this  manner  may  be  explained  the  peculiar  appearance 
so  frequently  to  be  seen  in  our  pine  and  oak  forests — the  appearance  of  huge 
tree-trunks  with  thick  woody  roots  springing  from  their  base  which  are  quite 
devoid  of  earth  on  their  upper  sides,  and  run,  half  underground,  in  snake-like 
coils  into  the  forest  ground. 

The  elevation  of  stems  by  means  of  roots  is  more  striking  in  tropical  man- 
groves even  than  in  our  native  trees.  After  the  seedling  has  fallen  from  the  tree 
and  bored  its  way  into  the  mud,  protuberances  arise  on  the  circumference  of  its 
lower  third  which  develop  into  roots  directed  obliquely  downwards.  Even  in  a 
few  months  the  buried  plant  is  raised  up  a  little  above  the  mud  by  the  lengthening 
of  these  roots,  so  as  now  to  look  as  if  it  were  supported  on  stilts. 

It  has  been  repeatedly  mentioned  that  the  primary  roots  of  the  embryo 
originate  from  places  on  the  hypocotyl  which  have  been  determined  beforehand. 
So  also  does  the  origin  of  roots  on  many  rhizomes,  runners,  and  on  climbing 
stems  seem  to  be  precisely  determined,  and  to  be  quite  independent  of  external 
influences.  For  example,  the  primary  root  of  mustard  and  numerous  other 
plants  is  developed  under  all  circumstances  from  one  pole  of  the  hypocotyl. 
The  runners  of  strawberry  plants  and  of  the  Creeping  Crowfoot  (Fragaria  vesca 
and  Ranunculus  repens)  develop,  without  any  external  stimulus,  a  group  of  from 
two  to  five  root-protuberances  on  the  stem-nodes,  and  the  bramble  branches, 
described  above,  curve  like  arches  to  the  ground,  forming  several  root-prominences 
at  definite  spots  near  the  apex  before  they  have  reached  it,  in  order  that  they  may 
take  root  in  the  soil.  In  many  epiphytic  aroids  and  orchids  the  places  of  origin 
of  the  roots  are  arranged  as  symmetrically  round  the  stem  as  are  those  of  leaves, 
and  many  other  examples  might  be  cited  from  which  it  follows  that  the  position 
of  part  of  the  roots  is  definitely  fixed  beforehand,  being  based  upon  the  specific 
constitution  of  the  protoplasm  of  the  species  in  question.  But  as  well  as  the 
roots  developing  in  the  manner  indicated  in  definite  positions,  others  are  formed 
which  require  for  their  development  a  special  stimulus  from  outside,  whose 
place  of  origin  is  not  determined  beforehand,  but  is  first  fixed  by  some  external 
agent.  To  this  category  belong  the  roots  arising  from  the  nodes  of  shrubs  which 
have  been  battered  down  on  the  ground,  and  from  stems  coming  in  contact 
with  damp  objects,  as  well  as  those  which  proceed  from  foliage-leaves,  and, 
finally,  the  wart-like  roots  of  parasites  known  as  haustoria.  When  shrubs 


772  REMARKABLE   PROPERTIES   OF   ROOTS. 

with  erect  stems  and  thick  stem-nodes,  e.g.  the  various  species  of  Galeopsis  or 
Polygonum,  are  extended  flat  on  the  ground  from  some  accidental  cause,  only 
a  part  of  the  stem  rises  up  again  after  a  time  by  a  right-angled  bend  at  one 
of  the  nodes,  the  part  next  the  free  apex  rising  up,  while  the  part  nearest  the 
attachment  remains  prostrate  on  the  ground.  Contact  with  the  soil  acts  as  a 
stimulus  to  the  formation  of  roots  on  this  latter  portion,  and  they  are  produced 
abundantly  near  the  node  from  the  knee-shaped  bent  portion,  and  penetrate 
into  the  ground,  functioning  as  absorbent  and  fixing  organs.  These  shrubby 
plants  would  not  have  developed  any  roots  on  their  stem-nodes  had  they  not  met 
with  the  accident  and  so  been  stretched  on  the  ground. 

Cut  branches  of  willow  placed  in  water,  wet  sand,  moistened  soil,  or  moss, 
develop  roots  in  about  a  week  at  the  place  where  they  are  in  contact  with  the 
water  or  damp  objects  mentioned;  roots  which  are  equally  useful  either  as  absorbent 
or  fixing  organs.  If  the  branches  had  not  been  cut  off  or  treated  in  this  way,  no 
roots  would  have  been  formed  on  them.  These  willow  branches  may  be  taken 
as  a  type  of  the  shoots  of  a  great  number  of  plants  which  all  readily  develop  roots 
from  the  stem  when  placed  in  damp  surroundings.  The  propagation  of  plants 
by  cuttings,  so  often  performed  by  gardeners,  depends  upon  the  fact  that  when 
branches  are  cut  off  from  a  plant  and  placed  in  damp  sand  they  "strike  root" 
in  it,  i.e.  they  send  out  roots  from  the  part  of  the  stem  situated  in  the  sandy  soil. 
Contact  with  damp  earth  operates  as  an  incitement  to  the  formation  of  roots  in  the 
aerial,  cord-like  roots  of  the  aroids  figured  on  p.  365,  just  as  in  these  cuttings.  The 
aerial  roots  descending  from  the  stems  of  these  aroids  do  not  develop  absorbent, 
lateral  roots  until  they  reach  the  soil;  but  they  have  scarcely  come  into  contact 
with  it  when  numbers  of  lateral  roots  arise  which  penetrate  into  the  ground  where 
they  can  suck  up  fluid  nourishment.  In  the  root-forming  leaves  of  species  of 
pepper,  of  begonias,  and  of  the  cuckoo  flower,  contact  with  damp  soil  stimulates 
the  production  of  roots — in  places,  too,  where  no  roots  would  have  been  formed 
without  this  contact.  If  a  pepper  or  begonia  leaf  is  cut  in  pieces,  and  each  piece 
laid  on  damp  sand  and  so  pressed  down  that  the  veins  projecting  from  the  lower 
side  are  embedded  in  the  sand,  roots  will  grow  out  of  the  parenchyma  adjoining  the 
veins,  and  turn  downwards,  while  above  they  develop  a  tissue-body  which  turns 
upwards  and  becomes  a  leafy  shoot,  being  provided  with  food  by  the  roots.  Long 
roots  arise  from  the  cellular  tissue  at  the  base  of  the  stalk  of  rank  ivy-leaves 
placed  in  wet  sand  or  in  water,  which  is  never  known  to  happen  when  the  ivy- 
leaves  are  growing  freely  in  the  air.  We  must  not  omit  to  mention  here  the  roots 
of  parasitic  plants  which  attach  themselves  to  the  living  tissue  of  other  plants  as 
so-called  haustoria;  these  only  arise  in  the  parts  of  the  parasite  which  come  directly 
into  contact  with  the  succulent  roots  of  the  living  host-plants. 

The  benefit  which  plants  derive  from  the  formation  of  these  roots  is  easily 
perceived.  In  the  stems  of  the  prostrated  shrubs  the  conduction  of  fluid  food  from 
the  ground  is,  no  doubt,  restricted  and  imperilled,  and  therefore  it  is  important  that 
the  part  of  the  shoot  again  rising  from  the  ground  should  be  provided  with  special 


REMARKABLE   PROPERTIES   OF   ROOTS.  773 

roots  at  the  node  where  the  knee-shaped  bending  takes  place,  in  order  to  conduct 
the  absorbed  nourishment  directly  to  the  foliage-leaves  on  the  upper  part  of  the 
shoot.  The  actual  existence  of  the  part  of  the  plant  in  question  depends  upon  the 
formation  of  such  roots  in  the  other  cases  enumerated  above.  The  cut  branches 
of  willows,  the  cut-up  foliage  of  begonias,  the  ivy-leaves  torn  from  their  stem, 
&c.,  would  all  die  if  they  did  not  provide  themselves  with  roots.  But  although 
it  is  easy  enough  to  perceive  the  benefit  ensuing  from  this  kind  of  root-formation, 
it  is  very  difficult  to  explain  how  the  mechanical  impulse  brings  about  this  new 
production.  It  has  been  shown  in  all  these  instances  cited  that  contact  with 
a  foreign  body  is  an  important  factor,  but  it  is  very  puzzling  to  understand  how 
the  deeper  cell-layers  are  stimulated  to  develop  roots  by  the  contact  of  the 
epidermis  with  damp  earth,  water,  and  the  like,  and  we  must  content  ourselves 
with  saying  that  the  contact  acts  as  a  stimulus,  which,  when  transmitted  to  the 
deeper  layers  of  cells,  stirs  them  up  to  construct  roots  as  a  deliverance  from  death. 
The  explanation  is  still  more  difficult  in  cases  where  the  cut  parts  of  the  plant 
develop  roots  to  preserve  their  life,  even  without  contact  with  a  foreign  body. 
Such  a  case  has  been  considered  earlier  (on  p.  89),  when  it  was  shown  that  on  cut 
shoots  of  various  species  of  stonecrop  (e.g.  Sedum  reflexum,  Boloniense,  elegana), 
which  are  hung  in  the  air  by  a  thread,  roots  will  develop  from  the  internodes 
between  the  foliage-leaves  in  places  where  no  roots  would  normally  have  arisen. 
They  grow  down  and  elongate  until  their  tips  come  in  contact  with  some  solid 
body.  Here  no  stimulus  could  have  acted  on  the  epidermis;  the  pendent  shoots 
stand  in  no  relation  to  the  surrounding  air  other  than  obtained  whilst  they  were 
still  united  to  the  rooted  plant,  i.e.  before  they  were  cut  off.  The  stimulus  to  root- 
formation  must,  therefore,  be  referred  to  the  separation  of  the  shoot  from  the  plant. 
We  must  not,  however,  imagine  the  action  to  be  merely  mechanical,  but  must  be 
concent  with  stating  that  the  living  shoot  hanging  in  the  air  can  only  save  itself 
from  death  by  developing  these  roots. 

To  the  most  remarkable  vital  phenomena  of  plants  belong  also  the  various 
bendings,  curvatures,  and  other  movements  performed  by  growing  roots.  Ap- 
parently every  root  tries  to  reach  a  definite  goal,  towards  which  it  directs 
its  way,  endeavouring  to  obtain  the  advantages  offered  by  it  with  as  little  expendi- 
ture as'  possible.  The  goal  which  growing  roots  strive  after  is  the  same  for  all, 
viz.  the  place  in  the  nourishing  substratum  best  adapted  to  them.  The  primary 
roots  of  plants  settled  on  the  bark  of  trees  as  epiphytes  or  parasites  direct  their 
tips  towards  the  axis  of  the  branch  of  the  tree  in  question,  land  plants  on  the 
other  hand,  towards  the  centre  of  the  earth,  and  the  primary  roots  proceeding 
from  seeds  lying  at  the  bottom  of  still  water  sometimes  direct  themselves  upward* 
and  grow  at  the  commencement  of  their  development  towards  the  surface 
the  water.  The  road  to  be  traversed  by  the  succeeding  roots,  from  whatevei 
part  of  the  plant  they  may  have  sprung,  is  apparently  not  so  clearly  determi 
but  on  a  closer  examination  it  is  found  that  they  too  strive  to  attain  to  places 
where  fluid  nourishment  abounds,  and  where  they  can  obtain  a  firm  hold. 


774  REMARKABLE   PROPERTIES   OF   ROOTS. 

soil  is  made  up  of  alternating  places  containing  a  larger  and  a  smaller  amount 
of  food-salts,  and  places  which  either  retain  water  badly  or  well.  In  one  place 
are  situated  nests  of  humus,  in  another  sharp-edged  stones,  and  it  is  only  natural 
that  these  inequalities  and  obstacles  in  the  path  pursued  by  the  roots  should  not 
be  without  effect  on  them.  As  a  matter  of  fact  manifold  contrivances  are  met  with 
for  preventing  the  roots  from,  so  to  speak,  blindly  passing  by  favourable  places  in 
the  soil  without  making  proper  use  of  them.  The  fact  that  the  tips  of  many  roots 
describe  oscillations  or  nutations,  not  unlike  those  which  are  observed  in  twining 
stems  and  in  certain  creepers,  is  an  instance  of  such  adaptation.  Roots  growing 
in  soil  are  of  course  much  more  restricted  in  their  movements  by  the  pressure 
of  their  environment  than  are  the  structures  which  circle  round  in  the  air,  but 
in  the  main  the  principle  is  the  same  in  both  cases.  The  path  travelled  by  the 
point  of  the  growing  root  is  most  accurately  depicted  by  a  spiral  line,  and  the  most 
important  advantage  obtained  by  following  such  a  path  lies  in  the  contact  of 
the  growing  root  with  as  large  a  portion  of  the  soil  as  possible.  A  root  growing 
in  a  straight  line  would  not  touch  half  as  many  points  as  that  following  a  spiral, 
and  since  the  likelihood  that  all  the  favourable  spots  will  not  be  left  on  one 
side  increases  with  the  number  of  points  of  contact,  the  spiral  movement  of 
the  roots  may  without  hesitation  be  regarded  as  a  contrivance  for  discovering 
the  best  sources  of  food  in  the  soil.  We  must,  of  course,  not  undervalue  various 
other  advantages  which  are  also  obtained  in  this  way,  in  particular,  the  greater 
ease  with  which  roots  following  a  spiral  line  can  bore  their  way  into  the  soil,  and 
the  better  hold  they  obtain. 

Although  the  root  follows  a  spiral  line  in  its  growth,  it  may  nevertheless 
maintain  a  straight  direction  on  the  whole;  this  is  actually  the  case  in  water 
and  in  a  homogeneous  and  uniformly  moistened  soil.  In  soil  differently  constituted 
and  unequally  moistened,  however,  a  diversion  takes  place  away  from  the  side 
where  the  conditions  are  unfavourable  to  the  root.  This  swerving  may  be  caused 
by  cold,  dryness,  by  chemical  conditions  of  the  soil,  and  by  pressure  and  injuries. 

It  is  well  known  that  in  the  far  north  the  ground  remains  always  frozen  below 
a  slight  depth.  During  the  short  summer  only  the  superficial  strata  are  thawed, 
but  below  this  the  "perpetual  ice"  stretches  to  an  immeasurable  extent.  A 
relatively  abundant  vegetation  develops  on  the  thawed  strata  under  the  warm  rays 
of  the  sun,  and  in  North  America  not  only  shrubs  and  low  bushes  but  also  colonies 
of  huge  fir-trees  grow  up.  The  roots  of  these  plants  penetrate  straight  down- 
wards and  grow  towards  the  centre  of  the  earth;  but  as  soon  as  they  come 
into  the  neighbourhood  of  the  ice  they  bend  aside,  curve  round,  and  continue  theii 
path  only  in  the  thawed  stratum.  The  diversion  is  usually  so  striking  that 
the  diverted  portion  is  sometimes  actually  at  right  angles  to  the  older  part  which 
grew  vertically  downwards. 

The  same  thing  happens  when  the  soil  is  moist  in  some  parts  and  dry  in  others. 
Here  again  the  growing  roots  seem  to  be  repelled  by  the  dry,  inhospitable  layers  of 
soil,  and  turn  towards  the  neighbouring  moister  region.  This  phenomenon  has  been 


REMARKABLE   PROPERTIES   OF   ROOTS.  775 

termed  hydrotropism.  It  frequently  happens  in  mountainous  districts  that  ai'i.-r 
heavy  downpours  of  rain  the  overflowing  streams  tear  deep  furrows  in  the  adjoin- 
ing steep  forest  lands,  and  root  up  the  ground,  throwing  everything  into  confusion 
and  depositing  below  on  the  valley-floor  a  mass  of  detritus  or  rubbish.  Usually 
numerous  organic  bodies,  blocks  of  wood,  pieces  of  turf,  leaves,  fir-cones,  and  the 
like  are  torn  away  by  these  turbulent  streams  with  the  stones  and  sand,  and  the 
deposit  is  therefore  studded  with  nests  and  strips  of  humus  which  owe  their  origin 
to  the  organic  fragments  mentioned.  Seeds  of  various  plants  from  the  neighbour- 
ing forest  are  swept  into  the  rubbish  heap,  and  among  them  those  which  only 
flourish  well  in  the  damp  humus  of  forest  soil.  These  seeds  germinate,  and 
their  roots  penetrate  downwards;  many  perish  at  once  in  the  inhospitable  soil, 
but  others  grow  excellently,  sending  up  a  vigorous  stem  and  unfolding  foliage 
and  flowers.  When  these  well-grown  plants  are  dug  up  in  order  to  see  the 
relation  of  their  roots  to  their  immediate  environment,  it  at  once  becomes  evident 
that  the  roots  in  their  downward  progress  have  curved  towards  the  nests  and 
veins  of  humus.  They  exhibit  the  most  wonderful  twists  and  bends,  and  look 
as  if  they  had  actually  been  attracted  by  the  humus  deposits.  Without  quite 
excluding  the  possibility  of  a  chemical  attraction,  we  must  regard  the  aversion 
of  the  roots  to  dryness  as  the  chief  cause  of  the  bending.  The  masses  of  humus 
embedded  in  sand  and  rubbish  retain  moisture  like  a  sponge,  and  when  the 
adjoining  sand -strata  have  been  for  long  dried  up  the  dark  nests  and  strips  still 
retain  their  saturated  condition.  When  a  root  shunning  the  dryness  turns  away 
from  the  sand,  and  in  continuing  its  growth  comes  to  a  deposit  of  humus  rich 
in  water,  it  finds  there  no  inducement  to  continue  bending,  and  so  grows  straight 
through  the  region  of  the  damp  layer.  When  in  its  further  growth  it  emerges 
from  the  ball  of  humus  and  enters  the  dry  sand,  it  of  course  again  bends  and 
curves  round  the  ball  of  humus,  or  wheels  round  in  a  half  circle  so  as  to  return  to 
the  moist  dark  clump  which  is  situated  like  an  oasis  in  the  dry  desert  of  sand. 

It  is  obvious  that  larger  pebbles  which  cannot  be  displaced  by  growing  roots 
must  cause  a  swerving;  the  root  whose  tip  is  in  contact  with  the  hard  stone 
bends  sideways  and  evades  the  insurmountable  obstacle  lying  in  its  path.  A  very 
noticeable  bend  ensues  when  the  growing  root  is  injured  on  one  side  of  its  tip, 
or  is  so  fastened  to  some  foreign  object  that  the  cells  at  the  place  of  contact 
are  damaged.  It  then  bends  away  from  the  injured  or  attached  side  and  assumes 
a  divergent  course. 

In  many  cases  it  might  be  thought  that  the  roots  were  not  repelled  by  the 
unfavourable  places  in  the  soil,  but  were  attracted  by  the  favourable  places,  and, 
as  already  stated,  the  possibility  of  an  attraction,  a  mutual  action  of  the  sap  of 
the  root  and  the  materials  contained  in  the  places  in  question  in  the  soil,  which 
might  find  expression  in  a  movement  of  the  growing  root-end,  is  not  entirely 
excluded,  although  it  has  not  hitherto  been  demonstrated  with  certainty. 

The  circling,  that  is,  the  spiral  movement  of  the  growing  root,  has  been  explained 
in  various  ways.  One  view  was  that  the  cylindrical  body  of  the  root  may  be 


776  REMARKABLE   PROPERTIES   OF   ROOTS. 

divided  up  into  longitudinal  strips,  all  of  which  were  supposed  not  to  grow  at 
the  same  time  or  to  an  equal  extent,  but  rather  that  the  wave  of  stronger  growth 
continually  passes  from  one  strip  to  the  next  one.  This  movement,  however,  like 
that  of  twining  stems  is  probably  an  alternate  bending  towards  the  different  radii 
of  a  circle  drawn  round  the  root,  and  since  it  is.  combined  with  an  elongation  of  the 
part  of  the  root  in  question,  the  growing  root-end  describes  a  spiral  line. 

The  bend  caused  by  the  diversion  of  the  root  is  either  produced  by  a  one-sided 
contraction,  or  by  a  one-sided  elongation.  Since  the  bend  occurs  in  the  growing 
portion  of  the  root,  a  more  vigorous  growth  on  one  side  may  be  regarded  as 
the  immediate  cause  of  the  bend,  and  every  impetus  which  would  promote  such 
unilateral  growth  would  also  cause  a  bending.  The  bending  of  roots  which 
shun  dry  places  may  perhaps  be  referred  to  a  withdrawal  of  water  from  one 
side  of  the  root-tip.  Thus  if  the  root  lies  imbedded  between  a  damp  and  a  dry 
layer,  that  side  which  abuts  on  the  dry  stratum  will  transpire  more  actively  than 
the  other,  and  it  has  been  suggested  that  this  active  transpiration  in  some  way 
promotes  an  increased  growth  in  length  in  that  half,  and  in  consequence  of  this 
unilateral  elongation  on  one  side,  the  other  half,  adjoining  the  damp  layer,  will 
become  concave. 

The  idea  that  the  curvature  is  not  produced  directly  in  the  place  where  the 
external  stimulus  operates,  but  in  the  growing  region  lying  behind  the  stimulated 
root-tip,  is  much  more  interesting  than  these  purely  mechanical  explanations. 
According  to  this  view  the  stimulus  is  transmitted  as  in  the  leaves  of  the  Sundew, 
Fly-trap,  Aldrovanda,  sensitive  plants,  and  many  other  cases.  The  active  stimuli 
may  be  afforded  by  pressure,  cold,  dryness,  and  probably  chemical  conditions  also. 
Gravity,  too,  may  be  looked  upon  as  a  stimulus,  indeed  as  one  which  influences 
the  direction  of  growth.  It  is  believed  that  gravity  acts  on  the  root-tip  as  a 
stimulus  to  growth  and  that  this  stimulus  is  conveyed  to  the  growing  region 
behind,  and  that  in  consequence  the  primary  roots  grow  down  towards  the  centre 
of  the  earth.  But  as  primary  roots  are  able  to  penetrate  into  mercury,  and  to  bore 
through  paper,  in  their  downward  growth,  something  more  than  mere  weight  oper- 
ates, since  this  would  not  be  the  case  if  the  roots  were  influenced  by  gravity  alone. 

The  part  of  the  growing  root  most  sensitive  to  stimuli  is — so  far  as  experi- 
mental evidence  points — the  tip,  and  the  phenomena  which  are  exhibited  in 
consequence  of  its  great  sensitiveness  are  so  astounding  that  Darwin  compared  the 
root- tip  to  the  brain  of  lower  animals.  He  writes,  "  it  is  hardly  an  exaggeration  to 
say  that  the  tip  of  the  radicle  thus  endowed,  and  having  the  power  of  directing 
the  movements  of  the  adjoining  parts,  acts  like  the  brain  of  one  of  the  lower 
animals;  the  brain  being  seated  within  the  anterior  end  of  the  body,  receiving 
impressions  from  the  sense-organs,  and  directing  the  several  movements  ". 

Remarkable  and  interesting  as  are  these  vital  phenomena  observed  in  roots, 
there  is  still  much  to  be  wished  for  in  the  matter  of  their  explanation  and  clear 
comprehension.  Here,  as  in  so  many  similar  cases,  a  phrase,  a  technical  term, 
a  word,  is  introduced  to  designate  the  process  observed,  and  not  infrequently 


REMARKABLE   PROPERTIES   OF   ROOTS.  777 

those  who  use  it  ultimately  come  to  think  they  have  given  an  explanation  of  the 
process,  while  really  they  have  only  stated  it.  This  is  especially  the  case  with  the 
term  "stimulus".  What  is  a  stimulus?  From  the  present  state  of  our  knowledge 
we  cannot  yet  give  a  concise  answer  to  this  question,  consequently  explanations  in 
which  this  word  is  inserted,  are,  as  explanations,  incomplete. 

In  these  remarks  there  is  no  desire  to  depreciate  the  results  obtained  by  the 
combined  efforts  of  so  many  indefatigable  investigators  of  past  and  modern  times; 
on  the  contrary,  we  may  regard  the  wealth  of  careful  observations  and  sagacious 
inferences  which  form  the  present  platform  of  our  knowledge,  and  which  have 
been  generally  reviewed  in  the  preceding  pages,  with  just  pride  and  satisfaction. 
But  this  pride  must  not  blind  us  to  the  recognition  of  the  fact  that  most  questions 
concerning  the  life  of  plants  are  as  yet  only  at  the  commencement  of  their  solution. 
Much  has  been  done,  but  much  is  still  reserved  for  the  future. 

"  Manchen  Flug  wagt  menschliches  Wissen  das  doch 

Kaum  ein  blatt  aufschlagt  in  dem  Buch  des  Weltalls." 
VOL.  i.  M 


END   OF  VOL.   1. 


RETURN  TO  the  circulation  desk  of  any 
University  of  California  Library 
or  to  the 

NORTHERN  REGIONAL  LIBRARY  FACILITY 
Bldg.  400,  Richmond  Field  Station 
University  of  California 
Richmond,  CA  94804-4698 

ALL  BOOKS  MAY  BE  RECALLED  AFTER  7  DAYS 
2-month  loans  may  be  renewed  by  calling 

(415)642-6233 
1-year  loans  may  be  recharged  by  bringing  books 

to  NRLF 
Renewals  and  recharges  may  be  made  4  days 

prior  to  due  date 


DUE  AS  STAMPED  BELOW 


JUN171988 


DEC/. 


APR  0  8  1991 


AU6 


96 

Kerner,  A. 

The  natural 
history  of  plants. 


QKU5 

Kii 

v.l 


LIBRARY 

UNIVERSITY   OF   CALIFORNIA 
DAVIS 


1175010859737 


